양자 기술

• Title: Efficient Measurement Schemes for Practical Quantum Computing

• Speaker: Dr. Daniel McNulty (University of Bari, Italy)

• Date & Time / Venue: July 29th (Tue.) 2:50 PM / #512, APCTP & Online via ZOOM

• ZOOM Link: https://us06web.zoom.us/meeting/register/98YoBRY7QiWL3nVE_Xt4ng

• Abstract
  : Efficient extraction of information from quantum computers is a critical bottleneck for realizing practical quantum advantages in areas such as computational chemistry, combinatorial optimization, and quantum simulation. A key challenge, especially for today’s small-scale quantum devices, is the inability to simultaneously measure many physically relevant observables—a fundamental limitation that hinders the performance of variational quantum algorithms, currently among the most promising approaches for near-term quantum speedups. In this talk, I will present my current research on developing efficient and noise-resilient measurement schemes aimed at improving the scalability and reliability of quantum algorithms on existing hardware. I will then outline future plans to develop a theoretical framework for characterising measurement resources in noisy quantum systems, with the goal of enabling quantum speedups in practical applications.

• Title: Spin Transport and Dynamics in Ferrimagnetic Materials

• Speaker: Dr. Duck-Ho Kim (KIST)

• Date & Time / Venue: July 3rd (Thu.) 4:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : The dynamic phenomena of topological spin objects in magnetic materials have been actively studied not only for their academic interest but also for their potential applications in next-generation memory devices [1-4]. Representative topological spin objects include chiral magnetic domain walls [1, 2] and magnetic skyrmions [3, 4]. It is possible to manipulate topological spin objects using electric current, and the underlying mechanism involves the conservation of angular momentum arising from the interaction between the electron's spin and the spin of the phase spin structure as the electron traverses the magnetic material [5, 6]. This interaction is known as spin torque and can be divided into spin-transfer torque and spin-orbit torque depending on the transmitting materials. Recently, ultrafast motion of topological spin objects has been predicted in antiferromagnetic materials (total magnetization = 0) through theoretical studies [7]. Unfortunately, antiferromagnetic materials are difficult to measure (total magnetization = 0) and challenging to control (due to strong coupling energy), making it difficult to experimentally verify the theory. However, in the case of ferrimagnetic materials, which have a finite magnetization at the angular momentum compensation point where the angular momentum is zero, it has been possible to investigate the spin dynamics of antiferromagnetic materials. With this possibility, ultrafast motion has been observed at the angular momentum compensation point when topological spin objects are driven by a magnetic field [8]. In this presentation, we will discuss the spin transfer torque phenomenon [9] and the phenomenon of skyrmion hall effect [10] at the angular momentum compensation point when topological spin structures is driven by electric current in ferrimagnetic materials.
  
  References
  [1] S. S. P. Parkin, M. Hayashi and L. Thomas, Science 320, 190 (2008).
  [2] S. P. Parkin, Sci. Am. 300, 76 (2009).
  [3] A. Fert, V. Cros and J. Sampaio, Nat. Nanotechnol. 8, 152 (2013).
  [4] J. Sampaio, V. Cros, S. Rohart, A. Thiaville and A. Fert, Nat. Nanotechnol. 8, 839 (2013).
  [5] G. Tatara and H. Kohno, Phys. Rev. Lett. 92, 086601 (2004).
  [6] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004).
  [7] T. Shiino et al. Phys. Rev. Lett. 117, 087203 (2016)
  [8] K.-J. Kim et al., Nat. Mater. 16, 1187 (2017).
  [9] T. Okuno et al., Nat. Electron. 2, 389 (2019).
  [10] Y. Hirata et al., Nat. Nanotechnol. 14, 232 (2019).

• Title: Uncertainty principle of mind waves and its applications to visual cognitive process

• Speaker: Prof. Hyun Myung Jang (POSTECH)

• Date & Time / Venue: May 14th (Wed.) 5:00 PM / Science Bldg I, #202

• Abstract
  : "Stream of consciousness," a narrative technique often used in the novels of James Joyce and Virginia Woolf, refers to the continuous occurrence and disappearance of modulated waves or wave packets that make up the "mind waves." In this lecture, I will mathematically address the temporal change of these modulated wave packets, ψ(x, t), using Fourier transformation of the amplitude in the momentum-space, φ(k). I will subsequently demonstrate that while these modulated waves continue to exist, there exists an uncertainty relationship between the position uncertainty (Δx) and the wave-number uncertainty (Δk), as stated in Heisenberg's uncertainty principle, i.e., ℏΔk·Δx ≈ ℏ. I will also show that a similar form of uncertainty exists between the uncertainty in the modulation frequency and the uncertainty in time, i.e., ℏΔω·Δt ≈ ℏ.
  In the latter part of the lecture, I will apply the uncertainty principle of modulated waves to the visual cognitive mind (眼識), responsible for visual perception, and obtain the following interesting results:
  (i) Our visual cognitive mind sees the incoming light frequency by an incredibly small factor of about 3/100 trillion (3 × 10⁻¹⁴).
  (ii) The incoming wavelength of light, λ₀ of ~500 nm, is perceived by the mind as roughly 1200 times larger than it actually is.
  (iii) Out of 20 billion incoming photons, only one is used for light detection, and the rest are discarded (i.e., We perceive the world by sampling only a very limited amount of information). If time allows, I will present an answer to how the visual cognitive mind (眼識) perceives an image in a plane direction orthogonal to the incoming light at a given pixel (rod cell). To do this, I will derive the susceptibility χ(k) in k-space, which has a Lorentzian structure and is mathematically the same as the spectrum I(ω) obtained using the Wiener-Khintchine theorem in ω-space.

• Title: Open Quantum Systems and Quantum Master Equations

• Speaker: Prof. Yeo Joonhyun (Konkuk Univ.)

• Date & Time / Venue: May 14th (Wed.) 4:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : A real physical system is open in the sense that it inevitably interacts with an uncontrollable environment. For classical systems, there are well established theoretical methods such as Langevin and Fokker-Planck equations to deal with the dynamics and thermodynamics of such open systems. For quantum mechanical systems, the situation is more complicated as the interaction with the environment is responsible for decoherence as well as dissipation. In this talk, we review theoretical approaches to study the dynamics and thermodynamics of open quantum systems using quantum master equations (QMEs). We introduce various QMEs such as the Lindblad equation and the Redfield equation with the approximations involved in their derivations and the shortcomings and strengths of these equations. We discuss the concept of complete positivity and the thermalization of an open quantum system with regard to QMEs.
  
  Biography:
  1988 B.S. Seoul National University
  1994 Ph. D. University of Chicago
  1994-1997 Postdoctoral Research Associate, University of Manchester
  1997-present Professor, Konkuk University

• Title: THz Nonlinear Photonics - from efficient THz wave generation to applications

• Speaker: Prof. Fabian Rotermund (KAIST)

• Date & Time / Venue: May 7th (Wed.), 2025, 4:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Electromagnetic radiation in the THz frequency range, located between the infrared and microwave regions of the electromagnetic spectrum, serves as a promising spectroscopic source. This range provides access to a variety of fundamental modes including the motion of free electrons, molecular rotations, crystal lattice vibrations, transitions in semiconductors, and spin precessions. Such characteristics make THz radiation highly valuable for probing the physical properties of diverse materials at a fundamental level. Over the years, significant advancements have been achieved in the efficient generation of broadband THz waves for a wide range of applications. This presentation will explore the recent progress in the development of highly nonlinear organic crystals applicable for efficient broadband THz wave generation. These organic crystals exhibit remarkable nonlinear optical properties, enabling the generation of high-intensity THz waves with broad spectral coverage.[1-3] Furthermore, the presentation will highlight the implementation of strong-field THz wave generation systems that utilize inorganic nonlinear crystals. These systems demonstrate superior performance in producing intense, coherent ultrashort THz waves, which are crucial for various advanced spectroscopic investigations. In addition to the discussion on efficient THz wave generation, several practical applications of broadband THz waves will be introduced. These include their use in nonlinear spectroscopy, where the interaction of intense optical or THz field with matter can reveal nonlinear optical phenomena, and time-resolved spectroscopy, which provides insights into useful characteristics and ultrafast dynamics in materials.[4-6]
  
  References:
  [1] B. J. Kang et al., Adv. Funct. Mater. 28, 1707195 (2018).
  [2] S.-J. Kim et al., Adv. Opt. Mater. 9, 2101019 (2021).
  [3] S.-J. Kim et al., Adv. Funct. Mater. 33, 2209915 (2023).
  [4] J.-H. Park et al., Nat. Commun. 13, 5530 (2022).
  [5] T. G. Park et al., ACS Nano 18, 30966 (2024).
  [6] J. Park et al., Adv. Energy Mater. 15, 2400225 (2025).

• Title: Terahertz Spectroscopy and Applications to Study of Quantum Materials

• Speaker: Prof. Jae Hoon Kim (Yonsei Univ.)

• Date & Time / Venue: February 26th (Wed.) 4:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Quantum materials lying at the frontier of condensed matter research call for access to increasingly low frequencies in the electromagnetic spectrum. In particular, the terahertz region is a rich arena for probing exotic quasiparticles associated with novel quantum phenomena. Ultrafast femtosecond lasers developed in the late 1990s brought an unexpected opportunity to implement terahertz time-domain spectroscopy (THz-TDS). Here, we introduce the standard method of THz-TDS and illustrate this technique by presenting several case studies in which the distinct advantages of THz-TDS were utilized to obtain key information on representative quantum materials such as topological insulators, superconductors, spin liquids, and low-dimensional van der Waals magnets.

• Title:Spintronics and iontronics for future data-centric technologies

• Speaker: Prof. Stuart S.P. Parkin (Max Planck Institute)

• Date & Time: February 10th (Mon.), 2025, 4:30 PM/ POSCO International Center Auditorium (1F)

• Abstract
  : Data-centric technologies are the lifeblood of today's digital world. In this talk I will discuss some of our recent work exploring thin film materials with novel properties that could be used to build advanced memory-storage and computing devices. In particular, I will focus on spintronic materials and phenomena that allow for high performance, non-volatile magnetic memories in the form of magnetic tunnel junctions and magnetic racetrack memory. The latter involves the current induced motion of nanoscopic domain walls along 2D magnetic conduits. Whilst most work to date on racetrack memory involves 2D racetracks, we have recently demonstrated the first 3D forms of racetrack memory using magnetic membranes formed using, on the one hand, water soluble sacrificial layers, and, on the other hand, a super-resolution 3D printer that we have built to create 3D scaffolds on which the racetracks are deposited. Typically, racetracks and magnetic tunnel junctions are formed from atomically engineered thin film magnetic heterostructures, but 2D van der Waals materials are highly interesting due to their near atomically perfect structures. I discuss some of our recent work on magnetic tunnel junctions formed from twisted CrSBr bilayers that have zero net magnetic moment and which display two or more non-volatile magnetic states in zero magnetic field. I also discuss the possibility of cryogenic racetrack memories that take advantage of a Josephson Diode effect and triplet supercurrents. Finally I will briefly discuss the control of properties via ionic gating that can lead to novel metasurfaces with exotic magnetic and optical properties.

• Title:Enhanced radiative cooling for deep sub-ambient temperature applications

• Speaker: Dr. Jaesuk Hwang (Centre for Quantum Technologies, National University of Singapore)

• Date & Time: January 22th (Wed.), 2025, 2:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : The sky is not only a source of heat and light, but also an abundant source of coldness. Radiative cooling, taking advantage of net thermal radiation towards the cold sky, can deliver a cooling power of around 100 W/m^2, comparable to that of a household refrigerator. Radiative cooling devices do not require any external energy source other than an open view to the sky and remove heat irreversibly from the earthly environment. Therefore, radiative cooling bears a potential to be a sustainable and renewable alternative to replace the conventional compression-based cooling systems, such as the refrigerators and air conditioners. However, current radiative cooling devices are far from reaching the theoretically predicted performance [1] and limited to the operation near the ambient temperature. To maintain the cooling power at a deep sub-ambient temperature, it is pointed out that a high degree of thermal insulation of the radiative cooling surfaces is crucial, such as obtained by a high-vacuum enclosure [1, 2]. As an exemplar case study, the first demonstration of sub-ambient radiative cooling in the equatorial tropics will be presented, where the humid air and the cloudy sky render radiative cooling the most challenging [2]. The future routes for the practical deployment of the enhanced radiative cooling devices, such as the use of aerogels, will be discussed [3].
  
  

[1] J. Hwang, “Climate-dependent enhancement of radiative cooling with mirror structures,” Journal of Photonics for Energy 14 028001 (2024).
[2] J. Hwang, “Daytime radiative cooling under extreme weather conditions,” Advanced Energy and Sustainability Research, 5 2300239 (2024).
[3] X.Y. Goh, J. Hwang, L.T. Nguyen, R.H. Ong, T. Bai, H.M. Duong, “Sub-ambient radiative cooling with thermally insulating polyethylene terephthalate aerogels recycled from plastic waste,” Solar Energy, 274 112544 (2024).

• Title:Renewable electricity harvesting from sky, day and night

• Speaker: Dr. Jaesuk Hwang (Centre for Quantum Technologies, National University of Singapore)

• Date & Time: January 21th (Wed.), 2025, 10:00 AM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : The sun is a vast thermodynamic resource at 6000K temperature. Considering sustainability and economic viability, solar photovoltaics is considered the main route in the transition to renewable energy. As the dependence on the solar energy increases, new types of sustainability challenges will arise. To this end, the complementary thermodynamic resource, outer space at 3K, can be exploited. First, since the solar energy is not available at night, the demand for the batteries will inevitably increase. To mitigate this issue, electricity can be harvested at night, based on the temperature gradient between the cold night sky and the ambient at the terrestrial level. Second, solar energy harvesting relies on the efficient absorption of the solar irradiation and a large fraction of the absorbed solar irradiation is released into the environment as heat, due to the limited conversion efficiency. To address this issue, the excess heat can be converted into thermal radiation towards outer space. The experimental test of the viability of these concepts is presented. Using a thermoelectric generator (TEG) as an example of the heat engine, it is demonstrated that electricity can be continuously generated throughout the day by a single device [1]. During the day, all the rejected heat from a solar thermoelectric generation is sent to outer space via radiation, to completely suppress the heat dissipation to the surroundings [1]. The routes for further optimisation for the nighttime electricity generation using the thermoelectric generator will be discussed [2].
  
  

[1] J. Hwang, “Harvesting solar energy without excess environmental heating,” Cell Reports Physical Science, 6, 102345 (2025).
[2] J. Hwang, D. Narducci, “Electrical and thermal impedance matching for thermoelectric generation via radiative cooling,” under review.

• Title: Aspects of quantum complexity in gravity and field theory

• Speaker: Dr. Arpita Mitra (POSTECH)

• Date & Time: January 20th (Mon.), 2025, 12:30 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : In recent years, there has been significant interest in applications of quantum information techniques in understanding quantum gravity. This was spurred by Susskind’s conjecture linking the notion of quantum complexity to black hole interior growth. I will discuss how a metric on a quantum state manifold can be identified with two dimensional Euclidean AdS, dS and flat geometries. We further associate a Jackiw-Teitelboim gravity model with these state manifolds and show that the dilaton can be interpreted as the quantum mechanical expectation values of the symmetry generators. On the other hand, we establish the equivalence between the classical solution of dilaton with the spread complexity of a target state. I will also discuss how some universal results of complexity change under perturbations of Hamiltonian.

• Title:Coherent control of quantum states in semiconductor quantum dot systems

• Speaker: Dr. Wonjin Jang (École Polytechnique Fédérale de Lausanne (EPFL), Switzerland)

• Date & Time: January 16th (Thu.), 2025, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : Gate-defined semiconductor quantum dots (QDs) offer intriguing avenues for exploring various spin and charge degrees of freedom of electrons (or holes) confined in the QDs, which can be harnessed for quantum computations and simulations. This talk aims to present the control and engineering of the quantum states in 1) semiconductor QDs, and 2) semiconductor QD-superconducting cavity hybrid architectures.
  The first part of the talk will focus on spin state manipulation in semiconductor QDs. Starting with a brief introduction to single-electron spin qubits in QDs, coherent control of the spin doublet states formed by three electrons will be mainly discussed. I will present a high-fidelity single-shot state detection scheme for the three-electron spin states, which facilitates coherent qubit operations based on exchange interactions. Moreover, I will show that the hyperfine interaction between the spin doublet and the environmental nuclear spins enables dynamic nuclear polarization (DNP), which allows bidirectional control over the nuclear Overhauser field. By tuning the local magnetic field experienced by the QDs via the DNP, a true decoherence-free-subspace (DFS) for spin qubit operations can be derived from the spin doublet structure. The true DFS is decoupled from both the long- and short-wavelength magnetic field noise, allowing highly coherent spin qubit operations.
  In the second part of the talk, the semiconductor-superconductor hybrid architecture will be discussed. Analogous to an atom in a cavity, a semiconductor QD embedded in a superconducting cavity can interact with the microwave photons in the cavity. This allows various applications such as microwave photon detection, quantum simulations, and long-range interactions between remote QD spins. Here, I will mainly present strong hole charge-photon interaction enabled by high-impedance Josephson junction array cavity. In particular, holes in QDs have emerged as a front-runner for QD-based quantum computation, mainly owing to their sizable spin-orbit interaction, and scalability. Exploiting the tunability of our resonator, the signature of spin doublet structure in the cavity will also be shown. These findings pave the way toward coherent hole spin-photon interface which is a prerequisite for large-scale hole-based quantum processors.

• Title:Quantum information processing using Kerr parametric oscillators

• Speaker: Dr. Sangil Kwon (Tokyo University of Science)

• Date & Time: January 13th (Mon.), 2025, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : A Kerr parametric oscillator(KPO) has been studied for decades, but only recently have its intrinsic quantum properties been recognised as useful for quantum computing, sparking a new wave of research. In this talk, I will introduce the unique properties of this exotic driven quantum system. I will present our experimental demonstration of entanglement-preserving conversion between Fock-state and cat-state encodings, as well as the efficient and straightforward implementation of a universal quantum gate set in the cat-state basis. Lastly, I will discuss our efforts to realisethe simplest autonomous quantum error correction scheme based on this KPO system.
  
  References:
  S.Kwon,S.Watabe, and J.-S. Tsai, npjQuantum Inf. 8, 40 (2022).
  D.Iyama, T. Kamiya, S. Fujiiet al., Nat. Commun. 15, 86 (2024).
  D.Hoshi,T.Nagase,S.Kwonetal.,arXiv:2406.17999.

• Title:Quantum Error Correction: Tackling the Error Problem in Quantum Technology

• Speaker: Dr. Seung Woo Lee (KIST)

• Date & Time: January 7th (Tue.), 2025, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : Quantum information science has advanced dramatically over the past decade, but significant challenges remain in achieving practical real-world quantum applications. Among these, the exponentially growing errors that arise as quantum computing systems scale have emerged as one of the most critical and urgent issues. Without addressing this error problem, no useful quantum algorithms can be executed, and practical applications of quantum computing as well as related other technology will remain unattainable. This seminar introduces Quantum Error Correction (QEC), a key technology aimed at fundamentally resolving the error problem in quantum information processing. It will cover the current status and trends in QEC research, including recent theoretical and experimental breakthroughs and the global race among leading quantum research groups and companies to develop QEC technologies. Finally, I will introduce hybrid and fusion QEC methods along with corresponding fault-tolerant architectures, discussing their potential advantages in building scalable quantum computing systems.

• Title:Advancing Thermal Energy Technologies: Thermoelectrics and Heat-Switching

• Speaker: Dr. Min Young Kim (The Ohio State University, USA)

• Date & Time: December 23th (Mon.), 2024, 2:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : The ongoing energy and climate crises present urgent challenges, driving the need for swift transition to sustainable energy sources. Among these, thermal energy stands out as a promising resource, capable of ensuring a stable energy supply while attracting growing interest for its wide-ranging applications. Recently, its utilization is extending to solid-state devices, driven by advancements in thermal nano-engineering, which enables precise manipulation of energy carrier interactions within solids. This seminar will spotlight thermoelectrics as a key example of thermal energy technologies, focusing on their ability to convert heat into electricity. Specifically, cutting-edge strategies for developing efficient thermoelectric materials will be discussed, with particular emphasis on exploiting the spin degree of freedom in magnetic materials and improving the performance of Seebeck, spin Seebeck and anomalous Nernst effects. Additionally, the seminar will explore in advancements in heat-switching technology, another innovation enabled by thermal nano-engineering. This approach addresses heat management challenges in solid-state devices by regulating heat flow through variable thermal conductance controlled by external fields. Particularly, a proof-of-concept heat-switching system based on topological insulators will be presented, demonstrating how interfacial thermal conductance can be modulated by topologically protected surface states. These findings open new avenues for the development of thermal circuit elements, including thermal resistances and switches, functioning analogously to electrical counterparts.

• Title: Parametric Circuits and Variational Methods

• Speaker: Prof. Seonggeun Park (Korea University)

• Date & Time / Venue: December 11th (Wed.), 2024,  4:00 PM / APCTP (HogilKim Memorial Bldg., #512)

• Abstract
  : This advanced session focuses on designing and implementing quantum algorithms using Qiskit. Participants will explore the creation of parametric quantum circuits, which enable dynamic adjustments during execution, and learn how to implement a simple Variational Quantum Algorithm (VQA). Additionally, the session introduces the broader Qiskit ecosystem, providing practical tips for utilizing its documentation effectively.

• Title: Running and Optimizing Quantum Circuits

• Speaker: Prof. Seonggeun Park (Korea University)

• Date & Time / Venue: December 10th (Tue.), 2024,  4:00 PM / APCTP (HogilKim Memorial Bldg., #512)

• Abstract
  : In the second session, participants will learn how to execute quantum circuits on real quantum hardware using Qiskit. The session covers key concepts such as Qiskit primitives, which simplify interactions with quantum systems, and the basics of the Qiskit transpiler for optimizing circuit performance. Through hands-on examples, attendees will explore the process of preparing circuits for execution and optimizing them for efficient results.

• Title: Fundamentals of Qiskit v1.0 and Quantum Circuit Design

• Speaker: Prof. Seonggeun Park (Korea University)

• Date & Time / Venue: December 9th (Mon.), 2024,  4:00 PM / APCTP (HogilKim Memorial Bldg., #512)

• Abstract
  :This seminar series offers a comprehensive introduction to Qiskit v1.0, encompassing fundamental principles and intermediate applications in quantum computing.
  The first session provides a comprehensive introduction to Qiskit and its foundational concepts, equipping participants with the skills to start designing quantum circuits. The session begins with a step-by-step guide to setting up the Qiskit development environment and an overview of the IBM Quantum platform. Participants will then learn the basics of quantum circuit construction, including how to incorporate classical feedforward mechanisms and control flow to enhance circuit functionality.

• Title:Novel magnetic imaging methods

• Speaker: Prof. Dong-Hun Lee (Korea Univ.)

• Date & Time: November 19th (Tue.), 2024, 4:00 PM/ Physics Seminar Room (Bldg.3, #302) & Online via ZOOM

• ZOOM Link : https://zoom.us/j/6978902944?pwd=A1FA5qzaerfEi20NpmKYJA0Jz8V2pt.1&omn=98510589492

• Abstract
  : Spin qubits are not only essential components for quantum information science and technologies, but also can be used as novel experimental tools to study magnetism. In this talk, we will introduce examples of the new opportunities using spin qubits, especially based on solid-state spin qubits; nitrogen-vacancy (NV) centers in diamond. We will focus on quantum sensing and imaging applications at two different length scales, i.e. nanometers, and micrometers. First, the NV center is combined with a scanning probe microscope to map out stray field from magnetic materials and current flows in transport devices at nanometer scale. Imaging of spin waves in YIG and other magnetic materials will be discussed as an imaging example. Second, ensemble of NV centers is combined with a wide field-of- view optical microscope to image magnetic nanowires and magnetic structures at micrometer scale. From the magnitude and spatial distribution of static magnetic field around nanowires, we are able to extract magnetization of individual ferromagnet nanowires which varies depending on relative size, material composition and so on. The novel imaging techniques based on spin qubits can provide quantitative analysis method of studying magnetism in various nanostructures.

• Title:Nanoscale Optics: Engineering Breakthroughs and Innovations

• Speaker: Prof. Myung-Ki Kim (Korea University)

• Date & Time: November 20th (Wed.), 2024, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : This presentation explores advanced techniques for engineering light at the nanoscale and the transformative potential they offer. By confining light within extremely small spaces, we enable breakthroughs in high-resolution imaging, precision lithography, ultra-dense optical storage, and high-performance optical communication devices. These miniaturized photonic systems deliver substantial improvements in integration, modulation speed, and energy efficiency, establishing a strong foundation for next-generation communication technologies. Nevertheless, overcoming the diffraction limit has long been a fundamental challenge in optics. Recent innovations in nanophotonics and plasmonics have introduced new methods to extend beyond this limitation. This talk explores sophisticated design strategies for controlling light near the diffraction limit, highlights cutting-edge advancements in on-chip integration, ultra-compact metasurface-based light control devices, and deep-subwavelength plasmonic systems. These technologies demonstrate significant promise for diverse applications, underscoring their potential to impact a wide range of scientific and technological fields.

• Title:Fluxoid-parity-controlled superconducting diode effect and Corbino–Josephson interferometry on an intrinsic topological insulator surface

• Speaker: Prof. Joon Young Park (Harvard University)

• Date & Time: November 29th (Fri.), 2024, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : The superconducting diode effect (SDE)—where supercurrent becomes asymmetric under applied current bias—can arise in Josephson junctions (JJs) when both time-reversal and inversion symmetries are broken. Recent studies suggest that changes in SDE polarity may signal topological phase transitions in linear JJs. In this talk, I will present the observation of a SDE alternating its sign for even- and odd-fluxoid states in Corbino-geometry topological insulator JJs. High-quality Nb JJs are fabricated on Sn-doped Bi1.1Sb0.9Te2S single crystals with extremely low bulk carrier concentrations, with their pristine surfaces preserved by Te films grown via ultrahigh-vacuum molecular beam epitaxy. Different Corbino-style geometries enable Josephson interferometry within a single junction, revealing the skewed current-phase relation of the highly transparent junctions. The even-odd SDE is attributed to a topological phase, with diode polarity directly reflecting the sign reversal in periodic boundary conditions for even/odd numbers of Josephson vortices. Additionally, I will discuss future directions to probe the non-Abelian statistics of MBSs in Corbino–Josephson trijunction hybrid devices.

• Title: No Need to Be Perfect: Quantum Photonics with Defective Materials and Bad Cavities

• Speaker: Prof. Je-Hyung Kim (UNIST)

• Date & Time: November 13th (Wed.), 2024, 4:00 PM/ Physics Seminar Room (Bldg.3, #302)

• Abstract
  : Solid-state quantum emitters have attracted much attention as an integrated source of photonic and spin qubits, which are basic elements for a range of quantum applications. Recent advances in solid-state quantum emitters demonstrate a variety of quantum resources, including bright quantum light sources, quantum memories, and spin-photon interfaces. However, such qubits in solid-state environments suffer from various unwanted interactions with phonons and charges, resulting in broad spectrum and decoherence. Therefore, researchers have sought to create faultless systems by improving the quality of quantum materials and photonic devices. However, these efforts to eliminate imperfections mostly require rigorous experimental conditions and sophisticated techniques, making the system less practical and scalable. In this talk, I will present recent approaches that leverage the benefits of crystal defects and low-Q cavities and show that imperfections can actually help address challenges in quantum materials and devices. In the first part I will discuss how 2D stacking faults can efficiently decouple electron-phonon interactions for a single point defect, leading to unexpected strong zero-phonon transition. In the second part, I will introduce how cavity dissipation loss can mediate collective interaction in quantum systems. In this system, we successfully demonstrate a steady-state subradiant state for the first time and observe unique features of giant photon bunching and suppressed single-photon decay, which are important signatures of a long-lived subradiant state. Our distinctive approaches that properly utilize imperfection in materials and devices could provide valuable quantum resources in a more efficient and practical manner, while also offering new insights into fundamental quantum light-matter interactions.

• Title: Ultracold molecules as a new quantum resource

• Speaker: Prof. Eunmi Chae (Korea University)

• Date & Time: November 26th (Tue.), 2024, 5:00 PM/ 이화여자대학교 연구협력관 지하 1층 주피터 회의실 & Online via ZOOM

• Abstract
  : Due to their complex internal structures and strong long-range interactions, diatomic molecules are expected to be a promising platform for quantum simulations/computations. The rotational states of the molecules trapped in an optical tweezer array form qubit states with long coherence time. Entanglement between the molecules can be generated using the strong electric dipole-dipole interactions.
  To achieve the molecular quantum platform, the molecules should be prepared at ultracold temperatures. One way to cool the molecules is laser-cooling, the workhorse technique to cool the atoms. Despite the complicated internal structures of molecules, laser-cooling and magneto-optical traps of molecules have been demonstrated for several species, reaching temperatures down to 5 uK.
  In this talk, two experiments using ultracold molecules will be introduced. I will first start with the CaF experiment at Harvard, where we confirmed dipolar spin exchange due to the strong interaction between the molecules. Here, rotational states of the molecules are adapted as the qubit states, and each molecule is trapped in an optical tweezer. A two-qubit gate to generate the entanglement is demonstrated between the two molecules with the fidelity of 0.89. In the second part of the talk, I will also briefly introduce a new MgF experiment at Korea University.

• Title: Quantum Engineering with Coherent Superconducting Devices and Circuits

• Speaker: Prof. Kaveh Delfanazari (University of Glasgow)

• Date & Time: October 21th (Mon.), 2024, 1:30 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : In this talk, I will first provide an overview of quantum computing and quantum communication activities at the University of Glasgow Centre for Quantum Technology (CQT). This will be followed by an update on our latest progress in developing quantum integrated circuits based on hybrid superconductor-semiconductor heterostructures, demonstrating coherent quantum transport and conductance quantization. In particular, the design, nanofabrication, and cryogenic measurements of large-scale hybrid superconductor-semiconductor field-effect conductance switches with novel chip architectures will be discussed with a focus on their electronic response, switching (on/off) statistics, quantum yield, and reproducibility [1-3]. If time allows, in the second part of the talk, our recent results on the fabrication and characterisation of high-Q (above million) CMOS-compatible superconducting microwave coplanar waveguide resonator arrays on tantalum (Ta), niobium nitride (NbN) and titanium nitride (TiN) material platforms and their microwave spectroscopy from many photon regimes to single photon regimes at millikelvin temperatures and high magnetic fields for their integrations with hybrid quantum circuits will be discussed [4-5]. Finally, light-matter interactions, and Josephson plasmonics in superconductor sources, and metamaterial arrays for future quantum communication networks will be discussed [6-11].
  
  [1] K Delfanazari et al., Physical Review Applied 21 (1), 014051 (2024)
  [2] K Delfanazari et al., Advanced Electronic Materials 10 (2), 2470006 (2023)
  [3] K Delfanazari et al., Advanced Materials 29 (37), 1701836 (2017)
  [4] P. Foshat et al., arXiv:2306.02356
  [5] S. Poorgholam et al., under review
  [6] K Delfanazari, Nature Photonics 18 (3), 214-215 (2024)
  [7] S Kalhor et al., Physical Review B 106 (24), 245140 (2022)
  [8] K Delfanazari, arXiv:2312.15515
  [9] K Delfanazari, arXiv:2106.11961
  [10] K Delfanazari, Advanced Photonics Research, 2400045 (2024)
  [11] M Zhang et al., IEEE Transactions on Quantum Engineering 4, 4100410 (2023)

• Title: All-Crystalline van der Waals Josephson Junctions for Enhanced Coherence and Scalability for Superconducting Qubits

• Speaker: Dr. Jinho Park  (Department of Mechanical Engineering, Columbia Univ.)

• Date & Time: October 21th (Mon.), 2024, 3:00 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Superconducting qubits, at the forefront of quantum computing, are currently limited by materials challenges that impede the enhancement of coherence times and hinder miniaturization for scalable quantum systems. Van der Waals (vdW) materials present a promising solution due to their intrinsic single-crystallinity, low defect densities, and absence of dangling bonds. In this talk, the DC electronic properties of vdW Josephson junctions (JJs) constructed from superconducting NbSe₂ electrodes and semiconducting WSe₂ barriers. We observed robust trends in switching current and normal resistance of JJs as a function of barrier thickness. Notably, we demonstrated a clear transition from proximity- to tunnel-Josephson coupling within the semiconducting Josephson junctions. Based on these findings we design and achieve dispersive coupling between the all-crystalline merged-element transmon and a microwave resonator. This advancement holds significant potential for the development of a new platform in superconducting quantum devices.

• Title: Plasma-based laser pulse compression

• Speaker: Prof. Hyyong Suk (GIST)

• Date & Time: September 11th (Wed.), 2024, 4:00 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Laser intensity has increased dramatically since the chirped-pulse amplification (CPA) technique was invented in 1985. Nowadays several PW (petawatt) level lasers are available around the world and they are used for basic physics research. However, people are not satisfied with the current level of PW and they want to have much higher peak power/intensity to explore some new regimes. For example, if the focused laser intensity reach the Schwinger limit of I=2.3×〖10〗^29  W/cm^2, the so-called vacuum breakdown effect can happen, where the vacuum will break down by the ultrastrong laser electric field and electron-positron pairs will be created in vacuum. To reach the Schwinger limit, however, we have to increase the present laser intensity by one million times, which is simply impossible with the current CPA method. In this talk, our efforts to achieve the very challenging and ambitious goal will be introduced.

• Title: 양자컴퓨터의 계산 실수? 오류 문제를 어떻게 해결할까

• Speaker: Dr. Seung-Woo Lee (한국과학기술연구원)

• Date & Time: August 19th (Mon.), 2024, 1:30 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : 양자 컴퓨팅, 양자 통신 등의 양자기술 개발의 가장 중요한 도전 과제는 정보가 입력된 양자 상태에 발생하는 정보의 손실과 잡음 그리고 연산 오류의 문제이다. 특히 실용적인 범용 양자컴퓨터 개발을 위해서는 양자오류정정(Quantum Error Correction)이 수행되는 양자컴퓨팅 시스템의 설계와 구현이 반드시 필요하다. 본 세미나에서는 양자 시스템 개발에서 잡음과 오류 문제 해결의 중요성과, 이를 해결하기 위해 개발된 여러 방법론에 대해서 소개하고, 양자컴퓨팅 개발에서 궁극적으로 양자오류정정이 수행되어야 하는 이유를 설명한다. 최근 다양한 플랫폼에서 진행되고 있는 양자오류정정 구현과 논리 큐비트(Logical qubit)의 개발, 그리고 결함허용 (Fault-tolerant) 또는 오류내성 범용 양자컴퓨팅 기술 개발의 동향을 소개한다. 마지막으로, KIST 이론 연구실의 양자오류정정 관련 최신 연구 결과를 소개한다.

• Title: Quantum Qubits, New Experimental Tools for Science: Diamond Magnetometry

• Speaker: Prof. Donghun Lee (Korea Univ.)

• Date & Time: May 22th (Wed.), 2023, 4:00 - 5:30 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Many breakthroughs and new findings in science have been accompanied with the invention and development of novel and precise experimental tools. Qubits are not only essential components for quantum information science and technologies, but also can be used as novel experimental tools for science. In this talk, we will introduce examples of the new opportunities using qubits, especially based on solid-state spin qubits; nitrogen-vacancy (NV) centers in diamond. We will focus on quantum sensing and imaging applications at two different length scales, i.e. nanometers, and micrometers. First, the NV center is combined with a scanning probe microscope to map out stray field from magnetic materials and current flows in transport devices at nanometer scale. Imaging of spin waves in YIG and other magnetic materials will be discussed as an imaging example. Second, ensemble of NV centers is combined with a wide field-of-view optical microscope to image magnetic nanowires and magnetic structures at micrometer scale. Finally, if time allowed, we will introduce our recent efforts of making miniaturized magnetic sensors and an education/training unit for basic qubit and quantum sensing experiment.
  Keywords: qubits, quantum sensing and imaging, diamond NV center, magnetometry

• Title: Quantum Computing with QuEra’s Neutral-Atom Quantum Computers

• Speaker: Dr. Tommaso Macri (QuEra Computing)

• Date & Time: February 14th (Wed.), 2023, 3:00 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : In this talk, I will explore the pioneering technology and strategic vision of QuEra, focusing on its neutral-atom quantum computers. QuEra's approach combines analog and digital quantum computing, leveraging the strengths of neutral atoms for high coherence and unique qubit manipulation. The presentation will cover QuEra's ambitious roadmap, which includes significant milestones in quantum error correction and scalability for gate-based computing, as well as recent applications obtained in the context of analog quantum computing for quantum simulation, optimization, and machine learning.

• Title: Toward quantum advantage with trapped ions

• Speaker: Prof. Kihwan Kim (Tsinghua Univ.)

• Date & Time: February 15th (Thu.), 2023, 4:00 PM/ Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : In this talk, I will describe our major efforts to achieve quantum advantage using trapped ions. Within the Noisy Intermediate-Scale Quantum (NISQ) realm, an important milestone is showing quantum advantage - outperforming classical computers on specific problems. So far, boson sampling with photons and random circuit sampling with superconducting qubits have achieved this. Here, I present two paths we are exploring to reach similar quantum advantages with trapped ions. First, we construct scalable phononic networks using the vibrational modes of trapped ions. We have realized a small-scale phonon sampler and shown this is an ideal testbed, since single phonons can be prepared and measured deterministically without loss - addressing key limitations of photonic systems [1]. Second, we have developed a trap capable of holding a 2D crystal of ions, demonstrating its equivalence to the standard linear ion chain for quantum simulation. Specifically, we achieved adiabatic ground state preparation for transverse Ising models with up to 10 ions [2]. We plan to scale up the number of ions to achieve quantum advantage. We will discuss the difficulties and our strategy for overcoming challenges towards this goal.
  [1] Wentao Chen, et al., Nature Physics 19, 877 (2023).
  [2] Mu Qiao, et al., Nature Physics, accepted

• Title: Towards Quantum Computing with Spins on Surfaces

• Speaker: Andreas Heinrich (IBS Center for Quantum Nanoscience (QNS) at Ewha Womans Univ.)

• Date & Time: November 16th (Thu.), 2023, 4:00 PM ~  5:00 PM / IBS POSTECH Campus, CGP Main Hall

• Abstract
  : There is a strong international research effort in the area of quantum information science. Here, the concepts of quantum coherence, superposition and entanglement of quantum states are exploited. These concepts were originally shown with photons as well as atoms and ions in vacuum traps. Over the past two decades, many advances at studying such quantum coherence in solid-state and molecular architectures have evolved [1]. 

In this talk we will focus on quantum-coherent experiments in Scanning Tunneling Microscopy (STM). STM enables the study of surfaces with atomic-scale spatial resolution and offers the ability to study individual atoms and molecules on surfaces. Here at Ewha, we have one of the world’s best facilities for such studies. STM can also be used to move atoms with atomic-scale precision, which enables us to build engineered nanostructures where each atom is in the exactly correct place. 

In order to study qubits with STM, we recently learned how to combine STM with electron spin resonance [2,3]. Spin resonance gives us the means to quantum-coherently control an individual atomic or molecular spin on a surface. Using short pulses of microwave radiation further enables us to perform qubit rotations and learn about the quantum coherence times of our spins [4]. Finally, we will finish with unpublished results on multi-qubit operations with spins on surfaces. 

1. Andreas J. Heinrich, William D. Oliver, Lieven M. K. Vandersypen, Arzhang Ardavan, Roberta Sessoli, Daniel Loss, Ania Bleszynski Jayich, Joaquin Fernandez-Rossier, Arne Laucht, Andrea Morello, “Quantum-coherent nanoscience”, Nature Nanotechnology, 16, 1318-1329 (2021). 

2. Susanne Baumann, William Paul, Taeyoung Choi, Christopher P. Lutz, Arzhang Ardavan, Andreas J. Heinrich, “Electron Paramagnetic Resonance of Individual Atoms on a Surface”, Science 350, 417 (2015). 

3. Yi Chen, Yujeong Bae, Andreas Heinrich, “Harnessing the Quantum Behavior of Spins on Surfaces”, Advanced Materials 2022, 2107534 (2022). 

4. Kai Yang, William Paul, Soo-Hyon Phark, Philip Willke, Yujeong Bae, Taeyoung Choi, Taner Esat, Arzhang Ardavan, Andreas J. Heinrich, and Christopher P. Lutz, “Coherent spin manipulation of individual atoms on a surface”, Science 366, 509 (2019).

• Title: Ultra-high-Q microcavities and their applications

• Speaker: Prof. Lee, Hansuek (KAIST)

• Date & Time: October 18th (Wed.), 2023, 4:00 PM ~  5:00 PM / Physics Seminar Room (Science Bldg Ⅲ, #302) 

• Abstract
  : Ultra-high-Q optical resonators have played an important role in a wide range of research fields including nonlinear optics, cavity optomechanics, quantum information science, and optical telecommunications. Recently, these essential components have been implemented on a chip with various kinds of materials bearing great inherent potentials utilizing micro-fabrication technology, which also allows precise control of optical properties such as free spectral range, dispersion, and coupling efficiency far beyond the traditional capability. In this talk, starting from the basics of microcavities, I will review the recent progress in this research field in terms of the advancement of the microcavity platform itself and the applications exploiting its enhanced strong-light matter interactions, which include optical frequency comb and narrow linewidth nonlinear lasers. In addition, the current efforts and future direction of my research group will be presented and discussed including the effort to expand the operation wavelength of the on-chip ultra-high-Q resonators from the near-IR to the mid-IR for the strong interaction with molecules.

• Title: Mechanical quantum oscillators

• Speaker: Prof. Suh, Junho (POSTECH)

• Date & Time: September 6th (Wed.), 2023, 4:00 PM ~  5:00 PM / 

• Abstract
  : Mechanical oscillators have been employed as precision sensors in a diverse range of physical measurements. These mechanical devices reached the quantum regime recently as they demonstrate operations near ground states and coherent couplings to other quantum systems. In this talk, I will discuss the principles of these mechanical quantum devices with examples from microwave cavity optomechanics experiments. The cavity optomechanical techniques enable us to explore and utilize the effects of quantum fluctuations in the measurement process. We recently focused on applying the cavity optomechanical techniques to develop mechanical quantum sensors. First, we demonstrate a niobium cavity electromechanical device operating up to 4 Kelvin and 0.9 Tesla, which suggest these devices as spin-phonon interfaces. Second, a nanowire mechanical oscillator realizes microwave bolometry that could be used for precision sensing of microwave powers at millikelvin temperatures. Our work illustrate potential routes to harness the versatility of mechanical quantum oscillators for quantum sensing and transduction in hybrid quantum systems.

• Title: From quantum materials to quantum devices

• Speaker: Dr. Kin Chung Fong

• Date & Time: August 24th (Thu.), 2023, 10:00 AM  (KST) / Physics Seminar Room (Science Bldg. Ⅲ, #302) & Online via ZOOM

• ZOOM link : https://us06web.zoom.us/meeting/register/tZ0kc-ihqD8rH9Z1mwswXD-VsyvUKMFoamSG

• Abstract
  : Technologies operating on the basis of quantum mechanical laws and resources such as phase coherence and entanglement are expected to revolutionize our future. However, fulfilling the promises of quantum technologies is not an easy task because of the need to initialize, manipulate, readout, and retain the information encoded in the quantum states. Interestingly, two-dimensional (2D) van der Waals (VdW) materials can play an important role to meet these challenges by exploiting the unique electronic properties of their heterostructures, the gate-tunability of the charge carrier density, and the atomically sharp, pristine interfaces to develop superconducting devices. In this talk, we will present our latest results on using 2D materials to develop single-photon detectors from infrared to microwave frequencies, and to miniaturize superconducting transmon qubits with vdW heterostructures. We will also discuss how the basic physics principles can impose fundamental limitations, such as sensitivity and bandwidth, to quantum devices as well as how the novel material properties can overcome conventional barriers to make new devices, such as kinetic inductance detectors.

References:

E. D. Walsh, et. al., "Josephson junction infrared single-photon detector," Science 372, 409 (2021).

A. Antony, et. al. "Miniaturizing transmon qubits using van der Waals materials," Nano Lett. 21, 10122 (2021). 

G.-H. Lee, al., "Graphene-based Josephson junction microwave bolometer," Nature 586, 42 (2020).

• Title: U(N) gauge theory in the strong coupling limit on D-wave quantum computer

• Speaker: Jangho Kim (Institute for Advanced Simulation, Germany)

• Date & Time / Venue: August 21th (Mon.), 2023, 10:00 AM / Physics Seminar Room (Science Bldg Ⅲ, #302)

• Abstract
  : Quadratic Unconstrained Binary Optimization (QUBO) problems can be addressed on quantum annealing systems. We reformulate the strong coupling lattice QCD dual representation as a QUBO matrix. We confirm that importance sampling is feasible on the D-Wave Advantage quantum annealer. We describe the setup of the system and present the first results obtained on a D-wave quantum annealer for U(N) gauge group. Furthermore, we outline a strategy for going to larger volumes and tackling the SU(N) gauge theory which has the sign problem 

• Title: A Qubit-Based Quantum Microscope for Space-Time Sensing

• Speaker: Prof .Wilson Ho  (University of California, Irvine)

• Date & Time / Venue: August 18th (Fri.), 2023, 10:30 AM / Auditorium(IBS POSTECH Campus, #104)

• Abstract
  : In contrast to all other microscopes, a qubit-based quantum microscope (QM) combining coherent light with the scanning tunneling microscope (STM) is unique in incorporating the quantum superposition principle in its operation. This QM uses the superposition of two levels in a single hydrogen molecule as the sensor to probe the electric fields of a sample’s surface. In a pilot study (Science 376, 401, 2022; Phys. Rev. Lett. 130, 096201, 2023) the QM demonstrates a 300-fold finer energy resolution and 0.1 angstrom spatial sensitivity of the sample’s near-field electrostatics, compared to microscopes not based on this quantum principle. Furthermore, the wave-particle duality, nonlinear Stark effects, superposition of multiple quantum states, and entanglement among adjacent two levels illustrate the sensitivity of the QM to a set of basic phenomena underlying quantum mechanics. This qubit-based QM advances precision measurement with space-time resolution by irradiating the STM junction with femtosecond THz radiation and recording in the time domain coherent oscillations of the light-induced rectified tunneling current. The common occurrence of systems with two levels within a double-well potential suggests a broad application of the QM in probing the heterogeneous distribution of static and dynamic properties of electrons in functional materials.

• Title: Evidence for the utility of quantum computing before fault tolerance

• Speaker: Ph.D. Youngseok Kim (IBM Quantum)

• Date & Time: August 30th (Tue.), 2023, 3:00 pm ~ / Science Bldg. I, #118

• Abstract
  : One of the biggest impediments for achieving useful quantum computing is noise. The widely accepted solution to this challenge is fault-tolerant quantum circuits, however, it is out of reach for currently available processors to achieve computation at utility scale. Instead, we argue that quantum error mitigation enables access to accurate expectation values even on existing, noisy quantum computers.

Establishing the applicability of these techniques at scales beyond those accessible to brute force classical methods is a crucial step toward probing a computational advantage with near-term noisy quantum computers. Here we experimentally demonstrate the efficacy of an error mitigation technique, zero-noise extrapolation, for quantum circuits using up to 127 qubits. The accuracy of the mitigated expectation values is greatly enhanced by novel advances in the coherence of large-scale superconducting quantum processors, and the ability to controllably scale noise at this scale. These experiments demonstrate an important tool for the realization of near- term quantum applications in a pre-fault tolerant era.

• Title: Rydberg atom quantum computer (리드버그 원자 양자컴퓨터)

• Speaker: Prof. Jaewook Ahn (KAIST)

• Date & Time: February 22th (Wed.), 2023, 4:00 pm~ / Physics Seminar Room (Bldg.3, #302)

• Abstract
  : As quantum computers continue to emerge, the era of quantum information technology is dawning. These machines utilize quantum entanglement of only 100 atoms to perform 100 nayuta (1 nayuta = 10^28) parallel calculations, seemingly impossible for classical digital computers. The extraordinary computing power of quantum computers engenders both expectations and concerns. In this talk, after we discuss the differences between quantum and classical information, as well as how quantum information is generated, processed, and measured differently from classical information, we introduce how quantum computers are developed and operate, with an example of Rydberg atom quantum computers.

양자컴퓨터들이 속속 개발되며 양자정보기술의 시대가 도래하고 있다. 양자컴퓨터는 양자얽힘을 이용하여 불과 100개의 원자로 100 나유타(1 나유타=10의 28제곱)가지의 계산을 병렬처리하는, 디지털컴퓨터로는 불가능해 보이는 계산능력을 보여준다. 양자컴퓨터의 이러한 계산능력에 대하여 기대와 우려가 공존하고 있다. 본 발표에서는 양자정보가고전정보와 어떻게 다른 지, 양자정보의 생성, 처리, 측정은 고전정보의 경우와 어떻게 다른 지, 그리고 리드버그 원자 양자컴퓨터 개발 이야기를 중심으로, 어떻게 양자컴퓨터가 만들어지고 작동하는지를 소개한다.

• Title: Non-abelian anyons and graph gauge theory on a superconducting processor

• Speaker: Prof. Eun-Ah Kim (Cornell University)

• Date & Time: Jan. 10th (Tue) 4:00 pm~

• Abstract
  : Topological quantum computation requires creating and braiding non-Abelian anyons. I will describe a simple and systematic approach to constructing effective unitary protocols for braiding, manipulation, and readout of non-Abelian anyons and preparing their entangled states on a NISQ device. Our approach is based on our new graph gauge theory on a planar qubit graph with vertices of degree 2, 3, and 4. I will discuss experimental observations guided by our theory.
  
  http://arxiv.org/abs/2210.09282
  http://arxiv.org/abs/2210.10255

• Title: Spin-phonon interactions and spin decoherence from first-principles

• Speaker: Dr. Jinsoo Park (Department of Applied Physics, California Institute of Technology)

• Date & Time: Dec. 29th (Thu) 3:00 pm~

• Abstract
  : Developing a microscopic understanding of spin decoherence is essential to advancing quantum technologies. Electron spin decoherence due to atomic vibrations (phonons) plays a special role as it sets an intrinsic limit to the performance of spin-based quantum devices. Two main sources of phonon-induced spin decoherence, the Elliott-Yafet (EY) [1] and Dyakonov-Perel (DP) mechanisms, have distinct physical origins and theoretical treatments. In this talk, I will present a rigorous framework that unifies their modeling and enables accurate predictions of spin relaxation and precession in semiconductors [2].  I compute the phonon-dressed vertex of the spin-spin correlation function with a treatment analogous to the calculation of the anomalous electron magnetic moment in QED [3]. These calculations show that the vertex correction provides a giant renormalization of the electron spin dynamics in solids, greater by many orders of magnitude than the corresponding correction from photons in vacuum. In summary, I will demonstrate a general approach for quantitative analysis of spin decoherence in materials, advancing the quest for spin-based quantum technologies.
  
  [1] J. Park, J.-J. Zhou, M. Bernardi, Phys. Rev. B 101, 045202 (2020)
  [2] J. Park, Y. Luo, J.-J. Zhou, and M. Bernardi, Phys. Rev. B 106, 174404 (2022)
  [3] J. Park, J.-J. Zhou, Y. Luo, and M. Bernardi, Phys. Rev. Lett. 129, 197201 (2022)  

• Title: Towards the creation of quantum degenerate molecular gases of NaK with long-range dipolar interactions

• Speaker: Dr. Young-Hoon Lim (POSTECH)

• Date & Time: Dec. 14th (Wed) 2:00 pm~

• Abstract
  : Ultracold atomic gases have provided platforms to explore quantum simulation of many-body physics with high level of control. However, exploring quantum physics with anisotropic and/or long-range interactions has been hindered since the interactions between ultracold atoms are only governed by the typical contact interaction. In this respect, a quantum gas of dipolar molecules will offer an exceptional toolbox to study novel dipolar matter, exotic quantum many-body physics, and processing of quantum information. Here, I will present our experimental progress in the creation of degenerate gas of strongly dipolar fermionic/bosonic NaK molecules. First, Na and K atoms are trapped in a dual magneto-optical trap, and simultaneously cooled into the degenerate regime by sympathetically cooling the mixture via rf evaporation of one of the species. From the atomic mixture of Na and K atoms, weakly bound NaK molecules are created using an s-wave Feshbach resonance. To employ stimulated Raman adiabatic passage (STIRAP) for the transfer of Feshbach molecules to the absolute rovibrational ground state without heating, a narrow linewidth Raman laser system is realized. I will also briefly give an outlook on quantum phenomena that can be explored with degenerate NaK molecular gases.

• 
  

1. 연사: 김은종 박사 (CALTECH)

2. 일시: 2022. 12. 5(월), 오후 4시 30분

3. 장소: 공학3동 302호 (세미나실)

4. 제목: Probing Many-Body Quantum Phenomena with Long-Range-Interacting Superconducting Quantum Devices

Experimental realizations of engineerable quantum systems—quantum simulators—provide insights into exotic quantum many-body concepts that are intractable with available classical methods. A key challenge in the development of modern quantum simulators is to maintain the level of connectivity and control during scale-up. While majority of scalable quantum simulation and computation architectures to date feature nearest-neighbor interactions limited by their local nature of coupling, long-range interacting quantum systems—exhibiting fast build-up of quantum correlation and entanglement—provide new approaches for studying quantum many-body phenomena and investigating quantum error-correction schemes in the near term. Utilization of extensible quantum bus such as a photonic waveguide provides a natural direction to investigate such many-body quantum systems where qubits interact non-locally by exchange of photons along the bus.

Following this idea, I will introduce our efforts toward constructing scalable superconducting quantum simulators with long-range connectivity and individual addressing. I will focus on two approaches for channeling photon-mediated interactions between qubits: (i) when the qubits are tuned inside photonic bands of a waveguide channel [1] and (ii) when the qubits are tuned inside bandgaps of photonic metamaterials [2]. Highlighting the advantages and limitations of these schemes, I will discuss experiments performed in such simulators at the scale of ten qubits. Our work enables realization of various regimes in many-body quantum dynamics, and more broadly, provides a novel class of accessible Hamiltonians for analog quantum simulation in superconducting circuits [3].

[1] M. Mirhosseini*, E. Kim* et al., Nature 569, 692-697 (2019)

[2] X. Zhang*, E. Kim* et al., arXiv:2206.12803 (2022)

[3] E. Kim*, X. Zhang* et al., Phys. Rev. X 11, 011015 (2021)

Magnetic Information Storage in Single Atoms

Harald Brune

Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL)

The magnetic properties of single surface adsorbed atoms became one of the core interests

in surface and nanoscience in 2003, where single Co atoms on Pt were reported to have 200

times the magnetic anisotropy energy of bulk Co [1]. Years later, even 1000 times this energy

was reached for single Co atoms on thin MgO films [2]. In a classical picture, this suggests that

these single atoms should be rather stable magnets. However, despite numerous efforts, the

magnetic quantum states of all investigated single surface adsorbed transition metal atoms

had very short magnetic relaxation times, below 1 μs.

Immediately after changing to rare-earth atoms, a few adsorbate/substrate combinations

could be identified, where the magnetization vector of a single atom is indeed stable over

hours in the absence of an external magnetic field [3,4]. Therefore, these systems are single

atom magnets and enable magnetic information storage in the smallest unit of matter.

We will give an overview over the present adsorbate/substrate systems exhibiting single atom

magnet behavior [3 – 7] and explain the essential ingredients for this surprising stability of

single spin systems that are exposed to numerous perturbations from the environment. These

atoms can be placed very close and still individually be addressed, conceptually enabling

information storage at densities by 3 orders of magnitude larger than presently used devices.

Now, the fundamental research in this field turns its attention to quantum coherent spin

operations in single surface adsorbed atoms. If they have long enough coherence times with

respect to the time it takes to perform a single quantum spin operation, these would be single

atom quantum bits. The requirements for long coherence times of the magnetic quantum

states are quite different from the ones of magnetic relaxation times. We will illustrate this

with a few examples and point out single rare-earth atom systems that lend themselves

already now as quantum repeaters in telecommunication [8], creating hope that single atom

qubits may indeed become reality in the near future.

[1] P. Gambardella et al. Science 300, 1130 (2003).

[2] I. G. Rau et al. Science 344, 988 (2014).

[3] F. Donati et al. Science 352, 318 (2016).

[4] R. Baltic et al. Nanolett. 16, 7610 (2016).

[5] A. Singha et al. Nat. Communic. 12, 4179 (2021).

[6] F. Donati et al. Nano Lett. 21, 8266 (2021).

[7] V. Bellini et al. ACS Nano 16, 11182 (2021).

[8] M. Zhong et al. Nature 517, 177 (2015).

IBS-CALDES SEMINAR (Zoom)

▶Zoom Link : https://us06web.zoom.us/j/89975791050?pwd=T0swSHJzbWZyMjZGUUJqUnBCU05Idz09

▶ID :  899 7579 1050  /  PW : 746176

Toward unambiguous identification of Majorana bound state in the vortex core

Tetsuo Hanaguri

RIKEN Center for Emergent Matter Science, Wako 351-0198, Japan

Majorana quasiparticles have attracted much attention because of their potential applications in fault-tolerant quantum computing. Although there are various theoretical proposals to realize the Majorana quasiparticles in solid, experimental implementations and verifications have been challenging. We argue how to identify the Majorana-bound state (MBS) in the vortex core of superconductors using scanning tunneling microscopy/spectroscopy (STM/STS). Since the Majorana particle is its own antiparticle, it should appear at exactly zero energy. This gives rise to the zero-energy peak in the tunneling spectrum. However, it is often difficult to distinguish the zero-energy MBS from the trivial low-lying states due to the limited energy resolution of STM/STS. Another issue is that the zero-energy peak is merely a necessary condition for the MBS. It is indispensable to search for a feature that is unique to the MBS. We show the results of ultrahigh energy resolution STM/STS in the vortex core of Fe(Se,Te) where we find a zero-energy state below any possible trivial-state energy [1]. We also discuss our attempts to clarify the Majorana character, a unique spin structure of the Majorana vortex core, using a Yu-Shiba-Rusinov-state-based spin-polarized STM/STS that can achieve 100 % spin polarization [2].

[1] T Machida et al., Nat. Mat. 18, 811 (2019).

[2] T. Machida, Y. Nagai, and T. Hanaguri, Phys. Rev. Research 4, 033182 (2022).

Speaker: Prof. Se Kwon Kim (KAIST)

Time: Sep 21 (Wed) 4 pm  

Place: Physics Seminar Room (Science Bldg. III, #302)

Title: Quantum Spintronics

abstract : 

Recent advancements in spintronic techniques originally developed for spin-based devices now enable us to study fundamental spin physics of various quantum materials with unprecedented spin-current control and measurement, opening a new area of theoretical and experimental investigation of quantum systems. In this talk, we will introduce this emerging research area of spin transport in quantum materials which is fueled by the global interest in quantum information science. As examples, we will discuss our discovery of magnonic topological insulators realized by 2D magnets [1-3], which shows how spintronic techniques can be used for probing elusive quantum materials, and our prediction of long-range spin transport mediated by a vortex liquid in superconductors [4], which shows that quantum materials can provide novel platforms for efficient spin-transport devices. We will conclude the talk by offering a future outlook on quantum spintronics. 

[1] S. K. Kim, H. Ochoa, R. Zarzuela, and Y. Tserkovnyak, “Realization of the Haldane-Kane-Mele Model in a System of Localized Spins,” Phys. Rev. Lett. 117, 227201 (2016) [2] G. Go, S. K. Kim, and K.-J. Lee, "Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers," Phys. Rev. Lett. 123, 237207 (2019) [3] S. Zhang, G. Go, K.-J. Lee, S. K. Kim, "SU(3) Topology of Magnon-Phonon Hybridization in 2D Antiferromagnets," Phys. Rev. Lett. 124, 147204 (2020) [4] S. K. Kim, R. Myers, and Y. Tserkovnyak, "Nonlocal Spin Transport Mediated by a Vortex Liquid in Superconductors," Phys. Rev. Lett. 121, 187203 (2018)

Speaker: Donghun Lee (Korea University) 

Time: August 4 (Thur) 2 pm 

Place: Online seminar

Zoom link:https://postech-ac-kr.zoom.us/j/92905571336?pwd=bjFldkt0azN4T0VuUjRJNlhUNnR1dz09

Meeting ID: 929 0557 1336

Password: 409985

Title: Sensing and imaging of magnetic field based on solid-state spin qubits in diamond (to be given in Korean)

Abstract: 

Nitrogen-vacancy (NV) centers are point defects in diamond and solid-state spin qubits that can probe magnetic field with high sensitivity and high spatial resolution. In this presentation, we will introduce the working principle and examples of sensing and imaging magnetic field with the diamond NV centers. We will focus on three imaging experiments based on the NV centers but with different image scale from nanometers to millimeters. First, the NV center is combined with a scanning probe microscope to map out stray field from magnetic materials and current flows in 2D materials at nanometer scale. Second, ensemble of NV centers is combined with a wide field-of-view optical microscope to image magnetic nanowires and magnetic structures at micrometer scale. Finally, we make a portable magnetometer based on the NV ensemble and image stray field from disk magnets and current circuits at millimeter scale using a 2D motorized translation stage.

Remark: This seminar is more like a tutorial. It is expected to be about two hours long.

Host: Hyun-Woo Lee

Shift current is a novel mechanism for a photovoltaic effect in noncentrosymmetric crystals and arises from shift of electron wavepacket upon photoexcitation, which has a close relationship to modern theory of electric polarization [1]. So far, shift current has been mostly investigated for electronic excitations in band insulators, for example in ferroelectric materials and topological semimetals. In contrast, shift current from other elementary excitations is less investigated so far. In this talk, I present such a mechanism for the photovoltaic effect, focusing on exciton polaritons that appear out of equilibrium [2]. We show that nonequilibrium steady states of polaritons exhibit photocurrent generation with breaking of inversion and time reversal symmetries. I will discuss possible experimental realization in transition metal dichalcogenides. 

[1] T. Morimoto, and N. Nagaosa, Sci. Adv. 2, e1501524 (2016).

[2] T. Morimoto, N. Nagaosa, Phys. Rev. B 102, 235139 (2020). 

■ ZOOM Webinar

1) Please register through this ZOOM link (password 0)

https://us06web.zoom.us/meeting/register/tZcoceuvpjgiHdTfgKd65mpFllDNta3r19hJ 

2) Join the webinar with a link generated after the registration 

3) Please rename your profile - E.g. Full name (affiliation) 

v  Title: A fast quantum algorithm for computing matrix permanent

v  Speaker: Prof. Joonsuk Huh (SKKU)

v  Place: Physics Seminar Room (Science Bldg. III, #302)

v  Date & Time: May 13th (Fri.), 11:00 AM ~

v  Title: 양자컴퓨팅/양자머신러닝 연구소개 및 최신동향

v  Speaker: Dr. Jeongho Bang (ETRI)

v  Place: Physics Seminar Room (Science Bldg. III, #302)

v  Date & Time: May 12th (Thu.), 04:00 PM ~

Atomic, molecular, and optical systems often exhibit long-range interactions, which decay with distance r as a power law 1/r^alpha. In this talk, we will discuss bounds on how quickly quantum information can propagate in such systems. We will then discuss applications of these bounds to numerous phenomena including quantum simulation, quantum supremacy, and the preparation of topologically ordered states 

■ ZOOM Webinar 1) Please register through this ZOOM link (password 0)

 https://us06web.zoom.us/meeting/register/tZIodeCrqD0sH9PFBWgi49T3HZ_yjh5MPcP9

2) Join the webinar with a link generated after the registration

3) Please rename your profile - E.g. Full name (affiliation) 

Recent advances open new possibilities to probe and control quantum materials on time and length scales that would have seemed the realm of science fiction just a few decades ago. In this talk I will begin by reviewing the key principles that govern the electronic properties of materials. I will then discuss how, by going beyond the traditional world of equilibrium physics, a wide range of new collective phenomena and opportunities for dynamical control of material properties become possible. As an illustration, I will demonstrate a novel route to spontaneous magnetism that occurs through symmetry breaking in the collective (plasmon) modes of a driven metallic system. The mechanism is general, and results from feedback of internal fields of the nonequilibrium system onto its own electronic structure. Estimates indicate that experimental realization should be within reach in present day high quality graphene devices. The approaches I will describe provide fertile ground for new fundamental studies of quantum many-body dynamics, as well as potential applications for example in electronics and information processing. 

■ ZOOM Webinar

1) Please register through this ZOOM link (password 0)

https://us06web.zoom.us/meeting/register/tZcuf-2rpjotG9H4NEBA1tDqNnggTwxuAZEY

2) Join the webinar with a link generated after the registration

3) Please rename your profile - E.g. Full name (affiliation) 

v  Title: High-fidelity iToffoli gate for fixed-frequency superconducting qubits

v  Speaker: Dr. Yosep Kim (KIST)

v  Place: Physics Seminar Room (Science Bldg. III, #302)

v  Date & Time: Apr. 12th (Thu.), 04:00 PM ~

v  Title: Nonclassical resources for quantum technologies

v  Speaker: Prof. Hyukjoon Kwon (KIAS)

v  ZOOM ID: 948 064 9013 / Password: 123456

v  Date & Time: Apr. 6th (Wed.) 4:00 pm ~

v  Zoom URL: https://us02web.zoom.us/j/9480649013?pwd=dmVqaWZ3bHozbjZXRjd0RllvVVVkdz09

* When entering the meeting, please rename your profile as your full name (affiliation)

Zoom ID: 993 9887 5694 / PW: 013949

The kagome lattice of transition metal atoms provides an exciting platform to study the interplay of electronic correlations and band topology. AV3Sb5 (A=K, Rb, Cs) is a recently discovered class of kagome metals that does not exhibit resolvable magnetic order, and yet, surprisingly, shows a large anomalous Hall response and superconductivity. In this talk, I will discuss our experiments on AV3Sb5 materials using low-temperature spectroscopic imaging scanning tunneling microscopy. In CsV3Sb5, we discover a cascade of different symmetry-broken electronic states as a function of temperature [1]. At a temperature far above the superconducting transition Tc ~ 2.5 K, we reveal a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe an additional breaking of the six-fold rotation symmetry, which persists through the superconducting transition. This rotation symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering attributed to the orbital-selective renormalization of the Vanadium kagome bands. I will conclude by discussing the symmetry of the 2a0 CDW state in KV3Sb5 and its response to externally applied magnetic field [2]. Our experiments reveal a complex landscape of electronic states that can co-exist on a kagome lattice, and provide intriguing parallels to high-Tc superconductors and twisted bilayer graphene. 

[1] Zhao, … Zeljkovic. Nature 599, 216–221 (2021) 

[2] Li, … , Zeljkovic. Nature Physics (2022)

#512 & Online via ZOOM

■ ZOOM Webinar

1) Please register through this ZOOM link

  https://us06web.zoom.us/meeting/register/tZEuduqgqT8qE9GB1DcvvO-fa44ySCxxJogC

2) Join the webinar with a link generated after the registration

3) Please rename your profile - E.g. Full name (affiliation)

■ Contact information

1) Host: Ryo Hanai (ryo.hanai@apctp.org)

2) Office: Research Support Team (ra@apctp.org)

Simulating non-equilibrium quantum spin dynamics beyond one-dimension remains a significant challenge, despite advances in computational methods over the last two decades. We tackle this issue by developing a method in which the time-evolution of  quantum observables is encoded by averages over a classical stochastic process. We demonstrate that the sampling efficiency can be greatly improved by sampling around mean-field spin configurations, providing results for moderately sized 2D systems. Furthermore, we exploit a gauge-freedom of the stochastic description to enhance sampling for generic quenches, which may not necessarily have a significant mean-field. Connections to phase space approaches are discussed, with emphasis placed on current and future research directions.

Phys. Rev. B 104, 024408 (2021) J. Phys. A 53 (2020) 50LT02