Poster Abstracts

Go here to vote for the best poster.

List of poster titles, full abstracts are below (alphabetical by presenter name):

Smart quantum sensors enhanced by machine learning. 

Dr. Muhammad Junaid Arshad, Heriot-Watt University

My poster explores the fusion of quantum sensors and machine learning, developing smart quantum sensors that enhance sensitivity, reduce noise, and automate operation, reducing complexity for the user. This can be a game changer in applications that require taking massive datasets and exploring a large parameter space; an example of this is nanoscale magnetic resonance imaging.

I will present our work on machine learning enhanced decoherence and frequency estimation using a single spin qubit in diamond. I will then present an outlook on future work and the need for optimised algorithms and hardware interfaces. Machine learning promises to transform scientific research and technology, deepening our understanding of fundamental phenomena by optimising quantum sensors.

------------------------------------------

Local symmetry breaking drives picosecond spin domain formation in polycrystalline halide perovskite films

Dr. Arjun Ashoka, University of Cambridge

Photoinduced spin–charge interconversion in semiconductors with spin–orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. However, in structurally disordered polycrystalline semiconductors, which are being widely explored for device applications, the presence and role of spin-associated charge currents remains unclear. Here, using femtosecond circular-polarization-resolved pump–probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin domains on the micrometre scale formed through lateral spin currents. Micrometre-scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin-domain formation is driven by the presence of strong local inversion symmetry breaking via structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum-locked currents, leading to local spin accumulation. Ultrafast spin-domain formation in polycrystalline halide perovskite films provides an optically addressable platform for nanoscale spin-device physics.

------------------------------------------

NV magnetometry to explore 2D Materials 

Belgacem Issam, Heriot-Watt University

The goal of my PhD is to apply sensors based on a single electronic spin to open problems in the physics of 2D materials. The sensor will be based on the Nitrogen-Vacancy center (NV center), a defect in diamond hosting an electronic spin that can be initialized and readout optically, and controlled with microwaves. By using a scanning probe system with an NV center on the tip, one can achieve nanoscale spatial resolution.  We are targeting different applications of scanning NV magnetometry to 2D materials. One example is the investigation of superconductivity, through the detection of the Meissner effect, i.e. the repulsion of a magnetic field by a superconductor. A different application is the study of ferro-electric domains in MoSe2.

------------------------------------------

All-optical phase rotation in strongly coupled light-matter systems. 

Dr. Fedor Benimetskiy, University of Sheffield

The rapid development of quantum technologies is driving the implementation of new photonic systems and prompting active research into new physical aspects within well-known photonic platforms. A key aspect of quantum photonics is the interaction of photons through optical nonlinearities, for example, cross-phase modulation. In this study, we demonstrate that exciton-polaritons in an open cavity with embedded quantum wells can provide the necessary nonlinearity. Using laser beams attenuated to the average intensity compared with the intensity of several photons, we observed cross-phase modulation of up to 17 ± 1 mrad per polariton. The optical phase shift that we demonstrate is five times larger compared to previous results observed in exciton-polariton systems.

------------------------------------------

Integrating spin centres with photonic structures in SiC based quantum devices. 

Shivani Bisht, Heriot-Watt University

The integration of spin centers with photonic structures, such as solid immersion lenses and nanopillars, and electronic devices such as p-i-n diode,  in silicon carbide-based quantum devices represents a promising frontier in quantum technology. This integration enables the precise control and manipulation of individual quantum states, leading to advances in quantum computing, communication, and sensing.

 In this poster, we outline our plans to integrate single spins in silicon carbide with electronic and photonic functionalities.  We discuss the potential applications of this technology and its significance in the realization of practical quantum devices for quantum sensing and computing applications. The synergy between silicon carbide's excellent material properties and photonic components opens up exciting possibilities for the future of quantum information processing and quantum-enhanced technologies.

------------------------------------------

Storing quantum coherence in a quantum dot nuclear spin ensemble for over 100 milliseconds. 

Harry Edward Dyte, University of Sheffield

Preserving coherent states over long timescales is a key requirement in quantum computing and communications. Nuclear spins benefit from well-defined interactions and good isolation from the surrounding environments, offering long coherence. However, nuclear spin coherence in optically-active group III-V quantum dots is limited to ~1 ms by the dipole-dipole interactions within the dense nuclear spin lattice. Here, we use nuclear magnetic resonance (NMR) dynamical decoupling to overcome the nuclear-nuclear interactions and extend the coherence time to a ~100 ms range. This is made possible using lattice-matched GaAs/AlGaAs QDs and strain engineering, which allows isolation of the spin ±1/2 subspace (Figure 1a). The smallness of inhomogeneous disorder of the spin ±1/2 states enables high-order multipulse CHASE dynamical decoupling (Figure 1b), which would be impossible in highly-strained Stranski-Krastanov QDs. These results demonstrate the potential to harness many-body nuclear spin baths present in these QDs as a form of optically-accessible long-lived quantum memory.

------------------------------------------

A new optically addressable electronic spin resonance in single defects in hexagonal boron nitride at room temperature 

Simone Eizagirre Barker, University of Cambridge

Single-photon emitting defects in hexagonal boron nitride (hBN) have emerged as a promising platform for quantum photonic applications, as they can be readily integrated into nanoscale devices and sensors thanks to their reduced dimensionality and show promising optical and spin properties at room temperature.

Recently, we showed the first demonstration of optically-detected magnetic resonance (ODMR) from single spin defects in a van der Waals material at room temperature, with ODMR contrasts of up to 30%, demonstrating their potential as solid-state qubits. In this contribution, we present our latest results towards determining the spin model of these singly addressable defects and further reveal their potential for applications in quantum information and sensing.

------------------------------------------

Bound States in Continuum in Tight-Binding Ladder Lattice Model

Oliver Fox, University of Exeter

------------------------------------------

Photophysics of single spin-active defects in hBN

Stephanie Fraser, University of Cambridge

Following recent discovery of bright and robust single photon emitters in hexagonal boron nitride (hBN), this material has emerged as a promising platform for solid-state quantum engineering. Quantum correlation measurements confirm the single photon purity of several emitters. We propose the magnetic field and excitation power-dependent variations in bunching dynamics may be explained by an electronic model featuring spin manifolds and a metastable dark state, describing a spin-active defect. However, dephasing mechanisms – including spectral diffusion – limit the optical (and spin) coherence of these defects. Room temperature photoluminescence emission spectra reveal a Lorentzian zero-phonon line centred around 580 nm. Our analysis suggests phonon broadening is the main source of linewidth broadening; though charge noise is likely to become dominant at cryogenic temperatures. Future work will involve fabrication of heterostructure devices to electrostatically gate hBN, with the aim of suppressing charge noise and hence limiting linewidth broadening.

------------------------------------------

Reversible spin-optical interface in luminescent organic radicals

Dr. Sebastian Gordon, University of Cambridge

Molecules present a versatile platform for quantum information science, and are candidates for sensing and computation applications.[1] Robust spin-optical interfaces are key to harnessing the quantum resources of materials. To date, carbon-based candidates have been non-luminescent,[2] which prevents optical read-out via emission. I will present the first organic molecules displaying both efficient luminescence and a high generation yield of excited states with S>1. This is achieved by designing an energy resonance between emissive doublet and triplet levels, here on trityl-carbazole radicals covalently linked to anthracene.[3] The doublet photoexcitation delocalises onto the acene within a few picoseconds and subse-quently evolves to a pure, strongly exchange-coupled state (quartet for monoradicals, quintet for biradical) of mixed radical-triplet character near 1.8 eV. These high-spin states are addressable with microwaves even at 295 K, with optical read-out enabled by reverse intersystem crossing to emissive states. Furthermore, the biradical ground state is spin polarised following excited state relaxation. Our approach simultaneously supports a high efficiency of initialisation, spin manipulations and light-based read-out at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies.

References

[1]        Wasielewski, M., Forbes, M., Frank, N., Kowalski, K., Scholes, G., Yuen-Zhou, J., Baldo, M., Freedman, D., Goldsmith, R., Goodson III, T., Kirk, M., McCusker, J., Ogilvie, J., Shultz, D., Stoll, S., Whaley, K.B. Exploiting chemistry and molecular systems for quan-tum information science. Nat Rev Chem, 4, 490-504 (2020). https://doi.org/10.1038/s41570-020-0200-5 

[2]        Quintes, T., Mayländer, M., Richert, S. Properties and applications of photoexcited chro-mophore–radical systems. Nat Rev Chem, 7, 75-90 (2023). https://doi.org/10.1038/s41570-022-00453-y 

[3]        Gorgon, S., Lv, K., Grüne, J., Drummond, B., Myers, W., Londi, G., Ricci, G., Valverde, D., Tonnelé, C., Murto, P., Romanov, A., Casanova, D., Dyakonov, V., Sperlich, A., Beljonne, D., Olivier, Y., ,Li, F., Friend, R.H., Evans, E.W. Reversible spin-optical interface in luminescent organic radicals. Nature, 620, 538-544 (2023). https://doi.org/10.1038/s41586-023-06222-1 

------------------------------------------

Quintet and Triplet Generation by Intramolecular Singlet Fission

Dr. Jeannine Grune, University of Cambridge

Singlet fission (SF) is a key concept for improving the efficiency of solar cells by enabling a multiplication of photoexcited states. The principle is based on a photoexcited singlet exciton rapidly decaying into spin-correlated triplet pairs that dissociate into free triplets. We combine the complementary techniques of optical spectroscopy, as transient absorption (TA), with electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) to monitor the intermediate states with different spin multiplicities. TA thereby enables to probe the rapid decay of singlet excitons into spin-correlated triplet pairs and the detection of long-lived triplet states. Complementary, EPR spectroscopy allows to indentify the involvement of exchange-coupled triplet pairs with quintet character (S=2), which dissociate into triplet excitons (S=1) via the singlet fission process. ODMR completes the picture by probing the high spin states participating in luminescence, enabling us to draw a picture of the exciton pathways in singlet fission. We focus on new concepts of intramolecular singlet fission (iSF) based on units of the SF-active chromophore diphenylhexatriene (DPH). We found that upon fast iSF to generate strongly exchange-coupled triplet pairs, the generation of quintet and triplet states strongly depends on the overall geometry and the number of molecular units. The characterization of different oligomers allows the study of the involved quintet and triplet dynamics to find a general recipe for efficient iSF materials.

------------------------------------------

Exploiting Scattering Scanning Near-field Optical Microscopy  (s-SNOM) for Nanoparticle Analysis

Xinyun Liu, University of Manchester

InN materials have been widely used in optoelectronic devices. However, the oxidization introduced by air exposure can change their bandgap and mobility drastically. In this study, we employed a new technique, scattering type near-field optical microscopy (s-SNOM) to investigate the oxidization level of InN nanoparticles and demonstrate how tapping amplitude and demodulation order affect the near-field response. Our nanoscopy shows the resonance modes of carbon and oxygen related bonds. Moreover, with tuning probing depth, we have confirmed the vertical inhomogeneity of this surface state. With theoretical model connecting sample dielectric property to optical response, we also retracted local dielectric function, correlating with resonances observed in optical spectra. This direct probing of InN surface states via s-SNOM provides the feedback to material growing process thus speeds up the optimization of InN materials.

------------------------------------------

Coherent spin-electric coupling in molecular nanomagnets

Junjie Liu, University of Oxford

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics because electric fields can be confined over shorter length scales than magnetic fields. This has important consequences for the design of spin-based information technologies: while the Zeeman interaction with a magnetic field provides a convenient tool for manipulating spins, it is difficult to achieve local control of individual spins on the length scale anticipated for useful quantum technologies. This motivates the study of spin-electric field couplings (SEC) in molecular nanomagnets.

Here we present the investigation of SECs in several molecular systems, ranging from transition metal/lanthanide-based molecular nanomagnets to light-induced radical pairs, by applying electric field pulses during time-resolved EPR measurements. By identifying molecules with a significant electrical polarisability and a spin spectrum that is highly sensitive to the structural degree of freedom, we demonstrate strong SECs that generate significant perturbations to Zeeman states. Importantly, the effect of the electric field does not affect the spin coherence, making the coupling suitable for quantum information processing applications. In HoW10 we demonstrate coherent electrical control of the molecular spin states and independent manipulation of the two magneti-cally indistinguishable inversion-related molecules in the unit cell of the crystal.

------------------------------------------

Room-temperature optically detected coherent control of molecular spins

Dr. Sarah Mann, University of Glasgow

Optically addressable molecular spins offer a promising platform for quantum technologies, particularly in the fields of imaging and sensing. Their ability to be tuned through synthetic chemistry, along with their flexible methods of deployment, combined with a spin-optical interface for initialisation, coherent control, and detection, offers the potential for customising quantum sensors to specific applications.

Achieving room-temperature optically detected coherent control of molecular spins would be advantageous for various applications, however to date this has remained challenging. Here we demonstrate key optically detected coherent control measurements—Rabi oscillations, Ramsey interferometry and Hahn echoes—obtaining photoluminescence contrasts exceeding 10% and microsecond coherence times at ambient temperatures in a molecular spin system. Additionally, we show how the simultaneous control of multiple spin transitions is as an effective strategy to enhance spin-optical contrast, and demonstrate the diverse deposition capabilities of molecular spin systems by extending the optical readout of coherent control of a thermally evaporated thin film. Overall, these results indicate promise for quantum sensing with molecular spins.

This work was supported by UK Research and Innovation [MR/W006928/1].

------------------------------------------

Terahertz Emission from Topological Insulators

Dr. Abdul Mannan, University of Manchester

Topological Insulators (TI) have attracted significant attention for spintronic applications due to the interesting properties of their topological surface states (TSS). However, to fully utilize the potential of TSS in TI materials, a contactless, non-destructive, and non-invasive characterization technique is required. [1] Topological materials have already shown terahertz (THz) emission capabilities due to the helicity-dependent photocurrents that link between their bulk and surface states. [2] Here, we have compared three types of topological materials: a) a Bi2Te3 thin-film b) a Cd3As2 nanocrystal and c) Cd3As2 nanowire ensemble using THz emission spectroscopy. We observe helicity dependent THz emission from all three materials. Although their crystal structures and band-structures differ significantly, the THz emission trends based on polarization and crystal orientation looks similar. Our result shows that TI materials are promising candidates for future terahertz devices and optoelectronic quantum devices. 

[1] Phys. Rev. B 96, 195407 (2017) [2] ACS Photonics 2023, 10, 5, 1473–1484 (2023)

------------------------------------------

Optical Sources for Atomic Sensors

James Meiklejohn, Cardiff University

Atomic sensors are clocks, magnetometers, and gyroscopes that use the electronic transitions of alkali metal vapours as a consistent physical reference to achieve extremely high precision and stability. They find a diverse range of uses in medical imaging, geological sensing, navigation,  and security, among other applications. The performance of the sensor is highly dependent on the quality of the laser source used, and a need for robust, commercially scalable and miniaturised atomic sensors leads to a requirement for suitable laser light sources.

A vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor diode laser. Highly reflective Bragg mirrors surround a cavity region containing a quantum dot or quantum well active region, with light emitted vertically from the top of the structure. Compared to edge-emitting diode lasers they are much smaller and can be produced at lower cost, but have lower output power. For atomic sensor applications there are strict requirements on the power, linewidth, beam profile, mode stability and wavelength.

We outline some of the facilities and techniques that are being used to develop VCSELs suitable for atomic sensor applications at Cardiff University, and some possible future directions for novel designs.

-----------------------------------------

Unveiling NV Charge Dynamics in Nanodiamonds through Redox Interactions

Dr. Shalini Menon, University of Nottingham

This investigation delves into the interaction between redox reactions and the charge dynamics of Nitrogen-Vacancy (NV) centers within fluorescent nanodiamonds (fNDs), with a focus on modulation mediated through interactions with a redox-active protein. Utilizing electrochemical methods, we probed the photoluminescence (PL) behavior of fNDs under varying electrochemical potentials. Our findings unveiled a notable modulation in PL intensity, indicative of NV charge state transitions corresponding to shifts in electrochemical potentials. Subsequent interactions with the redox protein exhibited inverse PL responses, showcasing electron transfer events and consequent NV charge state alterations. Further experiments with fNDs of different surface terminations were conducted to comprehend their influence on NV charge state modulation. This exploration elucidates the multifaceted relationship between redox reactions, surface attributes, and NV charge state dynamics in fNDs, and proposes a novel avenue for monitoring redox alterations, thereby expanding the potential applications of NV centers in sensing and beyond.

-----------------------------------------

The effect of alloying disorder and nanoscale strains in Al_yGa_(1−y)As/Al_xGa_(1−x)As quantum dots

Peter Milington-Hotze, University of Sheffield

The understanding of quantum dot (QD) strain in semiconductor nanostructures is crucial for their application to quantum logic gates, due to its effect on electron spin-qubit coherence times. Previous studies have estimated strain through lattice mismatches of the QD with its surrounding barrier, however determining how the strain is distributed within the QD has thus far not been achieved. Through a combination of nuclear magnetic resonance techniques we non-invasively probe the strain within a variety of GaAs, AlGaAs and InGaAs quantum dot systems. We find the addition of Al and In into the quantum dots cause visible broadening of the nuclear transition states, due to atomic scale strain effects . Increasing the content of these additions exaggerates the effect of this broadening for both As and Ga isotopes, with a greater effect occurring for the As due to the replacement of Ga in the zinc-blende lattice.

-----------------------------------------

Ab Initio Design of Molecular Qubits with Electric Field Control

William Morrillo, University of Manchester

Current commercial quantum computers require large footprints due to the size and scale of superconducting qubits. This imposes expensive running costs and restrictions of scale. Molecular qubits and qudits offer a promising alternative due to their atomic scale and the ability to tune their properties through chemical design [1]. The most commonly adopted technique for manipulating molecular qubits is the EPR experiment, which allows for the rotation of a half integer spin in the Bloch sphere using resonant microwave radiation. However, this technique does not offer the precision required for individual molecule manipulation. X-band EPR experiments have a wavelength of ≈ 3 cm, and such the experiment is performed on a ensemble of molecules. Electric fields offer a promising alternative due to their atomic scale precision and have been explored experimentally in recent literature [2–4]. In this work, we explore the theoretical explanation of electric field control of molecular qubits and outline ab initio driven methods to design molecules with enhanced electric field response as suitable candidates for molecular qubits and qudits. We identify how structural perturbations arising from applied ex- ternal electric fields generate coupling elements between near degenerate Kramer’s doublet ground states split by the Zeeman effect in the crystal field Hamiltonian. We demonstrate how the choice of coordination geometry and lanthanide (III) ions has an effect on the electric field susceptibility and the generation of coupling elements in the crystal field Hamiltonian. Finally, we present an analytical vibronic model of structural perturbations due to applied electric fields.

References

(1) R. E. Winpenny, Angewandte Chemie - International Edition, 2008, 47, 7992–7994.

(2) J. Liu, J. Mrozek, A. Ullah, Y. Duan, J. J. Baldov ́ı, E. Coronado, A. Gaita-Arin ̃o and A. Ardavan, Nature

Physics, 2021, 17, 1205–1209.

(3) S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan and A. J. Heinrich, Science, 2015, 350, 417–420.

(4) J. Robert, N. Parel, P. Turek and A. K. Boudalis, Journal of the American Chemical Society, 2019, 141, 19765–19775.

-----------------------------------------

Towards quantum-confined spin-qubits in monolayer, semiconducting WSe2

Eleanor Nichols, University of Cambridge

Monolayer transition metal dichalcogenides provide a promising platform for quantum communications due in large part to their 2D nature. Interface-free emission from the monolayer results in efficient light extraction. Meanwhile, large exciton binding energies ~100 meV due to low screening and confinement in the 2D plane are suitable for operation at elevated temperatures. Strong optical selection rules arising from the symmetry of the 2D lattice enable optical excitation of excitons at distinct valleys through control over the polarisation of optical excitation. Establishing a spin in the ground state with spin-dependent optical selection rules would enable an efficient mechanism for photon entanglement. Such a spin ground state is expected to have long coherence lifetimes (~40 ms) from the dilute nuclear spin bath and reduced dimensionality of the material. However, a local spin ground state has yet to be identified for these quantum emitters. Here, we present our on-going efforts to introduce a quantum emitter with a spin ground state in fully encapsulated, monolayer WSe2. Establishing a local spin qubit with optical selection rules will pave the way towards a coherent spin-photon interface in 2D materials.

-----------------------------------------

Masers-in-a-shoebox: Portable quantum sensors reaching 1 Kelvin noise temperatures, without cryogenics

Dr. Wern Ng, Imperial College London

Masers, the microwave analogue of the laser, have the ability to amplify the weakest electrical signals which gives them the potential to revolutionise medical sensors and frequency standards for GPS communication. However, all implementations of these masers to date are still unwieldy and laborious to transport, requiring large pump sources and magnets that weigh 100 kilograms. The maser needs miniaturisation, and so we present a room-temperature maser that has been made portable and weighs less than 5 kg. Not only is it trivial to transport compared with any maser ever demonstrated, but it also offers the highest output maser power recorded in the literature at -5 dBm, hence moving the technology forward in accessibility and capability. Beyond paving the way for the wider research community to make their own portable maser and test its sensing capabilities, our device would aid in the study of strong spin-photon coupling and cavity quantum electrodynamics, all at room temperature on the benchtop

-----------------------------------------

Monolithic Nanofabrication of Cavity-emitter System with Hexagonal Boron Nitride

Dr. Milad Nonahal, University of Manchester

Light–matter interactions in optical cavities underpin many applications of integrated quantum photonics. Among various solid-state platforms, hexagonal boron nitride (hBN) is gaining considerable interest as a compelling van der Waals host of quantum emitters. However, progress to date has been limited by an inability to engineer simultaneously an hBN emitter and a narrow-band photonic resonator at a predetermined frequency. In this work, we demonstrate deterministic fabrication of a monolithic, coupled cavity–emitter system with hBN. Specifically, we designed this system to host blue quantum emitters that has an emission wavelength of 436 nm and is induced deterministically by electron beam irradiation within the cavity hotspot. Our work constitutes a promising path to scalable on-chip quantum photonics and paves the way to quantum networks based on van der Waals materials.

-----------------------------------------

High Frequency Magnetometry with an Ensemble of Protected Spin Qubits in Hexagonal Boron Nitride

Charlie Patrickson, University of Exeter

The development of two-dimensional quantum sensors could provide exposure to novel systems with exceptional levels of sensitivity, as they would allow the probe to be placed in close proximity to the target. AC magnetometry is of particular interest, where current methods use the signal field to drive detectable changes in the spin state of crystalline defects. However, this approach presents a challenge in III-V material systems, as the non-zero magnetic moment of the host nuclei limits the coherence time of the spin states and thus the sensitivity. In this contribution we discuss the use of continuous concatenated dynamical decoupling (CCDD) schemes that simultaneously provide robust protection against these decoherence mechanisms, whilst also enabling high frequency magnetometry. The technique uses a strong continuous micro-wave drive to stabilise the spin vector and produces a splitting of the spin state energies which can be tuned into resonance with a signal field. Applying this approach to an ensemble of Boron vacancies in hexagonal Boron Nitride, we use amplitude and phase modulated CCDD to extend the ground state spin coherence time to 4us, which is an improvement of ~150 times[1]. Through manipulation of the CCDD drive terms we are able to tune the sensors spin state transitions, and subsequently the detection range, from 10 MHz to 3 GHz, demonstrating a sensitivity of 2uT/Hz^1/2 at ~2.5GHz. Additionally, we demonstrate amplitude detection by mapping the magnetic field from a 40um diameter wire loop, where we find good agreement between simulation, experimental and analytical results.

[1] A. J. Ramsay, R. Hekmati, C. J. Patrickson, S. Baber, D. R. M. Arvidsson-Shukur, A. J. Bennett, and I. J. Luxmoore, Coherence protection of spin qubits in hexagonal Boron Nitride, Nature Communications 2023 14:1 14, 1 (2023)

-----------------------------------------

2D Conductive Polymers for Quantum Electronic Devices

Rebecca Peake, Queen Mary University of London

Since the discovery of Single Layer Graphene in 2004, research has increased exponentially into the development of other 2D materials with similarly interesting electronic and chemical properties. Inorganic 2D materials provide promising candidates, such as Hexagonal Boron Nitride and Transition-metal dichalcogenides. However, these inorganic materials are limited as they lack flexibility in their design and therefore in their chemical and physical state. This is why the field of framework materials, including Metallic Organic Frameworks (MOF)’s and Covalent Organic Frameworks (COF’s) has attracted so much attention. The properties of constructed 2D COF’s can be adapted by choice of lattice symmetry and molecular orbital symmetry of constituent molecular building blocks. It is the aim of my PhD to establish clear relationships between the structure and electronic properties of a given set of conductive COF’s. In this poster I will give an overview on the experimental and theoretical methods driving this project.

-----------------------------------------

Purcell-Enhanced Single Photons at Telecom Wavelengths from a Quantum Dot in a Photonic Crystal Cavity

Dr. Catherine Philips, University of Sheffield

Quantum dots are promising candidates for telecom single photon sources due to their tunable emission across the different low-loss telecommunications bands, making them compatible with existing fiber networks. Their suitability for integration into photonic structures allows for enhanced brightness through the Purcell effect, supporting efficient quantum communication technologies. Our work focuses on InAs/InP QDs created via droplet epitaxy MOVPE to operate within the telecoms C-band. We observe a short radiative lifetime of 340 ps, arising from a Purcell factor of 5, owing to interaction of the QD within a low-mode-volume photonic crystal cavity. Through in-situ control of the sample temperature, we show both temperature tuning of the QD's emission wavelength and a preserved single photon emission purity at temperatures up to 25K. These findings suggest the viability of QD-based, cryogen-free, C-band single photon sources, supporting applicability in quantum communication technologies.

-----------------------------------------

Optical and spin characteristics of single-defect colour centres in hexagonal boron nitride

Oliver Powell, University of Cambridge

Colour centres in two-dimensional hexagonal boron nitride (hBN) are a promising candidate towards a 2D spin-photon interface, offering the advantage of room temperature single-photon emission combined with integration into scalable and compact hardware, thanks to their reduced dimensionality. Realizing a spin-photon interface in this material relies on identifying a defect species with highly coherent spin and optical degrees of freedom. Although both room-temperature access to single-defect spins and room-temperature Fourier-transform limited optical emission have been observed for spin defects in hBN, it is unclear if the defects in these separate investigations are related in structure. We present work on a spin-active carbon-related defect species showing promising optical properties at room temperature, with the goal of determining the limits of single-defect optical coherence. Via temperature-dependent spectroscopy of individual defects, we investigate the mechanisms responsible for broadening of the single-defect optical emission from room temperature down to cryogenic conditions. These results are an important step towards realizing coherent excitation of these defect centres. Additionally, understanding the photo-dynamics of these emitters provides additional information towards the identification of their microscopic structure, and will help unravel their potential as solid-state qubits and nanoscale quantum sensors.

-----------------------------------------

High Q Hybrid Mie-Plasmonic Resonances in Van der Waals Nanoantennas on Gold Substrate

Sam Randerson, University of Sheffield

Currently, dielectric nanoresonators are emerging as promising candidates for reducing the high losses associated with plasmonic devices, however they suffer from lower quality (Q) factor resonances. By fabricating a hybrid system of dielectric and metallic materials, one can retain the low losses of dielectric resonances whilst achieving stronger mode confinement with the metal, leading to improved Q factors. Multi-layered van der Waals materials are particularly suited to integration with metals owing to their weak attractive forces which do not require lattice matching. We therefore exfoliate high refractive index transition metal dichalcogenide (TMD) WS2 onto gold, to then fabricate and optically characterise a hybrid dielectric nanoantenna-on-metal system. We experimentally observe a variety of resonant modes within the structure, including a hybridisation of Mie resonances and surface plasmon-polaritons which are launched from the nanoantennas into the substrate. We name these hybrid modes Mie-plasmonic (MP), which exhibit Q factors of over 130 in experiment. We further show that such hybridised modes can be tuned to the strong coupling regime, showing signatures of a supercavity mode with a further enhanced experimental Q factor of over 260, two orders of magnitude higher than for Mie modes in nanoantennas on SiO2. We then simulate WS2 nanoantennas above gold with an hBN spacer in-between, resulting in calculated electric field enhancements exceeding 2600, and a Purcell factor of up to 713 for an emitter within the hBN, owing to the hybridised MP mode. The nanoantenna-hBN-gold structure also boasts a high directivity of the emitted light of up to 50 %, giving an overall light extraction enhancement of over 10^7 in simulation. Our results therefore demonstrate how multi-layer TMD nanoantennas can be combined in a variety of configurations with metals to access high-Q factor, strongly confined hybrid resonances, with the ability to greatly enhance coupled emitters such as excitons and single photon emitters placed within the nanoantenna-substrate gap.

----------------------------------------- 

Tuning of the qubit-register coupling in a central spin system via polarization locking

Noah Shofer, University of Cambridge

The central spin system, in which a single spin qubit interacts with a many-body register of spins, holds promise as a candidate for the exploration of quantum many-body physics and as a quantum computation platform. Previous work in InGaAs quantum dots (QDs) [1, 2] demonstrated the potential of optically active III-V semiconductor QDs as a central spin system. While InGaAs QDs are limited by a relatively short electron coherence time [3], nanodroplet-etched GaAs QDs benefit from much longer coherence times with similar optical properties to InGaAs dots [4]. In this work, we utilize a g-factor anisotropy in a GaAs QD to implement an electron-nuclear spin-flip interaction, realizing a central spin system consisting of an electron spin interacting with an ensemble of ∼105 nuclear spins. We find that by shifting the mean-field polarization of the nuclear bath by relatively small amounts using an in-situ algorithmic cooling routine we can change the strength of the spin-flip interaction. We then illustrate a potential application of this technique by tuning the Rabi rate of a collective nuclear spin flip as a function of the mean field polarization, and demonstrate that the scaling of the Rabi rate with polarization matches that of the spin-flip interaction. Overall, this work highlights the potential for engineering the mesoscopic environment of GaAs QDs for a range of quantum information purposes.

[1]  D. A. Gangloff, G. E ́thier-Majcher, C. Lang, E. V. Denning, J. H. Bodey, D. M. Jackson, E. Clarke, M. Hugues, C. Le Gall, and M. Atatu ̈re, Quantum interface of an electron and a nuclear ensemble, Science 364, 62 (2019).

[2]  D. M. Jackson, D. A. Gangloff, J. H. Bodey, L. Zaporski, C. Bachorz, E. Clarke, M. Hugues, C. Le Gall, and M. Atatu ̈re, Quantum sensing of a coherent single spin excitation in a nuclear ensemble, Nat. Phys. 17, 585 (2021).

[3]  R. Stockill, C. Le Gall, C. Matthiesen, L. Huthmacher, E. Clarke, M. Hugues, and M. Atatu ̈re, Quantum dot spin coherence governed by a strained nuclear environment, Nat Commun 7, 12745 (2016).

[4]  L. Zaporski, N. Shofer, J. H. Bodey, S. Manna, G. Gillard, M. H. Appel, C. Schimpf, S. F. Covre da Silva, J. Jarman, G. Delamare, G. Park, U. Haeusler, E. A. Chekhovich, A. Rastelli, D. A. Gangloff, M. Atatu ̈re, and C. Le Gall, Ideal refocusing of an optically active spin qubit under strong hyperfine interactions, Nat. Nanotechnol. 10.1038/s41565-022- 01282-2 (2023).

----------------------------------------- 

First-principles spin-phonon coupling calculations for the modeling of T1 relaxation in crystalline molecular magnetic materials

Jakob Staab, University of Manchester

Single-molecule magnets (SMMs) are an interesting research subject due to their potential applications as high density data storage units and solid-state qubits. Finite temperature spin dynamics driven by energy exchange with thermal vibrational energy present in the environment is detrimental to many of their desired properties, and harnessing the coupling of phonons to the electronic states is crucial to the design of the next generation SMMs.

The computational modelling of crystalline molecular magnetic materials is an important cornerstone in understanding relaxation and decoherence pathways on the atomic level,[1, 2] but the treatment of extended, infinitely periodic environments poses major challenges to contemporary quantum chemical protocols: (i) The efficient derivation of spin-phonon coupling parameters along tens of thousands of nuclear degrees of freedom; and (ii) the consistent inclusion of the long range electrostatic potential into correlated wave function methods. Recently, we implemented the fully analytical linear vibronic coupling method which enables the efficient and accurate evaluation of spin-phonon couplings in extended systems, where the condensed phase environment is described by point charges.[3] Using a finite cluster expansion of point charges coupled with a reaction field approach to recover the long-range electrostatics in correlated electronic structure calculations, we compute temperature-dependent magnetic relaxation rates in a Dy(III) based SMM in the solid state which show close agreement with experiment.[4]

(1) Mondal, S.; Lunghi, A. npj Comput. Mater. 2023, 9.

(2) Kragskow, J. G. C.; Mattioni, A.; Staab, J. K.; Reta, D.; Skelton, J. M.; Chilton, N. F. Chem. Soc. Rev. 2023.

(3) Staab, J. K.; Chilton, N. F. J. Chem. Theory Comput. 2022, 18, 6588–6599.

(4) Nabi, R.; Staab, J. K.; Mattioni, A.; Kragskow, J. G. C.; Reta, D.; Skelton, J. M.; Chilton, N. F. 2023, submitted.

-----------------------------------------

Microwave-based quantum control and coherence protection of tin-vacancy spin qubits in a strain-tuned diamond membrane heterostructure

Alexander Stramma, University of Cambridge

Robust spin-photon interfaces in solids are essential components in quantum networking and sensing technologies. Ideally, these interfaces combine a long-lived spin memory, coherent optical transitions, fast and high-fidelity spin manipulation, and straightforward device integration and scaling. The tin-vacancy center (SnV) in diamond is a promising spin-photon interface with desirable optical and spin properties at 1.7 K. However, the SnV spin lacks efficient microwave control and its spin coherence degrades with higher temperature. In this work, we introduce a new platform that overcomes these challenges - SnV centers in uniformly strained thin diamond membranes. The controlled generation of crystal strain introduces orbital mixing that allows microwave control of the spin state with 99.36(9) % gate fidelity and spin coherence protection beyond a millisecond. Moreover, the presence of crystal strain suppresses temperature dependent dephasing processes, leading to a considerable improvement of the coherence time up to 223(10) μs at 4 K, a widely accessible temperature in common cryogenic systems. Critically, the coherence of optical transitions is unaffected by the elevated temperature, exhibiting nearly lifetime-limited optical linewidths. Combined with the compatibility of diamond membranes with device integration, the demonstrated platform is an ideal spin-photon interface for future quantum technologies. 

-----------------------------------------

Correlative STEM/APT analysis of multiferroic Aurivillius-phase thin films

Geri Topore, Imperial College London

Functional properties in many technologically-relevant materials are highly dependent on their structure and composition at the nano and atomic scale. Improving the ways in which we probe materials at such length scales will allow for a better understanding of the interplay of structure, composition and processing for controlling material properties. Scanning Transmission Electron Micsroscopy (STEM) is one of the most powerful atomic-scale characterisation techniques, with the ability to record both structural information through imaging and diffraction, as well as composition and electronic structure through EDS and EELS. Recent advances in the form of segmented and pixelated detectors have led to the emergence of new information-rich techniques like 4D-STEM, where a variety of structural analyses can be conducted from a single dataset. Atom Probe Tomography (APT) is another technique that has enabled the compositional mapping of nano-scale volumes of materials in three dimensions with high elemental sensitivity, regardless of atomic weight. Combining STEM and APT analyses on the same sample has allowed the spatial correlation of structure and 3D composition at the atomic scale. However, we note that while this approach has largely been used in the study of metals, its place in the investigation of electronic materials such as oxide-based systems and semiconductors, as well as thin film heterostructures, is yet to be fully explored. Here a complementary and correlative analysis of a magnetoelectric Aurivillius-phase thin film using STEM/APT is demonstrated. EELS was used to map the fine structure and valency changes of magnetic cations (Mn and Fe) at the unit-cell length-scale, while APT is used to provide large scale compositional variations and segregations of these cations.

-----------------------------------------

Linear electric field effect in Mn(II) trigonal bipyramidal complexes

Mikhail Vaganov, University of Oxford

Molecular spins are among the candidate physical systems for the building blocks of quantum technologies. A key implementation challenge arises because magnetic fields, traditionally used to control quantum spins, are difficult to generate and localise on the molecular scale. A practical solution to this task is the use of molecular compounds whose spin Hamiltonian can be controlled via an electric field (E-field) which can easily be generated between compact and oppositely charged electrodes. This is possible when there is a strong coupling between the spin system and an E-field via the spin-orbit interaction or via crystal field parameter D. In particular, if the shape of the molecular compound changes in response to the application of an E-field, these changes affect the value of D along with the eigenvalues of the Hamiltonian, and are, consequently, rendered into the variation in the ESR frequency.

In this work we study three trigonal bipyramidal Mn(II) complexes where a single electropositive Mn ion with S=5/2 is juxtaposed to one of three different electronegative halogens (I, Br or Cl). An attractive feature of these compounds is the opportunity to change the halogen ion relatively easily: this allows one to study how the quantum properties and relaxation times of the molecules vary when just single ion is replaced.

-----------------------------------------

High nonlinearity of Mie-polariton with van der Waals heterostructures

Dr. Yadong Wang, University of Sheffield

-----------------------------------------

Heterogeneous integration of solid-state quantum  systems with a foundry photonics platform

Hao-Cheng Weng, University of Bristol

Nitrogen-Vacancy (NV) center is a promising candidate for quantum information processing and quantum communication interfacing with the optically addressable spin states. Recently, great efforts have been made to integrate diamond hosted NV centers with scalable quantum photonic platforms. Here, we show the first heterogeneous integrated device of nanodiamonds with foundry-optimized low-fluorescence silicon nitride photonics. Nanodiamonds are deterministically positioned over pre-defined sites on a waveguide during a single post-processing step. By an array of eight fibers, we excite NV centers selectively through the middle input ports and collect the photoluminescence (PL) with mode-matched grating couplers through output ports on the two ends. Experimentally, PL from NV centers is identified by the 6 ns-10 ns decay lifetime and the saturated emission accompanied by intermittency in the PL (blinking). We verify the single photon emission by an on-chip Hanbury Brown and Twiss experiment, where detection events from the two output ports are cross-correlated to show anti-bunching quantum statistics. Our work presents a simple but effective route to foundry compatible integration of NV centers at large scale.

-----------------------------------------

A quantum circuit architecture based on the integration of nanophotonic devices and two-dimensional molecular network

Dr. Wai Wu, University College London

Recently, spin-bearing molecules have experimentally been demonstrated to have great potentials as building blocks for quantum information processing, due to their tunability, portability and scalability. [1] Especially the optically addressable spin-bearing molecules are even more attractive for high-temperature operations. [2,3] Nanophotonic devices are important to program the quantum circuit to realize quantum gate operations. However, up to now, it is still elusive to design quantum circuit of this type. 

In this work, we have used cutting-edge hybrid-exchange density-functional theory to compute the exchange interactions [2]. In addition, we have performed the analysis for the time evolution of quantum states and the time-resolved electron paramagnetic spectra by using theory of open quantum systems [3].

Here we proposed a quantum computing architecture, by combining the two-dimensional molecular networks with nanophotonic devices. Our design is based on (1) the rigorous density functional theory calculations validated by the relevant experimental observations [4] and (2) an open quantum system simulation of the time-resolved electron paramagnetic resonance spectra with realistic experimental parameters. We take the biTYY-DPA molecule to compute the triplet-mediating exchange interaction between radicals [4]. The resulting optically driven ferromagnetic interaction is consistent with the previous experiments. We have also computed the time evolution of quantum states when the triplet is present and found that the coherence can be created by using the excited triplet state, as indicated by the non-zero off-diagonal terms in the reduced density matrix for radicals. We have also designed the quantum circuit architecture by integrating two-dimensional spin-bearing molecular network and nanophotonic devices such that we can realise programmable quantum gate operations. 

This work would therefore lay a solid theoretical cornerstone for optically driven quantum computing circuit in radical-bearing molecular network, thus towards high-temperature quantum information processing. In this way, we can not only scale up substantially the quantum circuits but also raise the operation temperature. 

References:

[1] S. L. Bayliss, et. al., Science 307, 309 (2020).

[2] J. Chang, et. al., arXiv: 2209.04835 (2022), accepted in NPG Asia Materials.

[3] L. Ma, et. al., NPG Asia Materials, 14:45 (2022).

[4] Y. Teki, et. al., J. Am. Chem. Soc. 122 984 (2000)

-----------------------------------------

Single photon sources in an all TMD material nanophotonic system

Dr. Panaiot Zotev, University of Sheffield

Transition metal dichalcogenide (TMD) single photon emitters (SPEs) have previously attracted a large interest due to their unique properties which offer numerous advantages for quantum information processing applications, such as deterministic positioning [1] and ease of integration into arrays via a twisted heterostructure [2]. While the nature of these quantum emitters is still under debate, strain in the host TMD is known to localize their formation [3]. Thus the integration of TMD monolayers with silicon dioxide (SiO2) nanopillars [1] or photonic nanoantennas in GaP has demonstrated not only spatial localization, but also resonant enhancement of their quantum efficiency [4]. These experimental approaches have previously required the integration of TMD monolayers witha different material system thereby constricting the choice of substrate. In our work, we demonstrate the formation of SPEs in WSe2 monolayers onto WS2 photonic nanoantennas with different substrates, namely SiO2 and Au, thereby achieving spatially localized and quantum efficiency enhanced SPEs in an all TMD material system with the additional advantage of arbitrary choice of substrate inherent to layered materials. We provide evidence for enhanced quantum efficiencies which reach an average of 21.3% for SPEs on WS2 fabricated on a SiO2 substrate with the possibility to further enhance this value by tuning the geometry of the nanoantennas. Measurements of time-resolved PL from the SPE system on an Au substrate yield a long risetime (> 1 ns) which linearly decreases with increasing excitation power to values above saturation of the emitter. This suggests that charge transfer from the TMD monolayer to the substrate [5] suppresses the Auger processes which are known to plague the saturation of TMD monolayer emitters [4]. This, therefore, enables us to record a trapping time of WSe2 dark excitons into a quantum state at excitation powers below saturation and a charge transfer dominated non-radiative decay time of the excitons at powers above saturation. Our work not only showcases the ability to form deterministically positioned SPEs with enhanced quantum efficiencies on different surfaces, but also provides evidence of the advantages offered by an insulating or metallic substrate thereby enabling the simple integration of two dimensional material single photon emitters with virtually any material systems and device.