Program

Schedule

Abstracts

Laura García-Álvarez (University of Basque Country)

Digital-analog quantum simulations of fermionic models with superconducting circuits

I will introduce digital, and digital-analog methods for quantum simulations of fermionic models in superconducting circuits. The digital approach allows the quantum simulation of complex fermionic interactions, the basis of condensed-matter models (1), and an AdS/CFT duality (2). The second approach provides a higher degree of scalability than purely digital or purely analog techniques, and it is suitable for implementing quantum simulations of interacting fermions and bosons, such as quantum field theory models (3), and quantum chemistry problems (4). I will provide examples related to these topics in the context of superconducting circuits, arguably the most advanced quantum platform for quantum computing and quantum simulations.

REFERENCES

(1) R. Barends et al., Nat. Commun. 6, 7654 (2015).

(2) L. García-Álvarez et al., arXiv:1607.08560.

(3) L. García-Álvarez et al., Phys. Rev. Lett. 114, 070502 (2015).

(4) L. García-Álvarez et al., Sci. Rep. 6, 27836 (2016).

Yuto Ashida (University of Tokyo)

Quantum many-body phenomena under continuous observation

Realizations of quantum gas microscopy have offered novel possibilities to measure quantum many-body systems at the single-particle level [1]. Further developments could allow us to perform a continuous monitoring of the many-body system [2,3]. In the first part of the talk, we discuss the influence of the measurement backaction on quantum criticality due to continuous observation. By analyzing the effective non-Hermitian Hamiltonian, we show that the measurement backaction can trigger (i) the bifurcation of critical exponents in Tomonaga-Luttinger liquid [4] and (ii) a new universality class of critical phenomena that has no counterpart in closed systems [5]. In the second part, we discuss the propagation of correlations under the measurement backaction [6]. Starting from the Lindblad master equation, we analyze the density matrix projected on the subspace containing a specific number of quantum jumps and find that there appear supersonic modes propagating beyond the generalized Lieb-Robinson bound. We discuss possible experimental realizations in ultracold atoms using controlled loss of atoms [7,8] together with quantum gas microscopy technique.

REFERENCES

[1] W. S. Bakr et al., Nature 462, 74-77 (2009).

[2] Y. A. Patil et al., PRL 115, 140402 (2015).

[3] YA and M. Ueda, PRL 115, 095301 (2015).

[4] YA, S. Furukawa, and M. Ueda, PRA 94, 053615 (2016).

[5] YA, S. Furukawa, and M. Ueda, Nat. Commun. 8, 15791 (2017).

[6] YA and M. Ueda, submitted.

[7] H. P. Luschen et al., PRX 7, 011034 (2017).

[8] T. Tomita et al., arXiv:1608.05061 (2017).

Aurelia Chenu (MIT)

From the dynamics of molecular aggregates to many-body decoherence, through portraits of thermal equilibrium

Nature has mastered the art of energy conversion, evolving an impressive diversity of photosynthetic light-harvesting complexes which are highly versatile and efficient. In a first part, I will focus on dynamical properties of the light-harvesting complexes, modeled as open quantum systems. I shall [1] present a theory explaining the origins of long-lived coherences observed in their 2D electronic spectra, and [2-3] present how to mimic natural excitation in the lab, i.e. how incoherent sunlight relates to coherent laser pulses. This leads to a new formalism, based on thermally excited wave packets, that provides the missing link for a continuous connection between classical and quantum representations of a thermal gas [4-5].

Then, I shall introduce a scheme for the quantum simulation of many-body decoherence, which is based on the unitary evolution of a stochastic Hamiltonian [6]. I will show how to simulate an effectively open dynamics governed by k-body Lindblad operator, following Markovian or non-Markovian dynamics, and provide the time scale governing the fidelity decay. Such scheme exhibits a strong signature of many-body decoherence, and can be readily implemented in current quantum platforms.

REFERENCES

[1] A. Chenu, N. Christensson, H. Kauffmann and T. Mancal, Sci. Rep. 3:2029 (2013)

[2] A. Chenu, A. Branczyk, G. Scholes and J. Sipe, Phys. Rev. Lett. 114:213601 (2015)

[3] A. Chenu and P. Brumer, J. Chem. Phys. 144:044103 (2016)

[4] A. Chenu, A. Branczyk, and J. Sipe, arXiv:1609.00014 (2016)

[5] A. Chenu and M. Combescot, Phys. Rev. A, 95:062124 (2017)

[6] A. Chenu, M. Beau, J. Cao and A. del Campo, Phys. Rev. Lett. 118:140403 (2017)

Hrvoje Buljan (University of Zagreb)

Engineering synthetic gauge fields, Weyl semimetals, and anyons

I will present a few topics of research in our group related to synthetic topological quantum matter [1]: topological phases in 3D optical lattices, more specifically Weyl semimetals in ultracold atomic gases [2], strongly interacting Bose gases in synthetic gauge fields [3], and one possible route to engineer anyons in a 2DEG in a strong magnetic field sandwiched between materials with high magnetic permeability, which induced e-e vector interactions to engineer charged flux-tube composites [4].

REFERENCES

[1] N. Goldman, G. Juzeliunas, P. Ohberg, I. B. Spielman, Rep. Prog. Phys. 77, 126401 (2014).

[2] Tena Dubček, Colin J. Kennedy, Ling Lu, Wolfgang Ketterle, Marin Soljačić, Hrvoje Buljan, Weyl points in three-dimensional optical lattices: Synthetic magnetic monopoles in momentum space, Phys. Rev. Lett. 114, 225301 (2015).

[3] Karlo Lelas, Nikola Drpić, Tena Dubček, Dario Jukić, Robert Pezer, and Hrvoje Buljan, Laser assisted tunneling in a Tonks-Girardeau gas, New J. of Phys. 18, 095002 (2016).

[4] M. Todorić, D. Jukić, D. Radić, M. Soljačić, and H. Buljan, The Quantum Hall Efect with Wilczek's charged magnetic flux tubes instead of electrons, in preparation

Pol Forn-Diaz (Barcelona Supercomputing Center )

Exploring the strongly driven spin-boson model in a superconducting quantum circuit

Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Such dissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the "spin", interacting with an environment formed by harmonic oscillators. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a superconducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to-incoherent transition can be achieved by tuning the drive amplitude.

Adolfo del Campo (UMass Boston)

Nonadiabatic Quantum Computation

The implementation of adiabatic protocols is often unfeasible or impractical. This motivates the study of intrinsically nonadiabatic quantum annealing schemes. To this end, heuristics derived from the Kibble-Zurek mechanism proves useful. Exploiting it, in the first part of the talk I derive a universal optimal stopping time for adiabatic quantum annealing in the presence of noise-induced stochastic Hamiltonian fluctuations. In the second part, I introduce a new class of algorithms that use spatial and temporal inhomogeneous protocols to drive strongly disordered quantum spin systems and minimize the residual energy of the final state.

REFERENCES

A. Dutta, A. Rahmani, A. Campo, Phys. Rev. Lett. 117, 080402 (2016)

A. Chenu, M. Beau, J. Cao, A.del Campo, Phys. Rev. Lett. 118, 140403 (2017)

M. M. Rams, M. Mohseni, A. del Campo, New J. Phys. 18, 123034 (2016)

M. M. Rams, A. del Campo, M. Mohseni, TBS

Matthew Bell (UMass Boston)

Symmetry Protected Quantum Bits

Recent advances in quantum information have resulted in superconducting qubits with relatively long coherence, suitable to demonstrate quantum error correction protocols. However, it is still a formidable challenge to realize a single logical qubit on the basis of conventional physical qubits - the number of faulty physical qubits required by the error correction protocols is very large and the control of these numerous physical qubits is very complex. In this talk experiments will be discussed which move toward implementing transformative ideas of improving the qubit coherence through symmetry-protection of a quantum state encoded in the parity of Cooper pairs in a superconducting circuit. We have studied the low-energy excitations in a minimalistic protected Josephson circuit which contains two basic elements (rhombi) characterized by the π periodicity of the Josephson energy. Novel design of these elements, which reduces their sensitivity to the offset charge fluctuations, has been employed. We have observed that the life time T_1 of the first excited state of this quantum circuit in the protected regime is increased up to 70μs, a factor of ∼100 longer than that in the unprotected state. The quality factor ω_{01} T_1 of this qubit exceeds 10^6 . Our results are in agreement with theoretical expectations; they demonstrate the feasibility of symmetry protection in the rhombi-based qubits fabricated with existing technology. It is expected that such circuits can demonstrate very long coherence in the protected state (the protection will have to be removed only during measurement of the qubit state). Even more important, the fault-tolerant gate operations can be performed on such qubits.

Norman Margolus (MIT)

Counting distinct states in physical dynamics

One of the great surprises in the history of physics was the discovery that finite physical systems have only a finite number of distinguishable states. This discovery revolutionized statistical mechanics and gave birth to quantum mechanics. One might wonder why, in a finite-distinct-state world, the basic quantities of macroscopic mechanics are not also counts of distinct states. The answer is that they are. Quantities such as energy and momentum are such counts. This is related to the finite symbol rate in bandlimited classical signals, and to new generalizations of uncertainty bounds. Counting distinct states clarifies relativistic relations, assigns an ideal (minimum realizable) energy to classical finite state dynamics, and relates continuous physics to finite-state dynamics.