JQI-QuICS-CMTC Seminar

We study a system of neutral two-level atoms prepared in a collective state coupled to an optical waveguide, particularly considering the regime where interatomic separations are comparable to the coherence length of a spontaneously emitted photon in the waveguide mode. Considering the atomic system interacting with the quantized electromagnetic field bath, we find that there is a departure from the Markovian dynamics due to the retardation effects in the field propagation and backaction of the bath on the system.

I will discuss a measurement scheme that validates the preparation of n-qubit stabilizer states. The scheme involves a measurement of n Pauli observables, a priori determined from the stabilizer state and which can be realized using single-qubit gates. Based on the proposed validation scheme, we derive an explicit expression for the worse-case fidelity, i.e., the minimum fidelity between the stabilizer state and any other state consistent with the measured data.

We study quantum information scrambling, specifically the growth of Heisenberg operators, in large disordered spin chains using matrix product operator dynamics to scan across the thermalization-localization quantum phase transition. We observe ballistic operator growth for weak disorder, and a sharp transition to a phase with sub-ballistic operator spreading.

We report on the fabrication of Josephson junctions using
the topological crystalline insulator Pb0.5Sn0.5Te as the weak link.
The properties of these junctions are characterized and compared to
those fabricated with weak links of PbTe, a similar material yet
topologically trivial. Most striking is the difference in the ac
Josephson effect: junctions made with Pb0.5Sn0.5Te exhibit a rich
subharmonic structure consistent with a skewed current-phase relation.

We investigate the momentum-space entanglement entropy and spectrum of several disordered one-dimensional free-fermion systems that circumvent Anderson localization, such as the random-dimer model, after a quantum quench. We numerically observe two different types of momentum-space entanglement entropy dynamics, an interesting slow logarithmic-like growth followed by saturation or rapid saturation. The type of dynamics one observes depends on the Fermi level of the intial state and the scattering matrix element structure in momentum-space.

Recent experiments on one-dimensional Rydberg simulators display quantum phase transitions to spatially ordered crystals with a period of N sites [1]. These experiments are capable of probing the universal Kibble-Zurek dynamics of the transitions, allowing experimental access to critical exponents [2]. For N>2, these transitions are in the same universality class as the Z_N chiral clock model, which exhibits a rich phase structure.

Physically motivated classical heuristic optimization algorithms such as simulated annealing (SA) treat the objective function as an energy landscape, and allow walkers to escape local minima. It has been argued that quantum properties such as tunneling may give quantum algorithms advantage in finding ground states of vast, rugged cost landscapes. Indeed, the Quantum Adiabatic Algorithm (QAO) and the recent Quantum Approximate Optimization Algorithm (QAOA) have shown promising results on various problem instances that are considered classically hard.

Progress in scaling up quantum computing implementations has led to a new set of technical challenges - development of classical control techniques to initialize and control these systems. Control problems in one such implementation, quantum dots defined in semiconductor heterostructures, currently rely on gross scale heuristics from experiments.

In this talk, I will present our recent work where we investigate many-body quantum chaos in an interacting disordered metal by computing the out-of-time-order correlators (OTOCs). We develop an augmented Keldysh version of Finkel’stein nonlinear sigma model to enable the computation of the regularized and unregularized OTOCs. Both correlators grow exponentially but with different Lyapunov exponents. The result of the regularized exponent is consistent with a previous diagrammatical perturbation study [1], and it obeys the Maldacena-Shenker-Stanford bound [2].

Topology in 2-D materials is important and cool; it explains the quantum Hall effect, and in the limit of really high magnetic fluxes (of order 10^4 Tesla for crystalline materials) gives rise to the fractal Hofstadter butterfly. These high fluxes are inaccessible in traditional condensed matter settings, but we engineered them in our effective 2-D lattice of ultracold 87Rb atoms.