Q-Farm Quantum Seminar Series presents TWO TOPICS

A quantum annealer with fully programmable all-to-all coupling via Floquet engineering -and- Spin squeezing in free-space atomic sensors
Wednesday, June 19, 2019 - 12:00pm
Y2E2 300
Peter McMahon; Zheng Cui
Abstract / Description: 

A quantum annealer with fully programmable all-to-all coupling via Floquet engineering
Peter McMahon

Quantum annealing is a promising approach to heuristically solving difficult combinatorial optimization problems. However, the connectivity limitations in current devices lead to an exponential degradation of performance on general problems. We propose an architecture for a quantum annealer that achieves full connectivity and full programmability while using a number of physical resources only linear in the number of spins. We do so by application of carefully engineered periodic modulations of oscillator-based qubits, resulting in a Floquet Hamiltonian in which all the interactions are tunable; this flexibility comes at a cost of the coupling strengths between spins being smaller than they would be had the spins been directly coupled. Our proposal is well-suited to implementation with superconducting circuits, and we give analytical and numerical evidence that fully-connected, fully-programmable quantum annealers with 1000 qubits could be constructed with Josephson parametric oscillators having coherence times of 500 microseconds, and other system-parameter values that are routinely achieved with current technology. Our approach could also have impact beyond quantum annealing, since it readily extends to bosonic quantum simulators and would allow the study of models with arbitrary connectivity between lattice sites. Describes work performed jointly with Tatsuhiro Onodera and Edwin Ng.

Spin squeezing in free-space atomic sensors
Zheng Cui

The compatibility of cavity-generated spin-squeezed atomic states with atom-interferometric sensors that require freely falling atoms is demonstrated. An ensemble of hundreds of thousands of spin-squeezed atoms in a high-finesse optical cavity with near-uniform atom-cavity coupling is prepared, released into free space, and measured using one of two different methods. The first method consists of recapturing the atoms in the cavity and probing them with the same QND measurement used to generate the initial entanglement among the atoms. Up to 9.8^{+0.5}_{-0.4} dB of metrologically-relevant squeezing is retrieved for few-hundred microseconds free-fall times, and decaying levels of squeezing are mapped out up to 3 milliseconds free-fall times. This protocol suffers of atom loss and atom-cavity coupling inhomogeneity after recapture. Fluorescence population spectroscopy is an alternative method when longer free-falls times are required. This method allows for the atom ensemble to free fall for up to 4 milliseconds without significant loss of squeezing nor quantum coherence. When operating as a microwave atomic clock with a 3.6 millisecond Ramsey time, a single-shot fractional frequency stability of 8.4(0.2)x10^{-12} is reported, 4.1(0.2) decibels below the quantum projection limit. The ability of the clock to utilize the maximum squeezing available is limited by microwave amplitude and phase noise, and external magnetic field fluctuations in the system. Free fall times of up to 8 milliseconds are also achieved, but at a loss of state coherence.