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Q-Farm Quantum Seminar Series presents Two Talks

Noise-resilient quantum circuits - AND - Spin squeezing in free-space atomic sensors
Wednesday, May 8, 2019 - 12:00pm
Physics/Astrophysics (Varian II) Building, Room 102/103
Isaac Kim (Stanford) - AND - Julian Martinez (Stanford)
Abstract / Description: 

A noisy quantum computer can simulate any noiseless quantum computation with an overhead that scales very modestly with the size of the computation, in the asymptotic limit in which the size of the computation becomes large. However, the overhead in practice is still too large even for state-of-the-art quantum computing devices. In order to circumvent this problem, we propose a specialized algorithm that remains robust in the presence of error even without error correction. Even though the size of the circuit increases with the problem size, the accumulated error on the answer is stabilized at a level comparable to the physical noise rate. This is possible because the errors introduced at earlier times are judiciously diluted more by the design of the circuit. This algorithm can dramatically speed up the existing (classical) computational methods to study strongly interacting quantum many-body systems. One may thus optimistically hope to accurately predict the properties of such systems with a noisy quantum computer, provided that the noise rate in these devices continues to get lower.

- and - 

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.