Quantum Noise: Can One Beat The "Standard Quantum Limit"

Daniele Cuomo

Alessandra Buonanno, Yanbei Chen

The uncertainty principle, applied naively to the test masses of a laser interferometer gravitational wave detector, produces a standard quantum limit (SQL) on the interferometer's sensitivity. It has long been thought that beating this SQL would require a radical redesign of interferometers. We show that no redesign is needed at all. LIGO-II interferometers, as currently planned, can beat the SQL by as much as a factor two over a bandwidth \\Delta f \\sim f, if their thermal noise can be pushed low enough. This is due to quantum correlations between photon shot noise and radiation pressure noise, produced by the LIGO-II signal-recycling mirror.

Kimura Kosuke, Daimon Shunsuke, W. Kada +6 more

Quantum entangled states have a potential to beat the standard quantum limit (SQL) and approach the Heisenberg limit (HL). We previously demonstrated that phase shifts induced by magnetic fields are coherently accumulated in a Ramsey protocol using an entangled Nitrogen-Vacancy (NV) pair. However, the sensitivity is limited to the order of ~µT/√Hz due to the various sources of error and noise, including classical readout noise, spin manipulation and initialization errors, and decoherence error. One of the most critical problems is that decoherence of entangled sensing proceeds more rapidly than ensemble-based sensing. When decoherence follows a Markovian process (i.e., when the environment noises behave as white noise without autocorrelation), the sensitivity of entangled sensing is limited to SQL3,4. Therefore, enhancement from entanglement cannot be achieved under the white noise conditions. To realize entanglement-enhanced sensing to beat SQL, it is essential to characterize the environment noise and prepare a suitable condition. In this study, we conducted noise spectroscopy to evaluate the environmental noise and demonstrated that diamond NV centers have the potential to beat SQL.

Michael A. Taylor

François Chapeau-Blondeau

Yuichiro Matsuzaki, Simon C. Benjamin, Joseph F. Fitzsimons

Entangled states can potentially be used to outperform the standard quantum limit by which every classical sensor is bounded. However, entangled states are very susceptible to decoherence, and so it is not clear whether one can really create a superior sensor to classical technology via a quantum strategy which is subject to the effect of realistic noise. This paper presents an investigation of how a quantum sensor composed of many spins is affected by independent dephasing. We adopt general noise models including non-Markovian effects, and in these noise models the performance of the sensor depends crucially on the exposure time of the sensor to the field. We have found that, by choosing an appropriate exposure time within the non-Markovian time region, an entangled sensor does actually beat the standard quantum limit. Since independent dephasing is one of the most typical sources of noise in many systems, our results suggest a practical and scalable approach to beating the standard quantum limit.