Counting Photons: Squeezing a Quantum Limit

Reeta Vyas, Surendra Singh

Anonymous

Haowei Shi, Quntao Zhuang

Abstract The nature of dark matter is unknown and calls for a systematical search. For axion dark matter, such a search relies on finding feeble random noise arising from the weak coupling between dark matter and microwave haloscopes. We model such process as a quantum channel and derive the fundamental precision limit of noise sensing. An entanglement-assisted strategy based on two-mode squeezed vacuum is thereby demonstrated optimal, while the optimality of a single-mode squeezed vacuum is found limited to the lossless case. We propose a “nulling” measurement (squeezing and photon counting) to achieve the optimal performances. In terms of the scan rate, even with 20-decibel of strength, single-mode squeezing still underperforms the vacuum limit which is achieved by photon counting on vacuum input; while the two-mode squeezed vacuum provides large and close-to-optimum advantage over the vacuum limit, thus more exotic quantum resources are no longer required. Our results highlight the necessity of entanglement assistance and microwave photon counting in dark matter search.

Arkadiusz Orłowski

Bahaa E. A. Saleh, Malvin Carl Teich

The channel capacity C of a light-wave communication system based on photoevent point process observations cannot be increased by use of photon-number--squeezed light. Under a constraint of maximum photon rate ${\ensuremath{\lambda}}_{\mathrm{max}}$, the channel capacity C=${\ensuremath{\lambda}}_{\mathrm{max}/\mathrm{e}}$ is achieved with a Poisson process. On the other hand, the channel capacity of a communication system based on photon counting can be increased by use of photon-number--squeezed light. The improvement vanishes in the limit of very small mean counts.

Carlos Sánchez Muñoz, Gaetano Frascella, Frank Schlawin

Two-photon absorption (TPA) is of fundamental importance in super-resolution imaging and spectroscopy. Its nonlinear character allows for the prospect of using quantum resources, such as entanglement, to improve measurement precision or to gain new information on, e.g., ultrafast molecular dynamics. Here, we establish the metrological properties of nonclassical squeezed light sources for precision measurements of TPA cross sections. We find that the Cram\'er-Rao bound does not provide a fundamental limit for the precision achievable with squeezed states in the limit of very small cross sections. Considering the most relevant measurement strategies---namely, photon-counting and quadrature measurements---we determine the quantum advantage provided by squeezed states as compared to coherent states. We find that squeezed states outperform the precision achievable by coherent states when performing quadrature measurements, which provide improved scaling of the Fisher information with respect to the mean photon number $\ensuremath{\sim}{n}^{4}$. Due to the interplay of the incoherent nature and the nonlinearity of the TPA process, unusual scaling can also be obtained with coherent states, which feature an $\ensuremath{\sim}{n}^{3}$ scaling in both quadrature and photon-counting measurements.