We have realized a high-detection-efficiency photon number resolving detector at an operating wavelength of about 850 nm. The detector consists of a titanium superconducting transition edge sensor in an optical cavity, which is directly coupled to an optical fiber using an approximately 300-nm gap. The gap reduces the sensitive area and heat capacity of the device, leading to high photon number resolution of 0.42 eV without sacrificing detection efficiency or signal response speed. Wavelength dependent efficiency in fiber-coupled devices, which is due to optical interference between the fiber and the device, is also decreased to less than 1% in this configuration. The overall system detection efficiency is 98%±1% at wavelengths of around 850 nm, which is the highest value ever reported in this wavelength range.

The most efficient modern optical communication is known as coherent communication, and its standard quantum limit is almost reachable with current technology. Though it has been predicted for a long time that this standard quantum limit could be overcome via quantum mechanically optimized receivers, such a performance has not been experimentally realized so far. Here we demonstrate the first unconditional evidence surpassing the standard quantum limit of coherent optical communication. We implement a quantum receiver with a simple linear optics configuration and achieve more than 90% of the total detection efficiency of the system. Such an efficient quantum receiver will provide a new way of extending the distance of amplification-free channels, as well as of realizing quantum information protocols based on coherent states and the loophole-free test of quantum mechanics.

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Abstract:We demonstrate a sub-shot-noise-limit discrimination of on-off keyed coherent signals by an optimal displacement quantum receiver in which a superconducting transition edge sensor is installed. Use of a transition edge sensor and a fiber beam splitter realizes high total detection efficiency and high interference visibility of the receiver and the observed average error surpasses the shot-noise-limit in a wider range of the signal power. Our technique opens up a new technology for the sub-shot-noiselimit detection of coherent signals in optical communication channels. 2728-2736 (1996). 5. M. Takeoka and M. Sasaki, "Discrimination of the binary coherent signal: Gaussian-operation limit and simple non-Gaussian near-optimal receivers," Phys. Rev. A 78, 022320 (2008). 6. R. S. Kennedy, "A near-optimal receiver for the binary coherent state quantum channel," Research

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