Interfacing a single photon with another quantum system is a key capability in modern quantum information science. It allows quantum states of matter, such as spin states of atoms [1,2], atomic ensembles [3,4] or solids [5], to be prepared and manipulated by photon counting and, in particular, to be distributed over long distances. Such light-matter interfaces have become crucial to fundamental tests of quantum physics [6] and realizations of quantum networks [7]. Here we report non-classical correlations between single photons and phonons -the quanta of mechanical motion -from a nanomechanical resonator. We implement a full quantu protocol involving initialization of the resonator in its quantum ground state of motion and subsequent generation and read-out of correlated photon-phonon pairs. The observed violation of a Cauchy-Schwarz inequality is clear evidence for the non-classical nature of the mechanical state generated. Our results demonstrate the availability of on-chip solid-state mechanical resonators as light-matter quantum interfaces. The performance we achieved will enable studies of macroscopic quantum phenomena [8] as well as applications in quantum communication [9], as quantum memories [10] and as quantum transducers [11,12].Over the past few years, nanomechanical devices have been discussed as possible building blocks for quantum information architectures [9,13]. Their unique feature is that they combine an engineerable solid-state platform on the nanoscale with the possibility to coherently interact with a variety of physical quantum systems including electronic or nuclear spins, single charges, and photons [14,15]. This feature enables mechanics-based hybrid quantum systems that interconnect different, independent physical qubits through mechanical modes.A successful implementation of such quantum transducers requires the ability to create and control quantum states of mechanical motion. The first step -the initialization of micro-and nanomechanical systems in their quantum ground state of motion -has been realized in various mechanical systems either through direct cryogenic cooling [16,17] or laser cooling using microwave [18] and optical cavity fields [19]. Further progress in quantum state control has mainly been limited to the domain of electromechanical devices, in which mechanical motion couples to superconducting circuits in the form of qubits and microwave cavities [15]. Recent achievements include single-phonon control of a micromechanical resonator by a superconducting flux qubit [16], the generation of quantum entanglement between quadratures of a microwave cavity field and micromechanical motion [20], * This work was published in Nature 530, 313-316 (2016 Interfacing mechanics with optical photons in the quantum regime is highly desirable because it adds important features such as the ability to transfer mechanical excitations over long distances [9,24]. In addition, the available toolbox of single-photon generation and detection allows for remote quantum state control [7]. However...

We have fabricated and tested superconducting single-photon detectors and demonstrated detection efficiencies of 57% at 1550-nm wavelength and 67% at 1064 nm. In addition to the peak detection efficiency, a median detection efficiency of 47.7% was measured over 132 devices at 1550 nm. These measurements were made at 1.8K, with each device biased to 97.5% of its critical current. The high detection efficiencies resulted from the addition of an optical cavity and anti-reflection coating to a nanowire photodetector, creating an integrated nanoelectrophotonic device with enhanced performance relative to the original device. Here, the testing apparatus and the fabrication process are presented. The detection efficiency of devices before and after the addition of optical elements is also reported.

No abstract

We investigate the source of large variations in the observed detection efficiencies of superconducting nanowire single-photon detectors between many nominally identical devices. Through both electrical and optical measurements, we infer that these variations arise from "constrictions:" highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. These constrictions limit the bias current density to well below its critical value over the remainder of the wire, and thus prevent the detection efficiency from reaching the high values that occur in these devices only when they are biased near the critical current density. PACS numbers: 74.76.Db, Superconducting nanowire single-photon detectors (SNSPDs) [1,2,3,4] provide a unique combination of high infrared detection efficiency (up to 57% at 1550nm [2] has been demonstrated) and high speed (<30 ps timing resolution [3,5], and few-ns reset times after a detection event [4]). Applications for these devices already being pursued include high data-rate interplanetary optical communications [6], spectroscopy of ultrafast quantum phenomena in biological and solid-state physics [7,8], quantum key distribution (QKD) [9], and noninvasive, high-speed digital circuit testing [10].In many of these applications, large arrays of SNSPDs would be extremely important [5]. For example, existing SNSPDs have very small active areas, making optical coupling relatively difficult and inefficient [7,11]. Their small size also limits the number of optical modes they can collect, which is critical in free-space applications where photons are distributed over many modes, such as laser communication through the atmosphere (where turbulence distorts the optical wavefront) and in fluorescence detection. Furthermore, it was shown in Ref.[4] that the maximum count rate for an individual SNSPD decreases as its active area is increased, due to its kinetic inductance, forcing a tradeoff between active area and high count rates. Count rate limitations are particularly important in optical communications and QKD, affecting the achievable receiver sensitivity or data rate [6,12]. Detector arrays could provide a solution to these problems, giving larger active areas while simultaneously increasing the maximum count rate by distributing the flux over many smaller (and therefore faster) pixels. Large arrays could also provide spatial and photon-number resolution. Although few-pixel detectors have been demonstrated [5,9,11,12], fabrication and readout methods scalable to large arrays have not yet been discussed.A first step towards producing large arrays of SNSPDs is to understand (and reduce) the large observed variation of detection efficiencies for nominally identical devices [2], which would set a crippling limit on the yield of efficient arrays of any technologically interesting size.In this Letter, we demonstrate that these detection efficiency variations can be understood in terms of what we call "constrictions:" highly localized, essential...

We measured the optical absorptance of superconducting nanowire single photon detectors. We found that 200-nm-pitch, 50%-fill-factor devices had an average absorptance of 21% for normally-incident front-illumination of 1.55-microm-wavelength light polarized parallel to the nanowires, and only 10% for perpendicularly-polarized light. We also measured devices with lower fill-factors and narrower wires that were five times more sensitive to parallel-polarized photons than perpendicular-polarized photons. We developed a numerical model that predicts the absorptance of our structures. We also used our measurements, coupled with measurements of device detection efficiencies, to determine the probability of photon detection after an absorption event. We found that, remarkably, absorbed parallel-polarized photons were more likely to result in detection events than perpendicular-polarized photons, and we present a hypothesis that qualitatively explains this result. Finally, we also determined the enhancement of device detection efficiency and absorptance due to the inclusion of an integrated optical cavity over a range of wavelengths (700-1700 nm) on a number of devices, and found good agreement with our numerical model.