Quantum-enhanced optical systems operating within the 2 μm spectral region have the potential to revolutionise emerging applications in communications, sensing and metrology. However, to date, sources of entangled photons have been realised mainly in the near-infrared 700-1550 nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting-nanowire single-photon detectors, we demonstrate two-photon interference and polarisation-entangled photon pairs at 2090 nm, i.e. in the mid-infrared and significantly beyond existing technology. These results open the 2 μm window for the development of optical quantum technologies such as quantum key distribution in nextgeneration mid-infrared fibre communications systems and novel Earth-to-satellite communications that exploits reduced atmospheric scattering in a spectral region with a reduced solar background.

Long-lifetime quantum storages accessible to the telecom photonic infrastructure are essential to long-distance quantum communication. Atomic quantum storages have achieved subsecond storage time corresponding to 1000 km transmission time for a telecom photon through a quantum repeater algorithm. However, the telecom photon cannot be directly interfaced to typical atomic storages. Solid-state quantum frequency conversions fill this wavelength gap. Here we report on the experimental demonstration of a polarization-insensitive solid-state quantum frequency conversion to a telecom photon from a short-wavelength photon entangled with an atomic ensemble. Atom–photon entanglement has been generated with a Rb atomic ensemble and the photon has been translated to telecom range while retaining the entanglement by our nonlinear-crystal-based frequency converter in a Sagnac interferometer.

Recent progress in the development of superconducting nanowire single photon detectors (SSPD or SNSPD) has delivered excellent performance, and has had a great impact on a range of research fields. Significant efforts are being made to further improve the technology, and a primary concern remains to resolve the trade-offs between detection efficiency (DE), timing jitter, and response speed. We present a stable and high-performance fiber-coupled niobium titanium nitride superconducting nanowire avalanche photon detector (SNAP) that resolves these trade-offs. We demonstrate afterpulse-free operation in serially connected two SNAPs (SC-2SNAP), even in the absence of a choke inductor, achieving a ~7.7 times faster response speed than standard SSPDs. The SC-2SNAP device showed a system detection efficiency (SDE) of 81.0% with wide bias current margin, a dark count rate of 6.8 counts/s, and full width at half maximum timing jitter of 68 ps, operating in a practical GiffordMcMahon cryocooler system.

We report the experimental demonstration of four-photon quantum interference using telecomwavelength photons. Realization of multi-photon quantum interference is essential to linear optics quantum information processing and measurement-based quantum computing. We have developed a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom wavelength region, and have demonstrated the quantum interference exhibiting the reduced fringe intervals that correspond to the reduced de Broglie wavelength of up to the four photon 'NOON' state. Our result should open a path to practical quantum information processing using telecom-wavelength photons.A variety of novel quantum optical technologies have been proposed for use in quantum information processing and quantum metrology [1,2]. Photons are the most promising and practical media to demonstrate such novel quantum technologies. Indeed, linear optical quantum computing [3] and measurement-based quantum computing [4] have attracted much attention. However, use of the latest quantum technologies requires a large number of multiple photons at the same time. Furthermore, the photons must be indistinguishable from each other because the quantum operations rely on quantum interference between photons. Thus, we need practical and efficient sources that can provide many, indistinguishable photons. To date, quantum processing including up to six photons has been demonstrated [5,6]. These demonstrations used near infrared (800-nm band) photons, for which efficient photon sources and reliable single-photon detectors are available. However, use of telecom-band (1.5-µm band) photons is desired for practical purposes.Here we report an experiment with four-photon quantum interference using telecom-wavelength photons, in which the photons exhibited reduced (∝ N −1 ) fringe intervals corresponding to the number (N ) of photons [7][8][9][10][11][12][13]. Combined with the recent developments of novel photon-detecting devices [14], our result should open a path to practical quantum information processing using telecom-wavelength photons.The most popular photon sources so far used for the demonstration of quantum information processing are based on spontaneous parametric down-conversion (SPDC), which generates twin (signal and idler) photons that can be used as entangled photons [15] or heralded single photons [16]. The linear optical quantum computing [3] and measurement-based quantum computing [4] both rely on quantum interference between photons to carry out quantum operations and quantum measurements. Photons generated by SPDC must be indistinguishable from each other to make them interfere. To do so, spectrotemporal purity of the photons is essential [17]. However, in general, photons generated by SPDC have spectrotemporal correlation [18] that destroys the purity of each photon. Spectral filtering has often been used to purify the spectrotemporal modes of photons; however, such filtering inevitably reduces the generation efficiency. Thus, efficient generation...

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