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.
We report on the direct measurement of the electron-phonon relaxation time, ! eph , in disordered TiN films. Measured values of ! eph are from 5.5 ns to 88 ns in the 4.2 to 1.7 K temperature range and consistent with a T -3 temperature dependence. The electronic density of states at the Fermi level N 0 is estimated from measured material parameters. The presented results confirm that thin TiN films are promising candidate-materials for ultrasensitive superconducting detectors.
We describe optical characterisation of a Strained Silicon Cold Electron Bolometer (CEB), operating on a 350 mK stage, designed for absorption of millimetre-wave radiation. The silicon Cold Electron Bolometer utilises Schottky contacts between a superconductor and an n ++ doped silicon island to detect changes in the temperature of the charge carriers in the silicon, due to variations in absorbed radiation. By using strained silicon as the absorber, we decrease the electron-phonon coupling in the device and increase the responsivity to incoming power. The strained silicon absorber is coupled to a planar aluminium twin-slot antenna designed to couple to 160 GHz and that serves as the superconducting contacts. From the measured optical responsivity and spectral response, we calculate a maximum optical efficiency of 50 % for radiation coupled into the device by the planar antenna and an overall noise equivalent power (NEP), referred to absorbed optical power, of 1.1 × 10 −16 W Hz −1 /2 when the detector is observing a 300 K source through a 4 K throughput limiting aperture. Even though this optical system is not optimised we measure a system noise equivalent temperature difference (NETD) of 6 mK Hz −1 /2 . We measure the noise of the device using a cross-correlation of time stream data measured simultaneously with two junction field-effect transistor (JFET) amplifiers, with a base correlated noise level of 300 pV Hz −1 /2 and find that the total noise is consistent with a combination of photon noise, current shot noise and electron-phonon thermal noise.
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