Upconversion detection near 2 μm at the single photon level

Polarization-independent photon up-conversion with a single lithium niobate waveguide

We propose a polarization-independent up-conversion protocol for single-photon detection at telecom band with a single thin-film periodically poled lithium niobate waveguide. By choosing the proper waveguide parameters, the waveguide dispersion can compensate the crystal birefringence so that quasi-phase-matching conditions for transverse electric and transverse magnetic modes can be simultaneously fulfilled with single poling period. With this scheme, randomly-polarized single photons at 1550 nm can be up-converted with a normalized conversion efficiency of 163.8%/W cm².

Optimizing up-conversion single-photon detectors for quantum key distribution

High-performance single-photon detectors (SPDs) at 1550-nm band are critical for fiber-based quantum communications. Among many types of SPDs, the up-conversion SPDs based on periodically poled lithium niobate waveguides are of great interest. Combined with a strong pump laser, the telecom single-photons are converted into short wavelength ones and detected by silicon-based SPDs. However, due to the difficulty of precise controlling waveguide profile, the direct coupling between a single-mode fiber and the waveguide is not efficient. Here by utilizing fiber taper with proper diameter, optimal mode-matching is achieved and coupling efficiency up to 93% is measured. With an optimized design, a system detection efficiency of 36% and noise counting rate of 90 cps are realized. The maximum detection efficiency is characterized as 40% with a noise counting rate of 200 cps. Numerical simulation results indicate that our device can significantly improve the performance of QKD and extend the communication distance longer than 200 km.

Two-photon quantum interference and entanglement at 2.1 μm

Quantum-enhanced optical systems operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 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 polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

Efficient C-band single-photon upconversion with chip-scale Ti-indiffused pp-LiNbO3 waveguides

Frequency upconversion for single photons at telecom wavelengths is important to simultaneously meet the different wavelength requirements for long-distance communications and quantum memories in a quantum nodal network. It also enables the detection for the telecom "flying qubit" photons with silicon-based efficient single-photon detectors with low dark count (DC) rates. Here, we demonstrate the frequency upconversion of attenuated single photons, using a low-loss titanium-indiffused periodically poled lithium niobate waveguide, pumped with a readily available erbium-doped fiber amplifier in the L-band. Internal and conversion efficiencies up to 84.4% and 49.9% have been achieved, respectively. The DC rates are suppressed down to 44 kHz at 13.9% end-to-end quantum efficiency (including full conversion and detection), enabled by our long-wavelength pump configuration and narrow 3.5-GHz bandpass filtering.

Quantum detector tomography of a single-photon frequency upconversion detection system

We experimentally presented a full quantum detector tomography of a synchronously pumped infrared single-photon frequency upconversion detector. A maximum detection efficiency of 37.6% was achieved at the telecom wavelength of 1558 nm with a background noise about 1.0 × 10^-3 counts/pulse. The corresponding internal quantum conversion efficiency reached as high as 84.4%. The detector was then systematically characterized at different pump powers to investigate the quantum decoherence behavior. Here the reconstructed positive operator valued measure elements were equivalently illustrated with the Wigner function formalism, where the quantum feature of the detector is manifested by the presence of negative values of the Wigner function. In our experiment, pronounced negativities were attained due to the high detection efficiency and low background noise, explicitly showing the quantum feature of the detector. Such quantum detector could be useful in optical quantum state engineering, quantum information processing and communication.

Integrated four-channel all-fiber up-conversion single-photon-detector with adjustable efficiency and dark count

Up-conversion single photon detector (UCSPD) has been widely used in many research fields including quantum key distribution, lidar, optical time domain reflectrometry, and deep space communication. For the first time in laboratory, we have developed an integrated four-channel all-fiber UCSPD which can work in both free-running and gate modes. This compact module can satisfy different experimental demands with adjustable detection efficiency and dark count. We have characterized the key parameters of the UCSPD system.

Near Infrared Spectroscopy—A Unique Window of Opportunities

It is our custom in NIR news to invite the THA winner to contribute a summary of their award lecture in an issue soon after the ceremony. Unfortunately the 2015 award article was overlooked but we are happy to have prevailed upon the winner, Søren Engelsen, to allow us to maintain this custom by writing something for us. In an exciting summary article, Søren initially recaps on some practical aspects of NIR spectroscopy and reminds us of why it is such a useful technique. In a look to the future, however, he holds out some tantalising glimpses of what developments might be just around the corner and how these might produce sea changes in our thinking and use of the method — how about instruments which require no calibration transfer operations? Read and enjoy! Oh and the lecture summary by the 2016 winner will appear in 2016!! Ed.

Low-noise mid-IR upconversion detector for improved IR-degenerate four-wave mixing gas sensing

We compare a nonlinear upconversion detector with a conventional cryogenic InSb detector for the detection of coherent infrared light showing near-shot-noise-limited performance in the upconversion system. The InSb detector is limited by dark noise, which results in a 500 times lower signal-to-noise ratio. The two detectors are compared for the detection of a coherent degenerate four-wave mixing (DFWM) signal in the mid-infrared, and applied to measure trace-level acetylene in a gas flow at atmospheric pressure, probing its fundamental rovibrational transitions. In addition to lower noise, the upconversion system provides image information of the signal, thus adding new functionality compared to standard point detection methods. We further show that the upconversion detector system can be implemented as a simple replacement of the cryogenic detector.