Single photon detectors are now known to possess a superior intensity sensitivity; however, the spectral resolution is usually lost after the detection event has occurred. This is a cause for concern regarding low signal infrared spectroscopy applications, as the ability to recover information about a photon’s frequency contribution is essential.
In an attempt to improve the performance of these detectors, a team of German and Russian Scientists have developed a highly efficient waveguide integrated superconducting single-photon detector for on-chip coherent detection.
Nanophotonic circuits are being used to realize complex functionality on a chip to enable the fabrication of functional devices with multiple optical components, all in a scalable fashion. Within these components, fast and efficient single-photon detectors are one of the essential building blocks, if on-chip quantum photonic circuits are to be realized. However, up until now they have suffered with memory recovery and have therefore not reached the point of commercialization.
The Researchers have developed a highly efficient waveguide integrated superconducting single-photon detectors for on-chip coherent detection. The device is a hybrid material composed of waveguides and superconducting nanowire detectors on silicon wafers. The device was fabricated using a combination of magnetron sputtering, electron-beam lithography, electron-beam physical vapor deposition (ePVD), reactive ion etching and O2 plasma cleaning methods.
The method involved heterodyne mixing with an embedded single-photon counting detector, utilizing a wave geometry approach. The Researchers also evanescently coupled the niobium nitride nanowires to a waveguided mode field.
The Researchers measured the single photon count by measuring the photon flux using a tunable laser source (New Focus TLB 6600), an optical attenuator (HP 8156A), a polarization controller (Thorlabs FPC032), a 50:50 beam splitter, a calibrated Lightwave multimeter (HP 8163A), a stable current source (Keithley 2400), bias tees (Mini-Circuits ZFBT-GW6+), low noise amplifiers (Mini-Circuits ZFL-1000LN+) and a 225 MHz counter (HP 53132A).
By using a single nanophotonic device, the Researchers were able to achieve both single photon counting and heterodyne coherent detection. The single photon counting was found to possess an on-chip detection efficiency of up to 86%, and the heterodyne coherent detection was found to have a spectral resolution f/∆f of at least 1011 at telecom wavelengths operated close to the shot-noise limit.
The Researchers mixed a local oscillator with the single photon signal field and observed a frequency modulation at the intermediate frequency, with an oscillating power in the femto-Watt range. The Researchers also optimized the geometry of the nanowires, as well the detection parameters and achieved quantum-limited sensitivity.
The Researchers also undertook further experiments to probe the heterodyne detection properties. The Researchers used an ultra-low oscillating power, of 105-109 ph/s, alongside a very weak signal photon flux (of the incident test signal) of 4 X 103- 109 ph/s to show a dependence between the conversion bandwidth and the geometry of the nanowire. It was found that shorter wires, which adopted a ‘U’ shape, showed a greater throughput than longer, ‘W’ shaped, wires. Such dependence is a major factor for a coherent detection where narrow-line observations are required in a wide band.
This research has allowed for the implementation and integration, of heterodyne nanophotonic device in the C-band range for both classical and quantum optics applications. This is a process where single-photon counting, as well as high spectral resolution, are required simultaneously. This marks a major advancement in nanophotonic circuits, and a major step towards commercial implementation.
Such advancements may find themself across multiple areas, including in Doppler shift detectors, weak signal frequency modulators and integrated quantum optics technologies in the context of correlation and spectral characterization of on-chip narrowband single-photon sources.
“On-chip coherent detection with quantum limited sensitivity”- Kovalyuk V., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-05142-1
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