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The European Physical Journal Applied Physics

Ultimate performance of a superconducting quantum detector

Alexei D. Semenov, Andreas Engel, K. Ilin, Gregory N Gol'tsman, M. Siegel, Heinz‐Wilhelm Hübers

Published Feb 27, 2003• 48 citations
Publisher

Abstract

We analyze the ultimate performance of a superconducting quantum detector in order to meet requirements for applications in near-infrared astronomy and X-ray spectroscopy. The detector exploits a combined detection mechanism, in which avalanche quasiparticle multiplication and the supercurrent jointly produce a voltage response to a single absorbed photon via successive formation of a photon-induced and a current-induced normal hotspot in a narrow superconducting strip. The response time of the detector should increase with the photon energy providing energy resolution. Depending on the superconducting material and operation conditions, the cut-off wavelength for the single-photon detection regime varies from infrared waves to visible light. We simulated the performance of the background-limited infrared direct detector and X-ray photon counter utilizing the above mechanism. Low dark count rate and intrinsic low-frequency cut-off allow for realizing a background limited noise equivalent power of 10−20 W Hz−1/2 for a far-infrared direct detector exposed to 4-K background radiation. At low temperatures, the intrinsic response time of the counter is rather determined by diffusion of nonequilibrium electrons than by the rate of energy transfer to phonons. Therefore, thermal fluctuations do not hamper energy resolution of the X-ray photon counter that should be better than 10−3 for 6-keV photons. Comparison of new data obtained with a Nb based detector and previously reported results on NbN quantum detectors support our estimates of ultimate detector performance.

DOI
10.1051/epjap:2003009

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We analyze the ultimate performance of a superconducting quantum detector in order to meet requirements for applications in near-infrared astronomy and X-ray spectroscopy. The detector exploits a combined detection mechanism, in which avalanche quasiparticle multiplication and the supercurrent jointly produce a voltage response to a single absorbed photon via successive formation of a photon-induced and a current-induced normal hotspot in a narrow superconducting strip. The response time of the detector should increase with the photon energy providing energy resolution. Depending on the superconducting material and operation conditions, the cut-off wavelength for the single-photon detection regime varies from infrared waves to visible light. We simulated the performance of the background-limited infrared direct detector and X-ray photon counter utilizing the above mechanism. Low dark count rate and intrinsic low-frequency cut-off allow for realizing a background limited noise equivalent power of 10−20 W Hz−1/2 for a far-infrared direct detector exposed to 4-K background radiation. At low temperatures, the intrinsic response time of the counter is rather determined by diffusion of nonequilibrium electrons than by the rate of energy transfer to phonons. Therefore, thermal fluctuations do not hamper energy resolution of the X-ray photon counter that should be better than 10−3 for 6-keV photons. Comparison of new data obtained with a Nb based detector and previously reported results on NbN quantum detectors support our estimates of ultimate detector performance.
We extend the concept of the superconducting quantum counter in order to develop a new quantum detector for submillimeter astronomy. The detector exploits a cumulative detection mechanism, in which the response appears due to successive formation of the normal spot and a resistive domain in a narrow strip carrying sub-critical supercurrent. The intrinsic recovery time of the counter is partly determined by diffusion of nonequilibrium electrons and, thus, depends on the energy of detected photons. Depending on the superconducting material used and operation conditions, such detector may have cut-off wavelengths for the single-photon regime ranging from terahertz waves to visible light and simultaneously provide a moderate energy resolution. We simulated performance of the background-limited submillimeter direct detector from Ti having the 100-micrometer cut-off wavelength, low dark count rate and intrinsic 10-21 W Hz-1/2 noise equivalent power for 4-K background radiation. We present first results obtained with a detector prototype fabricated from ultra-thin Nb film.
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We report on the energy resolution of a recently developed superconductor single-photon quantum detector. In a superconducting strip quasiparticles created by a single absorbed photon and a bias current jointly cause a normal domain and, subsequently, a voltage transient that manifests absorption of the photon. Given a constant optical coupling, the mechanism suggests a moderate to good energy resolution in the wavelength range from near-infrared to X-rays. We implemented a meander line from a 80-nm wide and 5-nm thick NbN strip to detect single near-infrared photons with the repetition rate 5•107 sec-1 and quantum efficiency of few per cent. Although with this detector operated at 2 K we have indeed observed photon-energy dependent detector response, the energy resolving capability appeared smaller than the detector model predicted. We suggest that the inconsistency owes to the influence of the bias current.
Thin film cryogenic bolometers are widely used as integrating detectors for infrared and millimeter wave radiation. A superconducting quantum detector based on a nanometer-scaled structure may be an alternative for future applications where energy resolved single-photon detection and fast response times are simultaneously required. The mechanism responsible for the single-photon detection capability is due to the combined action of a hot-spot formation after photon absorption and a subsequent normal domain caused by an applied bias current [1]. We present recent results on energy resolution and spectral sensitivity of infrared quantum detectors made of different materials (Nb and NbN). The spectral response of the detector exhibits a pronounced threshold at a particular photon energy E0 that depends on the material and detector geometry. According to the detector model, information about the photons energy is contained in the amplitude and duration of the voltage pulse. A reasonable energy resolution is expected only for photon energies E >> E0. Preliminary experiments performed near the threshold energy have shown that the time integral of the response pulses indeed varied with the photon energy. The observed energy resolution of about 20% at 0.6 eV was not sufficient for practical applications. However, these preliminary data validate the response mechanism and support the envisaged performance at higher photon energies. [1] Semenov A D, Gol'tsman G N and Korneev A A 2001 Physica C 351 349.
We have developed a new class of superconducting single-photon detectors (SSPDs) for ultrafast counting of infrared (IR) photons for secure quantum communications. The devices are operated on the quantum detection mechanism, based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The detectors are fabricated from 3.5-nm-thick NbN films and they operate at 4.2 K inside a closed-cycle refrigerator or liquid helium cryostat. Various continuous and pulsed laser sources have been used in our experiments, enabling us to determine the detector experimental quantum efficiency (QE) in the photon-counting mode, response time, time jitter, and dark counts. Our 3.5-nm-thick SSPDs reached QE above 15% for visible light photons and 5% at 1.3 - 1.5 μm infrared range. The measured real-time counting rate was above 2 GHz and was limited by the read-out electronics (intrinsic response time is <30 ps). The measured jitter was <18 ps, and the dark counting rate was <0.01 per second. The measured noise equivalent power (NEP) is 2 x 10-18 W/Hz1/2 at λ = 1.3 μm. In near-infrared range, in terms of the counting rate, jitter, dark counts, and overall sensitivity, the NbN SSPDs significantly outperform their semiconductor counterparts. An ultrafast quantum cryptography communication technology based on SSPDs is proposed and discussed.
We present a new class of single-photon devices for counting of both visible and infrared photons. Our superconducting single-photon detectors (SSPDs) are characterized by the intrinsic quantum efficiency (QE) reaching up to 100%, above 10 GHz counting rate, and negligible dark counts. The detection mechanism is based on the photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-wide superconducting stripe. The devices are fabricated from 3.5-nm-thick NbN films and operate at 4.2 K, well below the NbN superconducting transition temperature. Various continuous and pulsed laser sources in the wavelength range from 0.4 μm up to >3 μm were implemented in our experiments, enabling us to determine the detector QE in the photon-counting mode, response time, and jitter. For our best 3.5-nm-thick, 10×10 μm2-area devices, QE was found to reach almost 100% for any wavelength shorter than about 800 nm. For longer-wavelength (infrared) radiation, QE decreased exponentially with the photon wavelength increase. Time-resolved measurements of our SSPDs showed that the system-limited detector response pulse width was below 150 ps. The system jitter was measured to be 35 ps. In terms of the counting rate, jitter, and dark counts, the NbN SSPDs significantly outperform their semiconductor counterparts. Already identifeid and implemented applications of our devices range from noninvasive testing of semiconductor VLSI circuits to free-space quantum communications and quantum cryptography.