hen a photon strikes a superconductor, its energy cascades through the material via electron-phonon interactions, generating thousands of excitations called quasiparticles.Before this energy dissipates, these quasiparticles measurably change the kinetic inductance of the material, a property that researchers have used to make superconducting single-photon detectors (see Synopsis: Direct Detection of Quasiparticles).Now, Pieter de Visser at the Netherlands Institute for Space Research and colleagues have modified the design of such a detector to achieve a 2.5-fold increase in the precision with which the device can measure photon frequency [1].That precision increase could improve spectroscopy measurements of exoplanet atmospheres and fluorescence measurements of biological samples.Currently, operational superconductor-based photon detectors are made as follows: A series of superconducting circuits is deposited on a thick (> 300 µm) silicon or sapphire substrate, with each circuit making up one pixel.The frequency resolution
hen a photon strikes a superconductor, its energy cascades through the material via electron-phonon interactions, generating thousands of excitations called quasiparticles.Before this energy dissipates, these quasiparticles measurably change the kinetic inductance of the material, a property that researchers have used to make superconducting single-photon detectors (see Synopsis: Direct Detection of Quasiparticles).Now, Pieter de Visser at the Netherlands Institute for Space Research and colleagues have modified the design of such a detector to achieve a 2.5-fold increase in the precision with which the device can measure photon frequency [1].That precision increase could improve spectroscopy measurements of exoplanet atmospheres and fluorescence measurements of biological samples.Currently, operational superconductor-based photon detectors are made as follows: A series of superconducting circuits is deposited on a thick (> 300 µm) silicon or sapphire substrate, with each circuit making up one pixel.The frequency resolution
Abstract The fundamental excitations in superconductors—Bogoliubov quasiparticles—can be either a resource or a liability in superconducting devices: they are what enables photon detection in microwave kinetic inductance detectors, but they are a source of errors in qubits and electron pumps. To improve operation of the latter devices, ways to mitigate quasiparticle effects have been devised; in particular, combining different materials quasiparticles can be trapped where they do no harm and their generation can be impeded. We review recent developments in these mitigation efforts and discuss open questions.
While experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, these detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance, which can be measured by a sensitive SQUID readout. We modeled the kinetic inductance response of detectors made from disordered superconducting Nb, NbC, and MoRe films. Low phonon transparency of the interface between the superconductor and the substrate causes substantial re-trapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation-recombination noise can be as small as ∼10−19 W/√Hz at He4 temperatures.
Superconducting nanowires have been used to detect single photons for more than 20 years. The basic principle is that photons can perturb superconducting Cooper pairs and thereby create a resistive hotspot, which produces a current-crowding effect. This current-crowding effect reduces the superconducting energy gap near the hotspot, which results in the nanowire changing from a superconducting to a normal state near the hotspot. The resulting high impedance causes the current to be squeezed into a high frequency coaxial wire connected in parallel with it, which finally produces a measurable voltage pulse. Thanks to the maturity of current micro- and nano- fabrication techniques, high quality superconducting thin films and nano-wires have been realized in a variety of materials such as NbN, WSi, MoSi and others. These techniques include magnetron sputtering, physical vapor deposition, electron beam lithography, optical lithography, reactive ion etching, wafer cutting, etc. Measurements of superconducting single-photon detectors require cryogenic refrigeration systems, ranging from the initial liquid helium cold tank, to modern dry dilution refrigerators. On one hand, a superconducting device needs to be cooled down below its critical temperature to work. On the other hand, photon-detection events generate hot spots, which release heat to the device substrate, which the refrigerator needs to efficiently take away. Research on superconducting nanowire single-photon detectors (SNSPDs) has made significant advancements over the past two decades, including high detection efficiency, low dark counts, and broad wavelength range capabilities. Beyond improvements in traditional NbN, WSi and MoSi superconducting nanowires, there have been numerous efforts to develop high-temperature superconducting nanowires, such as MgB2 and Bi2Sr2CaCu2O8 (BSCCO), due to the operational limitations imposed by low-temperature superconducting materials. Typically, single-photon detectors require operation temperatures below the critical temperature. High-temperature superconducting nanowire single-photon detectors have currently reported to exhibit the highest operating temperature around 20 K in BSCCO, which is much lower than its critical temperature of 80 K, showing that large-gap superconductors have a comparably low sensitivity to low-energy photons. In recent years since 2018, several groups have reported that micrometer-wide superconducting wires can also be used for single-photon detection, unlike the conventional nanometer-wide superconducting wires. In general, as long as the bias current is uniformly distributed in superconducting wires and approaches the critical Cooper pair-breaking current, the hot spots generated by photon absorption can induce vortices and anti-vortices, and their motion under the action of the Lorentz force generates resistance and voltage, which ultimately produces a normal state region. An increase of the wire width to the micron scale offers more freedom to design new detectors based on current nano- and microfabrication techniques. In this doctoral dissertation, I will first introduce the basic Ginzburg-Landau theory of superconductivity and the measurement theory of superconducting circuits related to single-photon detection. I will then provide a detailed description of the equipment construction and sample fabrication techniques used in the experiment. Finally, I will present my two main studies: in one project I examined the photon-detection performance of superconducting micro-wires with a multi-pinhole structure, the correlation between dark counts and magnetic vortices, as well as the properties of critical current in magnetic fields. In a subsequent study I conducted numerical simulations using the time dependent Ginzburg-Landau equation, to analyze the dynamics of vortices in superconducting micro-strips with regular large-area perforations by pinholes as a function of magnetic field and bias current. The thus obtained predictions were then successfully confirmed in a detailed experimental study on a series of microwires.
Superconducting single-photon detector (SSPD) is a planar nanostructure patterned from 4-nm-thick NbN film deposited on sapphire substrate. The sensitive element of the SSPD is 100-nm-wide NbN strip. The device is operated at liquid helium temperature. Absorption of a photon leads to a local suppression of superconductivity producing subnanosecond-long voltage pulse. In infrared (at 1550 nm and longer wavelengths) SSPD outperforms avalanche photodiodes in terms of detection efficiency (DE), dark counts rate, maximum counting rate and timing jitter. Efficient single-mode fibre coupling of the SSPD enabled its usage in many applications ranging from single-photon sources research to quantum cryptography. Recently we managed to improve the SSPD performance and measured 25% detection efficiency at 1550 nm wavelength and dark counts rate of 10 s-1. We also improved photon-number resolving SSPD (PNR-SSPD) which realizes a spatial multiplexing of incident photons enabling resolving of up to 4 simultaneously absorbed photons. Another improvement is the increase of the photon absorption using a λ/4 microcavity integrated with the SSPD. And finally in our strive to increase the DE at longer wavelengths we fabricated SSPD with the strip almost twice narrower compared to the standard 100 nm and demonstrated that in middle infrared (about 3 μm wavelength) these devices have DE several times higher compared to the traditional SSPDs.
We present and theoretically analyze the performance of a nonlocal superconducting single-photon detector. The device operates due to the energy-to-phase conversion mechanism, where the energy of the absorbed single photon is transformed in a variation of the superconducting phase. Within this scope, the detector is designed in the form of a double-loop superconductor--normal-metal--superconductor (S-N-S) Josephson interferometer, where the detection occurs in a long S-N-S junction and the readout is operated by a short S-N-S junction. The variation of the superconducting phase across the readout junction is measured by recording the quasiparticle current flowing through a tunnel-coupled superconducting probe. By exploiting realistic geometry and materials, the detector is able to reveal single photons of frequency down to 10 GHz when operated at 10 mK. Furthermore, the device provides a value of the signal-to-noise ratio of up 104 in the range from 10 GHz to 10 THz by selection of the magnetic flux and the bias voltage. This device will hopefully find direct application as a single-photon detector in both basic science and quantum technology, while the energy-to-phase conversion mechanism could be the basis of nonlocal readout and memory architectures for superconducting qubits.
Nanowire superconducting single photon detectors (SSPDs) [1] are characterized by very high sensitivity in the near infrared (detection efficiency η up to 30%, for a dark count rate DK of few Hz), speed (up to ∼1 GHz repetition rate) and time resolution (jitter of 20 ps full width at half maximum, FWHM). They can be operated at temperatures near 4 K, so they can be packaged in cryogenic dipsticks or cryogen-free refrigerators. These features make SSPDs the most promising detectors for telecom-wavelength single-photon counting applications. The basic structure of an SSPD is a narrow (w=50 to 120 nm), thin (th∼4-10 nm) NbN superconducting nanowire folded in a meander pattern. The typical detector active area (i.e. the size of the pixel) is Ad=10 × 10 µm2 (which allows an efficient coupling with the core of optical fibers at telecom wavelengths) with filling factor (f, the ratio of the area occupied by the superconducting meander to the device total area) ranging from 40% to 60%. The meanders are embedded in a 50 Ω coplanar transmission line. At present, the SSPD detection efficiency is limited by its absorbance (α, the ratio of the number of photons absorbed in the nanowire to the number of incident photons on the device active area). Indeed, it has been shown that in the classic front-illumination configuration α cannot exceed 30%. Our approach to increase α consists in integrating SSPDs with advanced optical structures such as distributed Bragg reflectors (DBRs) and optical waveguides. This requires to transfer the challenging SSPD technology (i.e. the deposition of high-quality few-nm thick NbN films and the nano-patterning by electron beam lithography, EBL) from the usual comfortable substrates, i.e. sapphire and MgO, which are known to allow the deposition of few-nm thick NbN films of excellent quality, to an optical substrate like GaAs, on which DBRs and waveguides can be easily obtained. Our first task was then to optimize a process for the deposition of high-quality few-nm thick NbN films on GaAs and AlAs/GaAs-based DBRs. Because of the requirement of compatibility with GaAs, the substrate temperature used for the depositions is 400°C, in order to prevent As evaporation. As GaAs and DBRs are highly mismatched substrates, the deposition parameters were first optimized with respect to the superconducting properties of NbN films on MgO substrates, which allow the growth of high crystal quality NbN films at low temperature. This made easier to separate the influence of stoichiometry from that of microstructure. The optimized deposition parameters were then used to grow NbN films on GaAs and DBRs, under the reasonable assumption (later checked and confirmed) that changing the substrate would not produce a change in film stoichiometry, but only in its microstructure. NbN films ranging from 150nm to 3nm in thickness were then deposited on epitaxial-quality single crystal MgO, GaAs and DBRs structures. The deposition technique is the current controlled DC magnetron sputtering (planar, circular, balanced configuration) of Nb in an Ar + N2 plasma. NbN films deposited on MgO exhibit superconducting critical temperature ΤC=10 Κ, superconducting transition width ΔΤC=0.8 Κ and residual resistivity ratio RRR=R(20K)/R(300K)=0.8 for th=4 nm, which are state of the art values, proof of the excellent quality of our low-temperature deposition process. The quality of films deposited on GaAs and on DBRs is lower than that of NbN deposited on MgO, as for any thickness they systematically exhibit higher ΔΤC and lower ΤC and RRR. However, 5.5 nm-thick NbN films on GaAs still exhibit ΤC=10.7 Κ, ΔΤC=1.1 Κ and RRR=0.7, which compares with 4.5 nm thick films on MgO, making them suitable for device fabrication. To our knowledge, the growth of such high quality thin NbN films on GaAs and DBRs, has never been reported in literature. The degradation of the superconducting properties exhibited by NbN films on GaAs and DBRs was attributed to a highly defected microstructure, due both to a higher lattice misfit between NbN and GaAs and to a poorer quality of the substrate surface. Encouraging preliminary results show that the quality of these films can be improved either cleaning the GaAs/DBR substrate surface more effectively or adding an MgO buffer layer. SSPDs were fabricated on thin NbN films (th=3-7 nm) deposited under optimal conditions on MgO and GaAs by EBL and reactive ion etching. The geometrical parameters of our detectors are: Ad=5×5 µm2, w=60-200 nm, f=40%-60%. The devices were then characterized both electrically and optically. I-V curves of test structures were measured, from which it was possible to deduce important physical parameters used as figures of merit to estimate the superconducting properties of the nanowires, or for the design and the simulation of the devices. The quality of the devices fabricated on GaAs is poorer than those on MgO, most likely due to the lower quality of NbN films deposited on GaAs and to issues related to the EBL nano-patterning step. Measurements of η and of DK as a function of the bias current were performed on SSPDs fabricated on MgO and GaAs. The best performance was exhibited by a w=100 nm, f=40%, th=4 nm meander, showing η=20% and noise equivalent power NEP=10-16 W/Hz1/2 (at λ=1.3 µm and T=4.2 K), which are state of the art values. This result showed for the first time that high performance NbN SSPDs can be realized on a different substrate and from a deposition process at lower temperature than previously reported. High detection efficiencies could not be measured with SSPDs fabricated on GaAs, but it should be noted that at present only first-generation devices (fabricated on GaAs substrates of poor surface quality) have been tested. Better results are expected from devices fabricated on the improved NbN films grown on clean or MgO-buffered GaAs substrates. Although SSPDs on MgO have shown high detection efficiency, the fabrication yield of high performance detectors has to be improved. Variations of the critical current along a nanowire are responsible for the wide distribution in efficiency values of nominally identical SSPDs. In order to understand the physical origin of the nanowire constrictions (i.e. regions of suppressed superconductivity) we performed a spatially-resolved characterization of η of a long straight nanowire, followed by a high resolution SEM (scanning electron microscope) scan on its whole length. Two types of inhomogeneities were evidenced, corresponding to localized efficiency dips and peaks. The peaks likely correspond to constrictions. SEM observations did not evidence any width narrowing at the position of the efficiency peaks, which suggests that constrictions might be due to thickness or quality inhomogeneities of the film occurring during the film deposition or later in the process. On the other hand, the efficiency dips have been correlated with lithography problems discovered on SEM images. Finally, a new photon number resolving detector, the Parallel Nanowire Detector (PND), has been demonstrated, which significantly outperforms existing approaches in terms of sensitivity, speed and multiplication noise in the telecommunication wavelength range. In particular, it provides a repetition rate (80 MHz) three orders of magnitude larger than any existing detector at telecom wavelength, and a sensitivity (NEP=4.2×10-18 W/Hz1/2) one-two orders of magnitude better, with the exception of transition-edge sensors (which require a much lower operating temperature). An electrical equivalent model of the device was developed in order to study its operation. The modeling predicts a physical limit to the reset time of the PND, which is lower than initially estimated. Furthermore, the figures of merit of the device performance in terms of efficiency, speed and sensitivity were defined and their dependency on the design parameters analyzed. Additionally, we developed modeling tools to fully characterize the device and an algorithm to estimate the photon number statistics of an unknown light using the PND. The reconstruction proved to be successful only for low photon fluxes, most likely due to the limited counting capability and the poor calibration of the detector. The PND, with its high repetition rate and high sensitivity, is then suitable for measuring an unknown photon number probability distribution assuming accurate calibration and sufficient counting capability. ______________________________[1] G. N. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, Appl. Phys. Lett. 79, 705 (2001).