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arXiv:1711.10846 · Physical Review Applied

Proximity SQUID Single-Photon Detector via Temperature-to-Voltage Conversion

Paolo Solinas, Francesco Giazotto, Giovanni Piero Pepe

Published Aug 13, 2018• 15 citations
arXivHTMLPDF

Abstract

The authors discuss the possible implementation of a single-photon detector based on a superconducting quantum interference device, or SQUID. The SQUID is initialized in an unstable state through the magnetic flux piercing it. A photon absorbed in a proximized normal-metal link induces a change in the critical current of the link, and a voltage pulse at the extremes of the device. Pulses of several tens of microvolts could result. Such a device allows the discrimination of a vast range of photon frequencies and is robust against thermal noise, making it a valuable candidate for detecting terahertz photons in particular.

DOI
10.1103/physrevapplied.10.024015

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The authors discuss the possible implementation of a single-photon detector based on a superconducting quantum interference device, or SQUID. The SQUID is initialized in an unstable state through the magnetic flux piercing it. A photon absorbed in a proximized normal-metal link induces a change in the critical current of the link, and a voltage pulse at the extremes of the device. Pulses of several tens of microvolts could result. Such a device allows the discrimination of a vast range of photon frequencies and is robust against thermal noise, making it a valuable candidate for detecting terahertz photons in particular.
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 (SSS-NNN-SSS) Josephson interferometer, where the detection occurs in a long SSS-NNN-SSS junction and the readout is operated by a short SSS-NNN-SSS 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{10}^{4}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.
Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.
SQUIDs (superconducting quantum interference devices) have been in use for more than half a century and constitute one of the first macroscopic quantum devices. SQUIDs operating at millikelvin temperatures can act as qubits for quantum computers and the recent reports of more than 50 qubit circuits indicate how far the technology has developed. In this talk, the author will describe two other applications of SQUIDs as quantum detectors, focusing on single spin and single photon energy resolving detection. Most superconducting devices rely on tri-layer Josephson tunnel junctions which are not easily scalable to the nanoscale. We have developed a Josephson junction fabrication method, based on electron beam lithography or focused ion beam milling of a single thin film of superconductor (Nb generally) which can provide sizes down to 50 nm. These devices are particularly relevant for two main applications in quantum technology and metrology. First, by shrinking the size of the SQUID loop and the junctions to around 200 nm, the sensitivity of the SQUID for magnetization measurements is improved to the level where a single electron spin flip may be detected. This is possible at the relatively elevated temperature of 4K.  We are working with Surrey University to implant single magnetic ions within the SQUID loop to provide a platform to test this combination as the basis for a new form of qubit operating at higher temperatures than the conventional Transmon superconducting devices. A second SQUID based detector which we are developing is an inductive transition edge sensor device (ISTED) for energy resolving measurements of single photons. This is based on the development of conventional transition edge sensors where we detect the change of the penetration depth of a small thin film of superconductor when it absorbs a photon. In this way, a major source of noise in conventional TES may be avoided since the absorber remains in the superconducting state at all times. In this way we have demonstrated single photon detection at 633 nm with 0.1eV resolution at operating temperature of 7.5K.
We present a new type of transition-edge sensor for single-photon and macro-molecule detection. In our detector the absorber element is an isolated, passive absorber of extremely low thermal mass, maintained close to, but below, its superconducting–normal transition, and strongly inductively coupled to a SQUID sensor. Incoming particles or photons are sensed in terms of a transient change in the inductive coupling, rather than a change in resistance. The detector's energy sensitivity and response time can be defined by the thermal mass of the absorber and its thermal contact with a substrate, independently of any electrical connections. We have modelled the energy sensitivity of our inductive superconducting transition-edge sensor using a sub-micron SQUID as an inductive read-out device. An ultimate energy resolution of order 10 −25 J Hz −1 is theoretically estimated based on the properties of the read-out SQUID and the dimensions of the absorber. We also report our initial work on fabrication of the Nb nanoscale SQUID where we have used the same material deposited on top of the SQUID as a thin-film absorber.
Superconducting radiation detectors are promising for high sensitivity in the terahertz frequency range, as required for applications in astrophysics and quantum information processing. This study considers using a detector readout scheme based on a superconducting quantum interference proximity transistor (SQUIPT), with a detailed analysis of device physics and performance predictions. This interferometer-based scheme could also be used for other temperature-sensing purposes, and has the potential to improve the state of the art in ultrahigh-sensitivity superconducting single-photon detectors.
We have realized and tested a fast stroboscopic detector for magnetic flux measurements. The key element of our detector is a hysteretic dc superconducting quantum interference device (SQUID). Stroboscopic read-out of the magnetic flux coupled with the SQUID is accomplished by biasing the SQUID with fast current pulses. The shorter these pulses, the more stroboscopic and less invasive the measurement we are performing. In order to reduce the duration of the current pulses, we take advantage of the superconducting-normal transition induced by laser light in thin superconducting films. The interaction of laser light with superconducting thin films has been investigated thoroughly in the past and many applications have been proposed which rely on the fast typical times with which superconductivity is broken and a resistive behavior arises. We have measured a threshold resolution of 6.9mΦ0 at 4.2 K, and this value corresponds to the thermodynamic limit of the SQUID. The detector has been accurately characterized: An improved and more sensitive version might prove useful for quantum mechanics and quantum computation experiments, for example, in detecting the state of flux qubits.