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arXiv:0806.4205

The Quantum Capacitor Detector: A Single Cooper Pair Box Based Readout for Pair Breaking Photo-detectors

P. M. Echternach, Matthew D. Shaw, Juan Bueno, Peter K. Day, C. Matt Bradford

Published Jun 25, 2008• 0 citations
arXivHTMLPDF

Abstract

We propose a sensitive new detector based on Cooper pair breaking in a superconductor. The quantum capacitor detector (QCD) exploits the extraordinary sensitivity of superconducting single-electron devices to the presence of quasiparticles generated by pair-breaking photons. This concept would enable single-photon detection at far-IR and sub-millimeter frequencies with detector sensitivities that exceed that of transition-edge-sensor bolometers (TES), kinetic inductance detectors (KID), and superconducting tunnel junction detectors (STJ). The detectors we propose are based on the single Cooper pair box (SCB), a mesoscopic superconducting device that has been successfully developed at JPL for applications in quantum computing. This concept allows for frequency multiplexing of a large number of pixels using a single RF line, and does not require individual bias of each pixel. The QCD is ideal for the sensitive spectrographs considered for upcoming cold space telescopes, such as BLISS for SPICA in the coming decade, and for the more ambitious instruments for the SAFIR / CALISTO and SPIRIT / SPECS missions envisioned for the 2020 decade. These missions require large detector arrays (> 10,000 elements) which are limited by astrophysical background noise, corresponding to a noise-equivalent power (NEP) as low as 2x10-20 W / Hz1/2. Given its intrinsic response time, the QCD could also be used for energy-resolved visible photon detection, with estimated energy resolution > 100, enabling imaging low-resolution spectroscopy with an array of detectors.

DOI
10.48550/arxiv.0806.4205

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We propose a sensitive new detector based on Cooper pair breaking in a superconductor. The quantum capacitor detector (QCD) exploits the extraordinary sensitivity of superconducting single-electron devices to the presence of quasiparticles generated by pair-breaking photons. This concept would enable single-photon detection at far-IR and sub-millimeter frequencies with detector sensitivities that exceed that of transition-edge-sensor bolometers (TES), kinetic inductance detectors (KID), and superconducting tunnel junction detectors (STJ). The detectors we propose are based on the single Cooper pair box (SCB), a mesoscopic superconducting device that has been successfully developed at JPL for applications in quantum computing. This concept allows for frequency multiplexing of a large number of pixels using a single RF line, and does not require individual bias of each pixel. The QCD is ideal for the sensitive spectrographs considered for upcoming cold space telescopes, such as BLISS for SPICA in the coming decade, and for the more ambitious instruments for the SAFIR / CALISTO and SPIRIT / SPECS missions envisioned for the 2020 decade. These missions require large detector arrays (> 10,000 elements) which are limited by astrophysical background noise, corresponding to a noise-equivalent power (NEP) as low as 2x10-20 W / Hz1/2. Given its intrinsic response time, the QCD could also be used for energy-resolved visible photon detection, with estimated energy resolution > 100, enabling imaging low-resolution spectroscopy with an array of detectors.
According to a proposal, a phenomenon associated with excitation of quasiparticles in certain superconducting quantum devices would be exploited as a means of detecting photons with exquisite sensitivity. The phenomenon could also be exploited to perform medium-resolution spectroscopy. The proposal was inspired by the observation that Coulomb blockade devices upon which some quantum logic gates are based are extremely sensitive to quasiparticles excited above the superconducting gaps in their leads. The presence of quasiparticles in the leads can be easily detected via the charge states. If quasiparticles could be generated in the leads by absorption of photons, then the devices could be used as very sensitive detectors of electromagnetic radiation over the spectral range from x-rays to submillimeter waves. The devices in question are single-Cooper-pair boxes (SCBs), which are mesoscopic superconducting devices developed for quantum computing. An SCB consists of a small superconducting island connected to a reservoir via a small tunnel junction and connected to a voltage source through a gate capacitor. An SCB is an artificial two-level quantum system, the Hamiltonian of which can be controlled by the gate voltage. One measures the expected value of the charge of the eigenvectors of this quantum system by use of a radio-frequency single-electron transistor. A plot of this expected value of charge as a function of gate voltage resembles a staircase that, in the ideal case, consists of steps of height 2 e (where e is the charge of one electron). Experiments have shown that depending on the parameters of the device, quasiparticles in the form of "broken" Cooper pairs present in the reservoir can tunnel to the island, giving rise to steps of 1 e. This effect is sometimes called "poisoning." Simulations have shown that an extremely small average number of quasiparticles can generate a 1-e periodic signal. In a device according to the proposal, this poisoning would be turned to advantage. Depending on the wavelength, an antenna or other component would be used to couple radiation into the reservoir, wherein the absorption of photons would break Cooper pairs, thereby creating quasiparticles that, in turn, would tunnel to the island, creating a 1-e signal. On the basis of conservative estimates of device parameters derived from experimental data and computational simulations that fit the data, it has been estimated that the noise equivalent power of a device according to the proposal could be as low as 6 10(exp -22) W/Hz(exp 1/2). It has also been estimated that the spectroscopic resolution (photon energy divided by increment of photon energy) of such a device in visible light would exceed 100.
Single photon detectors (SPDs) at terahertz (THz) waveband are necessary for the THz applications. The superconducting quantum capacitance detectors (QCDs) are proved to be ultra-sensitive at THz waveband. 1.5 THz single photon detection has been achieved. Besides their sensitivity, large array of QCDs has also developed. Therefore, it is convenient of QCDs to achieve multiplexing through a microwave feedline like MKIDs. A QCD consists of a high quality factor (Q) microwave superconducting resonators and a single cooper-pair box (SCB). The microwave resonators are fabricated using tantalum (Ta) or niobium (Nb). The SCB is made of aluminum (Al). The ground electrode of the SCB is designed to be an absorber at THz waveband. The absorber is an inductive absorber consisting of orthogonal Al nanowires. Finally, we succeeded in measuring the periodic quantum capacitance signals. However, the responsivity is relatively low due to its weak coupling to the resonators of SCB. In the future, we need to optimize the performance of the detectors. QCDs are a prospect candidate of the next-generation astronomy observation and so on.
We present the optical characterization at 200 μm wavelength of an antenna coupled quantum capacitance detector (QCD), a cryogenic detector based on a single Cooper pair box (SCB). The response of the device to a cryogenic blackbody source and the device noise have been measured giving an optical noise-equivalent power (NEP) at a readout frequency of 10 kHz of 2 × 10−17 W/Hz1/2 at an optical loading of 17 fW. For optical loadings of a few pW, the optical NEP was on the order of 10−16 W/Hz1/2, demonstrating that QCDs could already work as detectors for far-infrared and submillimeter wave radiation in a ground based telescope.
We present a proposed design for a pair-breaking photodetector for far-infrared and submillimeter radiation. Antenna-coupled radiation generates quasiparticles in a superconducting absorber, the density of which are measured using a single Cooper-pair box. Readout is performed using an electromagnetic oscillator or a microwave resonator, which is well suited for frequency multiplexing in large arrays. Theoretical limits to detector sensitivity are discussed and modeled, with predicted sensitivities on the order of ${10}^{\ensuremath{-}21}\text{ }\text{W}/\sqrt{\text{Hz}}$. We anticipate that this detector can be used to address key scientific goals in far-infrared and submillimeter astronomy.
We are developing superconducting direct detectors for submillimeter astronomy that can in principle detect individual photons. These devices, Single Quasiparticle Photon Counter (SQPC), operate by measuring the quasiparticles generated when single Cooper-pairs are broken by absorption of a submillimeter photon. This photoconductive type of device could yield high quantum efficiency, large responsivity, microsecond response times, and sensitivities in the range of 10-20 Watts per root Hertz. The use of antenna coupling to a small absorber also suggests the potential for novel instrument designs and scalability to imaging or spectroscopic arrays. We will describe the device concept, recent results on fabrication and electrical characterization of these detectors, issues related to saturation and optimization of the device parameters. Finally, we have developed practical readout amplifiers for these high-impedance cryogenic detectors based on the Radio-Frequency Single-Electron Transistor (RF-SET). We will describe results of a demonstration of a transimpedance amplifier based on closed-loop operation of an RF-SET, and a demonstration of a wavelength-division multiplexing scheme for the RF-SET. These developments will be a key ingredient in scaling to large arrays of high-sensitivity detectors.