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Applied Physics Letters

Proof of concept of the quantum capacitance detector

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

Published Mar 8, 2010• 18 citations
Publisher

Abstract

We fabricated and tested a proof-of-concept quantum capacitance detector, a superconducting radiation detector described in a recent publication [Shaw et al., Phys. Rev. B 79, 144511 (2009)]. In this concept, quasiparticle tunneling in a single Cooper-pair box is used to measure the density of quasiparticles in a superconducting absorber. We measured and characterized the response of the device to electrical quasiparticle injection obtaining its sensitivity. Moreover, we have converted the sensitivity to electrical noise-equivalent power, which is in the 10−18 W/Hz1/2 range at loading powers between 10−15–10−16 W.

DOI
10.1063/1.3339163

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Publisher's Note: Quantum capacitance detector: A pair-breaking radiation detector based on the single Cooper-pair box [Phys. Rev. B79, 144511 (2009)]

Matthew D. Shaw, Juan Bueno, Peter K. Day +2 more

Physical Review B•May 13, 2009•1 citation
We fabricated and tested a proof-of-concept quantum capacitance detector, a superconducting radiation detector described in a recent publication [Shaw et al., Phys. Rev. B 79, 144511 (2009)]. In this concept, quasiparticle tunneling in a single Cooper-pair box is used to measure the density of quasiparticles in a superconducting absorber. We measured and characterized the response of the device to electrical quasiparticle injection obtaining its sensitivity. Moreover, we have converted the sensitivity to electrical noise-equivalent power, which is in the 10−18 W/Hz1/2 range at loading powers between 10−15–10−16 W.
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.
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.
Quasiparticle tunneling events are measured in real time using a quantum capacitance detector (QCD), allowing for the extraction of tunneling rates as a function of temperature and optical loading of radiation coming from a black body source filtered to 200 m. The measurements are used to corroborate the basic operating principles of the QCD. An estimate of the residual quasiparticle density is made, and the noise equivalent power (NEP) is assessed to be 7.2×10−20W/Hz1/2 at the lowest signal power of 9.2×10−20W. This NEP was higher than the photon noise by only a factor of 7 over a wide signal power range.
Superconducting quantum capacitance detectors (QCDs) have emerged as among the most sensitive terahertz (THz) photon detectors, positioning them as promising detector components for next-generation cryogenic telescopes. However, their limited dynamic range (DR, ∼32 dB) and low saturation power (∼0.1 fW) have hindered their broader applications, such as in THz astronomy, high-resolution spectroscopy, and microscopic imaging. Here, we propose a mechanism that enables controllable adjustment of the sensitivity and DR of THz QCDs, supported by both theoretical modeling and experimental validation. We developed a QCD design that features a fork-shaped gate capacitor and a high-efficiency grid absorber. The resulting devices exhibit a noise equivalent power of 5.0 × 10−20 W/Hz1/2 and a DR of 40 dB, providing a favorable balance between sensitivity and DR and advancing the practical application of QCDs in future THz applications.
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.