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arXiv:cond-mat/0408145 · Physical Review B

Quantum capacitive phase detector

Leif Roschier, Mika A. Sillanpää, Pertti Juhani Hakonen

Published Jan 31, 2005• 15 citations
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Abstract

We discuss how a single Cooper-pair transistor may be used to detect the superconducting phase difference by using the phase dependence of the input capacitance from gate to ground. The proposed device has a low power dissipation because its operation is in principle free from quasiparticle generation. According to the sensitivity estimates, the device may be used for efficient qubit readout in a galvanically isolated and symmetrized circuit.

DOI
10.1103/physrevb.71.024530

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We discuss how a single Cooper-pair transistor may be used to detect the superconducting phase difference by using the phase dependence of the input capacitance from gate to ground. The proposed device has a low power dissipation because its operation is in principle free from quasiparticle generation. According to the sensitivity estimates, the device may be used for efficient qubit readout in a galvanically isolated and symmetrized circuit.
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Abstract We demonstrate that the supercurrent branch of a single Cooper-pair transistor (SCPT) can be used for sensitive phonon detection. We built a theoretical model of phonon detection based on quasiparticle poisoning and demonstrated its operation with an SCPT and a superconducting quantum interference device as a phonon source both composed of mesoscopic Al/AlO x /Al junctions. The sensitivity of the SCPT detector turned out to be approximately 103 times as high as that of the traditional superconducting-tunnel-junction detector.
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 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.
Several physical realizations of quantum bits have been proposed. Of those, nano-electronic devices appear most suitable for large-scale integration and potential applications. We suggest to use low-capacitance Josephson junctions, exploiting the coherence of tunneling in the superconducting state combined with the possibility to control individual charges by Coulomb blockade effects (cond-mat/9706016,cond-mat/9808067). These systems constitute quantum bits, with logical states differing by one Cooper-pair charge. Single- and two-bit operations can be performed by applying a sequence of gate voltages. The phase coherence time is sufficiently long to allow a series of these steps. In addition to the manipulation of qubits the resulting quantum state has to be read out. This can be accomplished by coupling a single-electron transistor capacitively to the qubit (cond-mat/9801125). To describe this quantum measurement process we study the time evolution of the density matrix of the coupled system. Only when a transport voltage is turned on, the transistor destroys the phase coherence of the qubit; in this case within a short time. The measurement is accomplished after a longer time scale, when the signal resolves the different quantum states. At still longer times the measurement process itself destroys the information about the initial state. We present a suitable set of system parameters, which can be realized by present-day technology.
Among the potential physical implementations of quantum bits (qubits) solid state devices appear most promising for large scale applications and integration in electronic circuits. Devices based on low-capacitance Josephson junctions exploit the coherence of the superconducting state, combined with the possibility to control individual charges by Coulomb blockade effects. The logical states in these systems differ by one Cooper-pair charge. Single- and two-bit operations can be performed by applying a sequence of gate voltages. The phase coherence time is sufficiently long to allow a series of these steps. If, in addition, the strength of the Josephson coupling is controlled (e.g. by currents in SQUID loops) the system is ideal for quantum computation. Recent experiments by Nakamura et al. show the feasibility of this approach. In addition to the manipulations of qubits the resulting quantum state has to be read out. This quantum measurement process can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. We investigate the process by analyzing the time-evolution of the density matrix of the coupled system. The measurement process is characterized by three time scales: the dephasing time, which is fast when a dissipative current flows in the SET, a longer ‘measurement time’ needed to read out the signal, and a third, even longer ‘mixing time’, characterizing measurement induced transitions which destroy the information about the initial quantum state. The noise spectrum of the current in the SET displays these time scales and allows their direct observation.