Photon shot noise limited detection of terahertz radiation using a quantum capacitance detector

P. M. Echternach, K. J. Stone, C. M. Bradford, P. K. Day, D. W. Wilson, K. G. Megerian, N. Llombart, J. Bueno
2013 Applied Physics Letters  
We observed a sweep rate dependence of the quantum capacitance in a single Cooper-Pair box used as the readout of a Quantum Capacitance Detector. A model was developed that fits the data over five orders of magnitude in sweep rate and optical signal power and provides a natural calibration of the absorbed power. We are thereby able to measure the noise equivalent power of the detector as a function of absorbed power. We find that it is shot-noise-limited in detecting 1.5 THz photons with
more » ... d power ranging from 1 Â 10 À22 W to 1 Â 10 À17 W. V C 2013 AIP Publishing LLC. A number of approaches for photodetection of sub-mm and far-infrared radiation are being pursued for ground and space based applications. 1-6 For cooled space based telescopes, requiring photon noise limited detection, the required Noise Equivalent Powers (NEP) are extremely low. As an example, for a background load of 10 À19 W, the required NEP is of the order of 1 Â 10 À20 W/Hz 1/2 for 1.5 THz radiation. 7 No detector to date has demonstrated an NEP lower than 0.8 Â 10 À19 W/Hz 1/2 . 8-10 A detector based on semiconductor quantum dots has been reported to detect individual photons, but information on photon flux is not available to determine the actual NEP. 11 We have been developing a concept, the Quantum Capacitance Detector (QCD), 12-15 and in the work described here we demonstrated an NEP of 1.2 Â 10 À20 W/Hz 1/2 at 1.15 Â 10 À19 W and photon noise limited performance from 10 À22 W through 10 À17 W. As part of the characterization process, we have developed a detailed balance model of the quasiparticle population in the readout device, the Single Cooper-pair Box (SCB). 16 This model provided an insight on the mechanism for the so-called quasi-particle poisoning, a persistent problem in the development of quantum bits based on SCBs and other implementations, rendering this work relevant for research in quantum computation. [17] [18] [19] [20] [21] The QCD is based on the SCB, 16 a superconducting mesoscopic circuit consisting of a superconducting island connected to a lead electrode (or reservoir) via a small (100 Â 100 nm typically) tunnel junction, as shown in Fig. 1 . Radiation is coupled to the reservoir via the antenna. When a photon is absorbed in the reservoir, Cooper-pairs are broken, and quasiparticles are created above the superconducting gap edge and trapped inside the reservoir by the higher gap niobium plugs. Quasiparticles can then tunnel to the island, thereby changing the capacitance of the device by C Q , which in turn changes the resonance frequency of the half-wave resonator. This frequency change is sensed by a change in the phase of the microwave passing through the feedline. The island can be biased via a gate capacitor. The capacitance of the island consists of a periodic set of peaks of height C Q ¼ ð4E C =E J ÞðC 2 g =C R Þ above a constant level C geom determined by the dimensions of the junction. Here, C R is the sum of the junction and gate capacitance, C g is the gate capacitance, E C is the charging energy e 2 /2C R , and E J the tunnel junction Josephson energy. The peaks arise due to the quantized nature of the Cooper pair charges and are called the quantum capacitance. 17 When biased at a voltage corresponding to a peak (C g V g ¼ 1e, 3e...), assuming a quasiparticle tunnels to the island, the capacitance will drop from C Q þ C geom to C geom . To transform the SCB into a detector, the reservoir is connected to an antenna, which couples radiation in to break Cooper pairs, generating a population N qp of quasiparticles in the reservoir, which is a function of the optical signal power coupled to the reservoir. This establishes a tunneling rate C in onto the island proportional to N qp , while quasiparticles can tunnel out of the island with a rate C out , which is approximately independent of N qp . The probability of occupation of the island by one quasiparticle is given by P odd ðN qp Þ ¼ C in =ðC in þ C out Þ. The average value of the capacitance peak will then be given by C peak ¼ C geom þ C Q ð1 À P odd Þ and is a function of the optical signal power. In order to read out the capacitance, the island is coupled to a half wave resonator. A capacitance change on the island will cause a resonance frequency shift in the resonator. The resonator is coupled on the opposite end to a microwave feedline, and the transmitted power through the feedline is measured using a conventional in-phase-quadrature (IQ) mixer after amplification by a cold (4.2 K) low noise amplifier followed by room temperature amplifiers. The sample used in this work consisted of a 5 Â 5 array of detectors and is shown in Fig. 1 . The resonators for each pixel had a slightly different resonant frequency and were all connected to a single feedline, allowing for simple frequency multiplexing. The audio frequency bias was applied also through the microwave feedline. This eliminated an a)
doi:10.1063/1.4817585 fatcat:len6loyfsnb2xpjczr7qbmvmo4