Experimental observation of the breaking and recombination of single Cooper pairs

Abstract

We observe the real-time breaking of single Cooper pairs by monitoring the radio-frequency impedance of a superconducting double quantum dot. The Cooper pair breaking rate in the microscale islands of our device decreases as temperature is reduced, saturating at 2 kHz for temperatures beneath 100 mK. In addition, we measure in real time the quasiparticle recombination into Cooper pairs. Analysis of the recombination rates shows that, in contrast to bulk films, a multistage recombination pathway is followed. Unpaired electronic excitations-quasiparticles-play an important role in determining the behavior of superconducting electrical devices. They lead to even-odd parity effects in Coulomb blockade nanostructures [1,2]; they act as a source of decoherence in superconducting qubits [3]; they cause generation-recombination noise in superconducting resonators [4]; they may be important in superconductingnormal devices for Majorana fermionics [5]; and, importantly, they enable the detection of far-infrared light, for example, in kinetic inductance detectors [6]. In this Rapid Communication we investigate the generation and recombination of single quasiparticle pairs in a superconducting double dot (SDD), a Coulomb blockade nanostructure. Double quantum dots have been widely investigated in the context of semiconducting spin qubits where they enable electrostatic control and measurement over electron spins and spin pairs [7]. Previously, semiconductor double dots have been integrated with superconducting leads, allowing electrostatically tunable supercurrents [8] and the splitting of Cooper pair currents into spatially separated and correlated electron currents [9][10][11]. However, apart from an early study investigating the superconducting double quantum dot as a qubit architecture [12], there have been few studies of this system, thus motivating our current work.We investigate the quasiparticle dynamics in the SDD, therefore, our results are relevant to the long-standing quasiparticle poisoning problem in superconducting qubits [13,14]. It has long been known that incoherent quasiparticle tunneling interrupts the coherent tunneling of Cooper pairs. Quasiparticle poisoning is hence a serious issue in charge-based * ajf1006@cam.ac.uk superconducting qubits [15] and has recently been shown to be relevant in the case of low-charging energy transmon qubits [16]. Experiments on superconducting qubits have shown that by taking extreme care over filtering infrared radiation it is possible to extend coherence times, presumably because of the lower quasiparticle temperatures achieved [17]. Quasiparticle tunneling into a Cooper pair box has been used to detect far-infrared radiation from a blackbody source with a noise-equivalent power of less than 10 −19 W/Hz 1/2 , potentially providing a successor technology to kinetic inductance detectors [18]. In parallel, studies on superconducting resonators have shown a saturation of the quasiparticle population at a relatively high temperature of 140 mK [19]. It remains an experimenta...

Identifying single electron charge sensor events using wavelet edge detection

“…The rate of false positives may be reduced by including a prior knowledge of the detector sensitivity: edges can be rejected if the signal level does not change by the expected value in the vicinity of the edge. It should be noted that there are also many ways to improve the thresholding technique presented here, although at the cost of adding additional parameters that must be tuned; for example, local averaging to compensate for low-frequency signal drift [17], or using a Schmitt trigger to select events [42]. In general, these filters will make threshold edge detection more similar to the wavelet approach, which can be thought of as a filter with an excellent selectivity to sharp edges.…”

Quantum capacitance and charge sensing of a superconducting double dot

“…1(b)); and an anticrossing opens up between even parity states. These are analogous to the anticrossings observed in semiconductor double dots 15 , and similarly result in a change in ∂E/∂V s , leading to a non-zero quantum capacitance close to the anticrossing 16 .…”

“…Alternatively, if the splitting is driven by incident radiation it may be usable as a tunable detector for microwave light 16 . …”

Cooper pair tunnelling and quasiparticle poisoning in a galvanically isolated superconducting double dot

“…To support our estimates for the device parameters, we model the quantum capacitance of a GIDD as a function of the gate voltages. Quantum capacitance is the polarizability associated with the Cooper pair eigenstates and has been used to readout the quantum state of devices consisting of Josephson junctions 10,24,25 . The quantum capacitance is given by C Q = −∂ 2 E/∂V 2 g1 , where E correponds to the energy of the occupied eigenstate of the GIDD.…”

“…A possible approach to improve poisoning levels in this system is to include small quasiparticle traps for each island 28,29 . Finally, it would be advantageous to be able to probe the poisoning events in real time 10 ; by embedding the GIDD in a superconducting resonator, it should be possible to improve the sensitivity of our measurement in order to access this regime.…”