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

Abstract

We increase the isolation of a superconducting double dot from its environment by galvanically isolating it from any electrodes. We probe it using high frequency reflectometry techniques, find 2e-periodic behaviour, and characterise the energy structure of its charge states. By modelling the response of the device, we determine the quasiparticle poisoning rate and conclude that quasiparticle exchange between the dot and the leads is an important relaxation mechanism.The presence of excess quasiparticle excitations has a deleterous effect on many superconducting technologies, including resonators 1,2 , single photon detectors 3 , on-chip electronic refrigerators 4 , and superconducting qubits 5-7 . In qubits this is known as quasiparticle poisoning and is often the limiting factor for coherence times. Typically, there is a significant quasiparticle population even at dilution fridge temperature due to incomplete shielding of the qubit from non-equilibrium radiation 8,9 . Understanding and supression of this is therefore important for successful superconducting technologies.We have recently found the superconducting double dot (SDD) a useful platform to understand and exploit quasiparticles 10-12 . The superconducting double dot comprises two superconducting islands, with a charging energy comparable to the superconducting gap, coupled to each other by a Josephson junction. In our previous work, the islands have been connected via tunnel junctions to normal-metal leads, but here we study a galvanically isolated double dot (GIDD). This approach removes one of the key relaxation pathways for the SDD of quasiparticle exchange with the leads 10 , but isolated semiconductor qubits have been shown to have reduced electron temperatures 13 . It also prevents quasiparticle poisoning via tunnelling from the leads. Previous studies on a similar system measured via a charge sensor 14,15 found strictly e-periodic behaviour, implying complete poisoning of the device. This was ascribed to back action from the charge sensor. Here we revisit the system, using radio-frequency reflectometry and microwave spectroscopy to assess the poisoning rate.In Fig. 1(a) we show an SEM of the GIDD, made via double angle shadow mask evaporation 16 , with 20 nm thick aluminium forming each island. The SQUID-like geometry allows for the tuning of the Josephson energy of the junction between the two islands with a perpendicular magnetic field. Electrostatic gates allow control of the chemical potentials of the islands via V 1 and V 2 and the introduction of microwave frequency excitations (V MW ) and a probe tone for reflectometry (V RF ). The device is embedded in a tank circuit ( Fig. 1(b)) comprising a lumped element inductor (L = 560 nH) and its parasitic capacitance (C p = 0.33 pF), and cooled to a base temperature of 35 mK. The inductor has a resonant frequency of f 0 = 370.25 MHz, and, by homodyne detec-(a) (b) 1 um (c) 30 20 10 0 -10 -20 -30 20 10 0 -10 -20 -30 30 0.08 0.06 0.04 0.02 0.00 V RF Bias tee V 1 L C p C g1 V MW C m2 C m1 C g2 V 2 V...

Rapid microwave-only characterization and readout of quantum dots using multiplexed gigahertz-frequency resonators

“…We now move on to the capacitively coupled gate resonators and investigate DGS in the double quantum dot (DQD) regime [17,25,[43][44][45][46][47][48][49][50][51][52]. To tune the system into a DQD, the voltages of gates T4, T5, and T6 are each decreased into the tunneling regime.…”

Revealing charge-tunneling processes between a quantum dot and a superconducting island through gate sensing

“…More precisely, tunneling within these structures can impart a frequency shift on a resonant circuit that can be observed on short time scales with high accuracy. In this way, experiments have revealed coherent charge hybridization between superconductors [14][15][16] and semiconductor double quantum dots [17][18][19][20]. Moreover, capacitive RF sensing has been used to study charging of QDs connected to normal and superconducting reservoirs [21][22][23].…”

“…As we have seen, the main difference between the two devices is that the odd states of the SC can not be directly accessed in the regime E 0 > E S C . This changes in absence of lead reservoirs because quasiparticles that tunnel from the QD onto the SC are confined to the system [16]. The additional energy associated with decharging the QD makes Cooper pair tunneling energetically unfavorable when E 0 < E S C + E QD C .…”