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arXiv:1709.07815 · Applied Physics Letters

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

A. A. Esmail, Andrew J. Ferguson, N. J. Lambert

Published Dec 18, 2017• 4 citations
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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 time averaged probability that the device is poisoned by quasiparticles, and by comparing this with previous work, we conclude that quasiparticle exchange between the dots and the leads is an important relaxation mechanism.

DOI
10.1063/1.5009079

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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 time averaged probability that the device is poisoned by quasiparticles, and by comparing this with previous work, we conclude that quasiparticle exchange between the dots and the leads is an important relaxation mechanism.
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 to be significantly worse than that of a unisolated double dot, and conclude that quasiparticle exchange between the dot and the leads is an important relaxation mechanism.
One of the essential limitations for a physical realisation of topological-qubit based quantum computing is the problem of uncontrolled quasiparticle poisoning of the superconducting charge island. These quasiparticles have been previously reported as a known limiting factor in most of the superconducting architectures. To this end, we consider the importance of equilibrium quasiparticle poisoning in hybrid semiconducting-superconducting systems believed to be able to host Majoranas. We employed gate-based dispersive sensing by investigating a reflected radiofrequency (RF) signal from a resonator, which is capacitively coupled to one of the gate electrodes of the hybrid double dot realised in an InAs nanowire. This way, the quantum capacitance of the system can be probed, which in turn reveals charge tunnelling processes, and can be used as an indicator of single-electron tunnelling events. By operating the device in a so-called floating regime, where a double dot system is entirely decoupled from the leads, external non-equilibrium quasiparticle poisoning events can be exponentially suppressed. By appropriately tuning the chemical potentials on the quantum dot and the superconducting island, we attempt to scrutinise the dynamics of the quasiparticles innate to the superconductor. We investigated the dependence of quasiparticle tunnelling rates on the temperature of the system, as well as the applied RF power, as both are predicted to increase the average number of quasiparticles on the island. In both cases, we could find no observational proof of quasiparticle tunnelling events through the use of our measurement technique. We report an observation of a two-level resonator response, which we explain using a two-level fluctuator. Analysing telegraph measurements, we demonstrate the viability of the analysis tool, which allows us to determine the average occupation times of each state. In principle, for processes occurring at rates slower than the measurement frequency, this technique can be used to convert a\nrelatively faint difference between the “poisoned and unpoisoned state”, to the signal of the gate being on coulomb resonance versus on Coulomb blockade, which is a much clearer signal to distinguish. Both, the technique, as well as the expected rates of the physical processes within the island, are evaluated in the context of the experiment.
We demonstrate the operation of a differential single Cooper-pair box, a charge qubit consisting of two aluminum islands, isolated from ground, coupled by a pair of small-area Josephson junctions, and read out with a superconducting differential radio-frequency single electron transistor. We have tested four devices, all of which show evidence of quasiparticle poisoning. The devices are characterized with microwave spectroscopy and temperature dependence studies, and Coulomb staircases are shown to be e-periodic in all samples. However, coherent control is still possible with non-adiabatic voltage pulses. Coherent oscillation experiments and a relaxation time measurement were performed using a charge derivative readout technique.
We present experiments on single Cooper-pair transistors made of two different superconducting materials. We chose Ti and Al to create an energy gap profile such that the island has a higher gap than the leads, thereby acting as a barrier to quasiparticle tunneling. Our transport measurements demonstrate that quasiparticle poisoning is suppressed in all our TiAlTi structures (higher gap for the island) with clear 2e periodicity observed, whereas full quasiparticle poisoning is observed in all AlTiAl devices (higher gap for the leads) with e periodicity.
The work presented in this thesis investigates transitions between quantum states in superconducting double dots (SDDs), a nanoscale device consisting of two aluminium superconducting islands coupled together by a Josephson junction, with each dot connected to a normal state lead. The energy landscape consists of a two level manifold of even charge parity Cooper pair states, and continuous bands corresponding to charge states with single quasiparticles in one or both islands. These devices are fabricated using shadow mask evaporation, and are measured at sub Kelvin temperatures using a dilution refrigerator. We use radio frequency reflectometry to measure quantum capacitance, which is dependent on the quantum state of the device. We measure the quantum capacitance as a function of gate voltage, and observe capacitance maxima corresponding to the Josephson coupling between even parity states. We also perform charge sensing and detect odd parity states. These measurements support the theoretical model of the energy landscape of the SDD. By measuring the quantum capacitance in the time domain, we observe random switching of capacitance between two levels. We determine this to be the stochastic breaking and recombination of single Cooper pairs. By carrying out spectroscopy of the bath responsible for the pair breaking we attribute it to black-body radiation in the cryogenic environment. We also drive the breaking process with a continuous microwave signal, and find that the rate is linearly proportional to incident power. This suggests that a single photon process is responsible, and demonstrates the potential of the SDD as a single photon microwave detector. We investigate this mechanism further, and design an experiment in which the breaking rate is enhanced when the SDD is in the antisymmetric state rather than the symmetric state. We also measure the quantum capacitance of a charge isolated double dot. We observe 2e periodicity, indicating the tunnelling of Cooper pairs and the lack of occupation of quasiparticle states. This work is relevant to the range of experiments investigating the effect of non-equilibrium quasiparticles on the operation of superconducting qubits and other superconducting devices.
We study single-Cooper-pair-transistors in which the island is made from an ultra-thin (10nm) aluminum film. The value of the superconducting gap (2Delta) changes dramatically with film thickness therefore allowing different superconducting gaps for the islands and leads simply by using different film thicknesses. Devices engineered in this way can be made less susceptible to having a single quasiparticle trapped on the island (often known as the quasiparticle poisoning problem). This is an advantage for charge-based superconducting qubits where quasiparticle poisoning is a source of decoherence. We perform radio-frequency measurements and observe 2e-periodic supercurrent peaks as a function of gate charge. With the device biased on a supercurrent oscillation, a fast random-telegraph signal due to individual quasiparticle poisoning events is observed. By analyzing the statistics of the poisoning and un-poisoning events we determine the energetics and thermal activation of these processes