We report direct detection of charge tunneling between a quantum dot and a superconducting island through radio-frequency gate sensing. We are able to resolve spin-dependent quasiparticle tunneling as well as two-particle tunneling involving Cooper pairs. The quantum dot can act as an RF-only sensor to characterize the superconductor addition spectrum, enabling us to access subgap states without transport. Our results provide guidance for future dispersive parity measurements of Majorana modes, which can be realized by detecting the parity-dependent tunneling between dots and islands.
We report direct detection of charge tunneling between a quantum dot and a superconducting island through radio-frequency gate sensing. We are able to resolve spin-dependent quasiparticle tunneling as well as two-particle tunneling involving Cooper pairs. The quantum dot can act as an RF-only sensor to characterize the superconductor addition spectrum, enabling us to access subgap states without transport. Our results provide guidance for future dispersive parity measurements of Majorana modes, which can be realized by detecting the parity-dependent tunneling between dots and islands.
Spin and topological-qubit based quantum computers require an easily scalable and highly sensitive method for readout and manipulation, for which Dispersive Gate Sensing (DGS) is a promising candidate [1, 2]. DGS enables sensing of single electron tunneling events in a mesoscopic system by measuring a reflected Radio Frequency (RF) signal from a resonator capacitively coupled to one of the system’s gate electrodes. From the resonator’s perspective, the system is modeled as an effective ‘parametric capacitance’ [3]. Motivated by its viability for qubit readout and control, we measure transport characteristics of a semiconducting-superconducting hybrid charge island system using this method. Doing so, we observe a spin degeneracy modulation of interdot tunnel couplings resulting from hybridization of a dot orbital with a subgap state of the superconductor, measured entirely within Coulomb blockade. Without DC measurement, we also track a subgap state’s energy via voltage intervals between island charge states as a function of field. Subsequently, we attempt to correlate changes in the resonator’s internal dissipation with coherence of tunneling into the superconducting island. In particular, we collapse a superconducting island’s lowest energy state to the Fermi level by applying a magnetic field. Contrary to expectations, internal dissipation in the resonator did not change dramatically with field, in fact decreasing at charge degeneracy points at all field strengths. We corroborate measurements with existing theory, augmented by simple analytical and numerical models demonstrating that coherence factors of superconducting quasiparticle states and the degeneracy of dot orbitals modulate parametric capacitance. A master equation model of finite temperature interdot tunneling is solved, agreeing with experimental evidence that Sisyphus dissipation is negligible when a dot orbital is hybridized with multiple independent quasiparticle states. To be employed in quantum computers, a full understanding of all phenomena contributing to a DGS signal in these hybrid systems is critical. Hence, these results mark a step towards unambiguous readout of Majorana parity in topological qubits [4].
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.
Cooper pair splitters hold utility as a platform for investigating the entanglement of electrons in Cooper pairs, but probing splitters with voltage-biased Ohmic contacts prevents the retention of electrons from split pairs since they can escape to the drain reservoirs. We report the ability to controllably split and retain single Cooper pairs in a multi-quantum-dot device isolated from lead reservoirs, and separately demonstrate a technique for detecting the electrons emerging from a split pair. First, we identify a coherent Cooper pair splitting charge transition using dispersive gate sensing at GHz frequencies. Second, we utilize a double quantum dot as an electron parity sensor to detect parity changes resulting from electrons emerging from a superconducting island.
Quantum computers can solve some problems exponentially faster than classical computers. Unfortunately, the computational power of quantum computers is currently limited by the number of working qubits. It is difficult to scale up these systems, because qubits are easily affected by noise in their environment. This noise leads to decoherence: loss of the qubit’s encoded information. A possible solution to diminish decoherence is using Majorana box qubits, as these qubits are predicted to be insensitive to local noise. However, this promising type of qubit does not exist yet. With the research described in this thesis, we aim to contribute to the development of Majorana box qubits (MBQs). In these qubits, Majorana zero modes, the basic elements of MBQs, are contained within a superconducting island to suppress Majorana parity fluctuations caused by quasiparticle poisoning. To enable parity readout of the MBQ, these modes are coupled to quantum dots within a nanowire network. To help realize MBQs, we need a better understanding of quasiparticles in superconducting islands, parity-readout techniques, and ways to fabricate nanowire networks. These three aspects are the focus of the experiments presented in this thesis. To study superconducting islands and readout techniques, we used InAs semiconductor nanowires with an epitaxially grown Al shell. Majorana signatures have already been observed in such nanowires. We addressed quasiparticle dynamics in superconducting islands by measuring the gate-charge modulation of the switching current. We found a consistent 2e-periodic modulation at zero magnetic field, and an exponential decrease of parity lifetime with increasing magnetic field. We explored MBQ readout, using a quantum dot level as a proxy for a Majorana zero mode, and measured its charge hybridization with another dot using gate-based readout. We showed that we can rapidly discriminate between two settings with different tunnel couplings, demonstrating the potential of gate-based readout to measure MBQs. And, using gate-based readout, we could study charge-transfer processes occurring in hybrid structures of superconducting islands coupled to quantum dots. Finally, to find a good material platform for nanowire networks, we characterized two two-dimensional systems. We realized quantum point contacts in InSb, which we used to measure the g-factor anisotropy, and effective electron mass in this system. And, we studied the spin-orbit interaction in InAs/GaSb by extracting the difference in density between electrons with different spin orientations. This thesis finishes with a proposal for a series experiments to realize MBQs. These experiments make use of superconducting islands and the reflectometry setup we developed for gate-based readout.
Detecting quasiparticle tunneling events in superconducting circuits provides information about the population and dynamics of non-equilibrium quasiparticles. Such events can be detected by monitoring changes in the frequency of an offset-charge-sensitive superconducting qubit. This monitoring has so far been performed by Ramsey interferometry assisted by a readout resonator. Here, we demonstrate a quasiparticle detector based on a superconducting qubit directly coupled to a waveguide. We directly measure quasiparticle number parity on the qubit island by probing the coherent scattering of a microwave tone, offering simplicity of operation, fast detection speed, and a large signal-to-noise ratio. We observe tunneling rates between 0.8 and 7s−1, depending on the average occupation of the detector qubit, and achieve a temporal resolution below 10μs without a quantum-limited amplifier. Our simple and efficient detector lowers the barrier to perform studies of quasiparticle population and dynamics, facilitating progress in fundamental science, quantum information processing, and sensing.
We present a superconducting device showing a clear parity effect in the number of electrons, even when there is, on average, a single nonequilibrium quasiparticle present and the parity of the island switches due to quasiparticles tunneling in and out of the device at rates on the order of 100 Hz. We detect the switching by monitoring in real time the charge state of a superconducting island connected to normal leads by tunnel junctions. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kilohertz. We demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as a charge detector. With sufficiently low probing currents, our superconducting island is free of quasiparticles 97% of the time.