Microwave-coupled superconducting devices for sensing and quantum information processing: Dissertation

Visa Vesterinen

N. Sekiya, M. Mukaida, Atsushi SAITO +2 more

Matti Partanen, Kirill G. Fedorov, Stefan Pogorzalek +5 more

Kirill Petrovnin, Jiaming Wang, Gheorghe Sorin Paraoanu

Microwave photon detection is a key technology for low-temperature superconducting electronics and quantum information processing. A promising possibility is to use switching processes in parametric superconducting devices at criticality, which can be triggered by small perturbations. Here we demonstrate the unique sensing properties of the superconducting Kerr parametric resonator when operated in the proximity of the phase transition boundary. We utilize a semiclassical approximation to provide numerical and analytical results for the Heisenberg-Langevin and Fokker-Planck equations that describe the switching mechanism. We show that the probability of switching events is enhanced by probe input states with energies down to single quanta levels.

Matthias Mergenthaler

Circuit quantum electrodynamics has received an increasing amount of attention over the last decade in light of its potential use in quantum information processing. The underlying principle of this architecture is the coherent coupling between microwave photons stored in superconducting resonators and superconducting qubits made from non-linear, non-dissipative electrical circuits. This architecture can be extended to other solid-state systems with transition frequencies in the microwave regime, such as nuclear and electron spins or Andreev bound states. These hybrid devices would allow the coherent exchange of information between such systems and superconducting qubits mediated by a microwave resonator. This could lead to advances in quantum information processing and new insights in the physics governing these interacting systems. In this dissertation experimental work with different types of hybrid quantum circuits is presented. A first experiment investigates the coherent coupling of microwave photons with a spin ensemble in a correlated and uncorrelated state. The coupling strength and magnetic resonance is studied as a function of temperature and magnetic field direction. A pronounced temperature dependent anisotropy of the magnetic resonance is observed, which can be attributed to the onset of antiferromagnetic correlations in the spin ensemble when cooled below T=4 K. A simple one-dimensional model gives a good description of this anisotropy. A next experiment studies the transport characteristics of carbon nanotubes electrically accessed with normal and superconducting contacts. These devices were characterised using both DC and RF reflectometry techniques. The carbon nanotube devices with superconducting contacts exhibit transport characteristics of Josephson junctions with critical currents of up to Ic~18 nA, as well as multiple Andreev reflections. These semiconductor-superconductor hybrid Josephson junctions are then used to realise a carbon-nanotube-based superconducting qubit with voltage tunable transition frequency using a local electrostatic gate. Strong coupling (g~100 MHz) to a coplanar waveguide resonator is demonstrated via a resonator frequency shift dependent on applied gate voltage. Qubit parameters are extracted from spectroscopy and correspond well to the DC measurements of the carbon nanotube Josephson junctions. Qubit relaxation and coherence times in the range 10-100 ns are observed. The hybrid devices investigated in this work present potential building blocks for more extensive hybrid architectures for quantum information processing.

Lautaro Lucas Gabriel Labarca Guajardo

A leading platform for quantum information processing are superconducting circuits, which can now comprise tenths of qubits. One of the key challenges in the design of these systems is balancing the tradeoff between access to the qubits for readout and control versus environment induced decoherence. In other words, the same lines used to access the information on the system, unavoidable induce losses and destroy the quantum information store on it. To mitigate this issue, nonreciprocal devices are included between the superconducting qubits and the control electronics. Commercially available microwave nonreciprocal devices are bulky, and impractical for scaling these systems to the scales necessary for the implementation of useful quantum computing algorithms. For this reason, efforts are being directed to the development of on-chip microwave nonreciprocal devices. However, due to the large scale of these systems, their design and modelling is an ongoing challenge. In this thesis, we show how effective dynamical models of superconducting qubits can be obtained when these are coupled by nonreciprocal linear responses, extending the previous work to include on-chip nonreciprocal devices, and more formally all kinds of effectively linear lossless microwave devices. Notably, our formalism quantitatively captures the tradeoff between control and dissipation induced by coupling to the external electronics. In addition, we provide a modern presentation of the theoretical background underpinning circuit synthesis and modelling. Additionally, while scaling to thousands of qubits is necessary for the implementation of useful quantum computing algorithms, this is not the only route where quantum information processing systems can find applications in other fields. In particular, quantum metrology offers a venue where only a few well-tuned qubits and oscillators are needed for a quantum sensing advantage. Examples where these advantages have been realize include dark-matter searches, satellite-free navigational systems, imaging of biological tissues, among others. Despite their promise, quantum sensors must balance the tradeoff between sensitivity to external perturbations and robustness against noise sources. A pathway to limit the detrimental effects of noise and maximize the achievable sensitivity is given by reservoir-engineering and quantum error-correction. Thus, the development of quantum sensors robust against noise is a promising venue with broad applications. In our work, we show how recent protocols developed for the stabilization of bosonic codes can be adapted for quantum sensing. In particular, we show how stabilizing protocols for the Gottesman-Kitaev-Preskill code can be adapted for the quantum sensing of displacements. Quantum sensing of displacement may find applications in for example electric field sensing, force sensing and gravitational waves sensing. Remarkably, the protocol we study nearly saturates the sensitivity allowed by quantum mechanics.