In this dissertation, the use of on-chip superconducting (SC) resonators as a probe of spin-photon interactions as well as other superconducting phenomena will be presented. In particular, coherent Rabi oscillations are demonstrated for a spin s=1/2 ensemble in an ESR (Electron Spin Resonance) cavity as well as coherent multiphoton excitations. Applying techniques from the classical ESR measurements to an on-chip resonator is effectively shrinking the cavity volume and increasing vacuum field density. This in turn can greatly increase the sensitivity creating the possibility of probing the quantum to classical transition for spin-photon interactions. Additionally, an on-chip cavity provides the possibility of on-chip ESR measurements to be performed in a dilution refrigerator. Furthermore, progress has been made toward on-chip ESR type measurements. A highly sensitive heterodyne measurement setup has been developed, providing the capability of detecting signals as small as -125~dBm. Under the right conditions this setup may be able to detect single photons. To perform an on-chip ESR measurement, precise placement of the spin system at an anti-node of the magnetic field is required. For this, a technique for spin ensemble placement with micron scale precision is developed. Finally, performing ESR measurements requires the application of a (sometimes large) static field. Even a very small static field and can easily create vortices in a superconducting thin film when applied normal to the film plane. A precise alignment procedure is developed allowing for the operation of on-chip SC resonators to operate with an in-plane field up to 1~Tesla. This led to the first successful field-dependent observation of the nonlinear Meissner effect.
In this dissertation, the use of on-chip superconducting (SC) resonators as a probe of spin-photon interactions as well as other superconducting phenomena will be presented. In particular, coherent Rabi oscillations are demonstrated for a spin s=1/2 ensemble in an ESR (Electron Spin Resonance) cavity as well as coherent multiphoton excitations. Applying techniques from the classical ESR measurements to an on-chip resonator is effectively shrinking the cavity volume and increasing vacuum field density. This in turn can greatly increase the sensitivity creating the possibility of probing the quantum to classical transition for spin-photon interactions. Additionally, an on-chip cavity provides the possibility of on-chip ESR measurements to be performed in a dilution refrigerator. Furthermore, progress has been made toward on-chip ESR type measurements. A highly sensitive heterodyne measurement setup has been developed, providing the capability of detecting signals as small as -125dBm. Under the right conditions this setup may be able to detect single photons. To perform an on-chip ESR measurement, precise placement of the spin system at an anti-node of the magnetic field is required. For this, a technique for spin ensemble placement with micron scale precision is developed. Finally, performing ESR measurements requires the application of a (sometimes large) static field. Even a very small static field and can easily create vortices in a superconducting thin film when applied normal to the film plane. A precise alignment procedure is developed allowing for the operation of on-chip SC resonators to operate with an in-plane field up to 1Tesla. This led to the first successful field-dependent observation of the nonlinear Meissner effect.
Electron spin resonance (ESR) is a powerful spectroscopy method which allows to identify and characterize paramagnetic species. It usually relies on detecting microwave radiation absorbed or emitted by the spins into a microwave resonator tuned to their Larmor precession frequency. Because of the weak spin-microwave coupling, conventional ESR spectroscopy has a low sensitivity. Recent experiments have demonstrated that superconducting quantum circuits have the potential to enhance the spin detection sensitivity to single spin detection; by namely using high quality factor small mode volume resonator. However, these demonstrations have been realized in very restrictive conditions, using well-known spin systems. To probe samples coming from chemistry or biology, quantum-circuit enabled spectrometers have to be adapted to be robust to losses. They have typically two origins: the dielectric losses introduced by the samples, mostly aqueous, and losses coming from the strong magnetic field required to tune the electronic spins to the resonator frequency. This PhD work centered on implementing a resonator that would achieve the same ESR sensitivity while being robust to dielectric losses and strong magnetic fields. Using NbTiN films deposited on sapphire, microwave resonators were shown to have quality factors above Q=105 in magnetic field, even when samples with dielectric losses above tan d > 0.05 were introduced. A study of the resonator intrinsic losses showed marked drops at multiple specific magnetic fields: a signature of coupling to spurious systems. Over the course of the PhD, mitigation nanofabrication techniques for these contaminants were developed, such as hard mask etching and annealing, to remove all but one of this contaminants. Finally, test of the spectrometer on a benchmark species (BDPA, α,γ-Bisdiphenylene-β-phenylallyl, a known ESR marker) were realized.
This dissertation describes a set of experiments with the goal of creating a super-conductor-semiconductor hybrid circuit quantum electrodynamics architecture with single electron spins. Single spins in silicon quantum dots have emerged as attractive qubits for quantum computation. However, how to scale up spin qubit systems remains an open question. The hybrid architecture considered here could provide a route to realizing large networks of quantum dot–based spin qubit registers. The first experiment in this thesis is aimed at achieving strong coupling between a single electron spin and a single microwave photon. The electron is trapped in a gate-defined double quantum dot in a Si/SiGe heterostructure and the photon is stored in an on-chip superconducting high-impedance NbTiN cavity. The photon is coupled directly to the electron charge, and indirectly to the electron spin, mediated through a synthetic spin-orbit field. We observe a vacuum Rabi splitting that depends on the spin-charge hybridization. The ratio of spin-photon coupling strength to decoherence rates of the spin and cavity combined is larger than unity, confirming the strong coupling regime has been reached. In addition, we find an optimal degree of spin-charge hybridization for which this ratio is maximized. The demonstration of strong spin-photon coupling not only opens a new range of physics experiments, but fulfills also a crucial requirement for coupling spin qubits at a distance via a cavity. The second experiment is focused on spin readout with the on-chip cavity. Instead of the direct dispersive readout of a single spin, we use the cavity to detect whether the electron is allowed to tunnel between the two dots or not. We benchmark the charge sensitivity and bandwidth of the detector and find that rapid detection of the electron charge with high SNR is possible. In the two-electron regime, electron tunneling is contingent on the total spin state (Pauli spin blockade). This spin-to-charge conversion scheme enables single-shot detection of singlet states with high-fidelity. The demonstration of single-shot spin readout with a cavity is an essential step towards readout in dense spin qubit arrays, such as the crossbar network, where it is not possible to integrate electrometers and accompanying reservoirs adjacent to the qubit dots. In the third experiment, we develop on-chip microwave filters to suppress microwave photon leakage from the cavity through the gate electrodes that are necessary to form quantum dots. We introduce a new cavity design that is compatible with long-distance connectivity between spins, but is also more susceptible to microwave leakage. We test and compare two low-pass filter variations in terms of performance, footprint and integrability. They use the same nanowire inductor, but different implementations of the capacitor: one with a planar interdigitated capacitor and one novel design with an overlapping thin-film capacitor. We find that both approaches are effective against microwave leakage. However, the large footprint of the interdigitated capacitor makes this solution inconvenient as the number of gate lines increases. The thin-film capacitor, with its much smaller footprint, is better suited for our devices. The final part of this dissertation contains concluding remarks and possible future directions are proposed.
Precise detection of spin resonance is of paramount importance to achieve coherent spin control in quantum computing. We present a setup for spin resonance measurements, which uses a dc-SQUID flux detector coupled to an antenna from a coplanar waveguide. The SQUID and the waveguide are fabricated from a 20 nm Nb thin film, allowing high magnetic field operation with the field applied parallel to the chip. We observe a resonance signal between the first and third excited states of Gd spins S = 7/2 in a CaWO4 crystal, relevant for state control in multi-level systems.
Superconducting resonators are finding increasing applications in designing advanced quantum circuits for ongoing sensing, metrology, and computing technological revolution. A detailed knowledge of microscopic electromagnetic properties of superconducting resonators is directly relevant for their further improvements on circuitry design and device performance. Here, we introduce scanning-probe quantum microscopy to report nanoscale sensing of microwave and static magnetic field environment of an on-chip niobium (Nb) superconducting resonator. Taking advantage of Rabi oscillation measurements, we show that microwave magnetic fields generated by the superconducting resonator mode can be utilized to achieve coherent control of a quantum spin sensor. We further visualize static electromagnetic field response of the Nb resonator, showing magnetic field-induced formation, evolution, and depinning of superconducting vortices. Our results provide insights into future design, testing and evaluation of solid-state superconducting resonators, highlighting the potential of quantum sensors as a local probe to investigate electromagnetic properties of superconducting quantum circuits.
Superconducting resonators are finding increasing applications in designing advanced quantum circuits for an ongoing sensing, metrology, and computing technological revolution. A detailed knowledge of the microscopic electromagnetic properties of superconducting resonators is directly relevant for their further improvements on circuitry design and device performance. Here, we introduce scanning-probe quantum microscopy to report nanoscale sensing of the microwave and static magnetic field environment of an on-chip niobium (Nb) superconducting resonator. Taking advantage of Rabi oscillation measurements, we show that microwave magnetic fields generated by the superconducting resonator mode can be utilized to achieve coherent control of a quantum spin sensor. We further visualize the static electromagnetic field response of the Nb resonator, showing magnetic field-induced formation, evolution, and depinning of superconducting vortices. Our results provide insights into future design, testing, and evaluation of solid-state superconducting resonators, highlighting the potential of quantum sensors as a local probe to investigate electromagnetic properties of superconducting quantum circuits.
Microwave band on-chip microcoils are developed for the application to single electron spin resonance measurement with a single quantum dot. Basic properties such as characteristic impedance and electromagnetic field distribution are examined for various coil designs by means of experiment and simulation. The combined setup operates relevantly in the experiment at dilution temperature. The frequency responses of the return loss and Coulomb blockade current are examined. Capacitive coupling between a coil and a quantum dot causes photon assisted tunneling, whose signal can greatly overlap the electron spin resonance signal. To suppress the photon assisted tunneling effect, a technique for compensating for the microwave electric field is developed. Good performance of this technique is confirmed from measurement of Coulomb blockade oscillations.