The investigation of novel electronic phases in low-dimensional quantum materials demands for the concurrent development of measurement techniques that combine surface sensitivity with high spatial resolution and high measurement accuracy. We propose a new quantum sensing imaging modality based on superconducting charge qubits to study dissipative charge carrier dynamics with nanometer spatial and high temporal resolution. Using analytical and numerical calculations we show that superconducting charge qubit microscopy (SCQM) has the potential to resolve temperature and resistivity changes in a sample as small as $\Delta T\leq0.1\;$mK and $\Delta\rho\leq1\cdot10^{4} \,\Omega\cdot$cm, respectively. Among other applications, SCQM will be especially suited to study the microscopic mechanisms underlying resistive phase transition, such as the superconductor-insulator-transition in twisted bilayer graphene, to investigate novel topological boundary modes found in higher order topological insulators and to optimize the transport properties of nano- and mesoscopic devices.
The investigation of novel electronic phases in low-dimensional quantum materials demands for the concurrent development of measurement techniques that combine surface sensitivity with high spatial resolution and high measurement accuracy. We propose a new quantum sensing imaging modality based on superconducting charge qubits to study dissipative charge carrier dynamics with nanometer spatial and high temporal resolution. Using analytical and numerical calculations we show that superconducting charge qubit microscopy (SCQM) has the potential to resolve temperature and resistivity changes in a sample as small as ΔT≤0.1mK and Δρ≤1⋅104Ω⋅cm, respectively. Among other applications, SCQM will be especially suited to study the microscopic mechanisms underlying resistive phase transition, such as the superconductor-insulator-transition in twisted bilayer graphene, to investigate novel topological boundary modes found in higher order topological insulators and to optimize the transport properties of nano- and mesoscopic devices.
The investigation of novel electronic phases in low-dimensional quantum materials demands for the concurrent development of new measurement techniques that combine surface sensitivity with high spatial resolution and high measurement accuracy.We propose a quantum sensing imaging modality based on superconducting charge qubits to study dissipative charge-carrier dynamics with nanometer spatial and better than nanosecond temporal resolution.Using analytical and numerical calculations, we show that superconducting-charge-qubit microscopy (SCQM) has the potential to resolve temperature and resistivity changes in a sample as small as T 0.1 mK and ρ 1 × 10 4 cm, respectively.Among other applications, SCQM will be especially suited to study the microscopic mechanisms underlying interaction driven quantum phase transitions, to investigate the boundary modes found in novel topological insulators and, more broadly, to visualize dissipative charge-carrier dynamics occurring in mesoscopic and nanoscale devices.
The work involves a study of physical phenomena that take place at very small length scales, below one micro-meter. At temperatures roughly below one degree Kelvin, quantum-mechanical effects may rule in electronic transport. Macroscopic quantum-coherent effects that occur in metallic superconducting microstructures, are particularly intriguing. Large-scale quantum information processing is widely believed to be attainable utilizing such physical systems. This work concentrates on answering the question of how the described quantum-mechanical systems may be used as sensitive measuring devices. Considerable attention is paid to energy-storing metallic microstructures whose electrical properties resemble those of the familiar inductor or capacitor. This research involves primarily experimental investigations conducted around temperatures of 0.1 Kelvin. Methods both at low and at radio frequencies have been used. The experimental findings have been modelled theoretically, and theoretical concepts for new physical phenomena have been introduced. An inductively measured radio-frequency Cooper-pair transistor, the L-SET, has been developed and experimentally verified in this work. Being highly sensitive, fast, and non-invasive, the L-SET appears to be the most promising method for measuring electric charge. Sensitivity in charge measurements of 20 millionths of the electron charge (micro-e) within one second, and an input bandwidth of 100 MHz, have been demonstrated. It has been shown theoretically that the ultimate measurement accuracy is about 0.1 micro-e within a second. A new phase detector based on the Cooper-pair transistor has been proposed. This system has also been shown to be potentially usable as a quantum bit. A new type of radio-frequency single-electron transistor built using a multi-walled carbon nanotube has been fabricated and operated. Technologies have been developed in order to make the physical nano- or microstructures. A method has been presented to fabricate non-superconducting tunnel junctions. Consequences of the inverse superconducting proximity effect on the studied superconducting structures were considered. Measurement procedures were investigated for a new low-noise nanoamplifier, the Bloch-oscillating transistor. Single superconducting tunnel junctions were tested as detectors of energy states of the environment, or of noise.
Abstract Interest is growing in the development of quantum sensing based on the principles of quantum mechanics, such as discrete energy levels, quantum superposition, and quantum entanglement. Superconducting flux qubits are quantum two-level systems whose energy is sensitive to a magnetic field. Therefore, they can be used as high-sensitivity magnetic field sensors that detect the magnetization of a spin ensemble. Since the magnetization depends on temperature and the magnetic field, the temperature can be determined by measuring the magnetization using the flux qubit. In this study, we demonstrated highly sensitive temperature sensing with high spatial resolution as an application of a magnetic field sensor using the quantum coherence of a superconducting flux qubit. By using a superconducting flux qubit to detect the temperature dependence of the polarization ratio of electron spins in nano-diamond particles, we succeeded in measuring the temperature with a sensitivity of 1.3 µKµ Hz − 1 at T = 9.1 mK in the submicrometer range.
The topic of superconducting nanowires has recently been an interesting field of research which has included the study of the superconductor to insulator transition (SIT), the observation of macroscopic quantum behavior such as quantum phase slips (QPS), and the potential use of nanowires as qubits. Superconducting coplanar microwave waveguide resonators have also become a popular way of studying superconducting junctions and qubits, as they provide an extremely low noise environment. For example, superconducting two-dimensional Fabry-Perot resonators have been used by other groups to make non-demolition measurements of a qubit. The motivation of this thesis will be the merging of the fields of superconducting nanowires and the technique of using superconducting microwave resonators to study junctions by incorporating a nanowire into the resonator itself at a current anti-node. By doing this, the nonlinear effects of the nanowire can be studied which may find application in single photon detectors, mixers, and the readout of qubits. We also employ the technique of molecular templating to fabricate some of the thinnest superconducting nanowires ever studied (down to ~ 5 nm in diameter in some cases). In this thesis, we extend the understanding of the nonlinear properties of a nanowire- resonator system and investigate a new type of nonlinearity that involves a pulsing regime between the superconducting and normal phases of the nanowire. We develop a model, which describes the results quantitatively and by modeling the system, we are able to extract information regarding the relaxation time of the nanowire back into the superconducting state. We also study double nanowire-resonator systems where two closely spaced parallel nanowires interrupt the resonator center conductor and form a loop where vortex tunneling processes can occur. Using a double nanowire-resonator we are able to observe the Little-Parks effect at low temperatures (where the resistance of the wires is immeasurably low) and are able to confirm the multivalued nature of the current phase relationship (CPR) in nanowires. Additionally, we observe an anomalous transmission regime where the periodic pulsing is replaced by stochastic amplitude fluctuations. In addition to microwave measurements on nanowires, we also study the normal state in resistively shunted nanowires with dc measurements where the inclusion of a shunt resistor is observed to change the nature of the normal state from the Joule heated state to a state that preserves phase coherence. Finally, the statistics on switching current events in graphene proximity junctions are analyzed and compared to the well known results for Josephson junctions. Only thermal activation (and no macroscopic quantum tunneling) is observed in graphene proximity superconducting junctions down to temperatures of ~ 300 mK.
We have developed a low temperature (LT), ultra-high-vacuum (UHV) system that combines two complementary techniques, scanning probe microscopy (SPM) and electrical transport measurements, within a single platform. By providing simultaneous access to the atomic-scale surface landscape and the macroscopic device response of the same sample, the setup enables direct correlations between local structural/spectroscopic signatures and global electronic transport behavior in two-dimensional (2D) devices. The system allows experiments where atomic-scale modifications or controlled manipulations are performed while continuously monitoring their impact on device-scale performance. The setup consists of two interconnected UHV chambers: a dedicated preparation chamber and a separate measurement chamber that houses a liquid-helium cryostat and the SPM/transport stage. Base pressure is 1x10^-11 Torr. A key feature is direct optical access to the sample, enabling rapid and reliable tip positioning with an accuracy of 5 microns x 5 microns within 10 minutes. The SPM, operated using custom-built electronics, can track the exact same sample region across a temperature range from 2.9 K to 400 K, with mechanical stability below 1 pm. System performance is demonstrated on graphene devices, and bulk Pb is used to determine energy resolution. Using superconducting tips, scanning tunneling spectroscopy measures the superconducting gap with an energy resolution of 30 microV. Transport measurements track the temperature dependence of both resistivity and critical current across the superconducting transition of an in-situ prepared Pb nanowire.
A single-spin qubit placed near the surface of a conductor acquires an additional contribution to its 1/T 1 relaxation rate due to magnetic noise created by electric current fluctuations in the material.We analyze this technique as a wireless probe of superconductivity in atomically thin two-dimensional materials.At temperatures T T c , the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations.We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature.We show that scaling of the noise with sample-probe distance provides a window into the nonlocal quasistatic conductivity of superconductors, both clean and disordered.At low temperatures the quasiparticle fluctuations get suppressed, yet the noise can be substantial due to resonant contributions from collective longitudinal modes, such as plasmons in monolayers and Josephson plasmons in bilayers.Potential experimental implications are discussed.