A superconducting condensate is characterized by the emergence of macroscopic and collective order, established between its constituent electrons. The degree of correlation at a given spatial position is a complex scalar: it is characterized both by an amplitude (proportional to the minimal energy to generate a fundamental excitation) and a complex phase. Through its gradient, the latter enables the flow of the condensate, demonstrating the quantum fingerprint of superconductivity. Superconductors exhibit strong coupling to electromagnetic fields, so that phase-dependent dissipationless transport through “weak link” circuital elements is easily manipulated by applying voltage or magnetic flux bias to superconducting terminals and loops, respectively. For these reasons, superconducting electronics is nowadays a core technology to enable robust access and manipulation of the fundamental degrees of freedom in quantum devices, from ultrasensitive electromagnetic sensors to superconducting qubits. In this work, we explore different designs of micro-magnetometers based on superconducting interferometers. Differently from conventional designs based on Superconductor, Insulator, Superconductor (SIS) tunnel junctions, here the core elements are nanoscale diffusive metal wires acting as superconducting weak links. These consist in circuital dishomogeneities that can be fabricated over scales much smaller than typical superconductor coherence lengths, typically yielding unique response properties. On the other hand, their intrinsic transparency is usually associated with strong supercurrent concentration, which can severely limit their practicality due to superconducting depairing and thermal-driven hysteresis upon switching to the dissipative regime. In this thesis we demonstrate how a judicious use of nanofabricated designs makes it possible to counter these drawbacks and achieve complete phase polarization in interferometers based on diffusive weak links. The resulting micro-magnetometers are characterized by extremely high magnetic responsivity.
A superconducting condensate is characterized by the emergence of macroscopic and collective order, established between its constituent electrons. The degree of correlation at a given spatial position is a complex scalar: it is characterized both by an amplitude (proportional to the minimal energy to generate a fundamental excitation) and a complex phase. Through its gradient, the latter enables the flow of the condensate, demonstrating the quantum fingerprint of superconductivity. Superconductors exhibit strong coupling to electromagnetic fields, so that phase-dependent dissipationless transport through “weak link” circuital elements is easily manipulated by applying voltage or magnetic flux bias to superconducting terminals and loops, respectively. For these reasons, superconducting electronics is nowadays a core technology to enable robust access and manipulation of the fundamental degrees of freedom in quantum devices, from ultrasensitive electromagnetic sensors to superconducting qubits. In this work, we explore different designs of micro-magnetometers based on superconducting interferometers. Differently from conventional designs based on Superconductor, Insulator, Superconductor (SIS) tunnel junctions, here the core elements are nanoscale diffusive metal wires acting as superconducting weak links. These consist in circuital dishomogeneities that can be fabricated over scales much smaller than typical superconductor coherence lengths, typically yielding unique response properties. On the other hand, their intrinsic transparency is usually associated with strong supercurrent concentration, which can severely limit their practicality due to superconducting depairing and thermal-driven hysteresis upon switching to the dissipative regime. In this thesis we demonstrate how a judicious use of nanofabricated designs makes it possible to counter these drawbacks and achieve complete phase polarization in interferometers based on diffusive weak links. The resulting micro-magnetometers are characterized by extremely high magnetic responsivity.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. \n \nThe first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. \n \nWe then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. \n \nIn the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers.
The first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable.
We then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction.
In the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
Superconducting circuits combine zero electrical resistance with macroscopic quantum coherence, making them a versatile platform for both fundamental studies and technological applications. Modern thin-film deposition and lithography allow precise control over geometry, material composition, and dimensional scales, enabling the fabrication of complex superconducting structures with tailored properties. These capabilities give access to phenomena not achievable with conventional electronic components, establishing superconducting circuits as a unique tool for exploring quantum physics and developing devices with unique functionalities. In superconducting rings, macroscopic quantum coherence manifests as discrete circulating currents. Fluxoid quantization restricts these currents to discrete winding states. Transitions between winding states occur via phase-slip events. A phase slip event requires overcoming an energy barrier, which can be crossed either through thermal activation, resulting in thermally activated phase slips (TAPS), or via quantum tunneling, resulting in quantum phase slips (QPS) that have no classical analogue. Both processes depend sensitively on temperature, loop geometry, material properties, and the intrinsic properties of the superconductor. Embedding weak links or nanowires in a superconducting ring provides localized regions where phase slips preferentially occur, enabling controlled TAPS and QPS dynamics. When the QPS rate becomes sufficiently high, coherent tunneling between neighboring winding states generates an energy splitting, forming the basis of qubit states. The discrete winding states can thus be exploited for device applications: serving as robust memory elements when the energy barrier is high and phase slips are rare and well controlled, and as qubits when coherent quantum phase slips allow the system to occupy superpositions of distinct winding states, marking the transition from a classical bit to a quantum bit. Phase-slip dynamics in superconducting rings have traditionally been studied using switching-current measurements, in which the bias current is ramped until the ring or nanowire becomes resistive. While this approach provides statistical information about phase-slip rates, it is inherently dissipative and cannot detect events that do not drive the system in the normal state, leaving many phase slips hidden and preventing their unambiguous classification by origin or rate.This thesis develops a non-invasive, radio-frequency resonator-based platform for studying phase-slip dynamics in superconducting rings. Building on magnetooptical imaging of flux penetration in superconducting resonators, we first established methods to apply a local magnetic field to a loop without perturbing the resonator and couple it to the resonator, linking changes in circulating currents to shifts in the resonance frequency. This enabled continuous, timeresolved detection of individual phase-slip events on microsecond-to-nanosecond timescales. Using this platform, we systematically investigated how loop geometry, weak-link properties, kinetic inductance, and temperature govern phase-slip rates and dynamics. Deterministic control of discrete winding states in high-kinetic-inductance aluminum loops, our chosen material platform, was demonstrated, realizing a superconducting memory based on fluxoid quantization. By progressively reducing loop dimensions and introducing weak links, we observed quantum phase-slip events at millikelvin temperatures, providing the first experimental signatures of coherent tunneling between winding states, a first step toward realizing phase-slip-based qubits. Collectively, these results establish a robust experimental pathway from conventional DC measurements to non-invasive RF probing, provide a framework for controlled studies of superconducting phase-slip dynamics, and open the door to exploring and utilizing coherent phase slips in circuit-QED-compatible architectures with device geometries that are simple and easy to fabricate.
The research work discussed in this thesis deals with the study of superconducting Josephson qubits. Superconducting qubits are solid-state artificial atoms which are based on lithographically defined Josephson tunnel junctions properties. When sufficiently cooled, these superconducting devices exhibit quantized states of charge, flux or junction phase depending on their design parameters. This allows to observe coherent evolutions of their states. The results presented can be divided into two parts. In a first part we investigate operations of superconducting qubits based on the quantum coherence in superconducting quantum interference devices (SQUID). We explain experimental data which has been observed in a SQUID subjected to fast, large-amplitude modifications of its effective potential shape. The motivations for this work come from the fact that in the past few years there have been attempts to interpret the supposed quantum behavior of physical systems, such as Josephson devices, within a classical framework. Moreover, we analyze the possibility of generating GHZ states, namely maximally entangled states, in a quantum system made out of three Josephson qubits. In particular, we investigate the possible limitations of the GHZ state generation due to coupling to bosonic baths. In the second part of the thesis we address a particular cause of decoherence of flux qubits which has been disregarded until now: thermal gradients, which can arise due to accidental non equilibrium quasiparticle distributions. The reason for these detrimental effects is that heat currents flowing through Josephson tunnel junctions in response to a temperature gradient are periodic functions of the phase difference between the electrodes. The phase dependence of the heat current comes from Andreev reflection, namely an interplay between the quasiparticles which carry heat and the superconducting condensate which is sensitive to the superconducting phase difference. Generally speaking, the flux qubit states are characterized by different values of the phase difference through their Josephson junctions. Consequently, the phase-dependent thermal current through a device subject a temperature gradient is related to the phase-dependent qubit states. We study how the thermal currents change according to the state of the qubits hence yielding a measurement of the qubit state. This in turn leads to an impact of temperature gradient on the dynamics of the system. We show that flux qubits in the Delft qubit design can have limitations of the decoherence time to the order of microseconds as a result of this newly discovered source of decoherence. In contrast, the fluxonium qubit is found to be well protected due to its superinductance.
Over the past two decades the performance of superconducting quantum bits (qubits) has been improved tremendously: the coherence time of individual qubits has been increased by five orders of magnitude, from a few nanoseconds to >100 μs. The much-improved coherence, lithographic scalability, and compatibility with microwave control place superconducting qubits in the leading position in the race amongst several quantum systems being considered for quantum information processing. Despite this very impressive progress, experimental realization of quantum error corrections and demonstration of logical qubits remains a considerable challenge.One of possible solutions to this problem is the development of so-called protected qubits whose errors would be suppressed by special symmetries of the underlying Hamiltonian. The realization of such qubits requires elements not found in the conventional superconducting circuit toolbox, such as Josephson elements with cos(ϕ/2) and cos(2ϕ) dependences of the Josephson energy E_J on the phase difference ϕ, circuits with a very large kinetic inductance, and junctions with unusually low E_J.This thesis focuses on design, fabrication and characterization of circuits based on low-E_J Josephson junctions (JJs) and superinductors (SIs). By analyzing limitations on the junction performance imposed by the thermally activated phase slips, we observed a dramatic reduction of the critical currents in the regime E_J≤T which was accompanied by an increase of the zero-bias resistance R_0. The first part of this work provides practical considerations for the use of such junctions in quantum circuits. With the aim of improving elements with a very high kinetic inductance, in the second part we developed superinductors based on the granular Aluminum (grAl) films, in which Josephson junctions are realized between nanoscale grains. The circuits based on such SIs demonstrate low microwave losses at ultra-low temperatures. Superinductors are an essential element of a novel qubit that we have developed – the so-called bifluxon. The qubit consists of a Cooper-pair box (CPB) with low-E_J Josephson junctions shunted by a superinductor, thus forming a superconducting loop. When the loop is threaded by the magnetic flux Φ=Φ_0⁄2 where Φ_0 is the flux quantum, the qubit offers exponential suppression of energy decay from charge and flux noises, and dephasing from flux noise. In the last part of this work, we observed an increase of the energy relaxation time by two orders of magnitude, up to 100μs, by turning on protection in the bifluxon qubit.
Josephson junctions are the basis of superconducting qubits, amplifiers, and magnetometers. Historically, tunnel-style junctions have been most common. However, for some applications--those requiring a small, highly transparent, all-superconducting junction--nanobridge weak link junctions may be preferable. This thesis presents extensive characterization of aluminum nanobridge Josephson junctions. The junction behavior is simulated by numerically solving the Usadel equations. These simulations are then tested and confirmed via low- and microwave-frequency transport measurements. The data confirm that nanobridge junctions approach the ideal weak-link Josephson limit.Such a near-ideal junction can be used in a superconducting quantum interference device (SQUID) to form an ultra sensitive magnetometer. This thesis presents the first nanobridge-based dispersive SQUID magnetometer. The devices show near-zero dissipation, with bandwidth and sensitivity on par with the best reported results for any SQUID-based magnetometer. These magnetometers have several flexible modes of operation, allowing for optimization of sensitivity, bandwidth, or backaction. They also provide insight into the internal dynamics of dispersive measurement with nonlinear cavities, as the magnetometer backaction depends on the bias point and can readily be measured.Nanobridge junctions also provide a useful tool for diagnosing sources of decoherence in superconducting qubits. By replacing the usual tunnel junction with a nanobridge, the contribution of the junction to decoherence processes may be probed. In particular, the interaction of nanobridge junctions with quasiparticles provides information both about the junctions themselves and about quasiparticle generation and relaxation mechanisms. This thesis reports measurements of nonequilibrium quasiparticles trapping in phase-biased nanobridge junctions. By probing the quasiparticle-induced changes in resonant frequency of a high-Q nanoSQUID oscillator, one may measure the mean distribution of trapped quasiparticles and study its temperature dependence. These measurements also provide spectroscopy of the junctions' internal Andreev states and the dynamics of quasiparticle excitation and retrapping. The work presented here demonstrates the utility of nanobridge junctions as a tool for quantifying the population and distribution of quasiparticles in a superconducting circuit.