Abstract Applying a radian phase shift other than 2π is a key issue for superconducting circuits, such as flux qubits. The magnetic flux is useful when generating a phase shift. However, a quantized magnetic flux accompanying a trapped vortex in a superconductor does not have a phase shifter function. The magnetic flux generated by an external field generates noise. In this study, we propose a phase bias system that does not require an external field during operation. We confirm the phase shift of a direct current superconducting interference device (SQUID) placed on an ultrathin superconducting Nb bilayer with a through-hole by cooling it to a temperature below the superconducting transition temperature with an external field. Although the cause of the phase shift in our system is unclear, we believe that it may be caused by a fractional quantum in the bilayer. When the SQUID is replaced by a qubit, the phase shift can be applied to a phase bias.
Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.
Superconducting quantum interference devices (SQUIDs) are among the most sensitive detectors for out-of-plane magnetic field components. However, due to their periodic response with short modulation period
M=1Φ0, determined by the magnetic flux quantum
Φ0≈2.068×10−15Wb, it is difficult to infer the value of the magnetic flux unambiguously, especially in case the magnetic flux enclosed in the SQUID loop changes by many flux quanta. Here, we demonstrate that by introducing a second degree of freedom in the form of a second SQUID, we substantially enhance the modulation period
M of our device without sacrificing sensitivity. As a proof of concept, we implement our idea by embedding two asymmetric direct current SQUIDs into a common tank circuit. By measuring the reflection coefficient of the device, we extract the two lowest energy eigenfrequencies as a function of the external magnetic flux created by a superconducting field coil, from which we experimentally deduce a modulation period
M≥15Φ0, as well as the magnetic offset-field
B0=22nT present in our experiment.
Nano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5 K and magnetic flux densities up to over 20 mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements.
A rf--superconducting quantum interference device (SQUID) flux qubit that is robust against fabrication variations in Josephson-junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest-lying states, both in the coherent and incoherent regimes, are presented. These experimental results are shown to be in agreement with predictions of a quantum-mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rf SQUIDs reported in the literature thus far, with a
1/f flux noise spectral density at 1 Hz of ${1.3}_{\ensuremath{-}0.5}^{+0.7}\text{ }\ensuremath{\mu}{\ensuremath{\Phi}}_{0}/\sqrt{\text{Hz}}$. An explicit formula for converting the observed flux noise spectral density into a free-induction-decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented.
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.