Observation of Josephson Harmonics in Tunnel Junctions

Physical Review B, 1999

Journal of Physics: Condensed Matter, 2006

Nature, 1999

Low-capacitance Josephson junctions, where Cooper pairs tunnel coherently while Coulomb blockade effects allow the control of the total charge, provide physical realizations of quantum bits (qubits), with logical states differing by one Cooper-pair charge on an island. The single-and two-bit operations required for quantum computation can be performed by applying a sequence of gate voltages. A basic design, described earlier [1], is sufficient to demonstrate the principles, but requires a high precision time control, and residual two-bit interactions introduce errors. Here we suggest a new nano-electronic design, close to ideal, where the Josephson junctions are replaced by controllable SQUIDs. This relaxes the requirements on the time control and system parameters substantially, and the two-bit coupling can be switched exactly between zero and a non-zero value for arbitrary pairs. The phase coherence time is sufficiently long to allow a series of operations.

URSI Radio Science Letters

Josephson-junction circuits allow generation, mixing, detection, and parametric amplification in classical and quantum signal processing. General energy relations for Josephson junctions govern frequency conversion and AC-DC conversion. In this article, we review physical principles, properties, and applications of superconducting electronics based on Josephson parametric amplifiers, and give an overview of the development over the last 50 years.

At present, the performance of superconducting qubits is limited by decoherence. Strong decoherence of phase qubits is associated with spurious microwave resonators residing within the Josephson junction tunnel barrier . In this work, we investigate three different fabrication techniques for producing tunnel junctions that vary the properties of the superconductor-insulator interface. Through experimental measurements, we characterize the junction and corresponding qubit quality. We find that there is a strong correlation between the morphology of oxidized base electrodes and the lowering of subgap currents in the junction I-V characteristics, while there is no noticeable improvement in the performance of fabricated phase qubits. Thus, "traditional" indicators of junction performance may not be enough to determine qubit performance. However, truly crystalline insulating barriers may be the key to improving Josephson junction based qubits.

Physical Review B, 1987

We study a simple quantum-mechanical generalization of the resistively shunted junction model to describe the current-voltage characteristic of a single Josephson junction. An exact series representa- tion is given for the nonlinear resistance, which is valid for arbitrary temperature and dissipation and includes both classical and quantum phase slips. For sufficiently high temperature, the quantum effects disappear and A cancels from the corresponding series. The resulting expression is equivalent to classical Brownian motion in a periodic potential with friction coefficient y. We derive a rigorous expression for the leading quantum corrections to the classical result which are of order A, In the overdamped limit they are equivalent to a renormalization of the barrier, whereas for y~O they are bounded below by the effect of an additional bias of order A'y. An approximate continued fraction is derived for the resistance in the general case, which becomes exact in the classical high-damping lim- it. It is evaluated numerically and leads to reasonable qualitative results for small barriers, unless the temperature approaches zero. We discuss the possibility of seeing the quantum effects in junctions with small capacity, in particular the existence of a regime with negative differential resistance and the implications of our results to the quasireentrant behavior observed in thin superconducting granu- lar films.

Journal of Superconductivity - J SUPERCOND, 1999

In nanoscale Josephson junctions, the Josephson coupling energy is usually comparable with the charging energy of the junction and with the typical energy of thermal fluctuations. Under these circumstances, phase fluctuations imposed by the electromagnetic environment of the junction crucially affect the junction electrical behavior. In particular, they determine the maximum “supercurrent” the junction can sustain. We discuss this quantity in the case where the junction is not resistively shunted, so that the IV characteristics of the junction remains hysteretic. For a simple, yet realistic, unshunted junction model, we obtain detailed predictions of the shape of the supercurrent branch of the IV characteristic. Finally, we present experimental results supporting the theoretical analysis and which demonstrate that the supercurrent in an unshunted nanoscale Josephson junction can indeed be of the order of its critical current.

PRX Quantum, 2021

There are two elementary superconducting qubit types that derive directly from the quantum harmonic oscillator. In one, the inductor is replaced by a nonlinear Josephson junction to realize the widely used charge qubits with a compact phase variable and a discrete charge wave function. In the other, the junction is added in parallel, which gives rise to an extended phase variable, continuous wave functions, and a rich energy-level structure due to the loop topology. While the corresponding rf superconducting quantum interference device Hamiltonian was introduced as a quadratic quasi-one-dimensional potential approximation to describe the fluxonium qubit implemented with long Josephson-junction arrays, in this work we implement it directly using a linear superinductor formed by a single uninterrupted aluminum wire. We present a large variety of qubits, all stemming from the same circuit but with drastically different characteristic energy scales. This includes flux and fluxonium qubi...

arXiv (Cornell University), 2021

The discovery that a gate electrode suppresses the supercurrent in purely metallic systems is missing a complete physical understanding of the mechanisms at play. We here study the origin of this reduction in a Superconductor-Normal metal-Superconductor Josephson junction by performing, on the same device, a detailed investigation of the gate-dependent switching probability together with the local tunnelling spectroscopy of the normal metal. We demonstrate that high energy electrons leaking from the gate trigger the reduction of the critical current which is accompanied by an important broadening of the switching histograms. The switching rates are well described by an activation formula including an additional term accounting for the injection of rare high energy electrons from the gate. The rate of electrons obtained from the fit remarkably coincides with the independently measured leakage current. Concomitantly, a negligible elevation of the local temperature is found by tunnelling spectroscopy which excludes overheating scenarios. This incompatibility is resolved by the fact that phase dynamics and thermalization effects occur at different time-scales.

Physical Review Letters, 2006

The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single currentbiased Josephson junction (CBJJ) can be considered as a multiqubit register. It can be coupled to other CBJJs to allow the application of quantum gates to an arbitrary pair of qubits in the system. Our results indicate an alternative way to realize superconducting quantum information processing.

Reviews of Modern Physics, 2001

Unconventional superconductors exhibit an order parameter symmetry lower than the symmetry of the underlying crystal lattice. Recent phase sensitive experiments on YBa2Cu3O7 single crystals have established the d-wave nature of the cuprate materials, thus identifying unambiguously the first unconventional superconductor . The sign change in the order parameter can be exploited to construct a new type of s-wave-d-wave-s-wave Josephson junction exhibiting a degenerate ground state and a double-periodic current-phase characteristic. Here we discuss how to make use of these special junction characteristics in the construction of a quantum computer. Combining such junctions together with a usual s-wave link into a SQUID loop we obtain what we call a 'quiet' qubit -a solid state implementation of a quantum bit which remains optimally isolated from its environment.

New Journal of Physics, 2023

We treat the Cooper pairs in the superconducting electrodes of a Josephson junction (JJ) as an open system, coupled via Andreev scattering to external baths of electrons. The disequilibrium between the baths generates the direct-current bias applied to the JJ. In the weak-coupling limit we obtain a Markovian master equation that provides a simple dynamical description consistent with the main features of the JJ, including the form of the current-voltage characteristic, its hysteresis, and the appearance under periodic voltage driving of discrete Shapiro steps. For small dissipation, our model also exhibits a self-oscillation of the JJ's electrical dipole with frequency Ω=2eV/ℏ around mean voltage V. This self-oscillation, associated with "hidden attractors" of the nonlinear equations of motion, explains the observed production of monochromatic radiation with frequency Ω and its harmonics. We argue that this picture of the JJ as a quantum engine resolves open questions about the Josephson effect as an irreversible process and could open new perspectives in quantum thermodynamics and in the theory of dynamical systems.

Superlattices and Microstructures, 1999

When two superconductors are connected by a weak link a supercurrent flows determined by the difference in the macroscopic quantum phases of the superconductors. Originally, this phenomenon was discovered by Josephson [1] for the case of a weak link formed by a thin tunnel barrier. The supercurrent I is related to the phase difference ϕ through the Josephson current-phase relation, I = I c sin ϕ, with I c , the critical current, depending on the properties of the weak link. A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction [2]. In all cases, the phase difference ϕ = 0 when no supercurrent flows through the junction, and ϕ increases monotonically with increasing supercurrent until the critical current is reached. Using nanolithography techniques we have succeeded in making and studying a Josephson junction with a normal metal weak link, in which we have direct access to the microscopic current-carrying states inside the link. We find that the fundamental Josephson relation can be changed from I = I c sin ϕ to I = I c sin(ϕ + π), i.e. to a π-junction, by suitably controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as (quantum) logic circuits based on superconductors.

Physical Review B, 1990

We studied the effect of the distributed Josephson current of a large one-dimensional edge junction on macroscopic quantum tunneling. An explicit expression for the Lagrangian of the whole system was derived from which we obtained the reduced Euclidean action for the tunneling mode. The result can be used to calculate the quantum tunneling rates in various cases. The measurement of quantum tunneling in such a system was also discussed.