Large Josephson Junction Research Articles

Control of the Entanglement between Two Josephson Charge Qubits

We consider a model which consists of two coupled superconducting charge qubits by sharing a large Josephson junction. The time evolution of entanglement between two Josephson charge qubits is investigated by employing the concurrence. We examine the influence of the initial mean photon number, the relative phase and the amplitude of the two qubits on the time evolution of entanglement. The results show that the initial mean photon number, the relative phase and the amplitude of the two qubits play an important role in the evolution of entanglement.

Entanglement of two superconducting charge qubits with the Glauber–Lachs state

We investigate the entanglement properties of two superconducting charge qubits that share a large Josephson junction where the field is assumed to be prepared initially in a Glauber–Lachs state. We explore the increasing effect of the chaotic field due to the thermal photons on the entanglement properties of these two superconducting charge qubits. We show that the entanglement of the qubits is very sensitive to thermal photons. The chaotic effect of the field has a reducing effect on the entanglement degree of the qubits. Also, we draw a parallelism between the Glauber–Lachs state and the random telegraph noise by means of the entanglement.

The dynamics of the kink in curved large area Josephson junction

A formalism that allows description of the kink motion in an arbitrarily curved large area Josephson junction is proposed. A general formula for the lagrangian density that describes the curved Josephson junction, in small curvature regime, is obtained. Examples of propagation of the kink along the curved Josephson junction are considered.

Linear Entropy and Entanglement in Two-Charge-Qubit Circuits

We consider a model that contains two coupled superconducting charge qubits by sharing a large Josephson junction. We examine the dynamical properties of the linear entropy of two qubits and the probability of both qubits being in an excited state. The results show that the initial mean photon number, the initial phase of the field and the relative phase of the two qubits' levels play an important role in the evolution of the linear entropy of the two qubits and the probability.

Suppression of macroscopic quantum tunneling in a large Josephson junction coupled to a resonator

We employ a functional integral technique to calculate the macroscopic quantum tunneling rate at zero temperature of a current-biased Josephson junction weakly coupled to a resonator. We allow for the effects of environmental dissipation on both the junction and the resonator via baths of harmonic oscillators, and we consider both cases of weak and strong junction damping. The resonator can be, in principle, any quantum harmonic oscillator that couples to the junction bilinearly in either the coordinate or the velocity. The Josephson phase difference $\ensuremath{\phi}$ is the tunneling variable in this system, and the low-lying bound states in the junction's potential energy well are weakly metastable due to tunneling through a finite-width energy barrier. There has been some interest in using such biased junctions as qubits with the resonator providing multiqubit coupling. The main result of this work is that coupling to the resonator has a suppressive effect on the junction's tunneling: the stronger the coupling strength between the junction and resonator, the greater the reduction of the tunneling rate. Including damping to the junction also suppresses tunneling, a well-known result, but damping of the resonator actually reduces the magnitude of suppression; i.e., damping the resonator partially counteracts the suppression provided directly by the junction-resonator coupling. Details of the junction-resonator coupling yield interesting variations on this theme that may be useful for qubit design. For example, the coupling ${U}_{\mathit{int}}$ between a current-biased junction and an AlN dilatational resonator should depend on the angular frequency of the resonator ${\ensuremath{\omega}}_{R}$ in a power-law fashion [A. N. Cleland and M. R. Geller, Phys. Rev. Lett. 93, 070501 (2004)], specifically ${U}_{\mathit{int}}\ensuremath{\propto}{({\ensuremath{\omega}}_{R})}^{n}$, where $n=1∕2$. We find that for any value of $n$ in the range $0lnl1$ the junction's tunneling rate exhibits a nonmonotonic dependence on ${\ensuremath{\omega}}_{R}$, and for $n=1∕2$, the tunneling rate is maximally suppressed for ${\ensuremath{\omega}}_{R}∕{\ensuremath{\omega}}_{J}\ensuremath{\approx}1$, where ${\ensuremath{\omega}}_{J}$ is the bias current-dependent plasma frequency of the junction.

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective interqubit coupling controlled by the external magnetic flux. This switchable interqubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.

Microscopic origin of critical current fluctuations in large, small, and ultra-small area Josephson junctions

We analyze data on the critical current and normal-state resistance noise in Josephson junctions and argue that the noise in the critical current is due to a mechanism that is absent in the normal state. We estimate the noise produced by conventional two level systems in the insulating barrier and find that it agrees both in magnitude and in temperature dependence with the resistance fluctuations in the normal state but it is not sufficient to explain the critical current noise observed in large superconducting contacts. We propose a microscopic mechanism for the noise in the superconducting state in which the noise is due to electron tunneling between weak Kondo states at subgap energies. We argue that the noise produced by this mechanism gives temperature, area dependence, and intensity that agree with the data.

Development of RSFQ voltage drivers for arbitrary AC waveform synthesisers

We have realised RSFQ circuits for an integrated bit pattern generator to drive large Josephson junction arrays and synthesise arbitrary AC waveforms. We present test circuits to investigate the driving of junction arrays by sequences of voltage pulses. To elevate the level of voltage pulses, the circuits have been terminated by SQUID stack voltage drivers. Drivers of various bit lengths and different designs, in pure-serial and in combined parallel-and-serial SFQ pulse processing modes, were developed, fabricated and tested. Voltage amplification by factors four and eight were obtained. The circuits were realised in 4-µm Nb SIS trilayer technology with externally shunted Josephson junctions (critical current density: 1 kA/cm2).

Complex magnetic field dependence in large quantum well-coupled Josephson junctions

We study the dependence of the critical current on applied magnetic field in Josephson junctions with an InAs quantum well as the coupling medium between Nb superconductors. The junctions are mesoscopic in length, L, but not in width, W ≫ L. The critical current exhibits a complex dependence on applied field that does not resemble a Fraunhoffer pattern. We discuss the data in light of previous studies of self-field and non-local effects in planar junctions, effects of structural non-uniformity, and recent studies of flux trapping in Nb films.

Running-phase state in a Josephson washboard potential

We investigate the dynamics of the phase variable of an ideal underdamped Josephson junction in switching current experiments. These experiments have provided the first evidence for macroscopic quantum tunneling in large Josephson junctions and are currently used for state read-out of superconducting qubits. We calculate the shape of the resulting macroscopic wavepacket and find that the propagation of the wavepacket long enough after a switching event leads to an average voltage increasing linearly with time.

Quantum manipulation and simulation using Josephson junction arrays

We discuss the prospect of using quantum properties of large scale Josephson junction arrays for quantum manipulation and simulation. We study the collective vibrational quantum modes of a Josephson junction array and show that they provide a natural and practical method for realizing a high quality cavity for superconducting qubit based QED. We further demonstrate that by using Josephson junction arrays we can simulate a family of problems concerning spinless electron–phonon and electron–electron interactions. These protocols require no or few controls over the Josephson junction array and are thus relatively easy to realize given currently available technology.

Superconducting qubit network with controllable nearest-neighbour coupling

We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbour coupling. The qubit coupling is achieved by placing large Josephson junctions (JJs) at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional JJs included in the qubit loops for qubit readout can also be employed for qubit coupling.

A New Flux/Phase Qubit With Integrated Readout

We propose a new scheme for a flux/phase qubit, based on a double-SQUID modified to achieve a direct readout of the flux state. The readout is performed by inserting a large Josephson junction in the SQUID loop; the junction is biased by an external current and switches from the zero-voltage to the running state depending on the flux state of the qubit. We report recent results concerning the theoretical behavior and the experimental characterization at 4.2 K of the device, which has been realized with Nb/AlOx/Nb trilayer technology.

On Harmonic Approximation for Large Josephson Junction CouplingCharge Qubits

We revisit the harmonic approximation (HA) for a large Josephson junctioninteracting with some charge qubits through the variational approach for thequantum dynamics of the junction-qubit coupling system. By making use ofnumerical calculation and analytical treatment, the conditions under whichHA works well can be precisely presented to control the parametersimplementing the two-qubit quantum logical gate through the couplings to thelarge junction with harmonic oscillator Hamiltonian.

Quantum computation with Josephson qubits using a current-biased information bus

We propose an effective scheme for manipulating quantum information stored in a superconducting nanocircuit. The Josephson qubits are coupled via their separate interactions with an information bus, a large current-biased Josephson junction treated as an oscillator with adjustable frequency. The bus is sequentially coupled to only one qubit at a time. Distant Josephson qubits without any direct interaction can be indirectly coupled with each other by independently interacting with the bus sequentially, via exciting/de-exciting vibrational quanta in the bus. This is a superconducting analog of the successful ion trap experiments on quantum computing. Our approach differs from previous schemes that simultaneously coupled two qubits to the bus, as opposed to their sequential coupling considered here. The significant quantum logic gates can be realized by using these tunable and selective couplings. The decoherence properties of the proposed quantum system are analyzed within the Bloch-Redfield formalism. Numerical estimations of certain important experimental parameters are provided.

Spectroscopy of Three-Particle Entanglement in a Macroscopic Superconducting Circuit

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, a resonator coupling two large Josephson junctions, produced complex energy spectra well explained by quantum theory over a large frequency range. By tuning each junction separately into resonance with the resonator, we first observe strong coupling between each junction and the resonator. Bringing both junctions together into resonance with the resonator, we find spectroscopic evidence for entanglement between all 3 degrees of freedom and suggest a new method for controllable coupling of distant qubits, a key step toward quantum computation.

Fast entanglement of two charge-phase qubits through nonadiabatic coupling to a large Josephson junction

We propose a theoretical protocol for quantum logic gates between two Josephson junction charge-phase qubits through the control of their coupling to a large junction whose Josephson coupling energy is much larger than its Coulomb charge energy. In the low excitation limit of the large junction, it behaves effectively as a quantum data-bus mode of a harmonic oscillator. Our protocol can be fast since it does not require the data-bus to stay adiabatically in its ground state, as such it can be implemented over a wide parameter regime independent of the data-bus quantum state.

Optical solitons and quantum solitons

We return to the notion of optical solitons as these were first introduced—namely as exact soliton and multi-soliton solutions of the semi-classical self-induced transparency (SIT) equations. The SIT equations, even in their most general inhomogeneously broadened forms, are infinite dimensional, completely integrable Hamiltonian systems, and thus display much mathematical structure, yet their exact soliton solutions display well the nonlinear (self-induced) transparency features of the physical phenomenon. The mathematical feature of complete integrability is handed down from the SIT equations to the nonlinear Schrodinger equations and from this point of view the nonlinear Schrodinger (NLS) equations equations are studied first of all at classical level and then at quantum level. The 'quantum soliton' of the quantum attractive NLS model is defined and its properties investigated in detail, particularly in a comparison with the particular properties ascribed to the 'quantum soliton' as described in a recently published article (Abraham 1999 Phys. World (February) 21) with this title; as suggested in that article, the quantum NLS soliton is seen as a qubit for quantum information purposes. The covariant form of the quantum attractive NLS model is the quantum sine-Gordon model and this is briefly studied in the context of quantum information also; new results for large area Josephson junctions are reported in this context. The quantum soliton of the one dimensional attractive NLS model has apparently been observed in 2002 in a Bose condensed metal vapour (7Li, (Khaykovich et al 2002 Science 296 1290)) and a detailed calculation suggests that the condensate undergoes a 'collapse' from three to one space dimensions thus forming an exact soliton solution of the one dimensional quantum attractive NLS model.

Implementation of a combined charge-phase quantum bit in a superconducting circuit

We discuss a qubit circuit based on the single Cooper-pair transistor (which consists of two ultrasmall Josephson junctions in series) connected in parallel with a large Josephson junction. The switching of this junction out of its zero-voltage state is used to readout the qubit. We report measurements of the discriminating power of the readout process, and we discuss its back-action on the qubit.

Phase-locking Josephson junctions arrays

We demonstrate that a large area Josephson junction oscillating in the fluxon oscillator mode can be synchronized to other large junctions and simultaneously pump, by emitted radiation, a small area junction. We study the synchronization of the oscillations of the long junctions as a function of relevant experimental parameters such as bias current and stripline coupling characteristics. Experimental results obtained on coupled series arrays of Josephson junctions designed on the basis of our calculations are presented. We have fabricated coupled arrays containing each up to 1500 junctions in order to estimate the usefulness of our calculations for voltage standard devices.