Observation of excitation in asymmetric flux qubits coupled inductively

We measured magnetic flux produced by coupled flux qubits with a switching current measurement of a DC-SQUID. Both the circulating currents and the SQUID–qubit coupling were designed to be asymmetric. The experimental result exhibits a broad peak and dip, and does not agree with the thermally averaged magnetic flux calculated with the two qubit Hamiltonian. This disagreement can be explained in terms of nonthermal excitation of the coupled qubits, which is probably caused by the microwaves generated by the DC-SQUID in the voltage state.     

Quantum Entanglement Creating and Evolving in Two Charge Qubits Coupling to a Nanomechanical Resonator System

We investigate the coupling of two superconducting quantum interference devices (SQUIDs) via a metallic nanomechanical resonator (NAMR), and bring out the effective interaction between the two SQUIDs. By constructing the evolution operator, we also study the evolvement of entanglement in this composed system.     

On Harmonic Approximation for Large Josephson Junction Coupling Charge Qubits

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

On Bosonic Magnetic Flux Operator and Bosonic Faraday Operator Formula

In the literature about mesoscopic Josephson devices the magnetic flux is considered as an operator, the fundamental commutative relation between the magnetic flux operator and the Cooper-pair charge operator is usually preengaged. In this paper we show that such a relation can be deduced from the basic Bose operators＇ commutative relation through the entangled state representation. The Faraday formula in bosonic form is then equivalent to the second Josephson equation. The current operator equation for LC mesoscopic circuit is also derived.     

Entangled States in the Capacitance Coupling Double Josephson Junction Mesoscopic Circuit

The number-phase quantization scheme of the capacitance coupling double Josephson junction mesoscopic circuit is given. The eigenvalues and the eigenstates of the system are investigated. It is found that using this system the entangled states can be prepared.     

Teleportation for an Unknown Entangled State Without Any Joint Measurement via Josephson Charge Qubits

An experimentally feasible scheme for teleportation of an arbitrary unknown entangled state is proposed. Our scheme is based on Josephson charge qubits, where we do not need any joint measurement. Moreover the successful probability and fidelity of the teleportation can both reach 1.0. The current scheme can be realized within the current experimental technology.     

Multi-Qubits Entangled State Generation with Multiple Flux Qubits Coupled to a Coplanar Waveguide Resonator

We propose a scheme to efficiently implement multi-qubits entangled states with multiple low-decoherence superconducting flux qubits coupled with a coplanar waveguide resonator. By accurate controlling the evolution operator, the cluster states, Greenberger-Horne-Zeilinger (GHZ) states, and W-type states can be generated by our scheme, respectively.     

External electric field control in the charge qubits including three Josephson junctions

Based on the standard canonical quantization principle, this paper gives the quantization scheme for the charge qubits mesoscopic circuit including three Josephson junctions coupled capacitively. By virtue of the Heisenberg equation, the time evolution of the phase difference operators across the polar plates and the number operators of the Cooper-pairs on the island are investigated and the modification of the Josephson equation is discussed. The time evolution of the phase difference operators is analysed when the Josephson junctions are irradiated by the external electrical field, which is referred to as also the obtainable controlling parameter.     

Magnetic avalanches: Josephson, Bean, and Bak

Josephson junction arrays provide an ideal physical realization for studying the complex dynamics of the sort found in sandpile models. They provide a means of separately investigating the dual physical effects of nonlinearity and disorder, and hold promise as an example for establishing a rigorous connection between the governing differential equations and the corresponding cellular automaton.     

Efficient One-Step Generation of Cluster State with Charge Qubits in Circuit QED

t We propose theoretical schemes to generate highly entangled cluster state with superconducting qubits in a circuit QED architecture. Charge qubits are located inside a superconducting transmission line, which serves as a quantum data bus. We show that large clusters state can be efficiently generated in just one step with the longrange Ising-like unitary operators. The quantum operations which are generally realized by two coupling mechanisms： either voltage coupling or current coupling, depend only on global geometric features and are insensitive not only to the thermal state of the transmission line but also to certain random operation errors. Thus high-fidelity one-way quantum computation can be achieved.     

Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.     

Design of a gap tunable flux qubit with FastHenry

In the preparations of superconducting qubits, circuit design is a vital process because the parameters and layout of the circuit not only determine the way we address the qubits, but also strongly affect the qubit coherence properties. One of the most important circuit parameters, which needs to be carefully designed, is the mutual inductance among different parts of a superconducting circuit. In this paper we demonstrate how to design a gap-tunable flux qubit by layout design and inductance extraction using a fast field solver Fast Henry. The energy spectrum of the gap-tunable flux qubit shows that the measured parameters are close to the design values.     

Effects of Electric and Magnetic Fields on Pure Dephasing of Exciton Qubits

In a two-dimensional quantum dot （QD） with parabolic confinement potential, we investigate pure dephasing due to deformation potential exciton-bulk longitudinal acoustic phonons （LAP） interaction for exciton qubits under the influence of external static electric and magnetic fields by adopting the full quantum-mechanical method of Kunihiro Kojima and Akihisa Tomita. The wave function is found and the dependence of the pure dephasing factor on the confinement length of the QD and time and temperature is discussed. We find the external electric and magnetic fields have important effects on pure dephasing of exciton qubits because exciton-LAP interaction increases, leading to more pure dephasing.     

Short-Time Decoherence of Solid-State Qubit at Optimal Operation Points

We investigate the short-time decoherence of a solid-state qubit under Ohmic noise at optimal operation points. The decoherence is analyzed by maximum norm of the deviation density operator. It is shown that at the temperature T = 3 mK, the loss of the fidelity due to decoherence is much smaller than the DiVincenzo low decoherence criterion, which means that the mode/may be an optimal candidate of qubit for quantum computation.     

Short-Time Decoherence of Solid-State Qubit at Optimal Operation Points

We investigate the short-time decoherence of a solid-state qubit under Ohmic noise at optimal operation points. The decoherence is analyzed by maximum norm of the deviation density operator. It is shown that at the temperature T = 3 mK, the loss of the fidelity due to decoherence is much smaller than the DiVincenzo low decoherence criterion, which means that the model may be an optimal candidate of qubit for quantum computation.     

Quantum entanglement and control in a capacitively coupled charge qubit circuit

The macroscopic quantum entanglement in capacitively coupled SQUID (superconducting quantum interference device)-based charge qubits is investigated theoretically. The entanglement characteristic is discussed by employing the quantum Rabi oscillations and the concurrence. An interesting conclusion is obtained, i.e., the magnetic fluxes Ф_x1 and Ф_x2 through the superconducting loops can adjust the entanglement degree between the qubits.     

Progress in superconducting qubits from the perspective of coherence and readout

Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms.Benefiting from the well-developed technology of microfabrication and microwave engineering,superconducting qubits have great advantages in design flexibility,controllability,and scalability.Over the past decade,there has been rapid progress in the field,which greatly improved our understanding of qubit decoherence and circuit optimization.The single-qubit coherence time has been steadily raised to the order of 10 to 100μs,allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control.Here we review recent progress in the coherence and readout of superconducting qubits.     

噪声对超导量子比特测量系统的影响

详细介绍了噪声对超导量子比特测量系统的影响。测量系统通过对时间的测量来获得单个约瑟夫森结跳变电流统计分布P(I)。在测量电路中采取了一系列的降噪措施,使得实验所能测到的约瑟夫森结的最低等效温度进一步的降低。     

Topological Quantization of the Magnetic Flux

The quantization of the magnetic flux in superconducting rings is studied in the frame of a topological model of electromagnetism that gives a topological formulation of electric charge quantization. It turns out that the model also embodies a topological mechanism for the quantization of the magnetic flux with the same relation between the fundamental units of magnetic charge and flux as there is between the Dirac monopole and the fluxoid.