Josephson Charge Qubits Research Articles

Decoherence of Josephson charge qubit in non-Markovian environment

In this paper we investigate the decoherence of Josephson charge qubit (JCQ) by using a time-nonlocal (TNL) dynamical method. Three kinds of environmental models, described with Ohmic, super-Ohmic, and sub-Ohmic spectral density functions are considered. It is shown that the TNL method can effectively include the non-Markovian effects in the dynamical solutions. In particular, it is shown that the sub-Ohmic environment has longer correlation time than the Ohmic and super-Ohmic ones. And the Markovian and non-Markovian dynamics are obviously different for the qubit in sub-Ohmic environment.

Controllable Quantum State Transfer Between a Josephson Charge Qubit and an Electronic Spin Ensemble

We propose a theoretical scheme to implement controllable quantum state transfer between a superconducting charge qubit and an electronic spin ensemble of nitrogen-vacancy centers. By an electro-mechanical resonator acting as a quantum data bus, an effective interaction between the charge qubit and the spin ensemble can be achieved in the dispersive regime, by which state transfers are switchable due to the adjustable electrical coupling. With the accessible experimental parameters, we further numerically analyze the feasibility and robustness. The present scheme could provide a potential approach for transferring quantum states controllably with the hybrid system.

Subluminal and superluminal light propagation in a superconducting quantum circuit via Josephson coupling energy

We investigate the dispersion-group index, as well as the transmission coefficient properties of a weak probe field in a superconducting quantum circuit with a tunable V-type artificial molecule constructed by two superconducting Josephson charge qubits coupled with each other through a superconducting quantum interference device. It is realized that the slope of dispersion can be changed from negative to positive or vice versa through the ratio of the Josephson coupling energy to the capacitive coupling strength which provides an extra controlling parameter for controlling the slope of dispersion. The temporal behavior of the probe dispersion and the required switching time for switching the superluminal light propagation to the subluminal light propagation are also discussed. The results may be useful for understanding the switching feature of slow light-based systems and have potential application in optical information processing.

Transient absorption and lasing without inversion in an artificial molecule via Josephson coupling energy

This letter investigates the dynamical behavior of the absorption in a superconducting quantum circuit with a tunable V-type artificial molecule constructed by two superconducting Josephson charge qubits coupled with each other through a superconducting quantum interference device. It is found that the ratio of the Josephson coupling energy to the capacitive coupling strength provides an extra controlling parameter for manipulating transient absorption behaviors. It is also realized that in the presence of an incoherent pumping field, lasing without inversion can be obtained just through the joint effect of the Josephson coupling energy and the capacitive coupling strength. Results may provide some new possibilities for solid-state quantum information science.

Optical bistability via Josephson coupling energy in a superconducting quantum circuit

A novel configuration is proposed to study optical bistability (OB) and optical multistability (OM) in a superconducting quantum circuit with a tunable V-type artificial molecule constructed by two superconducting Josephson charge qubits coupled with each other through a superconducting quantum interference device. We find that the ratio of the Josephson coupling energy to the capacitive coupling strength has a significant impact on creating optical bistability. The influence of other system parameters on bistable behavior of the artificial medium is then discussed. In particular, it is found that applying an incoherent pumping field can noticeably reduce the bistable threshold. We also realize a switch from OB to OM through proper tuning of detuning parameters. The controllability of OB and OM of this artificial molecule may be useful in building logic-gate devices for optical computing and quantum information processing and provide some new possibilities for solid-state quantum information science.

Switching from positive to negative absorption with electromagnetically induced transparency in circuit quantum electrodynamics

We present a theoretical study of electromagnetically induced transparency (EIT) in a superconducting quantum circuit with a tunable V-shaped energy spectrum derived from two superconducting Josephson charge qubits coupled with each other through a superconducting quantum interference device. Using the density matrix formalism and the steady-state approximation, we obtain the analytical expressions of the first-order matrix element associated with the absorption and dispersion of the probe field for two different V-type schemes. Our results show that, for this superconducting quantum system, it is possible to realize a remarkable phenomenon that dynamic conversion between EIT and EIT with amplification without population inversion. Such a unique optical feature has potential applications in quantum optical devices and quantum information processing.

Tuning quantum discord in Josephson charge qubits system

A type of two qubits Josephson charge system is constructed, and properties of the quantum discord (QD) as well as the differences between thermal QD and thermal entanglement are investigated. A detailed calculation shows that the magnetic flux is more efficient than the voltage in tuning QD. By choosing proper system parameter the maximum QD can be realized in our two qubits Josephson charge system.

Electromagnetically Induced Transparency Using a Artificial Molecule in Circuit Quantum Electrodynamics

Electromagnetically induced transparency (EIT) having wide applications in quantum optics and nonlinear optics is explored ordinarily in various atomic systems. In this paper we present a theoretical study of EIT using supercon- ducting circuit with a V-type artificial molecule constructed by two Josephson charge qubits coupled each other through a large capacitor. In our theoretical model we make a steady state approximation and obtain the analytical expressions of the complex susceptibility for the artificial system via the density matrix formalism. The complex susceptibility has additional dependence on the qubit parameters and hence can be tuned to a certain extent.

Robust Quantum Gates in Decoherence-Free Subspaces with Josephson Charge Qubits

We propose a new and feasible scheme to implement quantum gates in decoherence-free subspaces (DFSs) with Josephson charge qubits situated in a circuit QED architecture. Based on the resonator-assisted interaction, the controllable interqubit couplings occur only by tuning the individual flux biases, by which we obtain the DFS-encoded universal quantum gates. Compared with the non-DFS situation, we numerically consider the robustness of the DFS-encoded scheme that can be insensitive to the collective noises. Thus the protocol may perform the fault-tolerant quantum computing with Josephson charge qubits.

Effect of control procedures on the evolution of entanglement in open quantum systems

The effect of a number of mechanisms designed to suppress decoherence in open quantum systems are studied with respect to their effectiveness at slowing down the loss of entanglement. The effect of photonic band-gap materials and frequency modulation of the system-bath coupling are along expected lines in this regard. However, other control schemes, like resonance fluorescence, achieve quite the contrary: increasing the strength of the control leads to a faster decay of entanglement. The effect of dynamic decoupling schemes on two qualitatively different system-bath interactions are studied in depth. Dynamic decoupling control has the expected effect of slowing down the decay of entanglement in a two-qubit system coupled to a harmonic oscillator bath under a nondemolition interaction. However, nontrivial phenomena are observed when a Josephson charge qubit, strongly coupled to a random telegraph noise bath, is subject to decoupling pulses. The most striking of these reflects the resonance fluorescence scenario, in that an increase in the pulse strength decreases decoherence but also speeds up the sudden death of entanglement. This demonstrates that the behavior of decoherence and entanglement in time can be qualitatively different in the strong-coupling non-Markovian regime.

Structure formation of entanglement entropy in a system of two superconducting qubits coupled with an LC-resonator

This paper investigates theoretically the evolutions of the entanglement entropy of a system of two coupled-charge-qubits interacting with an LC-resonator. It is found that when the initial states of the two qubits are prepared in a given superposition excited state, the evolution of the von Neumann entropy of the system depends significantly on the coupling strength between the two Josephson charge qubits. With the variation of the coupling strength, the evolution of the entanglement entropy of the system forms some structures, especially the periodically bistable properties, which are the first discovered for such a system to our knowledge. It is found that the relative entropy entanglement of the system is also sensitive to the variation of the coupling strength between the two charge qubits, some novel ‘collective oscillations’ of the relative entropy are found for the system.

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.

High-efficiency tomographic reconstruction of quantum states by quantum nondemolition measurements

We propose a high efficiency tomographic scheme to reconstruct an unknown quantum state of the qubits by using a series of quantum nondemolition (QND) measurements. The proposed QND measurements of the qubits are implemented by probing the the stationary transmissions of the dispersively-coupled resonator. It is shown that only one kind of QND measurements is sufficient to determine all the diagonal elements of the density matrix of the detected quantum state. The remaining non-diagonal elements of the density matrix can be determined by other spectral measurements by beforehand transferring them to the diagonal locations using a series of unitary operations. Compared with the pervious tomographic reconstructions based on the usual destructively projective (DP) measurements (wherein one kind of such measurements could only determine one diagonal element of the density matrix), the present approach exhibits significantly high efficiency for N-qubit (N > 1). Specifically, our generic proposal is demonstrated by the experimental circuit-quantumelectrodynamics (circuit-QED) systems with a few Josephson charge qubits.

Using Josephson charge qubits system to realize the transfer of a special kind of quantum state

A type of Josephson charge qubit circuit is constructed in this paper, and properties of four-qubit quantum state transfer through this system is also investigated. By modulating the external magnetic flux, we show that ideal transfer of an excited state 1〉 from the first qubit to the fourth qubit can be realized for both homogeneous and inhomogeneous transmission channels. Moreover, the average fidelity of state transfer across this system is also analyzed, the result revealed that it cannot be used to transfer an arbitrary quantum state.

Testing quantum contextuality with macroscopic superconducting circuits

We propose an experimental procedure to macroscopically test the Kochen-Specker theorem with superconducting qubits. This theorem, which has been experimentally tested with single photons and neutrons, elucidates the conflict between quantum mechanics and noncontextual hidden variable theories. Two Josephson charge qubits can be controllably coupled by using a two-level data bus built with a phase qubit. Then, by performing joint nondestructive quantum measurements of two distinct qubits, we show that the proposed circuits could demonstrate quantum contextuality at a macroscopic level.

Time-optimal performance of Josephson charge qubits: A process tomography approach

A process tomography based optimization scheme for open quantum systems is used to determine the performance limits of Josephson charge qubits within current experimental means. The qubit is modeled microscopically as an open quantum system taking into account state leakage, as well as environment-induced dephasing based on experimental noise spectra. Within time-optimal control theory, we show that the competing requirements for suppression of state leakage and dephasing can be met by an external control of the effective qubit-environment interaction, yielding minimal gate fidelity losses of around $\ensuremath{\Delta}F\ensuremath{\approx}{10}^{\ensuremath{-}3}$ under typical experimental conditions.

Robust control of Josephson charge qubits

Typical for qubit realizations are two performance-limiting shortcomings: the physical system is not a perfect two-level system and it couples to an environment. In this work we investigate the limits of performance of the Josephson charge qubit due to the interplay of leakage into unwanted states and dissipative effects due to environmental noise within optimal control theory. For the example of the Hadamard transformation, we show that external control field-gate bias and magnetic flux, which perform the operation for the ideal qubit perfectly, show different robustness regarding leakage into unwanted states and against dissipation. Proper selection of the temporal evolution of gate bias and magnetic flux leads to performance improvements of typically 20% regarding dissipative effects with a predicted optimal fidelity of typically 98%, presuming perfect initial-state preparation and switching times of the order of 500 ps.

Delayed sudden birth and sudden death of entanglement in Josephson-charge qubits

Distinctive features of the entanglement dynamics of a circuit containing two-coupled superconducting charge qubits are explored. The striking effects of junction coupling on the entanglement are presented in terms of the concurrence. We also point out the importance effects of initial state selection for obtaining different features of entanglement, such as, sudden death and sudden birth.

Josephson charge qubits: a brief review

The field of solid-state quantum computation is expanding rapidly initiated by our original charge qubit demonstrations. Various types of solid-state qubits are being studied, and their coherent properties are improving. The goal of this review is to summarize achievements on Josephson charge qubits. We cover the results obtained in our joint group of NEC Nano Electronics Research Laboratories and RIKEN Advanced Science Institute, also referring to the works done by other groups. Starting from a short introduction, we describe the principle of the Josephson charge qubit, its manipulation and readout. We proceed with coupling of two charge qubits and implementation of a logic gate. We also discuss decoherence issues. Finally, we show how a charge qubit can be used as an artificial atom coupled to a resonator to demonstrate lasing action.

TELEPORTATION OF AN ARBITRARY AND UNKNOWN JOSEPHSON CHARGE-QUBIT ENTANGLED STATE VIA A CLUSTER STATE

An experimentally feasible scheme for teleporting an arbitrary and unknown entangled state is proposed. In this paper, we use a cluster state as the quantum channel, and we do not need any joint Bell-state measurement (BSM). Our scheme is based on Josephson charge qubits, and the successful probability and fidelity of teleportation can both reach unity. Moreover, the current scheme can be realized within the current experimental technology.