Quantum State Transfer between Charge and Flux Qubits in Circuit-QED

We propose a scheme to implement quantum state transfer in a hybrid circuit quantum electrodynarnics （QED） system which consists of a superconducting charge qubit, a flux qubit, and a transmission line resonator （TLR）. It is shown that quantum state transfer between the charge qubit and the flux qubit can be realized by using the TLR as the data bus.     

Quantum Controlled-Not Gate Operation and Complete Bell-State Analysis Using Hybrid Quantum Circuits

Here we propose a hybrid quantum circuit for achieving the quantum controlled-not (CNOT) gate operation on a photon-spin hybrid state. The hybrid quantum circuit consists of a nitrogen-vacancy (N-V) center and microtoroidal resonator coupling system, and a single photon waveguide. We implement the complete Bell state analysis using the proposed circuit. This proposed hybrid quantum circuit could enable a high fidelity of qubit manipulation and allows the feasible with the current experimental technologies.     

Macroscopic Greenberg-Horne-Zeilinger state and W state in charge qubits based on Coulomb blockade

Based on Coulomb blockade, we propose a scheme to generate two types of three-qubit entanglement, known as Greenberg-Horne-Zeilinger (GHZ) state and W state, in a macroscopic quantum system. The qubit is encoded in the charge qubit in the superconducting system, and the scheme can be generalized to generate the GHZ state and W state in multi-partite charge qubits. The GHZ state and W state are the eigenstates of the respective idle Hamiltonian, so they have the long lifetime.     

A simple scheme to generate χ-type four-charge entangled states in circuit QED

We propose a simple scheme to generate χ-type four-charge entangled states by using SQUID-based charge qubits capacitively coupled to a transmission line resonator (TLR). The coupling between the superconducting qubit and the TLR can be effectively controlled by properly adjusting the control parameters of the charge qubit. The experimental feasibility of our scheme is also shown.     

Readout of superconducting flux qubit state with a Cooper pair box

We study a readout scheme of a superconducting flux qubit state with a Cooper pair box as a transmon. The qubit states consist of the superpositions of two degenerate states where the charge and phase degrees of freedom are entangled. Owing to the robustness of the transmon against external fluctuations, our readout scheme enables the quantum non-demolition and single-shot measurement of flux qubit states. The qubit state readout can be performed by using the nonlinear Josephson amplifiers after a π/2 rotation driven by an ac electric field.     

Scalable ion trap quantum computing without moving ions

A hybrid quantum computing scheme is studied where the hybrid qubit is made of an ion trap qubit serving as the information storage and a solid-state charge qubit serving as the quantum processor, connected by a superconducting cavity. In this paper, we extend our previous work [CITE] and study the decoherence, coupling and scalability of the hybrid system. We present our calculations of the decoherence of the coupled ion-charge system due to the charge fluctuations in the solid-state system and the dissipation of the superconducting cavity under laser radiation. A gate scheme that exploits rapid state flips of the charge qubit to reduce decoherence by the charge noise is designed. We also study a superconducting switch that is inserted between the cavity and the charge qubit and provides tunable coupling between the qubits. The scalability of the hybrid scheme is discussed together with several potential experimental obstacles in realizing this scheme.     

Controllably Coupling Superconducting Charge and Flux Qubits by Using Nanomechanical Resonator

We propose an effective mechanism to couple superconducting charge and flux qubits by using a quantized nanomechanical resonator. The coupling between the charge and flux qubits can be controlled by the external flux of the charge qubit. Under the strong coupling limit, an iSWAP gate can be generated by this scheme. The experimental feasibility in our scheme is also presented.     

Controllable cross-Kerr interaction between microwave photons in circuit quantum electrodynamics

We propose a scheme to enable a controllable cross-Kerr interaction between microwave photons in a circuit quantum electrodynamics(QED) system.In this scheme we use two transmission-line resonators(TLRs) and one superconducting quantum interference device(SQUID) type charge qubit,which acts as an artificial atom.It is shown that in the dispersive regime of the circuit-QED system,a controllable cross-Kerr interaction can be obtained by properly preparing the initial state of the qubit,and a large cross-phase shift between two microwave fields in the two TLRs can then be reached.Based on this cross-Kerr interaction,we show how to create a macroscopic entangled state between the two TLRs.     

Optically detected adiabatic fast passage and cross-relaxation of the N-V center in diamond

In this paper, we report on optically detected adiabatic fast passage transients for the N-V center in type Ib diamond. The results provide evidence for the triplet character (³A) of the ground state of the N-V center, and give further insight into the photo-kinetics of the N-V center. In addition measurements of optically detected cross-relaxation, involving the spin sublevels of the N-V center³A ground state, are discussed.     

Preparation of Macroscopic Entangled Coherent States in Nitrogen-Vacancy Centers Ensembles Coupled to a Superconducting Flux Qubit

We propose a potentially practical scheme for creating macroscopic entangled coherent state between two separate nitrogen-vacancy center spin ensembles placed near a superconducting flux qubit. Through the collective magnetic coupling and the in situ tunability of the flux qubit, the arbitrary entangled coherent states of spin ensembles can be achieved with high success possibilities under the influence from decoherence of the flux qubit and spin ensembles.The experimental feasibility and challenge are justified using currently available technology.     

Implementation of a Controlled-Phase Gate and Deutsch-Jozsa Algorithm with Superconducting Charge Qubits in a Cavity

Based on superconducting quantum interference devices （SQUIDs） coupled to a cavity, we propose a scheme for implementing a quantum controlled-phase gate （QPG） and Deutsch-Jozsa （D J） algorithm by a controllable interaction. In the present scheme, the SQUID works in the charge regime, and the cavity field is ultilized as quantum data-bus, which is sequentially coupled to only one qubit at a time. The interaction between the selected qubit and the data bus, such as resonant and dispersive interaction, can be realized by turning the gate capacitance of each SQUID. Especially, the bus is not excited and thus the cavity decay is suppressed during the implementation of DJ algorithm. For the QPG operation, the mode of the bus is unchanged in the end of the operation, although its mode is really excited during the operations. Finally, for typical experiment data, we analyze simply the experimental feasibility of the proposed scheme. Based on the simple operation, our scheme may be realized in this solid-state system, and our idea may be realized in other systems.     

Charge Qubit Storage and Its Engineered Decoherence via Microwave Cavity

We study the entanglement of the superconducting charge qubit with the quantized electromagnetic field in a microwave cavity. It can be controlled dynamically by a classical external field threading the SQUID within the charge qubit. Utilizing the controllable quantum entanglement, we can demonstrate the dynamic process of the quantum storage of information carried by charge qubit. On the other hand, based on this engineered quantum entanglement, we can also demonstrate a progressive decoherence of charge qubit with quantum jump due to the coupling with the cavity field in quasi-classical state.     

Eliminating interactions between non-neighboring qubits in the preparation of cluster states in quantum molecules

We propose a scheme to eliminate the effect of non-nearest-neighbor qubits in preparing cluster state with double-dot molecules. As the interaction Hamiltonians between qubits are Ising-model and mutually commute, we can get positive and negative effective interactions between qubits to cancel the effect of non-nearest-neighbor qubits by properly changing the electron charge states of each quantum dot molecule. The total time for the present multi-step cluster state preparation scheme is only doubled for one-dimensional qubit chain and tripled for two-dimensional qubit array comparing with the time of previous protocol leaving out the non-nearest-neighbor interactions.     

Preparing arbitrary mode superconducting LC entangled coherent state via a superconducting charge qubit

We design a pure solid-device comprised by LC circuits, which are coupled to superconducting charge qubit to entangle superconducting LC coherent modes. The operation time of the state of the system does not increase with the increase of the number of LC modes. Based on the measurement of charge states, an arbitrary mode of the state can be easily generated. A brief discussion about the experimental feasibility of the scheme is also shown.     

Quantum Information Transfer from a Double Quantum Dot System to a Cavity Field＊

In this paper, we propose a scheme to realize quantum information transfer from a double quantum dot （DQD） system to a quantized cavity field. The DQD and the cavity field are treated as a two-state charge qubit and a continuous-variable system, respectively. It is shown that quantum information encoded in the two-state DQD system can be transferred to quantum states of the cavity field with a continuous-variable basis through appropriate projective measurements with respect to the DQD.     

Quantum computing in decoherence-free subspaces with superconducting charge qubits

Taking into account the main noises in superconducting charge qubits (SCQs), we propose a feasible scheme to realize quantum computing (QC) in a specially-designed decoherence-free subspace (DFS). In our scheme two physical qubits are connected with a common inductance to form a strong coupling subsystem, which acts as a logical qubit. Benefiting from the well-designed DFS, our scheme is helpful to suppress certain decoherence effects.     

Raman-excited spin coherences in nitrogen-vacancy color centers in diamond

Raman-excited spin coherences were experimentally observed in nitrogen-vacancy (N-V) diamond color centers by means of nondegenerate four-wave mixing and electromagnetically induced transparency. The maximal absorption suppression was found to be 17%, which corresponds to 70% of what is possible given the random geometric orientation of the N-V center in diamond. In the context of quantum computing in solids, this level of transparency represents efficient preparation of quantum bits, as well as the ability to perform arbitrary single-quantum-bit rotations.     

Charge and current beats in T-shaped qubit–detector systems

The time evolution of a charge qubit coupled electrostatically with different detectors in the forms of single, double and triple quantum dot linear systems in the T-shaped configuration between two reservoirs is theoretically considered. The correspondence between the qubit quantum dot oscillations and the detector current is studied for different values of the inter-dot tunneling amplitudes and the qubit–detector interaction strength. We have found that even for a qubit coupled with a single QD detector, the coherent beat patterns appear in the oscillations of the qubit charge. This effect is more evident for a qubit coupled with double or triple-QD detectors. The beats can be also observed in both the detector current and the detector quantum dot occupations. Moreover, in the presence of beats the qubit oscillations hold longer in time in comparison with the beats-free systems with monotonously decaying oscillations. The dependence of the qubit dynamics on different initial occupations of the detector sites (memory effect) is also analyzed.     

Excited-state spectroscopy using single spin manipulation in diamond

We use single-spin resonant spectroscopy to study the spin structure in the orbital excited state of a diamond nitrogen-vacancy (N-V) center at room temperature. The data show that the excited-state spin levels have a zero-field splitting that is approximately half of the value of the ground state levels, a g factor similar to the ground state value, and a hyperfine splitting approximately 20x larger than in the ground state. In addition, the width of the resonances reflects the electronic lifetime in the excited state. We also show that the spin level splitting can significantly differ between N-V centers, likely due to the effects of local strain, which provides a pathway to control over the spin Hamiltonian and may be useful for quantum-information processing.     

Cavity-assisted quantum computing in a silicon nanostructure

We present a scheme of quantum computing with charge qubits corresponding to one excess electron shared between dangling-bond pairs of surface silicon atoms that couple to a microwave stripline resonator on a chip. By choosing a certain evolution time, we propose the realization of a set of universal single-and two-qubit logical gates. Due to its intrinsic stability and scalability, the silicon dangling-bond charge qubit can be regarded as one of the most promising candidates for quantum computation. Compared to the previous schemes on quantum computing with silicon bulk systems, our scheme shows such advantages as a long coherent time and direct control and readout.