Simultaneous implementation of n SWAP gates using superconducting charge qubits coupled to a cavity

Based on superconducting quantum interference devices (SQUIDs) coupled to a cavity, we propose a scheme for implementing n SWAP gates simultaneously. In our scheme, the SQUID works in the charge regime, the quantum logic gate operations are performed in the subspace spanned by two charge states |0〉 and |1〉. The interaction between the qubits and the cavity field can be achieved by turning the gate voltage and the external flux. Especially, the gate operation time is independent of the number of the qubits, and the gate operation is insensitive to the initial state of the cavity mode. We also analyze the experimental feasibility that the conditions of the large detuning can be achieved by adjusting the frequency of the cavity mode, and the operation time satisfies the requirement for the designed experiment by choosing suitable detuning and the quality factor of the cavity. Based on the simple operation, our scheme may be realized in this solid-state system, and our idea may be realized in other systems.     

Deterministic geometric quantum phase gates for two atoms in decoherence-free subspace

We propose a scheme for realizing conventional geometric quantum phase gates in the context of cavity QED. During the operation neither the atomic system nor the cavity mode is excited, which is important in view of decoherence. The scheme does not require detection of photons, so the gate operation is deterministic and the influence of photodetection imperfection is eliminated. Taking advantage of the geometric manipulation, the phase gate is resilient to fluctuations of experimental parameters.     

Comparing the weak and strong gate‐coupling regimes for nanotube and graphene transistors

We present an experimental and theoretical comparison of the weak and strong gate‐coupling regimes that arise for carbon nanotube (CNT) and graphene field‐effect transistors (FETs) in back‐gated and liquid‐gated configuration, respectively. We find that whereas the back‐gate efficiency is suppressed for a liquid‐gated CNT FET, the back gate is still effective in case of a liquid‐gated graphene FET. We calculate the gate‐induced Fermi‐level shifts and induced charge densities. In both strong and weak coupling regimes, nonlinearities occur in the gate dependence of these parameters, which can significantly influence the electronic transport. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantum phase gate in decoherence-free subspace with cavity QED system

We demonstrate how to perform quantum phase gate with cavity QED system in decoherence-free subspace by using only linear optics elements and photon detectors. The qubits are encoded in the singlet state of the atoms in cavities among spatially separated nodes, and the quantum interference of polarized photons decayed from the optical cavities is used to realized the desired quantum operation among distant nodes. In comparison with previous schemes, the distinct advantage is that the gate fidelity could not only resist collective noises, but also immune from atomic spontaneous emission, cavity decay, and imperfection of the photodetectors. We also discuss the experimental feasibility of our scheme.     

A simplified post-soft-breakdown current model for MOS devices

Based on the tunneling current model, a simplified current model is developed for MOS devices after soft breakdown (SBD). The post-soft-breakdown current consists of modified direct tunneling current and Fowler Nordheim (FN) tunneling current. Considering the change of gate oxide after soft breakdown, impacts of soft breakdown on the dielectric constant, and effective electron mass of the gate oxide are discussed, and their values are obtained by fitting simulation results to experimental data. It is found that the effective electron mass is decreased after soft breakdown due to damaged oxide, while the dielectric constant is increased after soft breakdown due to interface distortion. In this way, the leakage current after soft breakdown can be accurately calculated. The validity of the proposed model is confirmed by experimental results. Z.L. Li currently is with the Department of Electrical and Electronic Engineering, University of Hong Kong.     

Fast Rydberg gates without dipole blockade via quantum control

We propose a scheme for controlling interactions between Rydberg-excited neutral atoms in order to perform a fast high-fidelity quantum gate. Unlike dipole-blockade mechanisms already found in the literature, we drive resonantly the atoms with a state-dependent excitation to Rydberg levels, and we exploit the resulting dipole-dipole interaction to induce a controlled atomic motion in the trap, in a similar way as discussed in recent ion-trap quantum computing proposals. This leads atoms to gain the required gate phase, which turns out to be a combination of a dynamic and a geometrical contribution. The fidelity of this scheme is studied including small anharmonicity and temperature effects, with promising results for reasonably achievable experimental parameters.     

Realization of Three-Qubit Controlled-Phase Gate Operation with Atoms in Cavity QED System

We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.     

Toffoli gate made from a single resonant interaction with a trapped ion system

We propose a simple but practical scheme to implement a three-qubit Toffoli gate by a single resonant interaction in a trapped ion system. The scheme does not require two-qubit controlled-NOT gates but uses a three-qubit phase gate and two Hadamard gates, where the phase gate can be implemented by only a single resonant interaction of the trapped ions with the first lower vibrational sideband mode. Both the situations, with and without spontaneous ionic emission, are investigated. Discussions are made for the advantages and the experimental feasibility of our scheme.     

Gain-switched semiconductor laser amplifier as an ultrafast dynamical optical gate

A gain-switched semiconductor laser is shown to act as an optical gate with picosecond resolution and amplification for light pulses from another laser source. The amplification mechanism and the gate width change qualitatively when the gate laser undergoes a transition from a pumping rate slightly below the dynamic laser threshold to slightly above the dynamic threshold. If the gate laser is pumped below but close to its dynamical threshold, unsaturated amplification of an external signal pulse occurs over a delay time range between the external optical pulse and the electrical driving pulse of about 100–200 ps which is equivalent to the optical gate width. The signal amplification is observed to increase by two orders of magnitude and the gate width decreases by one order of magnitude if the gate laser is pumped slightly above the dynamical threshold. Amplification then occurs for input signals injected much earlier. A detailed theory of coherent, time-dependent amplification including the nonlinear dynamics of the semiconductor laser is shown to account for the observations. Both amplification regimes, below and above threshold, are reproduced in the numerical simulations. The extremely short and highly sensitive gate range above threshold is identified as being due to the gain maximum related with the first relaxation oscillation of the laser.     

Modelling of Hot-Electron Energy in Short-Channel MOSFETs by Electrical Method

Channel hot-electron （HE） energy in short-channel metal-oxide-semiconductor field-effect transistors （MOS-FETs） is estimated based on electrical characterization. The HE assisted gate leakage is monitored, and its energy dependent tunnelling probability is calculated, from which the excess energy of HE is estimated. The credibility of the proposed method is supported by the experimental and theoretical results, and its accuracy in ultra-small-feature-size device application is also discussed.     

Implementation of a Quantum Conditional Phase Gate for the Quantum Fourier Transform in Circuit QED

It is well known that multiple superconducting charge qubits coupled to a transmission line resonator can be controlled to achieve quantum logic gates between two arbitrary qubits. We propose a scheme to realize a quantum conditional phase gate with a geometric property by circuit electrodynamics, and it is applied naturally to reaJize the quantum Fourier transform with high fidelity. It is also demonstrated that the application is feasible and considerable under the present experimental technology.     

Scalable quantum computing in decoherence-free subspaces with Cooper-pair box qubits

We theoretically propose a feasible scheme to perform quantum computing in decoherence-free subspaces (DFSs) with Cooper-pair box (CPB) qubits arrayed in a circuit QED architecture. Based on the cavity-bus assisted interaction, the selective and controllable interqubit couplings occur only by adjusting the individual gate pulses, by which we obtain the scalable DFS-encoded universal quantum gates to resist certain collective noises. Further analysis shows the protocol may implement the scalable fault-tolerant quantum computing with current experimental means.     

Electron transport in a double quantum ring: Evidence of an AND gate

We explore AND gate response in a double quantum ring where each ring is threaded by a magnetic flux ?. The double quantum ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, namely, Va and Vb, are applied, respectively, in the lower arms of the two rings which are treated as two inputs of the AND gate. The system is described in the tight-binding framework and the calculations are done using the Green's function formalism. Here we numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strengths, magnetic flux and gate voltages. Our study suggests that, for a typical value of the magnetic flux ?=?₀/2 (?₀=ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears only if both the two inputs to the gate are high (1), while if neither or only one input to the gate is high (1), a low output current (0) results. It clearly demonstrates the AND gate behavior and this aspect may be utilized in designing an electronic logic gate.     

Majority carrier transistor based on voltage-controlled thermionic emission

A new type of transistor is proposed based on gate-controlled charge injection in unipolar semiconductor structures. Its design has some similarity with the recently fabricated triangular barrier diodes but contains an additional input circuit which allows an independent control of the barrier height for thermionic emission. This circuit is provided by a MOS gate on the semiconductor surface. In the proposed device the current flows perpendicular to the semiconductor surface over a planar potential barrier controlled by the gate. The static transconductance characteristics and dynamical response are analyzed. The characteristic response time is limited by the time of flight of electrons across the structure and can be in the picosecond range. The gate voltage required to switch the output current at room temperature is of order 0.2 V.     

Appropriate Gate Time for Single Molecular Photon Detection Based on Optimal Signal-to-Noise Ratio Analyses

Signal-to-noise ratio of fluorescence detection from a single molecule ＆ analysed by using time-gated techniques. It is found that the optimal signal-to-noise ratio can be obtained by choosing an appropriate gate time with a certain optical background. The dependences of molecular fluorescence lifetime and the optimal signal-to-noise ratio on the appropriate gate time are respectively discussed with two kinds of background sources~ chaotic state with uniform distribution and coherent state with exponential distribution in time domain. For chaotic state background we find that a certain range for appropriate gate time can be obtained with a definite fluorescence lifetime, larger fluorescence lifetime would lower the value of optimal signal-to-noise ratios. For coherent state background we find that there is also a narrow range of appropriate gate time when lifetime of single molecule is less than that of background photons.     

Implementation of a Controlled-NOT Gate Using Superconducting Quantum Interference Devices

A scheme is proposed for implementing a controlled-NOT gate via superconducting quantum interference device （SQUID） in cavity-QED. The controlled-NOT gate can be achieved by coupling the SQUID to a single-mode microwave cavity field or classical microwave pluses. The scheme may be experimentally realizable.     

Efficient implementation of a nonlocal gate with nonmaximal entanglement

We present two schemes for efficient implementation of a nonlocal gate with nonmaximal entanglement. The main strategy of the schemes is local conversion of pure states, which consists of a generalized measurement described by a positive operator-valued measure (POVM), one-way classical communication, and corresponding unitary operations. First, we discuss the way to generate determinately the nonlocal gate via any pure shared entangled state combined with entanglement-assistance. Then we propose the other way to generate probabilistically the nonlocal gate via any pure entangled state with the aid of ancillary particles.     

Experimental demonstration on 40 Gbit/s all-optical multicasting logic XOR gate for NRZ-DPSK signals using four-wave mixing in highly nonlinear fiber

We propose and demonstrate all-optical multicasting logic XOR gate for non-return-to-zero differential phase-shift keying (NRZ-DPSK) signals by using non-degenerate four-wave mixing (FWM) in a highly nonlinear fiber (HNLF). Theoretical analysis regarding the operation principle of NRZ-DPSK logic XOR gate is clearly described by deriving an analytical solution under the non-depletion approximation. The NRZ-DPSK logic XOR operation is attributed to the linear relationship of complex amplitudes between converted idlers and input NRZ-DPSK signals. By using three non-degenerate FWM processes in an HNLF, 40 Gbit/s all-optical multicasting logic XOR gate for NRZ-DPSK signals are successfully demonstrated in the experiment.     

One-Step Implementation of Mulitqubit Quantum Phase Gate in a Cavity QED System

An alternative protocol is proposed to implement three-qubit phase gate between photon and atoms in a high-Q bimodel optical cavity. The idea can be extended to directly implement N-qubit phase gate, and the gating time that is required to implement the protocol does not rise with increasing number of qubits. The influence of cavity decay and atomic spontaneous emission on the gate fidelity is also discussed.     

Ultra fast third-order non-linear response of amino-triazole donor-acceptor derivatives by optical Kerr effect

We report the study of the temporal dependence of the non-linear optical response of novel organic materials in solution. The experimental results of the optical Kerr gate using 70 fs pulses show a quasi-instantaneous response for three derivatives of an amino-triazole donor-acceptor system. The non-linearity of the compounds is identified as arising from the electronic contribution to the third-order non-linear susceptibility. The non-linear parameters of each sample were obtained using the optical Kerr response of CS₂ as reference.