Process tomography of ion trap quantum gates

A crucial building block for quantum information processing with trapped ions is a controlled-NOT quantum gate. In this Letter, two different sequences of laser pulses implementing such a gate operation are analyzed using quantum process tomography. Fidelities of up to 92.6(6)% are achieved for single-gate operations and up to 83.4(8)% for two concatenated gate operations. By process tomography we assess the performance of the gates for different experimental realizations and demonstrate the advantage of amplitude-shaped laser pulses over simple square pulses. We also investigate whether the performance of concatenated gates can be inferred from the analysis of the single gates.     

Pulse shaping and its characterization of mid-infrared femtosecond pulses: Toward coherent control of molecules in the ground electronic states

We have examined a technique of complex shaping of mid-infrared femtosecond laser pulses towards accurate and precise control of rovibrational wave packets of molecules in the ground electronic state. A Germanium acousto-optics modulator was used as a device for the pulse shaping. In order to characterize the shaped pulses precisely, sum-frequency cross-correlation frequency-resolved optical gating was introduced for the analysis of both amplitude and phase of the electric fields. The mid-infrared pulses were shaped and characterized with a frequency resolution better than 4.5 cm⁻¹. Such a resolution is supposed to be sufficient for the realization of quantum gate operations with high fidelity, which is one of the most challenging applications of rovibrational wave packet manipulation of molecules. In order to demonstrate the precision of our method of shaping and its characterization, we have generated shaped pulses that will realize Hadamard and NOT quantum gates with rovibrational states of a CO molecule.     

High-fidelity local addressing of trapped ions and atoms by composite sequences of laser pulses

A vital requirement for a quantum computer is the ability to locally address, with high fidelity, any of its qubits without affecting their neighbors. We propose an addressing method using composite sequences of laser pulses that dramatically reduces the addressing error in a lattice of closely spaced atoms or ions and at the same time significantly enhances the robustness of qubit manipulations. To this end, we design novel (to our knowledge) high-fidelity composite pulses for the most important single-qubit operations. In principle, this method allows one to beat the diffraction limit, for only atoms situated in a small spatial region around the center of the laser beam are excited, well within the laser beam waist.     

One‐Step Implementation of N‐Qubit Nonadiabatic Holonomic Quantum Gates with Superconducting Qubits via Inverse Hamiltonian Engineering

A protocol is proposed to realize one‐step implementation of the N‐qubit nonadiabatic holonomic quantum gates with superconducting qubits. The inverse Hamiltonian engineering is applied in designing microwave pulses to drive superconducting qubits. By combining curve fitting, the wave shapes of the designed pulses can be described by simple functions, which are not hard to realize in experiments. To demonstrate the effectiveness of the protocol, a three‐qubit holonomic controlled π‐phase gate is taken as an example in numerical simulations. The results show that the protocol holds robustness against noise and decoherence. Therefore, the protocol may provide an alternative approach for implementing N‐qubit nonadiabatic holonomic quantum gates.     

Remote Implementation of a Fredkin Gate via Virtual Excitation of an Atom-Cavity-Fiber System

Simple operations and robust results are always of interest for any quantum tasks. Herein, a novel scheme is proposed for implementing a Fredkin gate via the virtual excitation of an atom-cavity-fiber system. The scheme is to control the nonlocal state-swap of two spatially separated target atoms according to the state of the control atom at hand. In the scheme, only the control atom at hand needs the laser to drive and the virtual excitation of the atom-cavity-fiber system effectively suppresses the decoherence. By numerical simulations, appreciated parameters are chosen and it is shown that the Fredkin gate can be implemented with high fidelity. Although the operation time error has slightly stronger influence on the fidelity than atom-cavity coupling strength error, the robustness of the scheme can be effectively improved against the operation time error by adopting Gaussian pulse to replace the constant pulse. In addition, the scheme can be generalized to implement alternative Fredkin gates by controlling the non-local state-swap of two remote atoms or of two remote and spatially separated atoms, which will be undoubtedly of benefit to the distributed quantum computation and remote quantum information processing.     

Bidirectional transfer of quantum information for unknown photons via cross-Kerr nonlinearity and photon-number-resolving measurement

We propose an arbitrary controlled-unitary(CU) gate and a bidirectional transfer scheme of quantum information(BTQI) for unknown photons.The proposed CU gate utilizes quantum non-demolition photon-number-resolving measurement based on the weak cross-Kerr nonlinearities(XKNLs) and two quantum bus beams;the proposed CU gate consists of consecutive operations of a controlled-path gate and a gathering-path gate.It is almost deterministic and is feasible with current technology when a strong amplitude of the coherent state and weak XKNLs are employed.Compared with the existing optical multi-qubit or controlled gates,which utilize XKNLs and homodyne detectors,the proposed CU gate can increase experimental realization feasibility and enhance robustness against decoherence.According to the CU gate,we present a BTQI scheme in which the two unknown states of photons between two parties(Alice and Bob) are mutually swapped by transferring only a single photon.Consequently,by using the proposed CU gate,it is possible to experimentally implement the BTQI scheme with a certain probability of success.     

Characteristics of all-optical 3R regenerators using cascaded second-order nonlinear effect in quasi-phase matched lithium niobate devices

We numerically show that quasi-phase matched (QPM) lithium niobate (LN) devices employing the cascaded second-order nonlinear effect of second harmonic generation (SHG) and difference frequency mixing (DFM) have all-optical decision gate characteristics. The decision gate function is realized by a parabolic transmittance for a low-power region and a limiting characteristic for a high-power region. The limiter function is attributed to the large group-velocity mismatch between the fundamental and second harmonic pulses. This operation principle differs from those of other all-optical 2R (reamplification and reshaping) or 3R (2R and retiming) regenerators that have been proposed in the past. Furthermore, we show that an initial time offset between the signal and clock pulses can improve the output signal power or the switching efficiency of the device. Based on the numerical results, we propose a method for designing all-optical 3R regenerators using the cascade of SHG and DFM in the QPM-LN devices. Following the design method, all-optical 3R operation at the bit rate of 200 Gbps can be achieved using a 1-cm-long waveguide device.     

Detecting vacuum entanglement in a linear ion trap

We propose and study a method for detecting ground-state entanglement in a chain of trapped ions. We show that the entanglement between single ions or groups of ions can be swapped to the internal levels of two ions by sending laser pulses that couple the internal and motional degrees of freedom. This allows us to entangle two ions without actually performing gate operations. A proof of principle of the effect can be realized with two trapped ions and is feasible with current technology.     

Quantum controlled phase gate and cluster states generation via two superconducting quantum interference devices in a cavity

A scheme for implementing 2-qubit quantum controlled phase gate (QCPG) is proposed with two superconducting quantum interference devices (SQUIDs) in a cavity. The gate operations are realized within the two lower flux states of the SQUIDs by using a quantized cavity field and classical microwave pulses. Our scheme is achieved without any type of measurement, does not use the cavity mode as the data bus and only requires a very short resonant interaction of the SQUID-cavity system. As an application of the QCPG operation, we also propose a scheme for generating the cluster states of many SQUIDs.     

Implementing logic gates and the Deutsch-Jozsa quantum algorithm by two-dimensional NMR using spin- and transition-selective pulses

Quantum logical operations using two-dimensional NMR have recently been described using the scalar coupling evolution technique [J. Chem. Phys. 109, 10603 (1998)]. In the present paper, we describe the implementation of quantum logical operations using two-dimensional NMR, with the help of spin- and transition-selective pulses. A number of logic gates are implemented using two and three qubits with one extra observer spin. Some many-in-one gates (or Portmanteau gates) are also implemented. Toffoli gate (or AND/NAND gate) and OR/NOR gates are implemented on three qubits. The Deutsch-Jozsa quantum algorithm for one and two qubits, using one extra work qubit, has also been implemented using spin- and transition-selective pulses after creating a coherent superposition state in the two-dimensional methodology.     

Fast and Robust Quantum Information Transfer in Annular and Radial Superconducting Networks

In this paper, we propose a protocol to achieve fast and robustness quantum information transfer (QIT) in annular and radial superconducting networks, where each quantum node is composed of a superconducting quantum interference device (SQUID) inside a coplanar waveguide resonator (CPWR). The process is based on reversely constructing time‐dependent control Hamiltonian by designing evolution operator. With the protocol, the maximal population of lossy intermediate states and the amplitudes of pulses can be easily controlled by two corresponding control parameters. Therefore, one can design feasible pulses for QIT with great flexibility. Besides, the speed of the QIT here is much faster compared with that with adiabatic QIT. Moreover, numerical simulations show that the protocol still possesses high fidelity when lossy factors and imperfect operations are taken into account. Therefore, the protocol may provide a useful way to manipulate quantum information networks.     

Quantum State Transfer Between Any Pair of Qubits in a Quantum Network via Optical Fibers

We propose scheme for transferring quantum state between any pair of nodes in a quantum network. Each node consists of an atom and a cavity, with the atom acting as the quantum bit. Any two adjacent nodes are connected by an optical fiber. During the operation neither the atomic system nor the fibers are excited, which is important in view of decoherence. Under certain conditions, the probability that the cavities are excited is negligible. The method has an inherent robustness against the fluctuation perturbations in the classical control parameters and the randomness in the atomic position. The scheme can be generalized to implement quantum phase gate between any two remote qubits.     

Physical implementation of three-qubit gates on a separate quantum particle

A virtual spin formalism is suggested to demonstrate that a single quantum particle possessing eight suitable discrete energy levels can be used for storing three information qubits and organizing on them a universal set of logical operations that are necessary for constructing an arbitrary quantum algorithm. The formalism can be practically implemented on a nuclear spin 7/2 subjected to resonance rf pulses. A single-pulse realization is found for all quantum gates of a universal set, including a three-qubit gate.     

Time-varying coupling-induced logical stochastic resonance in a periodically driven coupled bistable system

Coupling-induced logical stochastic resonance(LSR) can be observed in a noise-driven coupled bistable system where the behaviors of system can be interpreted consistently as a specific logic gate in an appropriate noise level. Here constant coupling is extended to time-varying coupling, and then we investigate the effect of time-varying coupling on LSR in a periodically driven coupled bistable system. When coupling intensity oscillates periodically with the same frequency with periodic force or relatively high frequency, the system successfully yields the desired logic output. When coupling intensity oscillates irregularly with phase disturbance, large phase disturbance reduces the area of optimal parameter region of coupling intensity and response speed of logic devices. Although the system behaves as a desired logic gate when the frequency of time-periodic coupling intensity is precisely equal to that of periodic force, the desired logic gate is not robust against tiny frequency difference and phase disturbance. Therefore, periodic coupling intensity with high frequency ratio is an optimal option to obtain a reliable and robust logic operation.     

High-Fidelity Geometric Gates with Single Ions Doped in Crystals

Single rare-earth ions doped in solids are one kind of the promising candidates for quantum nodes towards a scalable quantum network.Realizing a universal set of high-fidelity gate operations is a central requirement for functional quantum nodes.Here we propose geometric gate operations using the hybridized states of electron spin and nuclear spin of an ion embedded in a crystal.The fidelities of these geometric gates achieve 0.98 in the realistic experimental situations.We also show the robustness of geometric gates to pulse fluctuations and to environment decoherence.These results provide insights for geometric phases in dissipative systems and show a potential application of high fidelity manipulations for future quantum internet nodes.     

POSITION SENSING BY CHARGE DIVISION METHOD IN SELF-QUENCHING STREAMER TUBE

The experimental results of charge division method with the self-quenching streamer pulses are reported. It is shown that the position resolution at the middle point of 2.5 m long tube can reach 3.3 mm even the signals are sent to ADC module by 60 m long cable. It is much better than using the proportional pulses, and the non-linearity is less than 0.14% of 2.5 m full scale. Some other aspects, such as ADC gate width and selecting of the decoupling capacitor etc. are also discussed, they would provide some practical basis for the design of the sample and hold system in the gas sampling shower counter of the Beijing e⁺e^－ collider spectrometer.     

All-optical Serial Data Transfer between Registers using optical non-linear materials

The inherent parallelism of optical signal is an advantageous feature for high-speed computations and other digital logic operations. Different techniques have been proposed for performing arithmetic, algebraic and logic operations using light as the information-carrier. Here we propose a new method for Serial Data Transfer between Registers using optical non-linear material. This system is all-optical in nature. Optical NAND gate and NOT gate are the basic building blocks of this system.     

基于高速肖特基二极管的100 ps瞬态取样门设计与仿真

 皮秒级瞬态取样门主要应用于激光聚变实验和高能物理实验中，对单次高速脉冲进行实时取样。提出了一种新颖的基于肖特基二极管桥的平衡取样门，给出其模型和具体电路设计。电路仿真结果表明，对称的选通设计保证了选通脉宽为100 ps时，取样间隔也为100 ps，取样门带宽为4.4 GHz，可应用于多路超短激光脉冲取样。     

Three‐Photon Polarization‐Spatial Hyperparallel Quantum Fredkin Gate Assisted by Diamond Nitrogen Vacancy Center in Optical Cavity

The construction of a near‐deterministic photonic hyperparallel quantum Fredkin (hyper‐Fredkin) gate is investigated for a three‐photon system with the optical property of a diamond nitrogen vacancy center embedded in an optical cavity (cavity‐NV center system). This hyper‐Fredkin gate can be used to perform double Fredkin gate operations on both the polarization and spatial‐mode degrees of freedom (DOFs) of a three‐photon system with a near‐unit success probability, compared with those on the double three‐photon systems in one DOF. In this proposal, the hybrid quantum logic gate operations are the key elements of the hyper‐Fredkin gate, and only two cavity‐NV center systems are required. Moreover, the possibility of constructing a high‐fidelity and high‐efficiency hyper‐Fredkin gate in the experimental environment of a cavity‐NV center system is discussed, which may be used to implement high‐fidelity photonic computational tasks in two DOFs with a high efficiency.     

A Scheme for Atomic Entangled States and Quantum Gate Operations in Cavity QED

We propose a scheme for controllably entangling the ground states of five-state W-type atoms confined in a cavity and realizing swap gate and phase gate operations. In this scheme the cavity is only virtually excited and the atomic excited states are almost not occupied, so the produced entangled states and quantum logic operations are very robust against the cavity decay and atomic spontaneous emission.