利用SPDC和PBS实现受控非门的有效方案

任意一个N量子比特逻辑运算可以由一系列单量子比特门和受控非门实现[1].因此,这两种量子逻辑门的实现是研究量子计算自然的目标.虽然单量子比特门易于实现,但是由于光子间的相互作用比较弱,所以很难实现受控非门的操作.本文基于T.B.Pittman[2]与A.L. Migdall[3]等人的工作,提出了利用自发参量下转换(SPDC)过程采用多点延时探测触发的方法获得高效单光子源,提高实现受控非门效率的理论方案.     

Experimental demonstration of a nondestructive controlled-NOT quantum gate for two independent photon qubits

Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum information processing. However, photons, one of the best candidates for qubits, suffer from a lack of strong nonlinear coupling, which is required for quantum logic operations. Here we show how this drawback can be overcome by reporting a proof-of-principle experimental demonstration of a nondestructive controlled-NOT (CNOT) gate for two independent photons using only linear optical elements in conjunction with single-photon sources and conditional dynamics. Moreover, we exploit the CNOT gate to discriminate all four Bell states in a teleportation experiment.     

A novel frequency encoded all optical CNOT gate exploiting difference frequency generation and implementation of fast binary adders using frequency encoding and nonlinear dielectric films

Optics has an important role in logic implementation and computation is established in two and half decades by many researchers. Recently frequency encoding technique is established. This technique does not suffer from intensity dependent loss problems like other schemes. Amorphous dielectric thin films with reflecting edges can also be used for logic realization and has very fast response speed. It also does not use any semiconductor device and simple to construct. In this communication the authors have proposed all optical CNOT gate using frequency encoded difference frequency generation exploiting nonlinear response of some material and implementation of binary adders by CNOT gate and dielectric thin film AND gate.     

Hyper CNOT and Hyper Bell-State Analysis Assisted by Quantum Dots in Double-Side Optical Microcavities

There are many important works about the construction of universal quantum logic gates which are key elements in quantum computation. However, most of them focus on quantum transformations on the same degree of freedom (DOF) of quantum systems. We propose a CNOT gate performed on the polarization DOF and spatial mode DOF of one photon system assisted by a quantum dot in double-side optical microcavities. This hyper CNOT gate is implemented by using spin selective photon reflection from the cavity, without auxiliary spatial modes or polarization modes. This interface can also be used to construct a hyper photonic Bell-state analyzer. The high fidelities of the hyper CNOT gates may be achieved with low side leakage and cavity loss.     

Experimental teleportation of a quantum controlled-NOT gate

Teleportation of quantum gates is a critical step for the implementation of quantum networking and teleportation-based models of quantum computation. We report an experimental demonstration of teleportation of the prototypical quantum controlled-NOT (CNOT) gate. Assisted with linear optical manipulations, photon entanglement produced from parametric down-conversion, and postselection from the coincidence measurements, we teleport the quantum CNOT gate from acting on local qubits to acting on remote qubits. The quality of the quantum gate teleportation is characterized through the method of quantum process tomography, with an average fidelity of 0.84 demonstrated for the teleported gate.     

Modified Khaneja--Glaser Decomposition and Realization of Three-Qubit Quantum Gate

Optimal implementation of quantum gates is crucial for realization of quantum computation. We slightly modify the Khaneja-Glaser decomposition （KGD） for n-qubits and give a new Cartan subalgbra in the second step of the decomposition. Based on this modified KGD, we investigate the realization of three-qubit logic gate and obtain the result that a general three-qubit quantum logic gate can be implemented using at most 73 one-qubit gates rotations with respect to the y and z axes and 26 CNOT gates.     

Quantum Circuit Synthesis using a New Quantum Logic Gate Library of NCV Quantum Gates

Since Controlled-Square-Root-of-NOT (CV, CV^?) gates are not permutative quantum gates, many existing methods cannot effectively synthesize optimal 3-qubit circuits directly using the NOT, CNOT, Controlled-Square-Root-of-NOT quantum gate library (NCV), and the key of effective methods is the mapping of NCV gates to four-valued quantum gates. Firstly, we use NCV gates to create the new quantum logic gate library, which can be directly used to get the solutions with smaller quantum costs efficiently. Further, we present a novel generic method which quickly and directly constructs this new optimal quantum logic gate library using CNOT and Controlled-Square-Root-of-NOT gates. Finally, we present several encouraging experiments using these new permutative gates, and give a careful analysis of the method, which introduces a new idea to quantum circuit synthesis.     

The one-way CNOT simulation

In this paper, we present the complete simulation of the quantum logic CNOT gate in the one-way model, which consists entirely of one-qubit measurements on a particular class of entangled states.     

Remote interactions between two d-dimensional distributed quantum systems： nonlocal generalized quantum control-NOT gate and entanglement swapping

We present a systematic simple method to implement a generalized quantum control-NOT （CNOT） gate on two d-dimensional distributed systems. First, we show how the nonlocal generalized quantum CNOT gate can be implemented with unity fidelity and unity probability by using a maximally entangled pair of qudits as a quantum channel. We also put forward a scheme for probabilistically implementing the nonlocal operation with unity fidelity by employing a partially entangled qudit pair as a quantum channel. Analysis of the scheme indicates that the use of partially entangled quantum channel for implementing the nonlocal generalized quantum CNOT gate leads to the problem of ＇the general optimal information extraction＇. We also point out that the nonlocal generalized quantum CNOT gate can be used in the entanglement swapping between particles belonging to distant users in a communication network and distributed quantum computer.[第一段]     

Engineering CNOT gate in cavity QED scenario

We present a quantum CNOT logic gate based on interaction of a three-level cesium atom with a two-mode electromagnetic field in a high-Q superconducting cavity. The three-level atom acts as a control qubit and the two-mode electromagnetic field serves as a target qubit. Presently available QED experiments make it feasible to realize the theoretical suggestion in the laboratory. We determine the feasibility of our proposal by calculating the fidelity.     

利用二粒子部分纠缠态实现开靶目标的非局域量子可控非（CNOT）门的受控操作

提出利用二粒子部分纠缠态概率性地实现开靶目标的非局域量子可控非（CNOT）门的操控方案.首先考虑利用3个二粒子部分纠缠态实现3个靶目标共享的非局域量子CNOT门的受控操作,然后将该方案推广到N个靶目标共享的情形. 在该方案中,控制端Alice的局域正定算符值测量（POVM）起着关键作用,给出了该测量算符的数学表式.值得注意的是, 用二粒子部分纠缠态可确定性地实现非局域CNOT门. 关键词： 二粒子部分纠缠态 非局域可控非门 开靶目标 正定算符值测量     

Decoherence control in quantum computing with simple chirped pulses

We show how the use of optimally shaped pulses to guide the time evolution of a system (‘coherent control’) can be an effective approach towards quantum computation logic. We demonstrate this with selective control of decoherence for a multilevel system with a simple linearly chirped pulse. We use a multiphoton density-matrix approach to explore the effects of ultrafast shaped pulses for two-level systems that do not have a single photon resonance, and show that many multiphoton results are surprisingly similar to the single-photon results. Finally, we choose two specific chirped pulses: one that always generates inversion and the other that always generates self-induced transparency to demonstrate an ensemble CNOT gate.     

Entanglement concentration for a non-maximally entangled four-photon cluster state

We present a scheme for locally concentrating a non-maximally entangled four-photon cluster state into a maximally-entangled four-photon cluster state. This scheme has a high success probability. The controlled-NOT （CNOT） gate is a crucial ingredient in this scheme, and we use a nearly deterministic CNOT gate, which is similar with that first introduced by Nemoto et al. （Phgs. Rev. Lett., 2004, 93： 250502）. This CNOT gate has a simple structure and does not need the strong nonlinearity.     

CNOT gate by adiabatic passage with an optical cavity

We propose a scheme for the construction of a CNOT gate by adiabatic passage in an optical cavity. In opposition to a previously proposed method, the technique is not based on fractional adiabatic passage, which requires the control of the ratio of two pulse amplitudes. Moreover, the technique constitutes a decoherence-free method in the sense that spontaneous emission and cavity damping are avoided since the dynamics follows dark states.     

Molecular prototypes for spin-based CNOT and SWAP quantum gates

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden.     

Minimum construction of two-qubit quantum operations

Optimal construction of quantum operations is a fundamental problem in the realization of quantum computation. We here introduce a newly discovered quantum gate, B, that can implement any arbitrary two-qubit quantum operation with minimal number of both two- and single-qubit gates. We show this by giving an analytic circuit that implements a generic nonlocal two-qubit operation from just two applications of the B gate. Realization of the B gate is illustrated with an example of charge-coupled superconducting qubits for which the B gate is seen to be generated in shorter time than the CNOT gate.     

Realization of quantum gates by Lyapunov control

We propose a Lyapunov control design to achieve specific (or a family of) unitary time-evolution operators, i.e., quantum gates in the Schrödinger picture by tracking control. Two examples are presented. In the first, we illustrate how to realize the Hadamard gate in a single-qubit system, while in the second, the controlled-NOT (CNOT) gate is implemented in two-qubit systems with the Ising and Heisenberg interactions. Furthermore, we demonstrate that the control can drive the time-evolution operator into the local equivalence class of the CNOT gate and the operator keeps in this class forever with the existence of Ising coupling.     

Implementation of Controlled‐NOT Gate by Lyapunov Control

A scheme to implement the controlled‐NOT (CNOT) gate for quantum systems is proposed, which is based on Lyapunov control. The scheme does not require precise control of the interaction time since the system is stable when the control fields vanish. In particular, the control fields can be easily obtained by most initial states. As an example, the CNOT gate is realized for two atoms trapped in an optical cavity by exploiting two disturbance cases. Compared to continuous disturbance, the fidelity of the CNOT gate is higher under impulsive disturbance, however, interaction times are much longer. Numerical simulations indicate that the scheme is robust against variations of control parameters and decoherence caused by atomic spontaneous emission and cavity decay. Therefore, the scheme may provide useful applications in quantum computation.     

Universal set of quantum gates for double-dot spin qubits with fixed interdot coupling

We propose a set of universal gate operations for the singlet-triplet qubit realized by two-electron spins in a double quantum dot, in the presence of a fixed inhomogeneous magnetic field. All gate operations are achieved by switching the potential offset between the two dots with an electrical bias, and do not require time-dependent control of the tunnel coupling between the dots. We analyze the two-electron dynamics and calculate the effective qubit rotation angle as a function of the applied electric bias. We present explicit gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT gate sequence, completing the universal gate set.     

Conditional quantum CNOT gate between two four-level atoms

We propose a scheme to implement a quantum controlled-NOT (CNOT) gate between two four-level atoms inside the detuned optical cavity. The system state is evolved inside the decoherence-free (DF) subspace through stimulated Raman processes, which yields the desired unitary evolution operation for the CNOT.Our scheme is immune to decoherence due to dissipation of cavity excitation and spontaneous emission from the excited atomic level.