由各向异性海森伯自旋环链组成的量子位及其通用量子逻辑门

研究了各向异性耦合的三粒子海森伯自旋环链团簇在随时间变化的磁场中的运动.该系统的哈密顿量具有SU(2)代数结构.用代数动力学方法对此系统进行求解，得到了严格的解析解.基于严格解, 可以构造一位量子逻辑门.通过调节磁场强度和频率, 就可以控制该量子逻辑门, 实现一位量子逻辑门的任何操作. 关键词： 代数动力学 自旋环链团簇 一位量子逻辑门     

Anisotropic Spin Cluster as a Qubit

We study an anisotropic spin cluster of 3 spin S=1/2 particles with antiferromagnetic exchange interaction with non-uniform coupling constants. A time-dependent magnetic field is applied to control the time evolution of the cluster. It is well known that for an odd number of sites a spin cluster qubit can be defined in terms of the ground state doublet. The universal one-qubit logic gate can be constructed from the time evolution operator of the non-autonomous many-body system, and the six basic one-qubit gates can be realized by adjusting the applied time-dependent magnetic field.     

Quantum computing with spin cluster qubits

We study the low energy states of finite spin chains with isotropic (Heisenberg) and anisotropic (XY and Ising-like) antiferromagnetic exchange interaction with uniform and nonuniform coupling constants. We show that for an odd number of sites a spin cluster qubit can be defined in terms of the ground state doublet. This qubit is remarkably insensitive to the placement and coupling anisotropy of spins within the cluster. One- and two-qubit quantum gates can be generated by magnetic fields and intercluster exchange, and leakage during quantum gate operation is small. Spin cluster qubits inherit the long decoherence times and short gate operation times of single spins. Control of single spins is hence not necessary for the realization of universal quantum gates.     

Preparation of Entanglement of Atoms and of Photons via Cavity Input-Output Process

We propose a method to prepare multipartite entangled states such as cluster states and graph states based on the cavity input-output process and single photon measurement. Two quantum gates, a controlled phase gate and a fusion gate between two atoms trapped in respective cavities, are proposed to prepare atomic cluster states and graph states with one and two dimensions. We also introduce a scheme that can generate an arbitrary multipartite photon duster state which uses two coherent states as a qubit basis.     

用腔场中的二能级势阱离子实现量子逻辑门

利用光腔中的势阱粒子同时与外激光场和腔场发生相互作用的特性，我们提出了一种量子逻辑门的实现方案。在该方案中，我们采用文献[10-12]中的模型。文献[11-12]中实现的逻辑门是以离子内态和运动态作为量子比特，腔态充当辅助比特在计算过程中保持在基态。而[10]要求离子内态保持为基态，利用离子运动态和腔态构成量子比特。与文献[10-12]不同的是，我们实现的量子逻辑门是以粒子内态和腔态作为比特，而势阱离子的运动态作为辅助比特始终保持在基态。而且，我们对该方案的实验要求进行了讨论。     

Anisotropic Spin Cluster as a Qubit

We study an anisotropic spin cluster of 3 spin S=1/2 particles with antiferromagnetic exchange interaction with non-uniform coupling constants. A time-dependent magnetic field is applied to control the time evolution of the cluster. It is well known that for an odd number og sites a spin cluster qubit can be defined in terms of the ground state doublet. The universal one-qubit logic gate can be constructed from the time evolution operator of the non-autonomous many-body system, and the six basic one-qubit gates can be realized by adjusting the applied time-dependent magnetic field.     

Quantum computational gates with radiation free couplings

We examine a generic three level mechanism of quantum computation in which all fundamental single and double qubit quantum logic gates are operating under the effect of adiabatically controllable static (radiation free) bias couplings between the states. Under the time evolution imposed by these bias couplings the quantum state cycles between the two degenerate levels in the ground state and the quantum gates are realized by changing Hamiltonian at certain time intervals when the system collapses to a two state subspace. We propose a physical implementation of the mechanism using Aharonov-Bohm persistent-current loops in crossed electric and magnetic fields, with the output of the loop read out by using a quantum Hall effect aided mechanism. Received 26 March 2002 / Received in final form 8 July 2002 Published online 19 November 2002     

Physical Realization of Quantum C-Not Gate

We investigate a Heisenberg spin cluster with two particles controlled by a time-dependent magnetic field. The system is controlled by tuning the amplitude, frequency, and interaction time of the three-step time-dependent magnetic field. Then we solve the time-dependent Schrodinger equation of the two-particle system, and obtain the time evolution operator. By the three-timestep interaction, the wavefunction evolves from the initial state to the final state, and the total evolution operator can be expressed as a product of the three evolution operators. By adjusting the physical parameters, the key two-qubit logic gate, the C-Not gate, can be realized physically.     

Quantum logic networks for probabilistic teleportation

By eans of the primitive operations consisting of single-qubit gates.two-qubit controlled-not gates,Von Neuman measurement and classically controlled operations.,we construct efficient quantum logic networks for implementing probabilistic teleportation of a single qubit,a two-particle entangled state,and an N-particle entanglement.Based on the quantum networks,we show that after the partially entangled states are concentrated into maximal entanglement,the above three kinds of probabilistic teleportation are the same as the standard teleportation using the corresponding maximally entangled states as the quantum channels.     

Ballistic Aharonov–Bohm quantum bits and quantum gates

We propose an architecture to perform quantum computation, using ballistic electrons as qubits and coupled quantum rings as quantum gates. In the proposed architecture two adjacent one-dimensional wires, creating a single qubit, are connected to two coupled quantum rings, where the required magnetic flux is provided by enclosed nano-sized magnets. The phase modulation of the wave function of the ballistic electrons under the Aharonov–Bohm effect is carefully designed to facilitate reprogrammable and dynamically controllable quantum gates. Arbitrary single-qubit quantum gates with high fidelity can be constructed on the basis of this architecture.     

Robust Quantum Computing in Decoherence-Free Subspaces with Double-Dot Spin Qubits

We present a scheme for implementing robust quantum gates in decoherence-free subspaces （DFSs） with double-dot spin qubits. Through the resonator-assisted interaction, the controllable interqubit couplings can be achieved only by adjusting the qubit transition frequencies. We construct a set of logic gates on the DFS-encoded qubits to eliminate the collective noise effects, and thus the gate fidelities can be enhanced remarkably. This proposal may offer a potential approach to realize the robust quantum computing with spin qubits.     

Quantum information,oscillations and the psyche

In this paper, taking the theory of quantum information as a model, we consider the human unconscious, pre-consciousness and consciousness as sets of quantum bits (qubits). We view how there can be communication between these various qubit sets. In doing this we are inspired by the theory of nuclear magnetic resonance. In this way we build a model of handling a mental qubit with the help of pulses of a mental field. Starting with an elementary interaction between two qubits we build two-qubit quantum logic gates that allow information to be transferred from one qubit to the other. In this manner we build a quantum process that permits consciousness to “read” the unconscious and vice versa. The elementary interaction, e.g. between a pre-consciousness qubit and a consciousness one, allows us to predict the time evolution of the pre-consciousness + consciousness system in which pre-consciousness and consciousness are quantum entangled. This time evolution exhibits Rabi oscillations that we name mental Rabi oscillations. This time evolution shows how for example the unconscious can influence consciousness. In a process like mourning the influence of the unconscious on consciousness, as the influence of consciousness on the unconscious, are in agreement with what is observed in psychiatry.     

Computing with spins: from classical to quantum computing

This article reviews the use of single electron spins to compute. In classical computing schemes, a binary bit is represented by the bistable spin polarization of a single electron confined in a quantum dot and subjected to a weak magnetic field. The spin orientation can be either parallel or anti-parallel to the field, so that it becomes a binary variable which can encode logic 0 and logic 1. Coherent superposition of these two polarizations can represent a qubit for quantum computing. By engineering the exchange interaction between closely spaced spins in neighboring quantum dots, it is possible to implement either classical or quantum logic gates.     

Logic Bell state concentration with parity check measurement

Logic qubit plays an important role in current quantum communication. In this paper, we propose an efficient entanglement concentration protocol (ECP) for a new kind of logic Bell state, where the logic qubit is the concatenated Greenber–Horne–Zeilinger (C-GHZ) state. Our ECP relies on the nondemolition polarization parity check (PPC) gates constructed with cross-Kerr nonlinearity, and can distill one pair of maximally entangled logic Bell state from two same pairs of less-entangled logic Bell states. Benefit from the nondemolition PPC gates, the concentrated maximally entangled logic Bell state can be remained for further application. Moreover, our ECP can be repeated to further concentrate the less-entangled logic Bell state. By repeating the ECP, the total success probability can be effectively increased. Based on above features, this ECP may be useful in future long-distance quantum communication.     

Optimal processing of reversible quantum channels

We consider the general problem of the optimal transformation of N uses of (possibly different) unitary channels to a single use of another unitary channel in any finite dimension. We show how the optimal transformation can be fully parallelized, consisting in a preprocessing channel followed by a parallel action of all the N unitaries and a final postprocessing channel. Our techniques allow to achieve an exponential reduction in the number of the free parameters of the optimization problem making it amenable to an efficient numerical treatment. Finally, we apply our general results to find the analytical solution for special cases of interest like the cloning of qubit phase gates.     

Bidirectional Controlled Teleportation via Six-Qubit Cluster State

We present a scheme for bidirectional controlled teleportation by using a six-qubit cluster state as quantum channel. Based on the C-not operation and single qubit measurements, Alice may transmit an arbitrary single qubit state of qubit A to Bob and Bob may transmit an arbitrary single qubit state of qubit B to Alice via the control of the supervisor Charlie.     

用激光-二能级原子系统实现一位通用量子逻辑门

使用代数动力学方法，对激光-二能级原子系统的含时薛定谔方程进行求解.该系统哈密顿量具有SU(2)代数结构，得到了严格解.基于严格解，适当调节单模激光场的频率和振幅以及短脉冲激光与二能级原子的二阶虚光子作用强度，可实现一位通用量子逻辑门. 关键词： 代数动力学 激光-二能级原子系统 一位量子逻辑门     

Quantum computing with particle in a driven square well

We show that a particle confined in a one-dimensional square well in the presence of an external periodic linear modulation undergoes coherent oscillations between the discrete levels analogous to an atom exposed to an electromagnetic field. We define the selection rules for state transitions based on the matrix elements of the perturbation potential. These selection rules contribute to the time evolution of the qubit which can partake efficiently in quantum informatics. Quantum logic gates are realized in the square-well potential by controlling the period of modulation.     

两粒子自旋系统量子纠缠的时间演化

研究了两个具有海森伯耦合的自旋为1/2的粒子在随时间变化的磁场中的运动情况.系统的哈密顿量具有SU（2）代数结构,运用代数动力学方法对此系统进行求解,得到了时间演化算子的严格解.基于严格解,求得两粒子体系随时间变化的波函数,从而计算得到两粒子体系的纠缠.对不同初始波函数,研究了系统纠缠随时间的变化情况.讨论了外场影响纠缠的条件. 关键词： 二粒子系统纠缠 代数动力学解法