Perfect quantum information processing with Dicke-class state

We investigate the possibility of implementing perfectquantum information processing with Dicke-class state. It is shownthat the symmetric Dicke state |D_N ^(m)〉 only has themaximal bipartite entanglement of one ebit when N = 2m andgenerally it is not maximally entangled for all bipartitions. Byadjusting the suitable weights and relative phases in the Dickestate |D_N ^(m)〉, we present a class of asymmetric Dickestates \(|\overline D_N^{(m)}\rangle\) which have the maximalbipartite entanglement of q (1 ≤ q ≤ m) ebits. We also obtainthe sufficient and necessary condition that the Dicke-class states\(|\overline D_{N}^{(m)}\rangle\) have the maximal bipartiteentanglement. We illustrate our idea using the four-qubit Dickestate with two excitations. It is shown that our proposedDicke-class states have distinct advantages over the symmetric Dickestate in perfect quantum information processing.     

发展固态量子信息与计算的实验研究

量子计算与量子信息是21世纪基础和应用科学研究的一大挑战.要实现实用意义上的量子信息和量子计算,必须解决量子比特系统的可拓展性问题.基于现代半导体技术的固态量子系统,其应用和最终产业化的可行性较高.然而,固态量子体系受周边环境的影响比较严重,控制其退相干,维持其量子状态的难度更高.实验固态量子计算的研究是个新的领域,尚无实用的技术和方法. 文章介绍了中国科学院物理研究所固态量子信息和计算实验室近几年来新开辟的自旋、冷原子、量子点(包括原子空位)、功能氧化物和关联体系等固态量子信息的新载体和同量子计算与量子信息相关的科学与技术难题的实验研究.     

发展固态量子信息与计算的实验研究

量子计算与量子信息是21世纪基础和应用科学研究的一大挑战. 要实现实用意义上的量子信息和量子计算,必须解决量子比特系统的可拓展性问题.基于现代半导体技术的固态量子系统,其应用和最终产业化的可行性较高. 然而,固态量子体系受周边环境的影响比较严重,控制其退相干,维持其量子状态的难度更高.实验固态量子计算的研究是个新的领域,尚无实用的技术和方法. 文章介绍了中国科学院物理研究所固态量子信息和计算实验室近几年来新开辟的自旋、 冷原子、 量子点(包括原子空位)、功能氧化物和关联体系等固态量子信息的新载体和同量子计算与量子信息相关的科学与技术难题的实验研究.     

Photonic quantum information processing

Information processing with light is ubiquitous, from communication, metrology and imaging to computing. When we consider light as a quantum mechanical object, new ways of information processing become possible. In this review I give an overview of how quantum information processing can be implemented with single photons, and what hurdles still need to be overcome to implement the various applications in practice. I will place special emphasis on the quantum mechanical properties of light that make it different from classical light, and how these properties relate to quantum information processing tasks.     

Quantum information processing through quantum dots in slow-light photonic crystal waveguides

We propose a scheme to realize controlled phase-flip gate between two single photons through a single quantum dot (QD) in a slow-light photonic crystal (PhC) waveguide. Enhanced Purcell factor and large β-factor lead to high gate fidelity over broadband frequencies compared to cavity-assisted system. The excellent physical integration of this PhC waveguide system provides tremendous potential for large-scale quantum information processing. Then we generalize to a multi-atom controlled phase-flip gate based on waveguide system in Sagnac interferometer. Through the Sagnac interferometer, the single photon adds the phase-flip operation on the atomic state without changing the photonic state. The controlled phase-flip gate on the atoms can be successfully constructed with high fidelity in one step, even without detecting the photon.     

Effective quantum efficiency in the pulsed homodyne detection of a n-photon state

Homodyne detection can be used to perform measurements on various quantum states of the light, such as conditional single photon states produced by parametric fluorescence processes. In the pulsed regime, the time and frequency overlap between the single photon wave packet and the local oscillator field plays a crucial role. We show in this paper that this overlap can be characterized by an effective quantum efficiency, which is explicitly calculated in various situations of experimental interest. Received 27 July 2000 and Received in final form 29 November 2000     

Distributed quantum information processing via quantum dot spins

We propose a scheme to engineer a non-local two-qubit phase gate between two remote quantum-dot spins. Along with one-qubit local operations, one can in principal perform various types of distributed quantum information processing. The scheme employs a photon with linearly polarisation interacting one after the other with two remote quantum-dot spins in cavities. Due to the optical spin selection rule, the photon obtains a Faraday rotation after the interaction process. By measuring the polarisation of the final output photon, a non-local two-qubit phase gate between the two remote quantum-dot spins is constituted. Our scheme may has very important applications in the distributed quantum information processing.     

Ion trapping for quantum information processing

In this paper we have reviewed the recent progresses on the ion trapping for quantum information processing and quantum computation. We have first discussed the basic principle of quantum information theory and then focused on ion trapping for quantum information processing. Many variations, especially the techniques of ion chips, have been investigated since the original ion trap quantum computation scheme was proposed. Full two-dimensional control of multiple ions on an ion chip is promising for the realization of scalable ion trap quantum computation and the implementation of quantum networks.      

Invariant information and quantum state estimation

The invariant information, introduced by C. Brukner and A. Zeilinger [Phys. Rev. Lett. 83, 3354 (1999)], is reconsidered from the point of view of quantum state estimation. We show that this quantity is directly related to the mean error of the standard reconstruction from the measurement of a complete set of mutually complementary observables. We give its generalization in terms of the Fisher information. Provided that the optimum reconstruction is adopted, the information loses its invariant character.     

Scalable quantum information processing and the optical topological quantum computer

Optical quantum computation has represented one of the most successful testbed systems for quantum information processing. Along with ion-traps and nuclear magnetic resonance (NMR), experimentalists have demonstrated control of qubits, multi-gubit gates and small quantum algorithms. However, photonic based qubits suffer from a problematic lack of a large scale architecture for fault-tolerant computation which could conceivably be built in the near future. While optical systems are, in some regards, ideal for quantum computing due to their high mobility and low susceptibility to environmental decoherence, these same properties make the construction of compact, chip based architectures difficult. Here we discuss many of the important issues related to scalable fault-tolerant quantum computation and introduce a feasible architecture design for an optics based computer. We combine the recent development of topological cluster state computation with the photonic module, simple chip based devices which can be utilized to deterministically entangle photons. The integration of this operational unit with one of the most exciting computational models solves many of the existing problems with other optics based architectures and leads to a feasible large scale design which can continuously generate a 3D cluster state with a photonic module resource cost linear in the cross sectional size of the cluster.     

Cavity quantum networks for quantum information processing in decoherence-free subspace

We give a brief review on the quantum information processing in decoherence-free subspace (DFS). We show how to realize the initialization of the entangled quantum states, information transfer and teleportation of quantum states, two-qubit Grover search and how to construct the quantum network in DFS, within the cavity QED regime based on a cavity-assisted interaction by single-photon pulses.      

Ion-trap quantum information processing:experimental status

Atomic ions trapped in ultra-high vacuum form an especially well-understood and useful physical system for quantum information processing. They provide excellent shielding of quantum information from environmental noise, while strong, well-controlled laser interactions readily provide quantum logic gates. A number of basic quantum information protocols have been demonstrated with trapped ions. Much current work aims at the construction of large-scale ion-trap quantum computers using complex microfabricated trap arrays. Several groups are also actively pursuing quantum interfacing of trapped ions with photons.      

Entangled coherent states for quantum information processing

Entangled coherent states (ECSs) with relative phase equal to the phase shift between two coherent states are constructed. We study the degree of entanglement and the nonclassical features exhibited by the so-constructed states keeping in view their role in quantum information processing (QIP).     

Transfer of entangled state, entanglement swapping and quantum information processing via the Rydberg blockade

We provide a scheme with which the transfer of the entangled state and the entanglement swapping can be realized in a system of neutral atoms via the Rydberg blockade. Our idea can be extended to teleport an unknown atomic state. According to the latest theoretical research of the Rydberg excitation and experimental reports of the Rydberg blockade effect in quantum information processing, we discuss the experimental feasibility of our scheme.     

Ion-trap quantum information processing: Experimental status

Atomic ions trapped in ultra-high vacuum form an especially well-understood and useful physical system for quantum information processing. They provide excellent shielding of quantum information from environmental noise, while strong, well-controlled laser interactions readily provide quantum logic gates. A number of basic quantum information protocols have been demonstrated with trapped ions. Much current work aims at the construction of large-scale ion-trap quantum computers using complex microfabricated trap arrays. Several groups are also actively pursuing quantum interfacing of trapped ions with photons.     

Continuous‐variable quantum information processing

Observables of quantum systems can possess either a discrete or a continuous spectrum. For example, upon measurements of the photon number of a light state, discrete outcomes will result whereas measurements of the light's quadrature amplitudes result in continuous outcomes. If one uses the continuous degree of freedom of a quantum system for encoding, processing or detecting information, one enters the field of continuous‐variable (CV) quantum information processing. In this paper we review the basic principles of CV quantum information processing with main focus on recent developments in the field. We will be addressing the three main stages of a quantum information system; the preparation stage where quantum information is encoded into CVs of coherent states and single‐photon states, the processing stage where CV information is manipulated to carry out a specified protocol and a detection stage where CV information is measured using homodyne detection or photon counting.     

Magneto-optical properties of self-assembled InAs quantum dots for quantum information processing

Semiconductor quantum dots have been intensively investigated because of their fundamental role in solid-state quantum information processing. The energy levels of quantum dots are quantized and can be tuned by external field such as optical, electric, and magnetic field. In this review, we focus on the development of magneto–optical properties of single In As quantum dots embedded in Ga As matrix, including charge injection, relaxation, tunneling, wavefunction distribution,and coupling between different dimensional materials. Finally, the perspective of coherent manipulation of quantum state of single self-assembled quantum dots by photocurrent spectroscopy with an applied magnetic field is discussed.     

High-fidelity quantum state transfer through a dissipative quantum data bus

We propose and analyze a robust quantum state transfer protocol through a scalable quantum data bus that consists of a network of controlled dissipative modules. In particular, we first demonstrate the ability to achieve perfect state transfer between two distinct quantum sites which are adiabatically coupled to the data bus in non-dissipative situation. We then consider the role of dissipation in adiabatic quantum state transfer via using Born–Markov master equation in the standard Lindblad form. Numerical simulation shows that the dissipation effect on the quality of transmission can be suppressed by engineering the network couplings of data bus properly.     

Long-distance quantum state transfer through cavity-assisted interaction

We propose a scheme for long-distance quantum state transfer between different atoms based on cavity-assisted interactions. In our scheme, a coherent optical pulse sequentially interacts with two distant atoms trapped in separated cavities. Through the measurement of the state of the first atom and the homodyne detection of the final output coherent light, the quantum state can be transferred into the second atom with a success probability of unity and a fidelity of unity. In addition, our scheme neither requires the high-Q cavity working in the strong coupling regime nor employs the single-photon quantum channel, which greatly relaxes the experimental requirements.