基于无消相干子空间的量子避错码设计

针对量子系统的联合消相干模型,可以找到一些不受消相干错误影响的系统状态,这种状态被称为相干保持态,所有相干保持态构成的空间就称为无消相干子空间(decoherencefreesubspace,缩写为DFS).利用DFS的特性可以实现自动容错的量子避错码.首先提出一种DFS的定义,并且以定理的形式证明其他DFS的定义都是等价的.然后给出了DFS的存在性定理.最后利用群论的方法设计一种构造DFS的简单方法 关键词： 相干保持态 无消相干子空间 量子避错码 容错量子计算     

Experimental investigation of a two-qubit decoherence-free subspace

We thoroughly explore the phenomenon of a decoherence-free subspace (DFS) for two-qubit systems. Specifically, we both collectively and noncollectively decohere entangled polarization-encoded two-qubit states using thick birefringent crystals. These results characterize the basis-dependent effect of decoherence on the four Bell states, the robustness of the DFS state against perturbations in the assumption of collective decoherence, and the existence of a DFS for each type of stable noncollective decoherence. Finally, we investigate the effects of collective and noncollective dissipation.     

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.      

A New Quantum Secure Direct Communication Protocol Using Decoherence-Free Subspace

A new quantum secure direct communication （QSDC） protocol is proposed by using decoherence free subspace （DFS） to avoid insecurity of the present QSDC protocols in a quantum noise channel. This protocol makes it easily for Bob and Alice to find eavesdropping in channel because the collective dephasing noise disappears in DFS. The probability of successful attack by Eve in this protocol is smaller than in BB84 protocol. Thus this protocol realizes secure QSDC and is feasible with present-day technology.     

Towards a novel simulation approach for the transport of atomic states through polarized photons

A novel approach for transferring logic states from one quantum node to other is proposed. Logic states ‘0’ and ‘1’ are represented by two subspaces of the hyperfine states space of rubidium atom (⁸⁷Rb). The atom, placed at the center of a two-mode cavity, is excited by simultaneous application of two laser beams, one for each subspace. Based on the logic state of the atom, it makes a transition to a higher energy level within the corresponding subspace. When the atom relaxes back to a lower state within the subspace, a left- or right-circularly polarized photon is emitted depending on whether the initial state was logic ‘0’ or logic ‘1’. The polarized photon leaks out of the cavity, reaches the receive node and gets detected therein. Simulation results show the efficacy of the approach.     

Constructing Four-Photon States for Quantum Communication and Information Processing

We provide a method for constructing a set of four-photon states suitable for quantum communication applications. Among these states is a set of concatenated quantum code states that span a decoherence-free subspace that is robust under collective-local as well as global dephasing noise. This method requires only the use of spontaneous parametric down-conversion, quantum state post-selection, and linear optics. In particular, we show how this method can be used to produce all sixteen elements of the second-order Bell gem , which includes these codes states and is an orthonormal basis for the Hilbert space of four qubits composed entirely of states that are fully entangled under the four-tangle measure.     

Quantum correlations support probabilistic pure state cloning

The probabilistic scheme for making two copies of two nonorthogonal pure states requires two auxiliary systems, one for copying and one for attempting to project onto the suitable subspace. The process is performed by means of a unitary-reduction scheme which allows having a success probability of cloning different from zero. The scheme becomes optimal when the probability of success is maximized. In this case, a bipartite state remains as a free degree which does not affect the probability. We find bipartite states for which the unitarity does not introduce entanglement, but does introduce quantum discord between some involved subsystems.     

Coherent quantum evolution via reservoir driven holonomies

We show that in the limit of a strongly interacting environment a system initially prepared in a decoherence-free subspace (DFS) coherently evolves in time, adiabatically following the changes of the DFS. If the reservoir cyclicly evolves in time, the DFS states acquire a holonomy.     

On the quantum link for transport of logic states

A novel approach for representing logic states in the quantum nodes and transferring the states from one node to another is proposed. Both transmit and receive nodes consist of a rubidium atom (⁸⁷Rb) placed at the center of a two-mode cavity. Representation of logic states by two subspaces of the space of ⁸⁷Rb atom hyperfine states eliminates the need for the transmitting node to change logic state during logic transfer through Raman process. The atom is excited by simultaneous application of two laser beams - one for each subspace. Based on the logic state, the atom emits a photon of appropriate frequency and polarization through Raman process within the corresponding subspace. The emitted photon leaks out of the cavity, reaches the receiving node, and initiates logic dependent transitions there. A simulation platform is developed through the system Hamiltonians for transmit and receive nodes followed by the formulation of the time evolution of the density matrices for the nodes. The efficacy of the simulation approach is emphasized.     

Detecting entanglement by the mean value of spin on a quantum computer

We implement a protocol to determine the degree of entanglement between a qubit and the rest of the system on a quantum computer. The protocol is based on results obtained in paper [Frydryszak et al. (2017) [23]]. This protocol is tested on a 5-qubit superconducting quantum processor called ibmq-ourense provided by the IBM company. We determine the values of entanglement of the Schrödinger cat and the Werner states prepared on this device and compare them with the theoretical ones. In addition, a protocol for determining the entanglement of rank-2 mixed states is proposed. We apply this protocol to the mixed state which consists of two Bell states prepared on the ibmq-ourense quantum device.     

Experimental realization of decoherence-free subspace in neutron interferometry

A decoherence-free subspace (DFS) is an important class of quantum-error-correcting (QEC) codes that have been proposed for fault-tolerant quantum computation. The applications of QEC techniques, however, are not limited to quantum-information processing (QIP). Here we demonstrate how QEC codes may be used to improve experimental designs of quantum devices to achieve noise suppression. In particular, neutron interferometry is used as a test bed to show the potential for adding quantum error correction to quantum measurements. We built a five-blade neutron interferometer that incorporates both a standard Mach-Zender configuration and a configuration based on a DFS. Experiments verify that the DFS interferometer is protected against low-frequency mechanical vibrations. We anticipate these improvements will increase the range of applications for matter-wave interferometry.     

Long-distance quantum communication in a decoherence-free subspace

We show how to realize long-distance quantum communication using a long-lived quantum memory, which is embedded in a decoherence-free subspace (DFS). Neutral atoms were used in the present scheme, whose interactions are catalyzed by single photons or weak coherent light. The generation, purification and swapping of logical entangled states are performed with help of cavity-assisted photon scattering which is robust to random variation in the atom-photon coupling rate.     

Improved secure quantum sealed-bid auction

The security of a secure quantum sealed-bid auction protocol using quantum secure direct communication based on GHZ states [M. Naseri, Opt. Commun. 282 (2009) 1939] is reexamined. It is shown that the protocol does not complete the task of a sealed-bid auction fairly. It is shown that a dishonest bidder can obtain all the other one’s secret bids by two special types of attack, i.e., double Controlled NOT attack or using fake entangled particles. Furthermore, a simple possible improvement of the protocol is proposed.     

Optimal control of fast and high-fidelity quantum state transfer in spin-1/2 chains

Spin chains are promising candidates for quantum communication and computation. Using quantum optimal control (OC) theory based on the Krotov method, we present a protocol to perform quantum state transfer with fast and high fidelity by only manipulating the boundary spins in a quantum spin-1/2 chain. The achieved speed is about one order of magnitude faster than that is possible in the Lyapunov control case for comparable fidelities. Additionally, it has a fundamental limit for OC beyond which optimization is not possible. The controls are exerted only on the couplings between the boundary spins and their neighbors, so that the scheme has good scalability. We also demonstrate that the resulting OC scheme is robust against disorder in the chain.     

Novel quantum steganography with large payload

In this paper, we propose a novel quantum steganography protocol based on quantum secure direct communication. By using entanglement swapping of Bell states, the protocol builds up hidden channel within the improved ping-pong protocol to transmit secret messages. Comparing with the previous quantum steganographies, its capacity of hidden channel is increased to four times, and the superposition channel can transmit more information than the original quantum channel. Imperceptibility of the hidden channel in this protocol is good, since its possibility of detection can be arbitrarily reduced by increasing the Bell state's number. Security of the secret messages is also proved to be reliable regardless of whether the hidden channel has been detected or not. In addition, our protocol has various applications in quantum communication.     

多粒子纠缠态QTDM通信方案及QMU协议

分析了未知的三个三态粒子纠缠态的隐形传送过程,结合纠缠源的特性,提出了量子信道时分复用方案.同时还提出了一种可传送多粒子纠缠态的量子时分多路通信方案和量子多用户协议.研究结果表明,该方案与采用BB84协议的量子通信系统相比,不仅能够实现量子多用户安全保密通信,而且误码率低.     

Cavity loss induced generation of W states

The existence of decoherence-free subspace （DFS） has been discussed widely. In this paper, we propose an alternative scheme for generating the four-atom W states by manipulating DF qubits. The atoms are divided into two pairs and trapped in two separate optical cavities. Manipulation of atoms within DFS may generate a two-atom maximally entangled state in an individual cavity, which is a stable state. After driving the system out of DFS, the atoms will interact resonantly with the cavity field. The photons leaking from the cavities interfere at the beamsplitter, which destroys which-path information, and are finally detected by one of the detectors, leading to the generation of a W state. In addition, the numerical simulation indicates that the fidelity of the prepared state can, for a very wide parameter regime, be very close to unity.     

Single-shot readout of electron spins in a semiconductor quantum dot

We report on a method for single-shot readout of spin states in a semiconductor quantum dot that is robust against charge noise and can be used even when the electron temperature exceeds the energy splitting between the states. The spin states are first correlated to different charge states using a spin dependence of the tunnel rates. A subsequent fast measurement of the charge on the dot then reveals the original spin state. The method is analyzed theoretically, and compared to a previously used method. We experimentally demonstrate the method by performing readout of the two-electron spin states, achieving a single-shot visibility of more than 80%. We find very long triplet-to-singlet relaxation times (up to several milliseconds), with a strong dependence on in-plane magnetic field.     

Cavity loss induced generation of W states

The existence of decoherence-free subspace (DFS) has been discussed widely. In this paper, we propose an alternative scheme for generating the four-atom $W$ states by manipulating DF qubits. The atoms are divided into two pairs and trapped in two separate optical cavities. Manipulation of atoms within DFS may generate a two-atom maximally entangled state in an individual cavity, which is a stable state. After driving the system out of DFS, the atoms will interact resonantly with the cavity field. The photons leaking from the cavities interfere at the beamsplitter, which destroys which-path information, and are finally detected by one of the detectors, leading to the generation of a $W$ state. In addition, the numerical simulation indicates that the fidelity of the prepared state can, for a very wide parameter regime, be very close to unity.     

Quantum repeaters based on CNOT gate under decoherence

In this paper, we study single-qubit and single-user quantum repeaters based on CNOT gates under decoherence using the Kraus-operator representations of decoherence. We investigate the influence of decoherence on the information-disturbance trade-off of quantum repeaters. It is found that decoherence may lead to the appearance of three subspaces, called as the normal subspace, the anomalous subspace, and the decoherence-free subspace (DFS), respectively. It is indicated that in the normal subspace decoherence decreases the transmission and estimation fidelities, in the anomalous subspace decoherence enhances these fidelities, and in the DFS these fidelities do not change. The concept of the quality factor is introduced to evaluate the quality of the quantum repeater. It is indicated that the quality factor can be efficiently controlled and manipulated by changing the initial state of the probe qubit. It is found that under certain conditions the quantum repeater can be optimal even in the presence of decoherence.