Large scale quantum information processing requires stable and long-lived quantum memories. Here, using atom-photon entanglement, we propose an experimentally feasible scheme to realize decoherence-free quantum memory with atomic ensembles, and show one of its applications, remote transfer of unknown quantum state, based on laser manipulation of atomic ensembles, photonic state operation through optical elements, and single-photon detection with moderate efficiency. The scheme, with inherent fault-tolerance to the practical noise and imperfections, allows one to retrieve the information in the memory for further quantum information processing within the reach of current technology.
Decoherence-free Behaviors of Quantum Emitters in Dissipative Photonic Graphene
School of Physics and Institute for Quantum Science and Engineering, Huazhong University of Science and Technology（华中科技大学物理科学与技术学院量子科学与工程研究院）；Wuhan institute of quantum technology（武汉量子技术研究院）；Quantum Computing Center, RIKEN（理研量子计算中心）；Department of Physics, The University of Michigan（密歇根大学物理系）
Based on the recent experiments on the hybrid quantum system of a superconducting microwave cavity coupling strongly to an inhomogeneous broadening spin ensemble under an external driving field, we use the exact master equation approach to investigate its non-Markovian decoherence dynamics. Here the spin ensemble is made by negatively charged nitrogen-vacancy (NV) defects in diamond. Our exact master equation theory for open systems depicts the experimental decoherence results and reveals the mechanism how the decoherence induced by the inhomogeneous broadening is suppressed in the strong-coup…
Decoherence-free subspaces in the noisy dynamics of discrete-step quantum walks in a photonic lattice
Univ. Lille, CNRS, UMR 8523 – PhLAM – Physique des Lasers Atomes et Molécules（里尔大学，法国国家科学研究中心，联合研究实验室8523 - 激光原子与分子物理实验室）；Institute of Fundamental Physics IFF-CSIC（西班牙国家科学研究委员会基础物理研究所）
It is commonly known that the dephasing in open quantum systems is due to the establishment of bipartite correlations with ambient environments, which are typically difficult to be fully characterized. Recently, a new approach of average over disordered Hamiltonian ensemble is developed and shown to be capable of describing the nonclassicality of incoherent dynamics based on inferring the nonclassical nature of the correlations. Here we further extend the approach of Hamiltonian ensemble in the canonical form to the realm of structural disorder. Under the variable separation of the probability…
Optical lattice clocks can now resolve the height difference below 1 mm within an atomic ensemble by means of gravitational redshift with integration over sufficient amount of time. Further improvement in the stability enables the clocks to resolve the height difference of subsystems within an atomic ensemble that is conventionally interrogated as a single coherent spin state in a single Ramsey sequence, resulting in the dephasing of the coherent spin state. This effect is observable with a clock of a stability of $\lesssim 10^{-21}$ by introducing a single-layer-resolved imaging system for a …
We consider an ensemble of atoms with $Λ$-type level structure trapped in a single-mode cavity, and propose a geometric scheme of coherent manipulation of quantum states on the subspace of zero-energy states within the quantum Zeno subspace of the system. We find that the particular subspace inherits the decoherence-free nature of the quantum Zeno subspace and features a symmetry-protected degeneracy, fulfilling all the conditions for a universal scheme of arbitrary unitary operations on it.
Classical random matrix ensembles were originally introduced in physics to approximate quantum many-particle nuclear interactions. However, there exists a plethora of quantum systems whose dynamics is explained in terms of few-particle (predominantly two-particle) interactions. The random matrix models incorporating the few-particle nature of interactions are known as embedded random matrix ensembles. In the present paper, we provide a brief overview of these two ensembles and illustrate how the embedded ensembles can be successfully used to study decoherence of a qubit interacting with an env…
Ensembles of solid-state optical emitters enable broadband quantum storage and transduction of photonic qubits, with applications in high-rate optical quantum networks for secure communications, global time-keeping, and interconnecting future quantum computers. To realize coherent quantum information transfer using ensembles, spin rephasing techniques are currently used to mitigate fast decoherence resulting from inhomogeneous broadening. Here we use a dense ensemble of neodymium rare-earth ions strongly coupled to a nanophotonic resonator to demonstrate that decoherence of a single photon exc…
Quantum many-body theory of qubit decoherence in a finite-size spin bath. II. Ensemble dynamics
自旋浴中中心自旋或量子比特的退相干本质上是由多体浴演化决定的。浴动力学可以从纯态开始，或者更一般地，从统计系综开始。在上一篇文章中[W. Yang和R. B. Liu，Phys. Rev. B \textbf{78}，085315（2008）]，我们已经开发了用于从可因子化纯态启动的所谓单样本浴动力学的簇相关展开（CCE）理论。在这里，我们提出了系综CCE理论，它基于单样本CCE的类似思想：浴演变被分解为所有可能的集群相关性的产物，其中每一个都占了真实的（不可因式分解的）一组浴自旋的集体激发，并且对于量子位退相干问题中的有限时间演化，通过保持簇相关性达到一定大小来截断系综CCE，可以获得收敛的结果。系综CCE和单样本CCE之间的区别在于，在后一种形式中，在前一种情况下，不可能对浴中自旋-自旋相互作用的对角部分进行平均场处理。系综CCE可以应用于不可因式分解的初始状态。系综CCE检查对XY纺丝浴模型的精确解。对于小的旋转浴，它表明，单样本动力学是敏感的采样的初始状态从热合奏，因此非常不同的合奏平均。
Incorporating protection against quantum errors into adiabatic quantum computing (AQC) is an important task due to the inevitable presence of decoherence. Here we investigate an error-protected encoding of the AQC Hamiltonian, where qubit ensembles are used in place of qubits. Our Hamiltonian only involves total spin operators of the ensembles, offering a simpler route towards error-corrected quantum computing. Our scheme is particularly suited to neutral atomic gases where it is possible to realize large ensemble sizes and produce ensemble-ensemble entanglement. We identify a critical ensembl…
Fragility of quantum correlations and coherence in a multipartite photonic system
众所周知，某些量子态在存在退相干的情况下特别脆弱，如薛定谔著名的盖丹肯猫实验所示。最近人们更好地认识到，量子态可以用诸如纠缠、量子关联和量子相干性等量子量的层次来表征。已经证明，这些量中的每一个在退相干的存在下都有不同程度的脆弱性。在这里，我们实验证实了这一猜想，准备三方光子态，并使它们受到控制量的退相。当对所有量子比特进行退相处理时，我们发现纠缠是最脆弱的量，其次是量子相干性，然后是互信息。这与人们普遍认为的多体量子关联是量子性的一种非常脆弱的表现的观点是一致的。我们还对星形和$ W \bar{W} $态的三个量子位中的一个进行了退相。在这里，状态中的相关性和相干性的分布相对于退相位置变得更加重要。
