Xiao-Ye Xu

Experimental investigation of classical and quantum correlations under decoherence

It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an all-optical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.

Experimental investigation of the entanglement-assisted entropic uncertainty principle

Heisenberg’s uncertainty principle limits the precision with which we can measure two complementary properties of a quantum system. Entanglement, it has previously been proposed, can relax these constraints. This idea is now demonstrated experimentally with the aid of polarization-entangled photons.

Experimental investigation of the non-Markovian dynamics of classical and quantum correlations

We experimentally investigate the dynamics of classical and quantum correlations of a Bell diagonal state in a non-Markovian dephasing environment. The sudden transition from a classical to a quantum decoherence regime is observed during the dynamics of a Bell diagonal state. Due to the refocusing effect of the overall relative phase, the quantum correlation revives from near zero and then decays again in the subsequent evolution. However, the non-Markovian effect is too weak to revive the classical correlation, which remains constant in the same evolution range. With the implementation of an optical {sigma}{sub x} operation, the sudden transition from a quantum to a classical revival regime is obtained, and correlation echoes are formed. Our method can be used to control the revival time of correlations, which would be important in quantum memory.

Demon-like algorithmic quantum cooling and its realization with quantum optics

A universal pseudo-cooling method based on a Maxwell-demon-like swapping sequence is proposed. A controlled Hamiltonian gate is used to identify lower energy states of the system and to drive the system to those states. An experimental implementation using a quantum optical network exhibits a fidelity higher than 0.978.

Experimental characterization of entanglement dynamics in noisy channels.

We experimentally characterize the bipartite entanglement under one-sided open system dynamics and verify the recently formulated entanglement factorization law [Nature Phys. 4, 99 (2008)]. The one-sided open system dynamics is realized by implementing a phase damping and an amplitude decay channel, respectively, acting on one of the qubits, by an all-optical setup. Our results greatly simplify the characterization of entanglement dynamics and will play an important role in the construction of complex quantum networks.

Robust bidirectional links for photonic quantum networks

Jin-Shi Xu, ∗ Man-Hong Yung, 3, ∗ Xiao-Ye Xu, Jian-Shun Tang, Chuan-Feng Li, † and Guang-Can Guo Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, 230026, People’s Republic of China Department of Chemistry and Chemical Biology, Harvard University, Cambridge MA, 02138, USA Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing, 10084, People’s Republic of China (Dated: May 8, 2013)Researchers experimentally realize a promising method for creating robust bidirectional quantum communication links. Optical fibers are widely used as one of the main tools for transmitting not only classical but also quantum information. We propose and report an experimental realization of a promising method for creating robust bidirectional quantum communication links through paired optical polarization-maintaining fibers. Many limitations of existing protocols can be avoided with the proposed method. In particular, the path and polarization degrees of freedom are combined to deterministically create a photonic decoherence-free subspace without the need for any ancillary photon. This method is input state–independent, robust against dephasing noise, postselection-free, and applicable bidirectionally. To rigorously quantify the amount of quantum information transferred, the optical fibers are analyzed with the tools developed in quantum communication theory. These results not only suggest a practical means for protecting quantum information sent through optical quantum networks but also potentially provide a new physical platform for enriching the structure of the quantum communication theory.

Heisenberg-scaling measurement of the single-photon Kerr non-linearity using mixed states

Improving the precision of measurements is a significant scientific challenge. Previous works suggest that in a photon-coupling scenario the quantum fisher information shows a quantum-enhanced scaling of N2, which in theory allows a better-than-classical scaling in practical measurements. In this work, utilizing mixed states with a large uncertainty and a post-selection of an additional pure system, we present a scheme to extract this amount of quantum fisher information and experimentally attain a practical Heisenberg scaling. We performed a measurement of a single-photon’s Kerr non-linearity with a Heisenberg scaling, where an ultra-small Kerr phase of ≃6 × 10−8 rad was observed with a precision of ≃3.6 × 10−10 rad. From the use of mixed states, the upper bound of quantum fisher information is improved to 2N2. Moreover, by using an imaginary weak-value the scheme is robust to noise originating from the self-phase modulation.Quantum metrology usually relies on entanglement or squeezing for pursuing Heisenberg-limited precision. In this work, instead, the authors demonstrate Heisenberg-scaling measurement of a single photon Kerr’s nonlinearity using less-demanding mixed states.

Measurement-induced quantum entanglement recovery

By using photon pairs created in parametric down-conversion, we report on an experiment, which demonstrates that measurement can recover the quantum entanglement of a two-qubit system in a pure dephasing environment. The concurrence of the final state with and without measurement is compared and is analyzed. Furthermore, we verify that recovered states can still violate the Bell inequality, that is, to say, such recovered states exhibit nonlocality. In the context of quantum entanglement, sudden death and rebirth provide clear evidence, which verifies that entanglement dynamics of the system is sensitive not only to its environment, but also to its initial state.

Demonstration of Einstein–Podolsky–Rosen steering with enhanced subchannel discrimination

Einstein–Podolsky–Rosen (EPR) steering describes a quantum nonlocal phenomenon in which one party can nonlocally affect the other’s state through local measurements. It reveals an additional concept of quantum non-locality, which stands between quantum entanglement and Bell nonlocality. Recently, a quantum information task named as subchannel discrimination (SD) provides a necessary and sufficient characterization of EPR steering. The success probability of SD using steerable states is higher than using any unsteerable states, even when they are entangled. However, the detailed construction of such subchannels and the experimental realization of the corresponding task are still technologically challenging. In this work, we designed a feasible collection of subchannels for a quantum channel and experimentally demonstrated the corresponding SD task where the probabilities of correct discrimination are clearly enhanced by exploiting steerable states. Our results provide a concrete example to operationally demonstrate EPR steering and shine a new light on the potential application of EPR steering.Quantum steering: subchannel discrimination for the verification of EPR steeringA quantum optics experiment has demonstrated that the task of subchannel discrimination is more likely to succeed when using steerable states, providing an operational verification of quantum steering. Kai Sun at the University of Science and Technology of China in Hefei and collaborators have encoded bipartite Werner states in the polarization of photons, and evolved them according to a quantum channel given as a sum of subchannels. Identifying the subchannel in which the quantum evolution had taken place, a task called subchannel discrimination, was shown to have a higher chance of success for steerable Werner states – as measured from their mixing parameter. Testing the success probability of subchannel discrimination for unknown states therefore provides an operational approach for detecting steerable states and distinguishing different kinds of quantum nonlocality, which is useful for secure communications protocols.

Quantitative Verification of the Kibble–Zurek Mechanism in Quantum Nonequilibrium Dynamics

The Kibble–Zurek mechanism (KZM) captures the key physics of nonequilibrium dynamics in second-order phase transitions, and accurately predicts the density of topological defects formed in such processes. However, the central prediction of KZM, i.e., the scaling of the density of defects with the quench rate still needs further experimental confirmation, particularly for quantum transitions. Here, we perform a quantum simulation of the nonequilibrium dynamics of the Landau–Zener model based on a nine-stage optical interferometer with an overall visibility of \(0.975 \pm 0.008\). The results support the adiabatic-impulse approximation, which is the core of Kibble–Zurek theory. Moreover, the developed high-fidelity multistage optical interferometer can support more complex linear optical quantum simulations.