Xu-Bo Zou

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.

Analog to multiple electromagnetically induced transparency in all-optical drop-filter systems

We theoretically study a parallel optical configuration which includes N periodically coupled whispering-gallery-mode resonators. The model shows an obvious effect which has a direct analogy with the phenomenon of multiple electromagnetically induced transparency in quantum systems. The numerical simulations illuminate that the frequency transparency windows are sharp and highly transparent. We also briefly discuss the experimental feasibility of the current scheme in two practical systems, microrings and microdisks.

Experimental demonstration of photonic entanglement collapse and revival.

We demonstrate the collapse and revival features of the entanglement dynamics of different polarization-entangled photon states in a non-Markovian environment. Using an all-optical experimental setup, we show that entanglement can be revived even after it suffers from sudden death. A maximally revived state is shown to violate a Bells inequality with 4.1 standard deviations which verifies its quantum nature. The revival phenomenon observed in this experiment provides an intriguing perspective on entanglement dynamics.

Experimental realization of optomechanically induced non-reciprocity

Non-magnetic non-reciprocal transparency and amplification is experimentally achieved by optomechanics using a whispering-gallery microresonator. The idea may lead to integrated all-optical isolators or non-reciprocal phase shifters.

Measuring non-Markovianity of processes with controllable system-environment interaction

Non-Markovian processes have recently become a central topic in the study of open quantum systems. We realize experimentally non-Markovian decoherence processes of single photons by combining time delay and evolution in a polarization-maintaining optical fiber. The experiment allows the identification of the process with strongest memory effects as well as the determination of a recently proposed measure for the degree of quantum non-Markovianity based on the exchange of information between the open system and its environment. Our results show that an experimental quantification of memory in quantum processes is indeed feasible which could be useful in the development of quantum memory and communication devices.

Linear optical implementation of the two-qubit controlled phase gate with conventional photon detectors

Thermal entanglement of a two-qubit Heisenberg chain in presence of the Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction and entanglement teleportation when using two independent Heisenberg chains as quantum channel are investigated. It is found that the DM interaction can excite the entanglement and teleportation fidelity. The output entanglement increases linearly with increasing value of input one, its dependences on the temperature, DM interaction and spin coupling constant are given in detail. Entanglement teleportation will be better realized via antiferromagnetic spin chain when the DM interaction is turned off and the temperature is low. However, the introduction of DM interaction can cause the ferromagnetic spin chain to be a better quantum channel for teleportation. A minimal entanglement of the thermal state in the model is needed to realize the entanglement teleportation regardless of antiferromagnetic or ferromagnetic spin chains.

Private capacity of quantum channels is not additive.

Recently there has been considerable activity on the subject of the additivity of various quantum channel capacities. Here, we construct a family of channels with a sharply bounded classical and, hence, private capacity. On the other hand, their quantum capacity when combined with a zero private (and zero quantum) capacity erasure channel becomes larger than the previous classical capacity. As a consequence, we can conclude for the first time that the classical private capacity is nonadditive. In fact, in our construction even the quantum capacity of the tensor product of two channels can be greater than the sum of their individual classical private capacities. We show that this violation occurs quite generically: every channel can be embedded into our construction, and a violation occurs whenever the given channel has a larger entanglement-assisted quantum capacity than (unassisted) classical capacity.

Topological superfluids with finite-momentum pairing and Majorana fermions

Majorana fermions (MFs), quantum particles that are their own antiparticles, are not only of fundamental importance in elementary particle physics and dark matter, but also building blocks for fault-tolerant quantum computation. Recently MFs have been intensively studied in solid state and cold atomic systems. These studies are generally based on superconducting pairing with zero total momentum. On the other hand, finite total momentum Cooper pairings, known as Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) states, were widely studied in many branches of physics. However, whether FF and LO superconductors can support MFs has not been explored. Here we show that MFs can exist in certain types of gapped FF states, yielding a new quantum matter: topological FF superfluids/superconductors. We demonstrate the existence of such topological FF superfluids and the associated MFs using spin-orbit-coupled degenerate Fermi gases and derive their parameter regions. The implementation of topological FF superconductors in semiconductor/superconductor heterostructures is also discussed.

Experimental violation of the Leggett-Garg inequality under decoherence

Despite the great success of quantum mechanics, questions regarding its application still exist and the boundary between quantum and classical mechanics remains unclear. Based on the philosophical assumptions of macrorealism and noninvasive measurability, Leggett and Garg devised a series of inequalities (LG inequalities) involving a single system with a set of measurements at different times. Introduced as the Bell inequalities in time, the violation of LG inequalities excludes the hidden-variable description based on the above two assumptions. We experimentally investigated the single photon LG inequalities under decoherence simulated by birefringent media. These generalized LG inequalities test the evolution trajectory of the photon and are shown to be maximally violated in a coherent evolution process. The violation of LG inequalities becomes weaker with the increase of interaction time in the environment. The ability to violate the LG inequalities can be used to set a boundary of the classical realistic description.

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.