arXiv subjects

Guang-Can Guo

Publications and source records attributed to Guang-Can Guo.

At least 757 records · Page 42Linked to original sources

arXiv2013

Single-Photon-Level Quantum Image Memory Based on Cold Atomic Ensembles

An important step towards the successful development of network that allows the distribution of quantum information is the storage of light in a matter at the single-photon level. Encoding photons in high-dimensional photonic states can significantly increase their information-carrying capacity. So far, there is no any report on the storage of an image at true single photon level, realizing a quantum memory that can record the shape of a single photon is a challenging work. In this work, we report on the first experimental realization of storing a true single photon carrying an orbital angular momentum via electromagnetically induced transparency in a cold atomic ensemble and retrieved later after a given storage time. We experimentally not only prove the preservation of the property of the single photon, but also show very good similarity on the spatial structures of the input and the retrieved photons. More importantly, we also demonstrate the preservation of the coherence of the single photon during the storage using a Sagnac interferometer. The demonstrated capability of storing a spatial structure at single-photon-level opens the possibility to the realization of a high-dimensional quantum memory.

quant-ph↗

arXiv2013

Exotic pairing states in a Fermi gas with three-dimensional spin-orbit coupling

We investigate properties of exotic pairing states in a three-dimensional Fermi gas with three-dimensional spin-orbit coupling and an effective Zeeman field. The interplay of spin-orbit coupling, effective Zeeman field and pairing can lead to first-order phase transitions between different phases, and to interesting nodal superfluid states with gapless surfaces in the momentum space. We then demonstrate that pairing states with zero center-of-mass momentum are unstable against finite center-of-mass momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, with the center-of-mass momentum of the pairs opposite to the direction of the effective Zeeman field. Unlike conventional FFLO states, these FFLO states are induced by the coexistence of spin-orbit coupling and Fermi surface deformation, and have intriguing features like first-order transitions between different FFLO states, nodal FFLO states with gapless surfaces in momentum space, and exotic fully gapped FFLO states. With the recent theoretical proposals for realizing three-dimensional spin-orbit coupling in ultracold atom gases, our work is helpful for the future experimental studies, and provides valuable information for the general understanding of pairing physics in spin-orbit coupled fermionic systems.

cond-mat.quant-gas↗

arXiv2013

Photonic Bound State in the Continuum for Strong Light-matter Interaction

The photonic bound state in the continuum (BIC) is discovered in a hybrid photonic circuit with low refractive index waveguide on a high refractive index thin membrane, where the optical dissipation is forbidden because of the destructive interference of different leakage channels. Based on the photonic BIC, the low mode area in a hybrid waveguide and high quality factor in a microresonator can be applied to enhance the light-matter interaction. Taking the fabrication-friendly polymer structure on diamond membrane as an example, those excellent optical performances can exist in a wide range of structure parameters with large fabrication tolerance and induce the strong coupling between photon and nitrogen-vacancy center in the diamond for scalable quantum information processors and networks. Such a fabrication-friendly structure with photonic BIC is also very promising in laser, nonlinear optical and quantum optical applications.

physics.optics↗

arXiv2013

Photonic realization of nonlocal memory effects and non-Markovian quantum probes

The study of open quantum systems is important for fundamental issues of quantum physics as well as for technological applications such as quantum information processing. Recent developments in this field have increased our basic understanding on how non-Markovian effects influence the dynamics of an open quantum system, paving the way to exploit memory effects for various quantum control tasks. Most often, the environment of an open system is thought to act as a sink for the system information. However, here we demonstrate experimentally that a photonic open system can exploit the information initially held by its environment. Correlations in the environmental degrees of freedom induce nonlocal memory effects where the bipartite open system displays, counterintuitively, local Markovian and global non-Markovian character. Our results also provide novel methods to protect and distribute entanglement, and to experimentally quantify correlations in photonic environments.

quant-ph↗

arXiv2013

Compact Bell inequalities for multipartite experiments

A method for construction of the multipartite Clauser-Horne-Shimony-Holt (CHSH) type Bell inequalities, for the case of local binary observables, is presented. The standard CHSH-type Bell inequalities can be obtained as special cases. A unified framework to establish all kinds of CHSH-type Bell inequalities by increasing step by step the number of observers is given. As an application, compact Bell inequalities, for eight observers, involving just four correlation functions are proposed. They require much less experimental effort than standard methods and thus is experimentally friendly in multi-photon experiments.

quant-ph↗

arXiv2013

Experimental Demonstration of Robust Bidirectional Quantum Optical Communications

We experimentally realized a new method for transmitting quantum information reliably through paired optical polarization-maintaining (PM) fibers. The physical setup extends the use of a Mach-Zehnder interferometer, where noises are canceled through interference. This method can be viewed as an improved version of the current decohernce-free subspace (DFS) approach in fiber optics. Furthermore, the setup can be applied bidirectionally, which means that robust quantum communication can be achieved from both ends. To rigorously quantify the amount of quantum information transferred, optical fibers are analyzed with the tools developed in quantum communication theory. These results not only suggests a practical means for protecting classical and quantum information through optical fibers, but also provides a new physical platform for enriching the structure of the quantum communication theory.

quant-ph↗

arXiv2013

Size consistency of tensor network methods for quantum many-body systems

Recently developed tensor network methods demonstrate great potential for addressing the quantum many-body problem, by constructing variational spaces with polynomially, instead of exponentially, scaled parameters. Constructing such an efficient tensor network, and thus the variational space, is a subtle problem and the main obstacle of the method. We demonstrate the necessity of size consistency in tensor network methods for their success in addressing the quantum many-body problem. We further demonstrate that size consistency is independent of the entanglement criterion, thus providing a general and tight constraint to construct the tensor network method.

cond-mat.str-el↗

arXiv2013

Experimental observation of optical precursors in optically pumped crystals

We experimentally observed optical precursors in optically pumped crystals using polarization-based interference. By switching the user-programmable medium among the fast light, slow light and no-dispersion regimes, we observed an unchanged polarization states for the wavefronts. The robust polarization-encoded information carried by wavefronts suggests that precursors are the preferred carriers for both quantum and classical information in communication networks.

quant-ph↗

arXiv2013

Significance of dressed molecules in a quasi-two-dimensional polarized Fermi gas

We investigate the properties of a spin-orbit coupled quasi-two-dimensional Fermi gas with tunable s-wave interaction between the two spin species. By analyzing the two-body bound state, we find that the population of the excited states in the tightly-confined axial direction can be significant when the two-body binding energy becomes comparable or exceeds the axial confinement. Since the Rashba spin-orbit coupling that we study here tends to enhance the two-body binding energy, this effect can become prominent at unitarity or even on the BCS side of the Feshbach resonance. To study the impact of these excited modes along the third dimension, we adopt an effective two-dimensional Hamiltonian in the form of a two-channel model, where the dressed molecules in the closed channel consist of the conventional Feshbach molecules as well as the excited states occupation in the axial direction. With properly renormalized interactions between atoms and dressed molecules, we find that both the density distribution and the phase structure in the trap can be significantly modified near a wide Feshbach resonance. In particular, the stability region of the topological superfluid phase is increased. Our findings are helpful for the experimental search for the topological superfluid phase in ultra-cold Fermi gases, and have interesting implications for quasi-low-dimensional polarized Fermi gases in general.

cond-mat.quant-gas↗

arXiv2013

Qunatum Parrondo's games constructed by quantum random walk

We construct a Parrondo's game using discrete time quantum walks. Two lossing games are represented by two different coin operators. By mixing the two coin operators $U_{A}(α_{A},β_{A},γ_{A})$ and $U_{B}(α_{B},β_{B},γ_{B})$, we may win the game. Here we mix the two games in position instead of time. With a number of selections of the parameters, we can win the game with sequences ABB, ABBB, \emph{et al}. If we set $β_{A}=45^{\circ},γ_{A}=0,α_{B}=0,β_{B}=88^{\circ}$, we find the game 1\emph{}with {\normalsize $U_{A}^{S}=U^{S}(-51^{\circ},45^{\circ},0)$, $U_{B}^{S}=U^{S}(0,88^{\circ},-16^{\circ})$ will win and get the most profit.}If we set $α_{A}=0,β_{A}=45^{\circ},α_{B}=0,β_{B}=88^{\circ}$ and{\normalsize{} the game 2 with $U_{A}^{S}=U^{S}(0,45^{\circ},-51^{\circ})$, $U_{B}^{S}=U^{S}(0,88^{\circ},-67^{\circ})$, will win most. And}game 1\emph{}{\normalsize is equivalent to the}game\emph{}2\emph{}with the changes of sequences and steps. But at a large enough steps, the game will loss at last.

quant-ph↗

arXiv2013

Quantum discord of ensemble of quantum states

We highlight an information-theoretic meaning of quantum discord as the gap between the ac- cessible information and the Holevo bound in the framework of ensemble of quantum states. This complementary relationship implies that a large amount of pre-existing arguments about the evaluation of quantum discord can be directly applied to the accessible information and vice versa. For an ensemble of two pure qubit states, we show that one can evade the optimization problem with the help of the Koashi-Winter relation. Further, for the general case (two mixed qubit states), we recover the main results presented by Fuchs and Caves [Phys. Rev. Lett. 73, 3047 (1994)], but totally from the perspective of quantum discord. Following this line of thought, we also investigate the geometric discord as an indicator of quantumness of ensembles in detail. Finally, we give an example to elucidate the difference between quantum discord and geometric discord with respect to optimal measurement strategies.

quant-ph↗

arXiv2013

The effect of channel decoherence on entangled coherent states: A theoretical analysis

We analyse decoherence properties of entangled coherent states (ECSs) due to channel losses, and compare to the performance of bi-photon Bell states. By employing the concept of "entanglement of formation" (EOF), the degradation of fidelity and degree of entanglement are calculated. Two situations are considered: (1) asymmetric and (2) symmetric noise channel. In the first case, only the lower bound of EOF is given. However, in the latter case, we have obtained an explicit expression of concurrence (and then EOF) and found surprisingly our result is just incompatible with that of [Phys. Rev. A \textbf{82}, (2010) 062325] in the limit of $α\rightarrow0$. We demonstrate that entangled coherent states with sufficient small amplitudes are more robust against channel decoherence than biphoton entangled states in both cases.

quant-ph↗

arXiv2013

Performance of various correlation measures in quantum renormalization-group method: A case study of quantum phase transition

We have investigated quantum phase transition employing the quantum renormalization group (QRG) method while in most previous literature barely entanglement (concurrence) has been demonstrated. However, it is now well known that entanglement is not the only signature of quantum correlations and a variety of computable measures have been developed to characterize quantum correlations in the composite systems. As an illustration, two cases are elaborated: one dimensional anisotropic (i) XXZ model and (ii) XY model, with various measures of quantum correlations, including quantum discord (QD), geometric discord (GD), measure-induced disturbance (MID), measure-induced nonlocality (MIN) and violation of Bell inequalities (eg. CHSH inequality). We have proved that all these correlation measures can effectively detect the quantum critical points associated with quantum phase transitions (QPT) after several iterations of the renormalization in both cases. Nonetheless, it is shown that some of their dynamical behaviors are not totally similar with entanglement and even when concurrence vanishes there still exists some kind of quantum correlations which is not captured by entanglement. Intriguingly, CHSH inequality can never be violated in the whole iteration procedure, which indicates block-block entanglement can not revealed by the CHSH inequality. Moreover, the nonanalytic and scaling behaviors of Bell violation have also been discussed in detail. As a byproduct, we verify that measure-induced disturbance is exactly equal to the quantum discord measured by σ_z for general X-structured states.

quant-ph↗

arXiv2013

Quantum discord in quantum random access codes and its connection with dimension witness

We exploit quantum discord (and geometric discord) to detect quantum correlations present in a well-known communication model called quantum random access codes (QRACs), which has a variety of applications in areas ranging form quantum communication complexity, network coding, information causality, to security proof of QKD protocol. In spite of the fact that there is no entanglement between the two parts involved in this model, analytical derivation shows that the quantum discord is nonzero and highlights that quantum discord might be regarded as a figure of merit to characterize the quantum feature of QRACs, since this model has no classical counterparts. To gain further insight, we also investigate the dynamical behavior of quantum discord under state rotations. As a comparison, the connection between quantum discord and dimension witness is graphically discussed. Interestingly, our results illustrate that these two quantities are monotonically related to each other, which implies that the dimension witness and nonclassical correlations may be intrinsically linked.

quant-ph↗

arXiv2013

Photonic Simulation of System-Environment Interaction: Non-Markovian Process and Dynamical Decoupling

The system-environment interaction is simulated by light propagating in coupled photonic waveguides. The profile of the electromagnetic field provides intuitive physical insight to study the Markovian and non-Markovian dynamics of open quantum systems. The transition from non-Markovian to Markovian process is demonstrated by increasing the size of environment, as the energy evolution changes from oscillating to an exponential decay, and the revival period increases. Moreover, the dynamical decoupling with a sequence of phase modulations is introduced to such a photonic open system to form a band structure in time dimension, where the energy dissipation can be significantly accelerated or inhibited. It opens the possibility to tune the dissipation in photonic system, similar to the dynamic decoupling of spins.

quant-ph↗

arXiv2013

Relation between semi- and fully-device-independent protocols

We study the relation between semi and fully device independent protocols. As a tool, we use the correspondence between Bell inequalities and dimension witnesses. We present a method for converting the former into the latter and vice versa. This relation provides us with interesting results for both scenarios. First, we find new random number generation protocols with higher bit rates for both the semi and fully device independent cases. As a byproduct, we obtain whole new classes of Bell inequalities and dimension witnesses. Then, we show how optimization methods used in studies on Bell inequalities can be adopted for dimension witnesses.

quant-ph↗

arXiv2013

Cavity-enhanced narrow bandwidth photon pairs generation at the telecom wavelength with a triple resonance configuration

In this paper, we present a narrow bandwidth source at the telecom wavelength based on type II PPKTP crystal inside an optical cavity. Simultaneous resonances of the pump and the two down-converted fields are achieved by special design of the cavity. This is a triple resonances optical parametric oscillator (OPO) far below threshold, which generates photon pairs at around 1560 nm with a bandwidth of 8 MHz. A coherence time of 27.7 ns is estimate by a time correlation measurement, and the estimated photon pairs generating rate is 134 s-1MHz-1mW-1. As entangled photon pairs in the telecom regime is suitable for long distance transmission and manipulation, this source is desirable for the demands of quantum communication.

quant-ph↗

arXiv2013

Multimode image memory based on a cold atomic ensemble

Quantum memory is one of key ingredients consisting of quantum networks, therefore storing light at single photon level is an important step for realizing quantum networks. A photon, encoded in a high-dimensional state, can significantly increase the information capacity that a photon can carry. Furthermore, quantum memories with the ability to store multiple optical modes offer advantages in speed and robustness over single mode memories in quantum communication and computation. So far, there is no any report about the storage and retrieval of multiple mode images at single photon level. Here, we give a first experimental demonstration to our best knowledge, that two spatial probe fields at single photon level, each imprinted a real image, can be stored and retrieved with a considerable visibility and similarity through electromagnetically induced transparency in cold atomic ensemble. The accomplishment of multiple modes quantum image memory would move closer towards high dimensional quantum networks.

quant-ph↗