arXiv subjects

Guang-Can Guo

Publications and source records attributed to Guang-Can Guo.

At least 595 records · Page 33Linked to original sources

arXiv2017

Indistinguishability-induced classical-to-nonclassical transition of photon statistics

Photon statistics is one of the key properties of the photon state for the study of quantum coherence and quantum information techniques. Here, we discuss the photon indistinguishability induced bunching effect which can significantly change photon statistics. Through the measurement of the second-order degree of coherence of a mixed photon state composed of a single-photon state and a weak coherent state, the statistical transition from a classical behavior to a nonclassical behavior is experimentally demonstrated by modifying the indistinguishability of the two-photon states. The study will help us to understand and control the photon statistics with a method for quantum optical coherence and quantum information applications. It also indicates that the photon indistinguishability is a key parameter for multipartite quantum coherence.

quant-ph↗

arXiv2017

A self-guide method to search maximal Bell violation for unknown quantum states

In recent decades, a great variety of researches and applications concerning Bell nonlocality have been developed with the advent of quantum information science. Providing that Bell nonlocality can be revealed by the violation of a family of Bell inequalities, finding maximal Bell violation (MBV) for unknown quantum states becomes an important and inevitable task during Bell experiments. In this paper we introduce a self-guide method to find MBVs for unknown states using a stochastic gradient ascent algorithm (SGA), by parameterizing the corresponding Bell operators. For all the investigated systems (2-qubit, 3-qubit and 2-qutrit), this method can ascertain the MBV accurately within 100 iterations. Moreover, SGA exhibits significant superiority in efficiency, robustness and versatility compared to other possible methods.

quant-ph↗

arXiv2017

A photonic quantum interface for hybrid quantum network

The hybrid quantum network, a universal form of quantum network which is aimed for quantum communication and distributed quantum computation, is that the quantum nodes in it are realized with different physical systems. This universal form of quantum network can combine the advantages and avoid the inherent defects of the different physical system. However, one obstacle standing in the way is the compatible photonic quantum interface. One possible solution is using non-degenerate, narrow-band, entangled photon pairs as the photonic interface. Here, for the first time, we generate nondegenrate narrow-band polarization-entangled photon pairs in cavity-enhanced spontaneous parametric down-conversion process. The bandwidths and central wavelengths of the signal and idler photons are 9 MHz at 935 nm and 9.5 MHz at 880 nm, which are compatible with trapped ion system and solid-state quantum memory system. The entanglement of the photon source is certified by quantum state tomography, showing a fidelity of 89.6% between the generated quantum state with a Bell state. Besides, a strong violation against Bell inequality with 2.36+/-0.03 further confirms the entanglement property of the photon pairs. Our method is suitable for the hybrid quantum network and will take a big step in this field.

quant-ph↗

arXiv2017

Proof-of-principle experimental realization of a qubit-like qudit-based quantum key distribution scheme

In comparison to qubit-based protocols, qudit-based quantum key distribution (QKD) ones gen- erally allow two cooperative parties to share unconditionally secure keys under a higher channel noise. However, it is very hard to prepare and measure the required quantum states in qudit-based protocols in general. One exception is the recently proposed highly error tolerant qudit-based proto- col known as the Chau15 [1]. Remarkably, the state preparation and measurement in this protocol can be done relatively easily since the required states are phase encoded almost like the diagonal basis states of a qubit. Here we report the first proof-of-principle demonstration of the Chau15 protocol. One highlight of our experiment is that its post-processing is based on practical one-way manner, while the original proposal in Ref. [1] relies on complicated two-way post-processing, which is a great challenge in experiment. In addition, by manipulating time-bin qudit and measurement with a variable delay interferometer, our realization is extensible to qudit with high-dimensionality and confirms the experimental feasibility of the Chau15 protocol.

quant-ph↗

arXiv2017

Tunneling frustration induced peculiar supersolid phases in the extended Bose-Hubbard model

By using a state of art tensor network state method, we study the ground-state phase diagram of an extended Bose-Hubbard model on the square lattice with frustrated next-nearest neighboring tunneling. In the hardcore limit, tunneling frustration stabilizes a peculiar half supersolid (HSS) phase with one sublattice being superfluid and the other sublattice being Mott Insulator away from half filling. In the softcore case, the model shows very rich phase diagrams above half filling, including three different types of supersolid phases depending on the interaction parameters. The considered model provides a promising route to experimentally search for novel stable supersolid state induced by frustrated tunneling in below half filling region with dipolar atoms or molecules.

cond-mat.str-el↗

arXiv2017

Vortex-core structure in a mixture of Bose and Fermi superfluids

We study a single quantized vortex in the fermionic component of a mixture of Fermi superfluid and Bose-Einstein condensate. As the density ratio between the boson and the fermion components is tuned, we identify a transition in the vortex-core structure, across which fermions in the vortex core become completely depleted even in the weak-coupling Bardeen-Cooper-Schrieffer regime. This is accompanied by changes in key properties of the vortex state, as well as by the localization of the Bose-Einstein condensate in the vortex core. The transition in the vortex-core structure can be experimentally probed in Bose-Fermi superfluid mixtures by detecting the size and visibility of the vortices.

cond-mat.quant-gas↗

arXiv2017

Symmetry-enriched Bose-Einstein condensates in spin-orbit coupled bilayer system

We consider the fate of Bose-Einstein condensation (BEC) with time-reversal symmetry and inversion symmetry in a spin-orbit coupled bilayer system. When these two symmetry operators commute, all the single particle bands are exactly two-fold degenerate in the momentum space. The scattering in the two-fold degenerate rings can relax the spin-momentum locking effect resulting from spin-orbit coupling, thus we can realize the spin polarized plane wave phase even when the inter-particle interaction dominates. When these two operators anti-commute, the lowest two bands may have the same minimal energy, which have totally different spin structures. As a result, the competition between different condensates in these two energetically degenerate rings can give rise to interesting stripe phases with atoms condensed at two or four colinear momenta. We find that the crossover between these two cases is accompanied by the excited band condensation when the interference energy can overcome the increased single particle energy in the excited band. This effect is not based on strong interaction, thus can be realized even with moderate interaction strength.

cond-mat.quant-gas↗

arXiv2017

Experimental nonlocal steering of Bohmian trajectories

Interpretations of quantum mechanics (QM), or proposals for underlying theories, that attempt to present a definite realist picture, such as Bohmian mechanics, require strong non-local effects. Naively, these effects would violate causality and contradict special relativity. However if the theory agrees with QM the violation cannot be observed directly. Here, we demonstrate experimentally such an effect: we steer the velocity and trajectory of a Bohmian particle using a remote measurement. We use a pair of photons and entangle the spatial transverse position of one with the polarization of the other. The first photon is sent to a double-slit-like apparatus, where its trajectory is measured using the technique of Weak Measurements. The other photon is projected to a linear polarization state. The choice of polarization state, and the result, steer the first photon in the most intuitive sense of the word. The effect is indeed shown to be dramatic, while being easy to visualize. We discuss its strength and what are the conditions for it to occur.

quant-ph↗

arXiv2017

Dove prism in single-path Sagnac interferometer for orbital-angular-momentum photon states

The degree of freedom of orbital angular momentum (OAM) is an important resource in high-dimensional quantum information processing, as the quantum number of OAM can be infinite. The Dove prism (DP) is a most common tool to manipulate the OAM light, such as in interferometers. However, the Dove prism does not preserve the polarization of the photon states and decreases the sorting fidelity of the interferometer. In this work, we analyze the polarization-dependent effect of the DP on single-path Sagnac interferometers. The results are instructive to quantum information processing with OAM light. We also proposed a modified single-path beam splitter Sagnac interferometer (BSSI), of which the sorting fidelity is independent on input polarization and can be 100\% in principle. The single-path BSSI is stable for free running. These merits are crucial in quantum information processing, such as quantum cryptography.

quant-ph↗

arXiv2017

A tunable hybrid qubit in a triple quantum dot

We experimentally demonstrate quantum coherent dynamics of a triple-dot-based multi-electron hybrid qubit. Pulsed experiments show that this system can be conveniently initialized, controlled, and measured electrically, and has good coherence time as compared to gate time. Furthermore, the current multi-electron hybrid qubit has an operation frequency that is tunable in a wide range, from 2 to about 15 GHz. We provide qualitative understandings of the experimental observations by mapping it onto a three-electron system, and compare it with the double dot hybrid qubit and the all-exchange triple-dot qubit.

cond-mat.mes-hall↗

arXiv2017

Phase-Controlled Phonon Laser

A phase-controlled ultralow-threshold phonon laser is proposed by using tunable optical amplifiers in coupled-cavity-optomechanical system. Giant enhancement of coherent photon-phonon interactions is achieved by engineering the strengths and phases of external parametric driving. This in turn enables single-photon optomechanics and low-power phonon lasing, opening up novel prospects for applications, e.g. quantum phononics and ultrasensitive motion detection.

quant-ph↗

arXiv2017

Round-robin-differential-phase-shift quantum key distribution with and without monitoring signal disturbance

Round-robin-differential-phase (RRDPS) quantum key distribution (QKD) protocol has attracted intensive studies due to its distinct security characteristic, e.g., information leakage in RRDPS can be bounded without learning error rate of key bits. Nevertheless, its implementation is still far from practical due to the complication of its measurement device. Moreover, on the theoretical side, its security is still not clear in view of optimal attack. Here, by observing a potential phase randomization of the encoding states and its connection with eavesdropper's ancilla, we develop a theory to bound information leakage quite tightly and differently. Our theory is applicable for both with and without monitoring signal disturbance scenarios, which is significant for the understanding of RRDPS. Based on our novel security proof, the practicality and performance of RRDPS can be both improved dramatically. Furthermore, we realize a proof-of-principle experiment up to 140 km fiber distance which is the longest achievable distance of RRDPS system until now, while the original security proof predicts no secret key can be generated in our experiment. Our results pave an avenue towards practical RRDPS.

quant-ph↗

arXiv2017

Taking tomographic measurements for photonic qubits 88 ns before they are created

We experimentally demonstrate that tomographic measurements can be performed for states of qubits before they are prepared. A variant of the quantum teleportation protocol is used as a channel between two instants in time, allowing measurements for polarisation states of photons to be implemented 88 ns before they are created. Measurement data taken at the early time and later unscrambled according to the results of the protocol's Bell measurements, produces density matrices with an average fidelity of $0.90 \pm 0.01$ against the ideal states of photons created at the later time. Process tomography of the time-reverse quantum channel finds an average process fidelity of $0.84 \pm 0.02$. While our proof-of-principle implementation necessitates some post-selection, the general protocol is deterministic and requires no post-selection to sift desired states and reject a larger ensemble.

quant-ph↗

arXiv2017

Effect of unbalanced and common losses in quantum photonic integrated circuits

Loss is inevitable for the optical system due to absorption of materials, scattering caused by the defects and surface roughness. In quantum optical circuits, the loss can not only reduce the intensity of signal, but also affect the performance of quantum operations. In this work, we divide losses into unbalanced linear loss and shared common loss, and provide a detailed analysis on how loss affects the integrated linear optical quantum gates. It is found that the orthogonality of eigenmodes and the unitary phase relation of the coupled waveguide modes are destroyed by the loss. As a result, the fidelity of single- and two-qubit operations decrease significant as the shared loss becomes comparable to the coupling strength. Our results are important for the investigation of large-scale photonic integrated quantum information process.

physics.optics↗

arXiv2017

Degenerate cavity supporting more than 31 Laguerre-Gaussian modes

Photons propagating in Laguerre-Gaussian modes have characteristic orbital angular momentums, which are fundamental optical degrees of freedom. The orbital angular momentum of light has potential application in high capacity optical communication and even in quantum information processing. In this work, we experimentally construct a ring cavity with 4 lenses and 4 mirrors that is completely degenerate for Laguerre-Gaussian modes. By measuring the transmission peaks and patterns of different modes, the ring cavity is shown to supporting more than 31 Laguerre-Gaussian modes. The constructed degenerate cavity opens a new way for using the unlimited resource of available angular momentum states simultaneously.

quant-ph↗

arXiv2017

Quantum Secure Direct Communication with Quantum Memory

Quantum communication provides an absolute security advantage, and it has been widely developed over the past 30 years. As an important branch of quantum communication, quantum secure direct communication (QSDC) promotes high security and instantaneousness in communication through directly transmitting messages over a quantum channel. The full implementation of a quantum protocol always requires the ability to control the transfer of a message effectively in the time domain; thus, it is essential to combine QSDC with quantum memory to accomplish the communication task. In this paper, we report the experimental demonstration of QSDC with state-of-the-art atomic quantum memory for the first time in principle. We used the polarization degrees of freedom of photons as the information carrier, and the fidelity of entanglement decoding was verified as approximately 90%. Our work completes a fundamental step toward practical QSDC and demonstrates a potential application for long-distance quantum communication in a quantum network.

quant-ph↗

arXiv2017

Gradient optimization of finite projected entangled pair states

The projected entangled pair states (PEPS) methods have been proved to be powerful tools to solve the strongly correlated quantum many-body problems in two-dimension. However, due to the high computational scaling with the virtual bond dimension $D$, in a practical application PEPS are often limited to rather small bond dimensions, which may not be large enough for some highly entangled systems, for instance, the frustrated systems. The optimization of the ground state using time evolution method with simple update scheme may go to a larger bond dimension. However, the accuracy of the rough approximation to the environment of the local tensors is questionable. Here, we demonstrate that combining the time evolution method with simple update, Monte Carlo sampling techniques and gradient optimization will offer an efficient method to calculate the PEPS ground state. By taking the advantages of massive parallel computing, we can study the quantum systems with larger bond dimensions up to $D$=10 without resorting to any symmetry. Benchmark tests of the method on the $J_1$-$J_2$ model give impressive accuracy compared with exact results.

quant-ph↗

arXiv2017

Revealing photons' past via quantum twisted double-slit experiment

Are quantum states real? This most fundamental question in quantum mechanics has not yet been satisfactorily resolved, although its realistic interpretation seems to have been rejected by various delayed-choice experiments. Here, to address this long-standing issue, we present a quantum twisted double-slit experiment. By exploiting the subluminal feature of twisted photons, the real nature of a photon during its time in flight is revealed for the first time. We found that photons' arrival times were inconsistent with the states obtained in measurements but agreed with the states during propagation. Our results demonstrate that wavefunctions describe the realistic existence and evolution of quantum entities rather than a pure mathematical abstraction providing a probability list of measurement outcomes. This finding clarifies the long-held misunderstanding of the role of wavefunctions and their collapse in the evolution of quantum entities.

quant-ph↗