Junxiang Zhang

Realization of Dirac point with double cones in optics

The Dirac point (DP) with a double-cone structure for optical fields can be realized in optically homogenous media. The condition for the realization of DP in optical systems is the varying of refractive index from negative to zero and then to positive. Our analysis verify that, similar to electrons in graphene, the light field near DP possesses of the pseudodiffusive property obeying the 1/L scaling law, where L is the propagation distance inside the media.

Bright two-color tripartite entanglement with second harmonic generation

The bright two-color tripartite entanglement is investigated in the process of type II second harmonic generation (SHG) operating above threshold. The two pump fields and the second harmonic field are proved to be entangled, and the dependence of the entanglement degree on pump parametersigma and normalized frequency Omega is also analyzed.

Quantum switch for continuous variable teleportation

We propose a quantum teleportation scheme in which a quantum state is teleported from the sending station (Alice) to either of two receiving stations (Bob1, Bob2). In this scheme, two pairs of EPR beams with identical frequency and constant phase relation are used to produce two pairs of conditional entangled beams by composing their modes on two beamsplitters. One output of a beamsplitter is sent to Alice and the two outputs of the other beamsplitter are sent to Bob1 and Bob2. Which receiving station actually receives the teleported state can be decided by correlating the in-phase or out-of-phase quadrature components of two two-mode squeezed vacuum states. The switch system manipulated by squeezed state light might be developed as a practical quantum switch device for the communication and teleportation of quantum information.

Small-displacement measurements using high-order Hermite-Gauss modes

We present a scheme for small-displacement measurements using high-order Hermite-Gauss modes and balanced homodyne detection. We demonstrate its use with experimental results of displacement measurements using fundamental transverse mode TEM00 and first order transverse mode TEM10 as signal modes. The results show a factor of 1.41 improvement in measurement precision with the TEM10 mode compared with that with the TEM00 mode. This scheme has potential applications in precision metrology, atomic force microscopy, and optical imaging.

Fidelity with quadrature component variances for continuous-variable quantum teleportation

The efficiency of continuous-variable quantum teleportation is typically quantified by fidelity especially in experiment. From the original definition of fidelity in two pure states, we give an expansion of fidelity which is related to the variances of output and input states. With a coherent or a squeezed input state, it is convenient to quantify the quantum teleportation experiment, since the variances of quadrature components of the input and output states are measurable. Furthermore, the fidelity was discussed when the quantum channel lies in the phase- and amplitude-noisy environment, which is unavoidable in experiment, and this showed that the effect of phase noise on teleportation is more sensitive than that of the amplitude-noisy environment. The classical fidelity limit of squeezed state quantum teleportation is also obtained when the entanglement resources do not exist.

Generating multiplexed entanglement frequency comb in a nondegenerate optical parametric amplifier

In this paper, we propose a scheme to produce a multiplexed entanglement frequency comb in a type II phase-matching nondegenerate optical parametric amplifier (NOPA) operating below threshold. The entanglement of the signal and idler frequency combs in the wide frequency range, which is limited by the phase-matching bandwidth of the NOPA, is investigated by the inseparability criterion. Furthermore, N Einstein–Podolsky–Rosen pairs can be created from the two combs without any reduction in the correlations by using frequency-dependent beam splitters.

Efficient reflection via four-wave mixing in a Doppler-free electromagnetically-induced-transparency gas system

We experimentally demonstrate the high-efficiency reflection of a probe field in {Lambda}-type three-level atoms of cesium vapor driven by two counterpropagating coupling fields. More than 60% of reflection efficiency is observed at the phase-matching angle. The underlying mechanism theoretically is investigated as the four-wave mixing is enhanced by the electromagnetically-induced transparency. Both of the two Doppler-free two-photon resonances (one for the probe and co-propagating fields, the other for the reflected and the counterpropagation fields) play an important role in satisfying the phase matching in the reflection direction. The phase compensation due to the anomalous dispersion and the decrease of effective absorption length in the atomic system allow the efficient reflection to be observed in a wide range of incident angles of the probe field and detunings of the coupling field.

Direct generation of spatial quadripartite continuous variable entanglement in an optical parametric oscillator

Multipartite entanglement is used for quantum information applications, such as building multipartite quantum communications. Generally, generation of multipartite entanglement is based on a complex beam-splitter network. Here, based on the spatial freedom of light, we experimentally demonstrated spatial quadripartite continuous variable entanglement among first-order Hermite-Gaussian modes using a single type II optical parametric oscillator operating below threshold with an HG0245° pump beam. The entanglement can be scalable for larger numbers of spatial modes by changing the spatial profile of the pump beam. In addition, spatial multipartite entanglement will be useful for future spatial multichannel quantum information applications.

Superluminal reflection and transmission of light pulses via resonant four-wave mixing in cesium vapor

We report the experimental manipulation of the group velocities of reflected and transmitted light pulses in a degenerate two-level atomic system driven by a standing wave, which is created by two counter-propagating light beams of equal frequencies but variable amplitudes. It is shown that the light pulse is reflected with superluminal group velocity while the transmitted pulse propagates from subluminal to superluminal velocities via changing the power of the backward coupling field. We find that the simultaneous superluminal light reflection and transmission can be reached when the power of the backward field becomes closer or equal to the forward power, in this case the periodical absorption modulation for photonic structure is established in atoms. The theoretical discussion shows that the anomalous dispersion associated with a resonant absorption dip within the gain peak due to four-wave mixing leads to the superluminal reflection, while the varying dispersion from normal to anomalous at transparency, transparency within absorption, and electromagnetically induced absorption windows leads to the subluminal to superluminal transmission.

Manipulation of squeezed state in electromagnetically induced transparency system via dynamic Stark effect

We calculate the delay time and noise spectrum of a squeezed state throughout an electromagnetically induced transparency medium with dynamic Stark splitting. It is shown that the noise spectrum splits into two parts with the same delay time, so that the delayed squeezing can survive well in two channels. Furthermore, we show that the two squeezing channels as well as the delay time can be manipulated via one-photon detuning and detection frequency such that the quantum state with high delay time and squeezing can be well preserved. This avoids the influence of large noise from laser near zero detection frequency.