Wei-Tao Liu

Double-grating polarizer for terahertz radiation with high extinction ratio

We propose a layout of a high extinction ratio polarizer in the terahertz (THz) domain. This polarizer is composed of two dense metal wire gratings separated in parallel, of which the grating constant is much smaller than the incident wavelength. Numerical analysis shows that, in the range of 0.3 THz-3 THz, the transmission of TM wave through this polarizer is higher than 97% and the extinction ratio achieved is about 180 dB--much higher than the conventional wire-grid polarizer.

Proof-of-principle experiment of a modified photon-number-splitting attack against quantum key distribution

Any imperfections in a practical quantum key distribution (QKD) system may be exploited by an eavesdropper to collect information about the key without being discovered. We propose a modified photon-number-splitting attack scheme against QKD systems based on weak laser pulses taking advantage of possible multiphoton pulses. Proof-of-principle experiments are demonstrated. The results show that the eavesdropper can get information about the key generated between the legitimate parties without being detected. Since the equivalent attenuation introduced by the eavesdropper for pulses of different average photon numbers are different, the decoy-state method is effective in fighting against this kind of attack. This has also been proven in our experiments.

Multi-scale Adaptive Computational Ghost Imaging

In some cases of imaging, wide spatial range and high spatial resolution are both required, which requests high performance of detection devices and huge resource consumption for data processing. We propose and demonstrate a multi-scale adaptive imaging method based on the idea of computational ghost imaging, which can obtain a rough outline of the whole scene with a wide range then accordingly find out the interested parts and achieve high-resolution details of those parts, by controlling the field of view and the transverse coherence width of the pseudo-thermal field illuminated on the scene with a spatial light modulator. Compared to typical ghost imaging, the resource consumption can be dramatically reduced using our scheme.

Imaging through scattering layers exceeding memory effect range with spatial-correlation-achieved point-spread-function

We propose to measure intensity transmission matrices or point-spread-function (PSF) of diffusers via spatial-correlation, with no scanning or interferometric detection required. With the measured PSF, we report optical imaging based on the memory effect that allows tracking of moving objects through a scattering medium. Our technique enlarges the limited effective range of traditional imaging techniques based on the memory effect, and substitutes time-consuming iterative algorithms by a fast cross-correlation deconvolution method to greatly reduce time consumption for image reconstruction.

Coprime frequency modulation on light field for correlation imaging

Correlation imaging experiments correlate the outputs from two photodetectors. The image can be obtained by calculating the correlation function between results of two detectors. To reconstruct image of the object, many frames of different speckles are required. Therefore, the speed of correlation imaging is strongly limited by the speed of modulation of the light field. Usually, we use Spatial Light Modulator (SLM） to load different random phase at different positions of a plane-wave light field. The refresh rate of the speckle fields is thus limited by the surface refresh rate of SLM. However, the response speed of each pixel when we control it independently is far greater than the refresh rate of the whole surface. Based on this fact, we propose to modulate each pixel independently with different sinusoidal signals, in order to improve the refresh rate of speckle field. To generate randomly fluctuations, the frequencies of different modulation signals are selected to be coprime. At the same time, we can know in advance the intensity distribution of the speckle field in every frame, since we know the phase of each pixel when every pulse light modulated by the SLM.

EXPERIMENTAL REALIZATION OF PROBABILISTIC REMOTE STATE PREPARATION

We experimentally demonstrate a protocol of probabilistic remote state preparation by virtue of entanglement, local operation and forward classical communications. Arbitrary polarization qubit state can be remotely prepared with this protocol. For both pure states and mixed states, the average efficiency is no less than 50% and the classical information cost is 1 bit. All 18 remotely prepared states were estimated with quantum state tomography system. The fidelities are all above 0.99, with average fidelity being 0.9956.