Dongxu Chen

Orbital angular momentum complex spectrum analyzer for vortex light based on the rotational Doppler effect

The ability to measure the orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there have been many studies on the measurement of OAM modes, it is difficult to quantitatively and instantaneously measure the power distribution among different OAM modes, let alone measure the phase distribution among them. In this work, we propose an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect. The original OAM mode distribution is mapped to an electrical spectrum of beat signals using a photodetector. The power and phase distributions of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement technique to other spatial modes, such as linear polarization modes. These results represent a new landmark in spatial mode analysis and show great potential for applications in OAM-based systems and optical communication systems with mode-division multiplexing.

Probing the topological charge of a vortex beam with dynamic angular double slits

When a vortex beam with a spiral phase structure passes through dynamic angular double slits (ADS), the interference pattern changes alternatively between destructive and constructive at the angular bisector of the ADS. This change is due to their phase difference. Based on this property, we experimentally demonstrate a simple method to precisely and efficiently determine the topological charge of vortex beams. Furthermore, this scheme allows for the simultaneous determination of the modulus and the sign of the topological charge of the vortex beams.

Measuring mode indices of a partially coherent vortex beam with Hanbury Brown and Twiss type experiment

It is known that the cross-correlation function (CCF) of a partially coherent vortex (PCV) beam shows a robust link with the radial and azimuthal mode indices. However, the previous proposals are difficult to measure the CCF in practical systems, especially in the case of astronomical objects. In this letter, we demonstrate experimentally that the Hanbury Brown and Twiss effect can be used to measure the mode indices of the original vortex beam and investigate the relationship between the spatial coherent width and the characterization of CCF of the PCV beam. The technique we exploit is quite efficient and robust, and it may be useful in the field of free space communication and astronomy which are related to the photons orbital angular momentum.

Probing the fractional topological charge of a vortex light beam by using dynamic angular double slits

Vortex beams with fractional topological charge (FTC) have many special characteristics and novel applications. However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit (ADS), the far-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.

Demonstration of quantum permutation algorithm with a single photon ququart

We report an experiment to demonstrate a quantum permutation determining algorithm by employing photon polarization and spatial modes. This work displays the remarkable speedup of quantum algorithm and strong universality in quantum computation.

Robust method to probe the topological charge of a Bessel beam by dynamic angular double slits

Precisely determining the topological charge (TC) of both integral Bessel beams (IBB) and fractional Bessel beams (FBB) is a key issue for their applications. However, most of the common methods could not probe both the IBB and FBB precisely and robustly. In this paper, we present a robust method to probe the topological charge of a Bessel beam by dynamic angular double slits (ADSs). We find that when a Bessel beam vertically illuminates on a dynamic ADS, the information of the TC can be retrieved from its Fourier transform patterns near the optical axis. Even though there is a small misalignment between the center of the beams and the ADS, the global variation tendency is still good enough to obtain the TC. Based on these properties, the dynamic ADS device combining the method of fitting the experimental data can be used to measure the TC of any Bessel light beam precisely and robustly. The error is less than 2% without the misalignment and is less than 6% with a small misalignment for our experimental data. This method paves a new way to measure the TC of vortex beams.

Four-state quantum key distribution exploiting maximum mutual information measurement strategy

We propose a four-state quantum key distribution (QKD) scheme using generalized measurement of nonorthogonal states, the maximum mutual information measurement strategy. Then, we analyze the eavesdropping process in intercept–resend and photon number splitting attack scenes. Our analysis shows that in the intercept–resend and photon number splitting attack eavesdropping scenes, our scheme is more secure than BB84 protocol and has higher key generation rate which may be applied to high-density QKD.