Orbital Angular Momentum Multiplexed Quantum Dense Coding

To beat the channel capacity limit of conventional quantum dense coding (QDC) with fixed quantum resources, we experimentally implement the orbital angular momentum (OAM) multiplexed QDC (MQDC) in a continuous variable system based on a four-wave mixing process. First, we experimentally demonstrate that the Einstein-Podolsky-Rosen entanglement source coded on OAM modes can be used in a single channel to realize the QDC scheme. Then, we implement the OAM MQDC scheme by using the Einstein-Podolsky-Rosen entanglement source coded on OAM superposition modes. In the end, we make an explicit comparison of channel capacities for four different schemes and find that the channel capacity of the OAM MQDC scheme is substantially enhanced compared to the conventional QDC scheme without multiplexing. The channel capacity of our OAM MQDC scheme can be further improved by increasing the squeezing parameter and the number of multiplexed OAM modes in the channel. Our results open an avenue to construct high-capacity quantum communication networks.

Figure 1

The detailed experimental setup for a OAM MQDC scheme. (a) OAM MQDC scheme. HWP: half-wave plate; PBS: polarization beam splitter; AOM: acousto-optic modulator; SLM: spatial light modulator; GL: Glan-Laser polarizer; Rb-85: hot ${}^{85}\mathrm{Rb}$ vapor cell; GT: Glan-Thompson polarizer; AM: amplitude modulator; PM: phase modulator; SS: signal source; BS1 and BS2: $50:50$ beam splitters; BHD1 and BHD2: balanced homodyne detections; HJ: hybrid junction; SA: spectrum analyzer; P: pump beam; Pr: probe beam; Conj: conjugate beam; LO: local oscillator. (b) Energy level diagram of the double-$\mathrm{\Lambda }$ scheme in the $D1$ line of ${}^{85}\mathrm{Rb}$. $\mathrm{\Delta }$: one-photon detuning; $\delta$: two-photon detuning.

Figure 2

The measured output noise power spectra. (a) and (b) show the measured noise power spectra for a coherent state scheme with $L{G}_1$ mode (blue and red traces) and a QDC scheme with the seeded probe beam coded on the $L{G}_1$ mode (orange and black traces). (c)–(f) show the measured noise power spectra for a coherent state scheme with $L{G}_1+L{G}_{-1}$ superposition mode (blue and red traces) and a OAM MQDC scheme with the seeded probe beam coded on the $L{G}_1+L{G}_{-1}$ superposition mode (orange and black traces). (a),(c),(e) show the measured noise power spectra of the amplitude quadrature, while (b),(d),(f) show the measured noise power spectra of the phase quadrature. The vertical scale is normalized to the SNL.

Figure 3

The channel capacities for different schemes vs topological charges $l$. The error bars are obtained from the standard deviations of multiple repeated measurements.