image: Fig.1 Schematic of the experimental setup for continuous-variable entanglement-assisted quantum comumication.Alice encodes classical signals on the ancilla beam by an amplitude modulator (AM) and a phase modulator (PM) and couples it with one of the entangled beams by a 99:1 fiber beamsplitter. Both the entangled beams and local beam are transmitted to Bob through fiber channels respectively. The combination of polarization controller (PC), polarization beam splitter (PBS) and photodetector (PD) is used to control the polarization of output beams at Bob’s station. The output beams after decoding are measured by two homodyne detectors.
Credit: Xiaolong Su et al.
Entanglement-assisted quantum communication has substantial advantages in surpassing the power of classical communication by utilizing the entangled state. As a typical entanglement-assisted quantum communication encoding scheme, quantum dense coding enables two communication parties to enhance the channel capacity with the shared quantum entanglement. In continuous-variable quantum dense coding, the classical signals are encoded on both amplitude and phase quadratures of one entangled beam. Owing to the deterministic advantage in the generation and detection of continuous-variable entangled states, the combination of continuous-variable quantum dense coding enables the implementation of deterministic entanglement-assisted quantum communication.
Since the first experimental demonstration of quantum dense coding with entangled photon pairs, it has been experimentally demonstrated in several physical systems, including optical system, nuclear magnetic resonance system, and atomic system. However, most demonstrations of entanglement-assisted quantum communication with dense coding still remain in proof-of-principle experiments. The implementation of quantum dense coding in practical fiber channels is of great significance for advancing the practicalization of quantum communication. Therefore, it is urgent to carry out research on entanglement-assisted quantum communication with quantum dense coding in fiber channels.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Xiaolong Su from State Key Laboratory of Quantum Optics Technologies and Devices, Institute of Opto-Electronics, Collaborative Innovation Center of Extreme Optics, Shanxi University, China and co-workers experimentally demonstrated the deterministic entanglement-assisted quantum communication in fiber channels. Compared to previous proof-of-principle experiments, they extended the transmission distance of deterministic entanglement-assisted quantum communication from meters to 20.121 km, which has potential applications in metropolitan quantum communication.
They proposed an improved continuous-variable dense coding scheme, where Alice (sender) adjusts the amplitude of the classical signals according to the transmission efficiency to ensure that the received signals can not be decoded by using the coherent state at Bob’s (receiver) station. Moreover, by transmitting entangled state and local oscillator beam independently to reduce the excess noise in the fiber channel, they experimentally demonstrated the deterministic entanglement-assisted quantum communication in 20-km fiber channels. They introduced the details of the experiment as follows.
“The experimental setup is illustrated in Fig. 1. At Alice's station, the generated Einstein-Podolsky-Rosen entangled state is first coupled into the fiber. Subsequently, Alice modulates two classical signals on an ancilla coherent beam simultaneously by the fiber amplitude and phase modulators, respectively, and then couples the modulated ancilla beam and one of the entangled states on a 99:1 fiber beamsplitter to realize the encoding process. Finally, Alice transmits the entangled states to Bob through two independent fiber channels. At Bob’s station, he couples the two entangled beams on a 50:50 beam splitter. The amplitude and phase quadratures of the output beams from the 50:50 beam splitter are simultaneously measured by two homodyne detectors to realize the decoding process”.
“We encode 10 weak classical signals with different frequencies, including 5 amplitude signals and 5 phase signals, by applying the frequency division multiplexing technique as shown in Fig. 2a. With the help of the continuous-variable quantum entanglement, we successfully decode the 10 weak classical signals simultaneously, as shown in Fig. 2b and 2c. Since the correlated noises of the entangled state are lower than the corresponding shot noise limit (vacuum noise), the noise background of entanglement-assisted quantum communication is decreased, the weak classical signals that submerged in the vacuum noise are decoded, thereby the signal-to-noise ratios are increased.”
“The channel capacity of deterministic entanglement-assisted quantum communication is shown in Fig. 3. The results show that the channel capacity of entanglement-assisted quantum communication with improved signals and entanglement-assisted quantum communication is increased compared to classical communication using coherent state. The channel capacities of entanglement-assisted quantum communication based on quantum dense coding at the different transmission distances are higher than that of classical communication with coherent state in the case of large average photon number.” they added.
Journal
Light Science & Applications
Article Title
Deterministic entanglement-assisted quantum communication over 20 km fiber channel