Entanglement-assisted quantum communication has substantial advantages in surpassing the power of classical communication by utilizing the entangled state. Up to now, most of entanglement-assisted quantum communications with dense coding are limited to the proof-of-principle experiments. Here, we experimentally demonstrate the deterministic entanglement-assisted quantum communication based on the continuous-variable (CV) entangled state over 20 km commercial fiber channels. We propose a new CV dense coding scheme with improved classical signals and show that the transmission distance of CV entanglement-assisted quantum communication can be extended compared with that using fixed classical signals. By applying the frequency division multiplexing technique, we simultaneously decode 10 classical signals submerged in the shot noise of coherent state with the help of CV entangled state after the transmission through a 20.121 km fiber channel. The results show that around 3 times of channel capacity in classical communication with coherent state are achieved in the CV entanglement-assisted communication with the frequency division multiplexing technique. Our result takes a crucial step towards realizing the deterministic metropolitan entanglement-assisted quantum communication in practical quantum channels.
Entanglement-assisted quantum communication has substantial advantages in surpassing the power of classical communication by utilizing the entangled state. Up to now, most of entanglement-assisted quantum communications with dense coding are limited to the proof-of-principle experiments. Here, we experimentally demonstrate the deterministic entanglement-assisted quantum communication based on the continuous-variable (CV) entangled state over 20 km commercial fiber channels. We propose a new CV dense coding scheme with improved classical signals and show that the transmission distance of CV entanglement-assisted quantum communication can be extended compared with that using fixed classical signals. By applying the frequency division multiplexing technique, we simultaneously decode 10 classical signals submerged in the shot noise of coherent state with the help of CV entangled state after the transmission through a 20.121 km fiber channel. The results show that around 3 times of channel capacity in classical communication with coherent state are achieved in the CV entanglement-assisted communication with the frequency division multiplexing technique. Our result takes a crucial step towards realizing the deterministic metropolitan entanglement-assisted quantum communication in practical quantum channels.
Quantum key distribution with continuous variables already uses advantageous high-speed single-mode homodyne detection with low electronic noise at room temperature. Together with continuous-variable information encoding to nonclassical states, the distance for secure key transmission through lossy channels can approach 300 km in current optical fibers. Such protocols tolerate higher channel noise and also limited data processing efficiency compared to coherent-state protocols. The secret key rate can be further increased by increasing the system clock rates, and, further, by a suitable frequency-mode-multiplexing of optical transmission channels. However, the multiplexed modes couple together in the source or any other part of the protocol. Therefore, multiplexed communication will experience cross talk and the gain can be minuscule. Advantageously, homodyne detectors allow solving this cross-talk problem by proper data processing. It is a potential advantage over protocols with single-photon detectors, which do not enable similar data processing techniques. We demonstrate the positive outcome of this methodology on the experimentally characterized frequency-multiplexed entangled source of femtosecond optical pulses with natural cross talk between eight entangled pairs of modes. As the main result, we predict the almost 15-fold higher secret key rate. This experimental test and analysis of frequency-multiplexed entanglement source open the way for the field implementation of high-capacity quantum key distribution with continuous variables.
Channel multiplexing quantum communication based on exploiting continuous-variable entanglement of optical modes offers great potential to enhance channel capacity and save quantum resource. Here, we present a frequency-comb-type control scheme for simultaneously extracting a lot of entangled sideband modes with arbitrary frequency detuning from a squeezed state of light. We experimentally demonstrate fourfold channel multiplexing quantum dense coding communication by exploiting the extracted four pairs of entangled sideband modes. Due to high entanglement and wide frequency separation between each entangled pairs, these quantum channels have large channel capacity and the cross talking effect can be avoided. The achieved channel capacities have surpassed that of all classical and quantum communication under the same bandwidth published so far. The presented scheme can be extended to more channels if more entangled sideband modes are extracted.
Entanglement offers substantial advantages in quantum information processing, but loss and noise hinder its application in practical scenarios. Although it has been well known for decades that the classical communication capacity of lossy and noisy bosonic channels can be significantly enhanced by entanglement, no practical encoding and decoding schemes are available to realize any entanglement-enabled advantage. Here, we report structured encoding and decoding schemes for such an entanglement-assisted communication scenario. Specifically, we show that phase encoding on an entangled two-mode squeezed vacuum state saturates the entanglement-assisted classical communication capacity of a very noisy channel and overcomes the fundamental limit on covert communication that exists without the assistance of entanglement. We then construct receivers for optimum hypothesis-testing protocols with discrete phase modulation and for optimum noisy phase-estimation protocols with continuous phase modulation. Our results pave the way for entanglement-assisted communication and sensing in the radio-frequency and microwave spectral ranges.
Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a $\ensuremath{-}5.9$-dB squeezed state at 1.3 $\text{\ensuremath{\mu}}\mathrm{m}$ through 3 km of optical fiber. We obtain a $\ensuremath{-}3.2$-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.
Entanglement distribution over deployed fiber links is essential for implementing quantum communication protocols. However, environmental noise in deployed fibers, such as classical signals, stray light, and Raman scattering, can significantly degrade entanglement, ultimately destroying the nonlocal correlations required for quantum communication. In this work, we demonstrate noise-resilient quantum communication over 40 km of intra-city deployed fiber, with a total transmission loss of 23.04 dB, by utilizing hyperentanglement in the time-bin and frequency degrees of freedom. By exploiting the frequency anticorrelation of the photon pairs and implementing frequency-to-time mapping using dense wavelength-division multiplexing combined with calibrated delay lines, we realize frequency-resolved detection that discriminates frequency-anticorrelated entangled photon pairs from uncorrelated noise. This enables reliable entanglement distribution with over an order-of-magnitude improvement in signal-to-noise ratio and a substantial increase in Bell-state fidelity, even in high-noise and high-loss conditions of deployed fibers. Moreover, entanglement—initially lost due to entanglement sudden death—is successfully restored, enabling a clear violation of the Bell inequality. These results establish a practical and scalable method for entanglement distribution in noisy and lossy environments, advancing the feasibility of real-world quantum communication over existing fiber infrastructure.
A real time deterministic quantum teleportation over a single fiber channel was implemented experimentally by exploiting the generated EPR entanglement at 1550 nm. A 1342 nm laser beam was used to transfer the classical information in real time and also acted as a synchronous beam to realize the synchronization of the quantum and classical information. The dependence of the fidelity on the transmission distance of the fiber channel was studied experimentally with optimizing the transmission efficiency of the lossy channel that was established to manipulate the beam of the EPR entanglement in Alice's site. The maximum transmission distance of the deterministic quantum teleportation was 10 km with the fidelity of 0.51 ± 0.01, which is higher than the classical teleportation limit of 1/2. The work provides a feasible scheme to establish metropolitan quantum networks over fiber channels based on deterministic quantum teleportation.