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Experimental long-distance quantum secure direct communication
Feng Zhu, Wei Zhang, Yubo Sheng, Yidong Huang
TL;DR
QSDC seeks secure direct information transmission without encryption or decryption, but prior demonstrations were table-top. This paper reports a long-distance entanglement-based experiment with security testing, polarization encoding, fiber transmission, and Bell-state decoding. After 0.5 km, the two Bell-state fidelities were 91% and 88%, while theory indicates several-tens-kilometer fiber links could be expected.
Problem
Prior QSDC experiments had demonstrated the principle only in table-top settings, motivating evidence for long-distance operation.
Method
The experiment tests the quantum channel, encodes information on entangled-photon polarizations, transmits through optical fiber, and decodes using Bell-state measurement.
Results
91% and 88% are the fidelities of the two polarization-entangled Bell states after 0.5 km fiber transmission.
Takeaways & Limitations
QSDC over fiber links of several tens kilometers could be expected under current optical communication technologies.
Takeaways & Limitations
The analysis assumes equal η_s at both sides and includes data-processing time for the security analysis.
Abstract
from arXiv · showhide
Quantum secure direct communication (QSDC) is an important quantum communication branch, which realizes the secure information transmission directly without encryption and decryption processes. Recently, two table-top experiments have demonstrated the principle of QSDC. Here, we report the first long-distance QSDC experiment, including the security test, information encoding, fiber transmission and decoding. After the fiber transmission of 0.5 km, quantum state fidelities of the two polarization entangled Bell states are 91% and 88%, respectively, which are used for information coding. We theoretically analyze the performance of the QSDC system based on current optical communication technologies, showing that QSDC over fiber links of several tens kilometers could be expected. It demonstrates the potential of long-distance QSDC and supports its future applications on quantum communication networks.
Introduction
QSDC enables secure information transmission without encryption or decryption, and prior experiments had remained table-top. This work reports long-distance entanglement-based QSDC and evaluates fiber transmission and expected reach.
- QSDC transmits information directly without encryption and decryption, avoiding key-management loopholes.
- Recent QSDC demonstrations were table-top experiments based on single photons and entanglement.
- The paper reports the first long-distance entanglement-based QSDC experiment.
- The experiment tests channel security, encodes information on photon-pair polarizations, transmits photons through 0.5 km of optical fiber, and decodes them using Bell-state measurement.
- 91% and 88% are the fidelities of the two polarization-entangled Bell states after fiber transmission.
- QSDC over fiber links of several tens kilometers could be expected under current optical communication technologies.
A. Entanglement-based QSDC protocol
The entanglement-based protocol tests the quantum channel before encoding, then sends encoded photons to Bob for Bell-state decoding. Alice and Bob retain partner photons in fiber-based memories during this sequence.
- Alice generates polarization-entangled Bell-state photon pairs and sends photon B to Bob through 500 m of fiber.
- Bob and Alice measure extracted photons and combine their results to perform a Bell-inequality security test.
- If the channel is secure, Alice encodes information on the polarization-entangled state of the remaining photons A.
- Alice represents ‘1’ with the minus Bell state and ‘0’ with the plus Bell state, then sends photons A to Bob.
- Bob performs a Bell-state measurement between received photons A and stored photons B to decode the information.
B. Experimental setup
The setup uses telecom-band entangled photons, fiber coils as quantum memories, and fiber-based polarization analysis. It measures entanglement quality and implements Bell-state measurement after transmission.
- A spontaneous-four-wave-mixing source generates telecom-band polarization-entangled photon pairs with both photons at 1549.32 nm.
- Bob sends photon B through 500 m of dispersion-shifted fiber, splits it for security measurement and storage, and returns detection records to Alice over a public channel.
- Alice stores photon A in a 2200 m dispersion-shifted-fiber memory for 11 μs before polarization encoding and transmission to Bob.
- Bob stores photon B in a second 11 μs fiber memory until encoded photon A arrives, then sends both photons into polarization Bell-state measurement.
- The Bell-state measurement uses a 50:50 fiber coupler, polarization beam splitters, polarization controllers, delay alignment, detectors, and time-correlated counting.
Results
The experiment separately demonstrates Bell-inequality security testing and Bell-state information encoding/decoding, including transmission through fiber and quantum memories. After transmission, the two Bell states retain fidelities of 91%±3% and 88%±3%, while limited fringe visibility constrains the system error rate.
- Security test: S=2.46±0.12 violates the Bell inequality by 4 standard deviations, demonstrating security of the quantum channel.The reported error includes counting statistics, HWP angular position, and PBS imperfections.
- Experimental setup: The two Bell states require separate M1 and M2 measurements because the BSM uses only two SPDs; four SPDs would permit direct discrimination.This is an equipment constraint of the demonstrated setup rather than a stated limitation of the protocol itself.
- Fiber transmission: 78% and 81% visibilities for M1 and M2 produce coincidence peaks and dips when the photons arrive simultaneously at the fiber coupler.These results are similar to the pre-transmission measurements and support the feasibility of the encoding and decoding processes.
- Fiber transmission: After 0.5 km transmission, the fidelities of |ψ−〉 and |ψ+〉 are 91%±3% and 88%±3%, respectively.The setup uses two 500-meter transmission fibers and two 2.2-kilometer fiber sections as quantum memories, for 2.7 kilometers before measurement.
- Limitations: Further work is required to improve fringe visibility, for example with narrower optical filters and temperature control of the quantum-memory fibers.The paper links visibility to the QSDC system error rate and attributes polarization variation in the fibers as a reduction target.
Discussion and conclusion
The paper demonstrates a fiber-based QSDC prototype and theoretically evaluates its performance using current optical communication technologies. The analysis indicates potential transmission over several tens of kilometers, while capacity and performance remain constrained by Bell-state usage, fiber loss, security-test requirements, and memory storage losses.
- Experimental prototype: The experiment provides a prototype of entanglement-based QSDC over optical fibers, carrying information on polarization-entangled photon pairs and decoding it with Bell-state measurement.Optical fibers serve as both transmission channels and quantum memories at the two sides.
- Theoretical evaluation: The theoretical model evaluates QSDC performance under typical parameters of current optical fiber components and superconducting nanowire single-photon detectors.The model includes photon-pair generation, fiber transmission, quantum-memory storage, polarization modulation, and Bell-state-measurement collection efficiency.
- Theoretical evaluation: Rmax is used to evaluate system performance because it determines the information-transmission capacity of the entanglement-based QSDC system.The maximum coincidence rate is calculated for different transmission distances using the protocol’s storage-time condition and security-test requirements.
- Performance limits: For a fixed photon-pair generation probability, Rmax decreases monotonically with transmission distance because of fiber loss, while excessive generation probability increases multi-pair generation and affects security.Higher generation probability can increase Rmax, but only within a security-compatible range.
- Performance limits: With m=1000, Rmax exceeds 100 kHz at lt=1 km and 3 kHz at lt=10 km; with m=100, it exceeds 100 kHz at lt=10 km and 1.7 kHz at lt=25 km.Higher required coincidence counts lengthen security testing, increasing memory storage time and associated losses.
- Discussion and conclusion: The system could approach security-key rates of current commercial QKD systems while directly transmitting information rather than only secure keys.Practical systems could be developed by replacing wave plates with polarization modulators and adding electrical control for dynamic operation.
Figure captions
The figures describe the experimental setup, Bell-state measurement results before and after fiber transmission with quantum memories, and modeled QSDC performance across link conditions.
- Figure 1: Figure 1 maps the source, switching, Bell-inequality testing, quantum memories, information encoding, and Bell-state measurement components.The setup uses a telecom-band polarization-entangled photon-pair source and fiber-based optical components.
- Figure 2: Figure 2 shows coincidence counts for |〉 and |〉 under measurement settings M1 and M2 without transmission fibers or quantum memories.Gaussian curves fit the coincidence-count data for both Bell states.
- Figure 3: Figure 3 shows coincidence counts for |〉 and |〉 under M1 and M2 with transmission fibers and quantum memories.The plotted data are again accompanied by Gaussian fitting curves.
- Figure 4: Figure 4 evaluates Rmax against lt while varying p at fixed m=1000 and varying m from 100 to 2000 at fixed p=0.03.These panels examine how modeled QSDC performance changes with link length and system parameters.
Tables
The tables present the Bell’s inequality test results and the main parameters used to evaluate the QSDC system’s performance.
- Table 1: Table 1 reports the experimental results of the Bell’s inequality test.The table accompanies the experiment’s Bell-test analysis.
- Table 2: Table 2 lists the main parameters used for the performance evaluation of the QSDC system.The parameters are associated with the modeled system evaluation.