Synchronized distribution of quantum entanglement coexisting with high-rate, broadband classical optical communications over a real-world fiber link

TL;DR

This work addresses the challenge of distributing quantum entanglement over deployed fiber while coexisting with high-rate classical telecom traffic. It implements O-band polarization-entangled photons distributed across 24.4 km of real fiber alongside fully-loaded C-band data (1.6 Tbps total) and an L-band synchronization clock, and analyzes SpRS to guide wavelength allocation. The experiment achieves high entanglement fidelity under coexistence (about 0.942 to the target Bell state, with 0.988 relative fidelity to the dark-fiber reference) and demonstrates picosecond-scale timing accuracy, validating the practicality of quantum-classical networks integrated into current telecom infrastructure. These results signal a viable path toward scalable, real-world entanglement-based quantum networks embedded within high-capacity classical networks, with potential to scale bandwidth and distance further along metropolitan-to-intercity links.

Abstract

Compatibility with existing classical network infrastructure offers a scalable path towards deploying largescale quantum networks. Here, we demonstrate O-band polarization-encoded quantum entanglement distribution over an installed 24.4-km fiber while coexisting with a state-of-the-art fully-loaded C-band classical communications line system and a picosecond-level precision L-band synchronization signal. The classical system carries two 800-Gbps channels while the remainder of the C-band is filled with amplified spontaneous emission as is standard for such state-of-the-art communications systems. We examine the spontaneous Raman scattering spectrum generated from this broadband C-band light and offer insights into wavelength allocation for O-band quantum channels. Optimal wavelength selection and narrow filtering enable well-preserved Bell state fidelity when coexisting with 21.4-dBm aggregate launch power across the C-band suitable for 36 Tbps transmission. To the best of our knowledge, this is the first implementation of entanglement-based quantum communications between two remote nodes coexisting with independent classical communications traffic. We demonstrate coexistence of quantum entanglement with ultra-high power levels and record classical bandwidth, offering promise for real-world entanglement-based networking integrated within high-capacity communications infrastructure.

Figures (7)

• Figure 1: Entanglement is distributed over 24.4 km of deployed fiber from Evanston to Chicago. The quantum signal is multiplexed to propagate with a state-of-the-art classical communications system and optical synchronization clock. (EPPS = entangled photon pair source, OLS Tx (Rx) = classical optical line system transmitter (receiver), SNSPD = superconducting nanowire single photon detector, TDC = time-to-digital converter, WR = White Rabbit synchronizer, MUX = wavelength-division multiplexer, BW = bandwidth)
• Figure 2: Optical spectrum analyzer (OSA) measurement of the Ciena classical OLS (green) and WR synchronization signal (red) with 0.01 nm resolution. The peak at 1511 nm is the OSC on the OLS and the oscillations at 1541.3 nm and 1557.4 nm are the 800-Gbps data channels. Relative powers of the WR clock and OLS do not factor in experimental insertion losses. (WR = White Rabbit, OLS = optical line system, OSC = optical supervisory channel, ASE = amplified spontaneous emission)
• Figure 3: (a) Left axis (red dots): Single photon counts across the O-band from spontaneous Raman scattering (SpRS) over the 24.4-km deployed fiber due to the classical OLS at 18.3 dBm. Counts are reported in kilo-counts per second (kilo-cps) normalized by classical launch power and tunable bandpass filter bandwidth. Right axis (black stars): Transmission loss over the 24.4-km deployed fiber as a function of wavelength. (b) Left axis (black dots): Signal-to-noise ratio (SNR) as a function of wavelength simulated from the measured SNR at 1290 nm (black star), loss spectrum, and SpRS noise across the O-band. Right axis (red diamonds): Simulated visibility as a function of wavelength due to measured changes in loss and SpRS noise.
• Figure 4: Histograms of photon arrival time difference relative to an identical signal copy for either one TDC (black stars) or two TDCs synchronized by White Rabbit (red dots). Inserts show diagrams of the experimental setups for jitter measurements on (a) one TDC and (b) two synchronized TDCs on a 50:50 split 50-MHz RF signal. (b) is synchronized by White Rabbit (WR) with a 25-km fiber spool between TDCs. Measured root-mean-square timing jitters are $\sigma_{1TDC} = 3.5$ ps and $\sigma_{2TDC} = 4.6$ ps. From this, the increase in jitter due to WR synchronization is calculated to be $\sigma_{WR} = 3.0$ ps swabian_app_note. (TDC = time-to-digital converter, RF = radio frequency)
• Figure 5: Diagram of experimental implementation of entanglement distribution. The Sagnac loop source generates polarization entangled photon pairs at 1290 nm and 1310 nm by second harmonic generation cascaded with non-degenerate spontaneous parametric down-conversion. The 1310-nm photon is kept in Evanston, where it is filtered to ensure isolation from the pump and its polarization is characterized before it is detected. The 1290-nm photon is multiplexed with the classical OLS and synchronization clock and then distributed over a 24.4-km deployed fiber to Chicago. The classical signals are de-multiplexed and the 1290-nm photon is filtered to remove induced SpRS noise before its polarization is characterized. Photon detections are analyzed for coincidences to verify the entangled state by two TDCs synchronized by the White Rabbit optical clock. (CWDM = coarse wavelength-division multiplexer (WDM), DWDM = dense WDM, FWDM = O-Band/C-band WDM, CIRC = circulator, FPC = fiber polarization controller, PBS = polarizing beam splitter, PBS* = PBS with 180$^{\circ}$ phase flip on the V path, $\lambda/2$ = half-waveplate, $\lambda/4$ = quarter-waveplate, LCR = liquid crystal retarder, PPLN = periodically-poled lithium niobate waveguide, SNSPD = superconducting nanowire single photon detector, FPE = Fabry-Pérot etalon, C = common port, P = pass port, R = reflect port, Tx = transmitter, Rx = receiver, OLS = classical optical line system, WR = White Rabbit synchronization, TDC = time-to-digital converter, pps = pulse per second, ASE = amplified signal emission, PDFA = praseodymium-doped fiber amplifier, MZM = Mach-Zehnder modulator, POL = linear polarizer, RF = radio frequency modulation, CW = continuous wave)