Coexistence of High-Bit-Rate Quantum Key Distribution and Data on Optical Fiber

Quantum key distribution (QKD) uniquely allows the distribution of cryptographic keys with security verified by quantum mechanical limits. Both protocol execution and subsequent applications require the assistance of classical data communication channels. While using separate fibers is one option, it is economically more viable if data and quantum signals are simultaneously transmitted through a single fiber. However, noise-photon contamination arising from the intense data signal has severely restricted both the QKD distances and secure key rates. Here, we exploit a novel temporal-filtering effect for noise-photon rejection. This allows high-bit-rate QKD over fibers up to 90 km in length and populated with error-free bidirectional $\mathrm{Gb}/\mathrm{s}$ data communications. With a high-bit rate and range sufficient for important information infrastructures, such as smart cities and 10-Gbit Ethernet, QKD is a significant step closer toward wide-scale deployment in fiber networks.

Popular Summary

In the world of traditional communications, it is impossible for one person who receives a set of encryption keys from another to know whether or not the keys have been seen by the eyes of an eavesdropper. However, quantum key distribution (QKD), which encodes keys in the quantum states of a medium, can achieve the impossible by exploiting the fundamental quantum property that any eavesdropping on a quantum state always leaves a trace. In practice, secure QKD requires the use of only faint light pulses to exchange secret digital encryption keys over optical telecommunication fibers and has to, therefore, operate typically in a low-noise environment such as an expensive, dedicated fiber link. This is a severe bottleneck to wider adoption of QKD on a commercially viable scale. New methods that can integrate QKD into existing fiber infrastructures where data signals, which are millions of times brighter, are transmitted will represent breakthroughs. This paper demonstrates experimentally a paradigm-shifting method that allows fast and reliable quantum key distribution to go hand in hand with the transmission of bright optical data in conventional fibers over a record distance of 90 km.

A main difficulty for quantum key and optical data transmission sharing the same fiber arises from signal cross contamination, not unlike the difficulty of communicating over a long distance using only a faint torchlight under broad daylight. The intense data signals generate stray light which may spectrally overlap with the QKD signal and therefore cannot be filtered out by spectral filters. As the severity of contamination increases significantly with communication distance, past demonstrations of QKD were limited to a distance of 50 km, which is inadequate for extensive metropolitan coverage. Our approach to overcoming such signal contamination shifts from the spectral-filtering paradigm to a temporal-filtering one using fast-gated photo detectors. Noise photons enter the detectors at random times, while quantum-key signals arrive regularly. By gating the detectors only during the arrivals of quantum-key signals, noise photons arising from the data signal can be filtered out with a rejection efficiency nearing 90%. With this approach, we have achieved QKD in Gbit-per-second data fibers with lengths up to 90 km and with a transmission rate of secure keys that is more than 3 orders of magnitude higher than the current rates.

We expect that the temporal filtering will become a technique indispensable to integrating quantum-information technologies into today’s data communication networks.

Figure 1

Experimental setup. (a) Schematics for multiplexing of quantum, clock, and data channels. (b) Quantum transmitter. (c) Quantum receiver. SD-APD: self-differencing avalanche photodiodes, att.: optical attenuator, CWDM: coarse wavelength division multiplexer, NBF: narrow bandpass filter (0.56 nm).

Figure 2

Raman noise and its rejection. (a) Spectrum of backscattered Raman noise measured over 80 km with a 0-dBm launch laser at a wavelength of 1611 nm; also shown are spectra after a CWDM filter only, and a combination of a CWDM coupler and a NBF filter. (b) Measured (symbols) and calculated (solid lines) Raman noise power into the quantum receiver through Bob’s CWDM coupler. (c) Bit error rate measured over a fiber link of 80 km for the 1611-nm receiver as a function of receiving power. (d) Single-photon detection efficiencies as a function of gate delay of the gated detector under synchronized (circles) and nonsynchronized (squares) illuminations. The illumination source is a pulsed laser clocked at 1 GHz with an average flux of 0.02 photons per pulse.

Figure 3

Raman noise into the quantum receiver. (a) Measured (symbol) and calculated (solid line) Raman noise after Bob’s CWDM. (b) Raman noise after the narrow band pass filter (NBF). In both (a) and (b), two 0-dBm data lasers of 1591 and 1611 nm are launched simultaneously at Alice and Bob, respectively. (c) Reduced Raman noise after lowering the laser-launch powers. (d) Effective Raman noise received within the active time of the gated detectors. Solid lines (b)–(c) are calculated results.

Figure 4

QKD performance with error-free bidirectional $\mathrm{Gb}/\mathrm{s}$ data channels. (a) Calculation (line) and measurement (symbols) of sifted and secure key rates as a function of fiber length. (b) Calculation (line) and measurement (symbols) of QBER. Also shown is the calculation of the QBER without contribution from data lasers (dashed line).

Figure 5

Timing jitter vs received clock power measured between Alice and Bob over a fiber link of 80 km. Also shown is Bob’s cycle-cycle jitter.