Background

With decades of development progress in quantum science and technology, we are at the dawn of entanglement-based quantum networks [1]. Theoretical research has gone deep into the usage of a potential future entanglement-based internet with applications like distributed quantum computing [2] and entanglement-enhanced metrology [3]. As a basic application of a network, communication would also be much different from its current form. To extend the communication distance, we do not necessarily trust intermediate relays for secure communication. Instead, entanglement allows us to use teleportation to establish fully quantum repeaters without leaking information [4]. Pushing security to a new level, we can build on entanglement distribution to accomplish device-independent quantum cryptographic tasks, where the privacy relies solely on quantum nonlocality [5,6].

For an entanglement-based network to be of practical use, the first issue in engineering is the speed of communication. Taking the speed of classical optical communication as a benchmark, we can set a goal of 1 Gbps for the key rate of quantum key distribution (QKD) — but this is not an easy task. The reported highest key rate in a realistic QKD link is only 26.2 Mbps [7], which utilizes high-dimensional information carriers to increase the key rate. A basic communication setting involves three components: a photon source, a channel linking the source to the users, and photon detectors [8,9]. The recent progress in free-space experiments has provided an effective new approach to lowering loss and noise in the channel link [10]. Before the work by Neumann et al. [11], the bottlenecks were commonly considered to be the photon source and detection. In general, we desire high-fidelity entangled photon pairs to be produced and collected at a high speed.

In the literature, there are new entangled-photon sources developed with various good characteristics. These sources have played a role in a list of novel quantum information processing tasks. A remarkable example is the high-quality entanglement sources for loophole-free Bell tests. To illustrate a nonlocal behaviour of Bell-inequality violation that refutes local realism, one should close possible experimental loopholes simultaneously to rule out any potential local hidden variables. Among these loopholes, the most challenging one is the efficiency loophole. If the end-to-end efficiency from photon generation to detection is not high enough, classical physics could in principle fake Bell-inequality violation. For this purpose, one needs to make sure that every generated photon counts, that is, the source should have a high collection efficiency. Continuous efforts have been devoted to addressing this problem, including a most remarkable contribution by the laureates of the Nobel Prize in Physics 2022 [12]. Not long ago, photonic platforms have finally been successfully used to certify Bell nonlocality in a loophole-free manner [13,14]. With recent developments in device-independent cryptography tasks [15,16,17], the most important applications of Bell nonlocality, well-calibrated entanglement sources achieve record high collection efficiencies of up to $84.1\%$ [18]. For other applications, scientists and engineers have established entangled-photon sources that are very bright [19,20] or enjoy near-unity visibility for detection [21,22]. Though practical QKD links may not require single parameters to be as impressive as these reported ones, a photon source that possesses sufficiently good qualities simultaneously is necessary. Specifically, as the premise for a high key rate, the source should have high overall brightness and collection efficiency. Furthermore, if the visibility of the source is sufficiently high, we have more room to tolerate other origins of loss and noise in the QKD link. Till now, there was no such source with all these features yet.

Results

In the work by Neumann et al. [11], the authors set up an entangled photon source that simultaneously exhibits a high pair-creation rate, broad bandwidth, excellent state fidelity, and low intrinsic loss. The authors deploy the type-0 spontaneous parametric down-conversion (SPDC) and apply a Sagnac-loop configuration to generate polarization-entangled photon pairs. In comparison to the other two phase-matching types, the type-0 SDPC has an advantage of high brightness. In addition, the authors apply a narrow-band continuous-wave laser as the pump. In this way, maximum power of 500 mW is obtained without damaging the crystal. Though the configuration choice has the natural advantage of increasing the brightness, due to the intrinsic Poisson distribution of the SPDC process, there is a fundamental limit to the amount of generated entanglement. To overcome this obstacle, the authors further apply the trick of wavelength division multiplexing (WDM). With more wavelength channels working in parallel, the overall entanglement generation rate is effectively boosted. One of the most significant points of the source is that most of these wavelength channels exhibit sufficiently good parameters. For the listed wavelength channels of entanglement pairs, the collection efficiencies are all above $20\%$ and the visibility stays above $99.2\%$. The highest spectral brightness reaches $4.17\times10^6$ cps/mW/nm, showing the effectiveness of WDM. Besides the design, the establishment of the light source also requires extraordinary experimental techniques. There are fundamental trade-offs among the key parameters. For instance, a smaller waist in the Gaussian beam is good news for the generation rate of entangled photons, yet it brings with it a larger beam divergence that can be a catastrophe for the subsequent fibre-coupling and photon collection. Essentially, one needs a delicate design and optimisation for the crystal and the Sagnac loop.

Although a real-life field test is missing, building on the theoretical model to optimize QKD key rates with continuous-wave pumped entangled-photon sources [23], the authors show the feasibility to reach a secure QKD key rate of 1 Gbps for the first time. Operating the source with standard off-the-shelf 100 GHz WDM channels at 400 mW pump power, the simulation exhibits a key rate as high as 1.2 Gbps. Note that the QKD protocol in use is the Bennett-Brassard-Mermin-1992 protocol with polarization-entangled photons [24] — one does need to seek aid from high-dimensional entanglement. If the detection devices can match the high quality of the entanglement source, from the authors’ simulation, a maximum QKD key rate of 3.6 Gbps is possible with their entangled-photon source. Let us compare it with the state-of-the-art satellite-to-ground entanglement-based QKD experiment [25]. In this experiment, a continuous-wave pump was also applied. The reported entangled source generated entangled photon pairs at a speed of 5.9 MHz at the pump power of 30 mW and the collection efficiency was about $1\%$. Under these parameters, the QKD key rate was approximately 3.5 bps. In future upgrading works, if one applies an entangled-photon source as good as the one reported by Neumann et al. [11], the key rate can be boosted by more than two orders of magnitude, approaching kilobits per second. Other than using all the wavelength channels for a single link, one can also apply the source to a multi-user quantum network [26]. With a well-designed WDM device, different users can operate their own communication tasks in separate wavelength channels without cross-talks.

Outlook

With this best entanglement source ever, can we say that entanglement-based quantum communication becomes as practical as its classical counterpart? Not exactly. The bottleneck of practical quantum communication now moves to the detection side. In the simulation, the maximum detector count rates are set as 200 MHz — just within the reach of state-of-the-art experiments [27]. For short-distance QKD, such as metropolitan communication, the detection speed can hardly match the great potential of the source. For long-distance communication, the detection rate is usually not a big problem due to the high loss accumulated in the transmission. But the presence of jittering effects from electronics and fibers would result in erroneous detection clicks and hence affect the key rate. In summary, the game is still on for a practical entanglement-based quantum network.

Acknowledgement

I would like to thank Guoding Liu and Xingjian Zhang for detailed comments on this manuscript.

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