Quantum key distribution (QKD), a kind of quantum cryptography technology, has been theoretically proven to be able to provide unconditional secure communications. In the past several decades, QKD has been studied and can be deployed in real-world fiber networks [1–3]. Integrated and miniaturized QKD systems provide a constructive solution to realize highly stable, low-cost, portable, compact, and robust global ultralarge-scale quantum communications networks [4–8]. The key to the integrated QKD is the integration of a light source, a linear element, and a detector. The integration of quantum photonic components has already been reported [9–14], and a QKD system integration has been initially demonstrated, which means there is movement toward a fully integrated QKD system [15].

Generally, there are two categories of QKD schemes: discrete-variable QKD (DV-QKD) and continuous-variable QKD (CV-QKD). The integration of DV-QKD has made significant progress, and integrated DV-QKD systems are mainly divided into two categories. The first category includes those with integrated light sources, such as an indium phosphide (InP) transmitter chip including an on-chip DBR laser [coherence time>1.5ns, side-mode suppression ratio (SMSR)>50dB, operating wavelength of 1550 nm with ∼10nm tuning range] and a silicon nitride receiver chip, which achieves 568 kb/s secure key rate for an emulated 20 km fiber link [9]. Measurement device independent (MDI)-QKD has been demonstrated between two independent InP transmitters with on-chip lasers (linewidth of 30 pm, SMSR of 50 dB, and a tuning capability of 10 nm in the C-band), which achieves 12 kb/s over a 25 km emulated fiber link and 1 kb/s over a 100 km emulated fiber link [11]. Very recently, a fully integrated QKD system has been proposed with a phase-seeded QKD transmitter including two on-chip diode lasers (coherence time ∼100ns, linewidth <4.5MHz). This QKD system can achieve 248 kb/s over a secure transmission distance up to 50 km on standard fibers [16,17]. We can clearly see that the maximum transmission distance of all the integrated (fully and partially integrated) DV-QKD systems with on-chip lasers is currently no more than 50 km. In the second type, only the passive device is integrated without the on-chip light source. A silicon photonic polarization encoder with an off-chip laser is demonstrated, which achieves 157 kb/s in an intercity metropolitan test on a 43 km fiber (with a 16.4 dB loss) [10]. A silicon chip-based polarization-encoded MDI-QKD system with two transmitter chips based on off-chip lasers achieves a finite-key secret rate of 31 b/s over 36 dB channel loss and 497 b/s over 140 km commercial fiber spools [12]. A high-speed QKD transmitter based on silicon photonic integrated circuits and an off-chip laser is demonstrated with 329 kb/s over a 20 km fiber link [13]. A silicon polarization-encoded QKD transmitter with an off-chip laser is presented, which was used in a proof-of-concept demonstration of the BB84 QKD protocol with 0.95 kb/s over a 5 km long fiber link [14]. For CV-QKD, an integrated CV-QKD system with an off-chip light source, has been verified in principle recently under laboratory conditions [8]. This work demonstrates the feasibility of a CV-QKD on an integrated chip platform.

A light source generating designated quantum states is the critical component for an integrated QKD system. Although integrated light sources have been demonstrated in DV-QKD systems [9,11], they have not been studied in CV-QKD systems since the proposal of the GG02 protocol [18,19], especially with a local local oscillator (LLO) scheme. The current integrated CV-QKD is based on the transmitted local oscillator (TLO) scheme, which can guarantee the consistency of the LO wavefront and signal to achieve stable homodyne detection [8]. However, the crosstalk from the LO limits the development of long-distance transmission and causes potential practical security problems. Fortunately, a CV-QKD system based on the LLO scheme has been proposed, which can ensure that the quantum detection reaches the shot-noise limit under long-distance transmission conditions, and significantly eliminates the signal’s crosstalk by a more robust LO [20,21]. Meanwhile, since the LO is generated locally at the receiver, potential security loopholes are eliminated from the source. Therefore, a CV-QKD system based on an LLO scheme provides an up-and-coming solution for integrated CV-QKD systems.

However, an integrated LLO CV-QKD system has extremely high requirements for light sources. Most QKD realization requires only one laser source, but an LLO scheme requires two. The three main requirements for on-chip lasers are: (1) an accurate wavelength tunability to meet the alignment of the center wavelengths of two lasers; (2) a sufficient optical output power to ensure shot-noise-limited detection; and (3) a narrow linewidth and low noise to guarantee low untrusted excess noise and secure key generation. Therefore, it is still challenging to have two on-chip light sources with narrow linewidths, low noise, fine tunability, and simultaneously sufficient power, which is crucial for high-performance LLO CV-QKD realization.

In this paper, we implemented a compact, high-performance III-V/Si3N4 external cavity laser (ECL) that features high output, narrow linewidth, broad wavelength tunability, and high SMSR. A complete LLO CV-QKD system with a secure transmission distance exceeding 100 km is realized. This work has taken a crucial step to solve the bottleneck of the integrated on-chip laser source in high-performance integrated QKD systems, especially the integrated LLO CV-QKD systems in the past two decades since the proposal of the GG02 protocol [18,19]. We perform almost perfect correction and compensation for the frequency offset and phase drift, and finally achieve the secure key of 0.75 Mb/s under 50 km. This integrated CV-QKD system based on the LLO scheme solves both the upper limit of long-distance transmission and the practical security issues. Meanwhile, we have overcome the problems of high-performance on-chip laser integration commonly faced by an integrated QKD system. We extend the applicability of the III−V/Si3N4 ECL to CV-QKD and the potential to other quantum information applications. This system perfectly complements previous work on CV-QKD systems on a chip, paving the way for the full integration of metro CV-QKD networks.

In terms of low excess system noise, minor frequency offset between a signal and an LO, shot-noise-limited detection, and accurate detection requirements of a high-performance fully integrated LLO CV-QKD system, the on-chip quantum light source should simultaneously meet four important conditions: narrow linewidth, high tunability, sufficient output optical power, and high frequency stability. Therefore, we created a design to solve the quantum light source integration problem in a fully integrated CV-QKD system for high performance. The inset of Fig. 1(a) illustrates the full schematic structure of the two ECLs used in the system. The laser consists of an InP reflective semiconductor optical amplifier (RSOA) chip butt-coupled with a low-loss silicon nitride cavity extension chip. The rear side of the RSOA is coated with a high-reflection (HR) film to serve as the back mirror of the ECL. The waveguide is slanted by 8° at the front side and coated with an anti-reflection (AR) film to reduce the interface reflection. The coupling loss between the RSOA and external cavity is about 2.5 dB.

To meet the low excess noise requirements, the on-chip laser should have a narrow linewidth (low frequency noise). Traditional monolithic III-V lasers such as DFB lasers, sampled-grating DBR (SG-DBR) lasers, and vertical cavity surface-emitting lasers (VCSELs), have a relatively large linewidth due to the short cavity and large internal loss [22]. Compared to these lasers, ECLs composed of III-V gain and passive circuits are a solution to narrow the linewidth. To reduce the intrinsic linewidth to a kHz-level (coherence time of 0.1–1 ms), we use the low loss Si3N4 waveguide (0.3 dB/cm) to implement the external cavity with an effective length of about 26.8 mm. The low-loss long laser cavity extends the photon lifetime. Moreover, a well-known negative feedback loop (detuned loading effect) stabilizes the laser [23]. Both contribute to a narrower linewidth, resulting in a significant reduction in the excess noise in the CV-QKD system (see Appendix A).

The minor frequency offset between the signal and the LO demanded in the system means that the on-chip laser should provide high tunability. We designed a high-performance mode selection filter in the passive cavity composed of a Vernier filter based on two microrings (MRRs) to provide high SMSR and a wide wavelength tuning range. The circumferences of the two MRRs are 583 μm and 565 μm, corresponding to free spectral ranges (FSRs) of 1.93 nm and 1.99 nm, respectively. The extended FSR can exceed 60 nm. A phase shifter is also integrated in the cavity to align the wavelength with the passband center of the filter. The Vernier filter ensures high-performance, single-mode lasing and attenuates the side modes and amplified spontaneous emission noise (ASE) prior to the lasing output. The single lasing mode is controlled by aligning the resonances of two MRRs by tuning one MRR. The coarse tuning range measured by an optical spectrum analyzer (AQ6370D-12, 0.02 nm wavelength resolution; Yokogawa Electric) is more than 70 nm, as shown in Fig. 2(d). The wide wavelength tuning range is suitable for high-capacity quantum communications, wavelength division multiplexing systems, and more potential quantum information applications [24,25]. The lasing frequency can be continuously tuned over a long range by synchronously driving the MRRs and the phase shifter. The resulting fine-tuning range is about 30 GHz (limited by the maximum allowed heating power of the phase shifter) depicted in Fig. 2(b), which ensures the minor frequency offset between nonhomologous lasers. Additionally, Fig. 2(c) shows that the maximum SMSR of this ECL is up to 75 dB.

The shot-noise-limited detection requirements in a high-performance integrated CV-QKD system require the on-chip lasers to provide sufficient optical power. The output power of the ECL partially depends on the loss of the external cavity. Si3N4 has proven to have a lower linear and nonlinear transmission loss compared to silicon waveguides, and thus can achieve a higher optical power and a narrower linewidth [26–28]. The relatively large index contrast of Si3N4 waveguides also ensures the small bending radius of Vernier MRRs in the design. LiNbO3-on-insulator (LNOI) is a rising platform for high-performance, low-loss optical devices [29,30]. However, the photorefractive effect in LNOI potentially degrades the output power stability of LNOI-based ECLs. Therefore, we chose to implement Si3N4 waveguides as passive extension chips to provide sufficient optical power. We also designed a tunable Sagnac loop (TSL) with tunable reflectivity as another ECL mirror. The reflectivity can be tuned from 0 to 1 in the 70 nm wavelength range by controlling the phase difference of the Mach–Zehnder interferometer (MZI) arms (see Appendix A). Both the low loss and tunable reflectivity contribute to a maximum on-chip output of 47.3 mW, as shown in Fig. 2(a), which meets the requirements of shot-noise-limited detection under long distance and fully guarantees the practical security of CV-QKD in an implementation.

Accurate detection requires the ECL to have a high frequency stability. Because thermal management of side-placed chips is easier than 3D integration [31,32], a high frequency stability is expected. In our experiment, a thermoelectric cooler (TEC) is placed under the chips to dissipate the heat produced by the laser. During the operation, the TEC temperature was fixed at 20°C.

Figure 1(b) shows the ECL packaged in butterfly shells. Figure 1(c) is a microscope photo of the integrated laser. With the high-performance on-chip ECL suitable for the CV-QKD system, we will demonstrate its superior performance in the LLO CV-QKD system for practical long-distance fiber transmission.

We experimentally demonstrate a pilot-assisted LLO CV-QKD in a 50–100 km long fiber link with well-designed chip sources. Figure 1 shows the experimental setup. At the transmitter, Alice first uses one isolator and one attenuator (ATT) to control the optical power from the CW on-chip laser, and then employs a high performance LiNbO3 amplitude modulator (AM) to produce a series of coherent state pulses. The AM’s bias voltage is controlled by a high-precision DC power supply. Then a beam splitter (BS) is used to split this state into two portions with an intensity ratio of 99:1. The small part is used for optical monitoring, while the large part generates the quantum signal and the pilot signal. Subsequently, an IQ modulator (IQ MOD) modulates the signal, and two electronic signals carrying Gaussian-distributed random numbers are generated by an arbitrary waveform generator (AWG) and amplified by a microwave amplifier. The coherent states are modulated to |xA+ipA⟩, where xA and pA follow a centered Gaussian distribution and then are adjusted to a suitable variance of VA by tuning another ATT. The pilot-data channel pair is transmitted through the fiber link on one polarization. At the receiver, with the help of a manual polarization controller (PC), the time-multiplexing pilot-data pulses are injected into an integrated coherent receiver (ICR) for detection. In addition, an intense LO is generated by another on-chip laser, and its polarization is adjusted by another PC. A photon detector monitors the LO intensity, and finally, the LO is injected into the ICR and mixed with the signal. The phase information of the pilot is used to compensate for the raw data of the signal, from which the final key can be distilled.

To keep the frequency offset minor between the signal light and LO light, we adjusted the two lasers simultaneously to make the spectral peaks distinguishable, as shown in Fig. 3(a). Then we connected these two lasers to the QKD system. We adjusted the phase shifter to reduce the beat frequency until it was below 20 MHz. The beat frequency drifts over time. Figure 3(b) shows the histogram of the measured beat frequency after a period of time. The change of the central wavelength of the laser leads to a drift of beat frequency, which may originate from two sources. The output of the voltage source gives ripple noise and, as the TEC is placed on the lower surface of the chip, the local temperature of the chip is affected by the surrounding environment and is different from TEC. The frequency instability may introduce a certain amount of excess noise in the preparation of quantum states, thereby degrading the system performance. Therefore, we adopted a frame-by-frame processing algorithm in post-processing, and the beat frequency in each frame remains almost unchanged, which greatly reduces the impact of this frequency instability. As depicted in Fig. 3(c), we measured the frequency noise spectrum of the operating laser using an optical noise analyzer (A0040A, SYCATUS). The intrinsic linewidths of the signal and LO are 1.6 kHz and 3.2 kHz, respectively, calculated by multiplying the white frequency noise level by π in the high-frequency range (see Appendix A).

The quantum efficiency and electronic noise of this QKD system were calibrated in advance. The quadrature selection is achieved by maximizing the cross-modulation peak-to-peak difference. Both the output signal and the input signal for x and p quadrature modulation are recorded, and the data are collected for 0.5 μs with a sampling frequency of 10 Gb/s. The output signal on Bob’s side is synchronized with Alice’s modulation signal by measuring their cross-correlation. Figure 4(a) shows the normalized cross-correlation measurement between the heterodyne detector output and the corresponding modulation signal, and the inset in Fig. 4(a) shows the raw secret key shared by Alice and Bob. All signals are synchronized based on the cross-correlation and pass through a digital lowpass filter. Next, the filtered signals are demodulated and downsampled to 0.5 Gb/s to generate a set of correlated Gaussian keys that are shown in Fig. 4(a) with Alice’s key as the x coordinate and Bob’s key as the y coordinate. These plots confirm that Bob’s key only correlates to one of Alice’s keys with the same measured quadrature. Information reconciliation is then applied to the correlated Gaussian key. Figure 4(b) shows the excess noise performance of the system.

The secure key rate at a longer distance is calculated based on the assumption of a collective attack under the trusted device scenario, which means an eavesdropper cannot access the noise from Bob’s apparatus. Note that a coherent attack is an optimal attack, and it has proven to be equivalent to a collective attack under asymptotic conditions. The total losses consist of the losses on the transmission line and Bob’s equipment, while the losses on Alice’s side do not affect the final security key. Table 1 shows the main experimental parameters in the LLO-CV-QKD system with chip light sources under 50 and 100 km standard fiber links. These parameters can be achieved using the current post-processing scheme with the same SNR. The proportion of raw data used for the parameter estimation is 50%. Considering the unqualified excess noise (less than zero caused by statistical error or greater than the secret threshold), a part of the data is excluded through the post-selection. For a distance of 50 km, the qualified frame rate (QFR) is around 80%; for 100 km, around 10% of the frame can be used for subsequent data processing. With these remaining data, the secure key rate of the current CV-QKD system is estimated. Figure 4(c) shows the Shannon raw secret key rate. (From top to bottom, the solid black line represents the PLOB bound, the solid red line represents the expected LLO CV-QKD bound with a chip source, the orange pentagram represents experimental results of our LLO CV-QKD system with a chip source, the blue square represents the distance and secret key rate that have been achieved in the integrated TLO CV-QKD experiment [8], and the others represent related integrated DV-QKD works [9,10,13,14,17].)Table 1.

Summary of Experimental Parameters

To finally generate a secret key at such a lower SNR regime, multidimensional reconciliation and low-density parity check (LDPC) error correction codes are used to perform key extraction offline. The multidimensional reconciliation adopts the rotation algorithm to convert the Gaussian distributed data into binary distributed data [33]. The LDPC code adopts the rate-adaptive algorithm, the advantage of which is that the reconciliation efficiency can be maintained under a slight SNR fluctuation [34]. Through the parameter adjustment, we finally obtained the final secure key at 50 km and 100 km, respectively, and completed the whole process of key distribution. Such a result confirms the effectiveness of the integrated laser source in CV-QKD.

In conclusion, we have reported the use of on-chip III-V/Si3N4 external cavity lasers for a hundred-kilometer level, high-performance LLO CV-QKD. By giving the on-chip laser wavelength tunability, a narrow linewidth, and many other functional advantages, we creatively solved the challenge of integrating on-chip lasers that can truly support the long-distance practical metro optical fiber transmission networks of a CV-QKD system since the proposal of the GG02 protocol. Based on this on-chip quantum source, future demonstrations will focus on a fully integrated high-performance LLO-CV-QKD system that eliminates the security flaws in the physical mechanism of the previous integrated CV-QKD system based on the TLO. Moreover, it technically realizes the experimental verification in the actual optical fiber transmission environment of metropolitan application scenarios.