High-Performance Heterodyne Receiver for Quantum Information Processing in a Laser Written Integrated Photonic Platform

TL;DR

This work tackles the need for scalable, low-loss, polarization-insensitive receivers for continuous-variable quantum information. It introduces femtosecond laser micromachining on borosilicate glass to create photonic integrated circuits that form a tunable, low-loss heterodyne front-end suitable for CV-QKD and CV-QRNG. The glass PIC achieves a CMRR above $73$ dB and implements a configurable $90^{\circ}$ optical hybrid, enabling a record SDI-QRNG rate of about $43$ Gbps and a CV-QKD SKR of $3.2$ Mbit/s under realistic conditions. The platform shows strong stability and compatibility with fiber lasers and detectors, highlighting its potential for scalable, space-resilient quantum communication systems and future integration of active components.

Abstract

Continuous-Variable Quantum Key Distribution (CV-QKD) and Quantum Random Number Generation (CV-QRNG) are critical technologies for secure communication and high-speed randomness generation, exploiting shot-noise-limited coherent detection for their operation. Integrated photonic solutions are key to advancing these protocols, as they enable compact, scalable, and efficient system implementations. In this work, we introduce Femtosecond Laser Micromachining (FLM) on borosilicate glass as a novel platform for producing Photonic Integrated Circuits (PICs) realizing coherent detection suitable for quantum information processing. We exploit the specific features of FLM to produce a PIC designed for CV-QKD and CV-QRNG applications. The PIC features fully adjustable optical components that achieve precise calibration and reliable operation under protocol-defined conditions. The device exhibits low insertion losses ($\leq 1.28$ dB), polarization-insensitive operation, and a Common-Mode Rejection Ratio (CMRR) exceeding 73 dB. These characteristics allowed the experimental realization of a source-device-independent CV-QRNG with a secure generation rate of 42.74 Gbps and a QPSK-based CV-QKD system achieving a secret key rate of 3.2 Mbit/s. Our results highlight the potential of FLM technology as an integrated photonic platform, paving the way for scalable and high-performing quantum communication systems.

Figures (10)

• Figure 1: FLM-written PIC layout. (a) Conceptual layout of the tunable optical hybrid, where we can independently control the phase shift $\Delta\theta_{\rm LO}$ and the splitting ratio of the variable beam splitters (vBSs). The two red arrows indicate the inputs for the signal and lo, which are then processed in balanced beam splitters (BSs), waveguide crossing, and variable beam splitters (vBSs). When the phase $\Delta\theta_{\rm LO}$ is $\pi/2$ and the vBSs are perfectly balanced, the device acts as a $90°$ optical hybrid. (b) Schematic layout of the FLM-written photonic device. The dashed rectangle on the left highlights the mode mixing, where the two central modes swap by crossing one over the other (see left inset) thanks to the three-dimensional capabilities of . The central dashed rectangle evidences the area where the thermal shifter tunes the $\Delta\theta_{\rm LO}$ phase shift. The two dashed rectangles on the right mark the locations of the vBSs, implemented through reconfigurable Mach-Zehnder interferometers, used to balance the device's outputs. The thermo-optic phase shifters (associated to resistances R1, R2, and R3) are depicted in yellow, and the dark gray rectangles represent the trenches fabricated by alongside the waveguides to provide increased thermal insulation (see right inset), thereby reducing power dissipation per micro-heater and minimizing thermal cross-talk.
• Figure 2: Experimental setup. The figure details the optical components used for both the characterization and the implementation of the quantum protocols. The optical source provides a coherent beam with fixed power and polarization. Such optical power is split and undergoes different paths, generating the and the phase-modulated quantum signal $\ket{\alpha}$. These modes are coupled at the 's inputs and then converted into electrical signals by two bpd to measure the quadratures of the quantum signal. The configuration of the as an optical heterodyne receiver consists of balancing the chip's outputs and setting the tunable optical hybrid as a 90° one. While the balancing procedure is performed just with the optical beam, setting the hybrid required a phase-modulated classical signal along with the . Once configured, the device is exploited as the optical receiver for - and - protocols. awg: awg, ISO: optical isolator, pc: pc, pbs: pbs, bs: bs, pm: pm, fpga: fpga, AMP: RF amplifier, voa: voa, ATT: optical attenuator, bpd: bpd.
• Figure 3: Characterization of the optical configuration. (a) response of the variable beam-splitter vBS2 with respect to the voltage applied to R2 . The experimental data closely match the theoretical model described in Eq. (\ref{['eq: CMRR_model']}). By fine-tuning $V_{R2}$, the device achieved a as high as $\sim73.8\dB$. The vBS1 tunable displays a similar trend and performance. (b) Phase shift applied to the by R3 as a function of the driving voltage $V_{\rm R3}$. In the region of $\Delta\theta_{\rm LO}\approx 90°$, we estimate from the fitted linear coefficient a phase-voltage sensitivity of 0.14°mV, quantifying the TOPS response to the applied voltage.
• Figure 4: Temporal stability of the heterodyne configuration. a) The stability is shown for both the vBS. For quantum protocols implementations, a lower threshold is set to $40\dB$, since higher values do not lead to a significant improvement in receiver performance, as we will discuss in Section \ref{['Sec:QRNG']} and shown in Fig. \ref{['fig: min_entropy_calib_line']}. Higher values are consistently measured throughout the test duration. b) Stability of the 90° phase shift. The plot compares the phase stability (in green) with the $\pm 5°$ phase error commonly reported for commercial $90°$ optical hybrid devices (in blue). Apart from temperature stabilization, no active mechanism was employed to maintain the device configuration.
• Figure 5: performance and receiver characterization as a function of the receiver's . a) The blue curves represent the classical min-entropy $H_{min}(X)$ and the lower-bound on the quantum conditional min-entropy $H_\text{min}(X \vert E)$ for different values. In red, the clearance of the two output channels of the heterodyne receiver are shown. Data were collected at the highest LO power compatible with detector linearity. The green region highlight CMRR values $\geq 40~dB$, which we identified as the operational range suitable for randomness generation. b) Receiver characterization obtained for different values. The plot shows the characterization of output channel CH2, related to vBS2. Higher values expand the exploitable linear region for randomness generation, allowing for higher clearance and rate. For CMRR values above $40~dB$, no further expansion of the linear region is observed. The noise variance data are extracted from heterodyne detections filtered in the $0.5-2.3~GHz$ frequency range, satisfying the flat-spectra requirement for secure randomness generation.