High-resolution tunable frequency beamsplitter enabled by an integrated silicon pulse shaper

TL;DR

This work presents a silicon-on-chip quantum frequency processor that uses a six-channel integrated pulse shaper to realize high-fidelity, tunable frequency-bin beamsplitters with ultrafine spectral spacing down to $2\mathrm{GHz}$. The platform achieves near-ideal Hadamard gate performance, with fidelity $F>0.9995$ and modified success probability $\widetilde{\mathcal{P}}>0.9621$ across spacings from $2$ to $5\ \mathrm{GHz}$ and even with only four spectral channels. By adjusting spectral phase $\alpha$ or modulation index $\theta$, the system supports arbitrary beamsplitter ratios and rapid reconfiguration, enabling densely parallel single-qubit operations and multidimensional gate implementations in the frequency domain. These results demonstrate a scalable, resource-efficient path toward integrated frequency-bin quantum photonics and highlight opportunities for quantum transduction between optical and RF domains, as well as integration with on-chip photonic components to reduce SWaP and improve performance.

Abstract

We demonstrate high-fidelity, tunable, and ultrafine-resolution on-chip frequency beamsplitters using a quantum frequency processor based on an integrated pulse shaper with six spectral channels. Near-ideal Hadamard gate performance is achieved, with fidelity F > 0.9995 and modified success probability P > 0.9621 maintained across frequency spacings from 2-5 GHz and down to as few as four spectral pulse shaper channels. The system's support of frequency spacings as narrow as 2 GHz significantly surpasses prior bulk demonstrations and enables arbitrary splitting ratios via spectral phase or modulation index control. These results establish a scalable and resource-efficient platform for integrated frequency-bin quantum photonics, opening new directions in quantum information processing, including densely parallel single-qubit operations and multidimensional gate implementations.

Figures (7)

• Figure 1: (a) Conceptual illustration of a frequency beamsplitter, the spectral-domain analogue of a spatial beam splitter. (b) Schematic of the QFP (top) and its spectro-temporal configuration (bottom). The pulse shaper is programmed with six transmission channels spaced by $\Delta f\equiv \Delta\omega/2\pi$, with spectral phase settings $(0,0,0,\alpha,\alpha,\alpha)$. The two EOPMs are driven by out-of-phase RF signals with a common modulation index $\theta$.
• Figure 2: Experimental setup. EOIM: Electo-optic intensity modulator; EOPM: Electo-optic phase modulator; OSA: optical spectrum analyzer. Solid blue lines represent the optical path, while dashed pink lines indicate the RF path. The EOIM is activated only during dual-line testing. Phase Shifter 1 can be used to adjust the relative phase between the two input spectral lines. The QFP is composed of two EOPMs interleaved with an on-chip pulse shaper. A variable RF attenuator balances the modulation indices of the two EOPMs. Shifters 2 and 3 are tuned to ensure out-of-phase modulation between EOPM 1 and EOPM 2. The output spectrum is recorded by a high-resolution OSA.
• Figure 3: Experimentally measured spectral outputs for the frequency-domain Hadamard gate with 3 GHz mode spacing. (a) Single-bin input $|0_{\omega_0}, \alpha_{\omega_1}\rangle$. (b) Single-bin input $|\alpha_{\omega_0}, 0_{\omega_1}\rangle$. (c) In-phase two-bin input $|\alpha_{\omega_0},\alpha_{\omega_1}\rangle$. (d) Out-of-phase two-bin input $|\alpha_{\omega_0},-\alpha_{\omega_1}\rangle$. (e) Relative output intensities in bins 0 and 1 for general two-bin input $|\alpha_{\omega_0}, e^{i(\phi_i + \phi_s)} \alpha_{\omega_1}\rangle$, as the adjustable phase $\phi_s$ is swept over $2\pi$.
• Figure 4: (a) Fidelity $\mathcal{F}$ and (b) success probability $\widetilde{\mathcal{P}}$ as functions of the (even) number of spectral channels in the pulse shaper. Solid lines represent theoretical curves, while experimental data are shown as markers. Insets show enlarged views of the data. Note that the error bars indicate standard deviations and do not imply that the measured $\mathcal{F}$ exceeds the theoretical upper bound of unity.
• Figure 5: Gate performance as a function of frequency spacing. Experimentally measured (a) fidelity $\mathcal{F}$ and (b) success probability $\widetilde{\mathcal{P}}$ overlaid with corresponding theoretical values (dashed lines). Error bars reflect measurement uncertainty. (c) Average optical transmission across the six spectral channels, normalized by the $\sim$8.5 dB fiber-to-chip coupling loss.