Reconfigurable quantum photonic circuits based on quantum dots

TL;DR

This work investigates using quantum dots as reconfigurable phase shifters in photonic quantum circuits. It combines a chirally coupled quantum-dot model with a unitary-decomposition mesh (Clements scheme) and a detailed imperfection model to quantify performance and optimization; phase settings are computed for ideal unitaries and then adjusted under realistic losses, dephasing, and spectral diffusion. Across circuits up to 10 modes, the study finds that unitary infidelity remains below $0.001$, and two-qubit gates like CZ and CNOT can achieve fidelities approaching $0.9998$ with appropriate optimization, despite nonidealities. The results support emitter-driven, cryogenically compatible, fast, low-loss reconfigurable quantum photonic circuits, with demonstrated potential to scale and achieve high-fidelity quantum information processing on-chip.

Abstract

Quantum photonic integrated circuits, composed of linear-optical elements, offer an efficient way for encoding and processing quantum information on-chip. At their core, these circuits rely on reconfigurable phase shifters, typically constructed from classical components such as thermo- or electro-optical materials, while quantum solid-state emitters such as quantum dots are limited to acting as single-photon sources. Here, we demonstrate the potential of quantum dots as reconfigurable phase shifters. We use numerical models based on established literature parameters to show that circuits utilizing these emitters enable high-fidelity operation and are scalable. Despite the inherent imperfections associated with quantum dots, such as imperfect coupling, dephasing, or spectral diffusion, our optimization shows that these do not significantly impact the unitary infidelity. Specifically, they do not increase the infidelity by more than 0.001 in circuits with up to 10 modes, compared to those affected only by standard nanophotonic losses and routing errors. For example, we achieve fidelities of 0.9998 in quantum-dot-based circuits enacting controlled-phase and -not gates without any redundancies. These findings demonstrate the feasibility of quantum emitter-driven quantum information processing and pave the way for cryogenically-compatible, fast, and low-loss reconfigurable quantum photonic circuits.

Figures (6)

• Figure S1: Imperfect circuit transfer matrix generation with example quantum dot loss distribution. (a) $4 \times 4$ MZI mesh circuit depicting QD loss for each quantum dot in the MZIs. The color in the top-left branch of each MZI indicates the loss for the $\phi$ phase shifter, and the color in the top-right branch of each MZI indicates the loss for the $2\theta$ phase shifter. If the path is black this indicates no QD loss in that region of the circuit. (b) Normal distribution for QD loss with a central value of $17\%$ and a standard deviation of $5\%$ of $17\%$. (c) MZI with transfer matrices for the two QD phase shifters with losses $\gamma_{1}$ and $\gamma_{2}$ as colored, the two beam splitters with reflectivities $r_{1}$ and $r_{2}$ and nanophotonic loss per MZI of $L$.
• Figure S2: Coherent phase shift probability ($\left|\alpha_{\mathrm{co}}\right|^2$) maps spanning dephasing ($0\Gamma$ to $0.5\Gamma$) and directionality ($0$ to $1$) with coupling of $\beta = 0.9$ (a) $\Delta_{\mathrm{P}} = 0\Gamma$ (b) $\Delta_{\mathrm{P}} = 0.3\Gamma$.
• Figure S3: Spectral Diffusion effects on detuning and phase shift for $\sigma_{SD} = 0.06\Gamma$. (a) Normal distribution for detuning shift from spectral diffusion. (b) Phase shift distribution example for initial phases of $\pi$ and $\pi/2$ (inset). (c) Example MZI with initial phases of $\phi = \pi$ and $2\theta = \pi/2$ before and after sampling spectral diffusion.
• Figure S4: Unheralded CNOT performance for nanophotonic, state-of-the-art, and typical imperfections. (a) Matrix infidelity as a function of dephasing. The nanophotonic infidelities with no dephasing are shown as horizontal lines across the figure. (b-d) Optimized nanophotonic, state-of-the-art, and typical post-selected $4 \times 4$ computational basis matrices for the unheralded CNOT gate, with conditional output state fidelities listed.
• Figure S5: Matrix infidelity histogram over 100 Haar random $N = 4$ circuits for a beam splitter error of $2.3\%$, with no other imperfections. Here we show the mean of the data (green), a beta fit (orange), and a normal fit (blue), to the data.