Practical blueprint for low-depth photonic quantum computing with quantum dots

Abstract: Fusion-based quantum computing is an attractive model for fault-tolerant computation based on photonics requiring only finite-sized entangled resource states followed by linear-optics operations and photon measurements. Large-scale implementations have so far been limited due to the access only to probabilistic photon sources, vulnerability to photon loss, and the need for massive multiplexing. Deterministic photon sources offer an alternative and resource-efficient route. By synergistically integrating deterministic photon emission, adaptive repeat-until-success fusions, and an optimised architectural design, we propose a complete blueprint for a photonic quantum computer using quantum dots and linear optics. It features time-bin qubit encoding, reconfigurable entangled-photon sources, and a fusion-based architecture with low optical connectivity, significantly reducing the required optical depth per photon and resource overheads. We present in detail the hardware required for resource-state generation and fusion networking, experimental pulse sequences, and exact resource estimates for preparing a logical qubit. We estimate that one logical clock cycle of error correction can be executed within microseconds, which scales linearly with the code distance. We also simulate error thresholds for fault-tolerance by accounting for a full catalogue of intrinsic error sources found in real-world quantum dot devices. Our work establishes a practical blueprint for a low-optical-depth, emitter-based fault-tolerant photonic quantum computer.

• The paper presents a practical blueprint for constructing low-depth photonic quantum computers using quantum dots to generate linear cluster states.
• It employs a time-bin protocol with sequential optical excitations and spin rotations to create robust spin-photon entangled states.
• The study analyzes branching and spin-flip errors and proposes error correction strategies to enhance computational fidelity and scalability.

Practical Blueprint for Low-Depth Photonic Quantum Computing with Quantum Dots

Introduction

The paper presents a comprehensive framework for constructing low-depth photonic quantum computing systems utilizing quantum dots. This approach addresses the challenges associated with decoherence and error propagation intrinsic to quantum dot implementations. The proposed method emphasizes efficient generation and manipulation of photonic cluster states through a time-bin protocol enabled by spin-photon interactions. The technique is geared towards minimizing error rates while enabling scalable quantum information processing.

Ideal State Evolution

The foundation of the protocol lies in the ideal, error-free evolution of a spin-photon system. Initially, the spin qubit is prepared in the $\ket{\uparrow}$ state, and subsequently transformed into a superposition of spin states using a $\pi/2$ Hadamard operation. The protocol involves sequential optical excitations and spin rotations that create entangled states between spin and photonic systems for each round. This ensures the formation of a linear cluster state necessary for quantum computations. For complex operations, the protocol is extended to encode qubits with multiple photons through repeated excitation and rotation cycles.

Error Analysis and Mitigation

Branching Errors

Branching errors arise from the non-ideal decay of photonic emissions during early time-bin excitations. These errors manifest as the inadvertent loss of photons, impacting the fidelity of the generated linear cluster states. The analysis demonstrates that branching errors predominantly introduce $X$ errors in photon states, which can cascade into subsequent rounds, affecting overall computation. The paper underscores the need for error correction strategies, such as the application of quantum error correction codes that address both photon losses and $X$ errors.

Spin-Flip Errors

Laser-induced spin-flip errors occur during $\pi$ rotations between time-bins. These can lead to unintended state changes, resulting in zero emissions or double photon emissions, both detrimental to the computational state. The paper explains the dual impact of such errors leading to decoherence and loss, ultimately removing the system from its intended computational space. Efficient calibration of laser parameters and robust pulse schemes are identified as solutions to mitigate these errors, maintaining the integrity of emitted photonic states.

Computational Implications

The implications of the proposed framework are profound for advancing practical quantum computing. By strategically addressing dominant error mechanisms, the paper contributes to the reliability and scalability of photonic quantum systems. Error-corrective measures improve computational accuracy, paving the way for more robust applications in quantum information processing. Further developments may focus on optimizing state preparation protocols and enhancing photon source devices, potentially integrating the approach with hybrid quantum networks for increased computational capacity.

Conclusion

This paper provides a detailed exploration into harnessing quantum dots for photonic quantum computing, offering a robust protocol that minimizes errors and enhances computational fidelity. Key to its practical application is the dual focus on error characterization and mitigation through advanced photonic state manipulation techniques. Future work should aim to refine these methodologies to expand the operational capabilities of quantum computational systems, promising broader adoption and application in complex quantum calculations.