Loss tolerance in fusion-based photonic quantum computing

Short answer: Fusion-based quantum computing (FBQC) is a model for fault-tolerant quantum computing built from entangling measurements called fusions on small entangled resource states. A 2023 Nature Communications paper presents a ballistic scheme that its authors say tolerates a 10.4% probability of photon loss in each fusion, and a 2024 analysis reports that fault tolerance would need single-photon creation and detection with a total marginal efficiency above 0.973. These are simulation and theory results, not hardware demonstrations.

What is the core mechanism of FBQC?

The FBQC model relies on entangling measurements known as fusions. These operations are performed on qubits within small, constant-sized entangled resource states. Unlike deterministic unitary entangling gates, which are not natural operations in many photonic systems, fusions are physical primitives readily accessible in photonics. The model addresses the probabilistic nature of photonic gates and associated errors directly through quantum error correction protocols. This approach allows for a framework for universal quantum computing without requiring deterministic entangling operations,.

What are the specific loss tolerance thresholds?

The 2023 authors report that FBQC can achieve a higher threshold than schemes reported in the literature. A ballistic scheme presented in recent work can tolerate a 10.4% probability of photon loss in each fusion. This corresponds to a 2.7% probability of loss for each individual photon.

What hardware requirements are necessary for implementation?

The 2024 analysis reports that fault-tolerant quantum computing in the FBQC model is possible with currently achievable levels of optical losses in an integrated photonic implementation, provided an efficiency condition is met. The authors say single photons for the resource states must be created and detected with a total marginal efficiency higher than 0.973. This requirement applies to the linear-optical realization of fusions. The analysis numerically evaluates the requirements on losses for every component of these circuits. Meeting this efficiency threshold allows errors in fusion networks caused by optical losses to be corrected. The 2023 authors describe the architecture as highly modular.

How does FBQC compare to other photonic approaches?

The 2023 authors present FBQC as an alternative to existing protocols. It is constructed from physical primitives that are readily accessible in photonic systems, avoiding the need for deterministic entangling operations. Previous approaches often faced large resource and technical overhead due to the probabilistic nature of computation. The authors say probabilistic gates and errors are dealt with directly by the error correction protocol. The authors describe the architecture as highly modular with reduced classical processing requirements compared to previous photonic architectures.

FAQ

Is FBQC currently deployed in commercial systems?

No. The papers describe FBQC as a research-stage computational model, and they present analysis and numerical evaluation, not deployed systems.

Does FBQC require deterministic entangling gates?

No. FBQC is constructed from entangling measurements called fusions. Deterministic unitary entangling gates are not natural operations in many photonic systems. FBQC uses probabilistic operations handled by error correction.

What is the minimum photon efficiency required?

The 2024 analysis says a total marginal efficiency higher than 0.973 for creating and detecting single photons in the resource states is needed for fault tolerance in its linear-optical fusion circuits.

Can FBQC tolerate high loss rates?

The 2023 authors report a ballistic scheme that can tolerate a 10.4% probability of photon loss per fusion. This corresponds to a 2.7% loss probability per individual photon. They say the error correction protocol deals with these losses.