Improved error correction and switching for photonic quantum computers

Error-protected qubits in a silicon photonic chip

This paper just published in Nature Physics reports the experimental implementation of photonic graph and hyper-graph states to carry out some basic quantum information processing tasks (single qubit rotations, state teleportation) using error-protected logical qubits and measurement-based quantum computation.

In the measurement-based quantum computation approach, one starts with a large-scale entangled state of light, which can be represented as a graph encoding the entanglement between the different qubits. Quantum gates are then implemented by applying a sequence of measurements and measurement-dependent (feed-forward) operations on the qubits. 

The combination of measurement and feed-forward operations allows for errors to be detected and compensated for as the computation is carried out. However, the correction of physical errors requires additional qubits. This is a bottleneck limiting the scalability of photonic quantum computers, which are built on the probabilistic generation of few-photon entangled states. Therefore it is important to reduce error rates to minimise this overhead.

In this study, the authors demonstrate how clever encoding schemes (encoding multiple qubits onto a single photon and using optimal graph states) allows one to reduce the logical error rates of basic quantum operations. As an example, the success probability of a small-scale phase estimation algorithm in increased from 63% to 93%.

The article conclusion does stress some ongoing challenges that need to be addressed in order for photonic quantum computers to reach a useful size. Many new challenges emerge when going from proof-of-principle demonstrations based on a few photons to large-scale circuits with thousands or millions of photons, including the need to integrate near-deterministic photon sources and detectors, and the development of ultra-fast low-loss integrated optical switches.

Switch networks for photonic fusion-based quantum computing

Improving the performance of integrated optical switching is the subject of a preprint that just appeared on arXiv by PsiQ. PsiQ is taking an ambitious approach to build a photonic quantum computer, aiming to skip noisy-intermediate scale quantum devices completely and build a million-qubit processor, partnering with GlobalFoundries to develop mass-producible components. 

I like the approach taken by PsiQ because it addresses the elephant in the room plaguing all the companies pursuing devices based on superconducting qubits - how do you fit millions of qubits into a helium refrigerator?. 

For photonic quantum computers to be viable it will be essential to minimise losses. One significant source of losses is in active components such as electrically-controlled fast optical switches. 

This preprint analyzes improved schemes for creating different kinds of large-scale photonic switches using Mach-Zehnder interferomters and spatial and temporal multiplexing.