It is crucial to address experimental errors in quantum circuits at every stage, particularly when implementing quantum computation. Quantum error correction generally requires more qubits in order to complete the correction operation.

The fault-tolerant method is another option. It allows logical qubits to be encoded with multiple physical qubits. If the error in the physical spaces is unacceptable, it is expected that the correction will not be made. This excludes the qubit with errors from encoded space.

To be more specific, logical qubits could be output with a higher probability in fault-tolerant encoded circuits based on the exact same hardware than in non-encoded ones when the error rate falls below the threshold. Furthermore, the fault tolerance circuit could be verified using a small system that has several qubits. The threshold, which is explicit evidence supporting the fault-tolerant method's success, could be found by comparing the output probabilities for encoded and non-encoded systems.

A team of scientists led by Professor Chuan Feng Li, CAS Key Laboratory of Quantum Information University of Science and Technology of China has published a paper in Light Science & Application. They have used the spatial modes of two entangled photos to create an experimental platform and observed the fault tolerance threshold for the investigated quantum systems.

Two logical qubits, which are the coincident counts for the spatial modes of each photon in the physical qubits, can be encoded and manipulated using the corresponding operations on physical qubits. We could artificially import the error rate with a high level of accuracy and scan the range of error rates that covers the threshold. If the success rate of an encoded circuit exceeds that of a non-encoded one, it is possible to confirm the value of the threshold. This is supported by strong results, including single-qubit as well as two-qubit operations in logical space.

This work not only facilitates the investigation of fault-tolerant quant computation in scalable system, but also helps with other quantum information tasks such as entanglement purification or long-distance QI communication.

We could observe the error rate threshold to understand the details of fault-tolerant protocols, and assess their success. These scientists summarize the optical platform's performance.

"We build the setup using the spatial modes for two photons. This has the following advantages: (1) High-accuracy operation, which is the strict requirement of fault-tolerantcircuit; (2) Easy to import the artificial error rate and adjust it; (3) Present the straight pattern for every step of the fault-tolerant process; (4) Easy to implement fault-tolerant encoded and non-encoded.

This experimental platform could be used to investigate other error types in a universal fault tolerance protocol. The scientists state that the extension of the experimental platform based upon the optical spatial model from single-photon to two-entangled photon framework in this research could allow for further investigation of the nonlocal error effect in fault-tolerant quantum computation.