Optical demonstration of quantum fault-tolerant threshold

Area of Science:

• Quantum Information Science
• Experimental Quantum Computing
• Quantum Error Correction

Background:

• Quantum computation is susceptible to environmental errors, necessitating fault-tolerant schemes.
• Current noisy intermediate-scale quantum (NISQ) technology faces challenges in implementing full fault-tolerant protocols due to strict encoding requirements.
• Experimental verification of the error threshold for fault-tolerant quantum computation remains elusive.

Purpose of the Study:

• To experimentally demonstrate the existence of an error threshold for fault-tolerant quantum computation.
• To validate the effectiveness of fault-tolerant protocols in mitigating errors below a critical rate.
• To provide a platform for investigating error propagation in complex quantum circuits.

Main Methods:

• Utilized an all-optical setup representing four physical qubits as spatial modes of two entangled photons.
• Encoded two logical qubits using the physical qubits.
• Implemented fault-tolerant gates and compared circuit output probabilities with non-encoded circuits under varying error rates.

Main Results:

• Demonstrated that below the experimentally verified threshold, fault-tolerant circuits yield a higher probability of correct logical qubit output compared to non-encoded circuits.
• Observed no advantage for fault-tolerant implementation when the error rate exceeded the threshold.
• The high-accuracy optical system successfully showed the predicted behavior of fault-tolerant protocols relative to error rates.

Conclusions:

• The experimental results confirm the existence of a critical error threshold for the efficacy of fault-tolerant quantum computation.
• Fault-tolerant protocols offer a significant advantage in quantum computation accuracy only when operating below this identified threshold.
• The developed optical system serves as a robust platform for future research into error dynamics in advanced quantum circuits.