In January 2024, a collaboration led by Akira Furusawa (University of Tokyo) and Peter van Loock (Johannes Gutenberg University Mainz), with researchers from Palacký University Olomouc, NICT, and RIKEN, demonstrated the world's first GKP logical qubit in propagating light. The work was published in Science (Vol. 383, Issue 6680, pp. 289–293, 2024-01-18) as Konno et al., "Logical states for fault-tolerant quantum computation with propagating light."

Prior to this, GKP encoding had only been realized in superconducting circuits (ETH Zürich, 2019) and trapped ions (NIST, 2019). The optical domain presented a decades-long barrier due to the absence of strong optical nonlinearities required for state preparation.

The team overcame this by preparing Schrödinger cat states and shaping them into GKP states using only linear optical elements. Projective measurements were performed with superconducting nanowire single-photon detectors (SNSPDs) co-developed with NICT (quantum efficiency ~75%, timing jitter 70 ps). The system operates entirely at room temperature using telecom-wavelength light, requiring no cryogenic cooling.

The experiment encodes 1 logical qubit from a single light pulse. However, the authors explicitly state that error rates are "not sufficient for fault-tolerant quantum computation" — the logical structure is correct, but error rates exceed the fault-tolerance threshold. This result is classified as a below-threshold logical qubit.