Logical states for fault-tolerant quantum computation with propagating light

Abstract

To harness the potential of a quantum computer, quantum information must be protected against error by encoding it into a logical state that is suitable for quantum error correction. The Gottesman-Kitaev-Preskill (GKP) qubit is a promising candidate because the required multiqubit operations are readily available at optical frequency. To date, however, GKP qubits have been demonstrated only at mechanical and microwave frequencies. We realized a GKP state in propagating light at telecommunication wavelength and verified it through homodyne measurements without loss corrections. The generation is based on interference of cat states, followed by homodyne measurements. Our final states exhibit nonclassicality and non-Gaussianity, including the trident shape of faint instances of GKP states. Improvements toward brighter, multipeaked GKP qubits will be the basis for quantum computation with light. Editor’s summary Quantum computers under development are at the intermediate size scale and are already demonstrating quantum advantage over classical systems for specific tasks. Going to larger scale systems is challenging for some solid-state platforms. Optics provides a possible route, but optical systems require engineered photonic states to mitigate for loss and to run error correction codes. Konno et al. demonstrate the generation of Gottesman-Kitaev-Preskill states, or grid states, in which the wavefunction resembles a sharp-peaked two-dimensional array (see the Perspective by Pfister). These states have been predicted to be fault tolerant, allowing quantum error correction codes to be readily implemented. Realizing such engineered photonic states will be important in the development of large-scale optical quantum computers. —Ian S. Osborne Engineered photonic states required for fault-tolerant optical quantum computing were generated.