Generation of Flying Logical Qubits using Generalized Photon Subtraction with Adaptive Gaussian Operations

The generation of a logical qubit called the Gottesman-Kitaev-Preskill qubit in an optical traveling wave is a major challenge for realizing large-scale universal fault-tolerant optical quantum computers. Recently, probabilistic generation of elementary GKP qubits has been demonstrated using photon number measurements and homodyne measurements. However, the generation rate is only a few Hz, and it will be difficult to generate fault-tolerant GKP qubits at a practical rate unless success probability is significantly improved. Here, we propose a method to efficiently synthesize GKP qubits from several quantum states by adaptive Gaussian operations. In the initial state preparation that utilizes photon number measurements, an adaptive operation allows any measurement outcome above a certain threshold to be considered as a success. This threshold is lowered by utilizing the generalized photon subtraction method. The initial states are synthesized into a GKP qubit by homodyne measurements and a subsequent adaptive operation. As a result, the single-shot success probability of generating fault-tolerant GKP qubits in a realistic scale system exceeds 10$\%$, which is one million times better than previous methods. This proposal will become a powerful tool for advancing optical quantum computers from the proof-of-principle stage to practical application.Comment: 9 pages, 3 figure

Single-shot single-mode optical two-parameter displacement estimation beyond classical limit

Uncertainty principle prohibits the precise measurement of both components of displacement parameters in phase space. We have theoretically shown that this limit can be beaten using single-photon states, in a single-shot and single-mode setting [F. Hanamura et al., Phys. Rev. A 104, 062601 (2021)]. In this paper, we validate this by experimentally beating the classical limit. In optics, this is the first experiment to estimate both parameters of displacement using non-Gaussian states. This result is related to many important applications, such as quantum error correction.Comment: 5 pages, 4 figure

Propagating Gottesman-Kitaev-Preskill states encoded in an optical oscillator

A quantum computer with low-error, high-speed quantum operations and capability for interconnections is required for useful quantum computations. A logical qubit called Gottesman-Kitaev-Preskill (GKP) qubit in a single Bosonic harmonic oscillator is efficient for mitigating errors in a quantum computer. The particularly intriguing prospect of GKP qubits is that entangling gates as well as syndrome measurements for quantum error correction only require efficient, noise-robust linear operations. To date, however, GKP qubits have been only demonstrated at mechanical and microwave frequency in a highly nonlinear physical system. The physical platform that naturally provides the scalable linear toolbox is optics, including near-ideal loss-free beam splitters and near-unit efficiency homodyne detectors that allow to obtain the complete analog syndrome for optimized quantum error correction. Additional optical linear amplifiers and specifically designed GKP qubit states are then all that is needed for universal quantum computing. In this work, we realize a GKP state in propagating light at the telecommunication wavelength and demonstrate homodyne meausurements on the GKP states for the first time without any loss corrections. Our GKP states do not only show non-classicality and non-Gaussianity at room temperature and atmospheric pressure, but unlike the existing schemes with stationary qubits, they are realizable in a propagating wave system. This property permits large-scale quantum computation and interconnections, with strong compatibility to optical fibers and 5G telecommunication technology.Comment: 11 pages, 5 figure

Propagating Gottesman-Kitaev-Preskill states encoded in an optical oscillator

<p>Gottesman-Kitaev-Preskill (GKP) qubit in a single Bosonic harmonic oscillator is an efficient logical qubit for mitigating errors in a quantum computer. The entangling gates and syndrome measurements for quantum error correction only require noise-robust linear operations, a toolbox that is naturally available and scalable in optical system. To date, however, GKP qubits have been only demonstrated at mechanical and microwave frequency in a highly nonlinear stationary system. In this work, we realize a GKP state in propagating light at the telecommunication wavelength and demonstrate homodyne measurements on the GKP states without loss corrections. Our states do not only show nonclassicality and non-Gaussianity at room temperature and atmospheric pressure, but the propagating wave property also permits large-scale quantum computation with strong compatibility to telecommunication technology.</p><p>Funding provided by: Japan Society for the Promotion of Science<br>Crossref Funder Registry ID: https://ror.org/00hhkn466<br>Award Number: 23K13040</p><p>Funding provided by: Japan Society for the Promotion of Science<br>Crossref Funder Registry ID: https://ror.org/00hhkn466<br>Award Number: 21J11615</p><p>Funding provided by: Czech Science Foundation<br>Crossref Funder Registry ID: https://ror.org/01pv73b02<br>Award Number: 22-08772S</p><p>Funding provided by: Czech Science Foundation<br>Crossref Funder Registry ID: https://ror.org/01pv73b02<br>Award Number: 21-13265X</p><p>Funding provided by: Japan Science and Technology Agency<br>Crossref Funder Registry ID: https://ror.org/00097mb19<br>Award Number: JPMJPR2254</p><p>Funding provided by: Moonshot Research and Development Program<br>Award Number: JPMJMS2064</p><p>Funding provided by: Moonshot Research and Development Program<br>Award Number: JPMJMS2066</p><p>Funding provided by: UTokyo Foundation<br>Award Number: </p><p>Funding provided by: Nichia Corporation<br>Award Number: </p><p>Funding provided by: Research Foundation for OptoScience and Technology<br>Award Number: </p><p>Funding provided by: CLUSTEC<br>Award Number: 101080173</p><p>Funding provided by: EU H2020-WIDESPREAD-2020-5<br>Award Number: NONGAUSS (951737)</p><p>Funding provided by: Federal Ministry of Education and Research<br>Crossref Funder Registry ID: https://ror.org/04pz7b180<br>Award Number: </p><p>This is the data of the quadrature values of the generated states which are obtained by postprocessing of the homodyne detector data collected via oscilloscope. </p&gt