Continuous-variable quantum computing exploits the modes of electromagnetic fields to encode information. Multiplexing generates massive entangled states, called cluster states. These structures support measurement-based quantum computing. Purely Gaussian operations restrict computational power. The Gottesman-Knill theorem demonstrates that a classical algorithm efficiently simulates a Gaussian optical circuit. Quantum advantage requires the introduction of non-Gaussian processes. The generation of Schrödinger kitten states answers this need. These non-classical states authorize the realization of non-linear logic gates. They provide the fundamental resources for Gottesman-Kitaev-Preskill (GKP) error correction codes. The production of these resources constitutes a physical problem. Pure states undergo rapid decoherence under the effect of optical losses. Attenuation alters the quantum properties. The Wigner function loses its negativity. The state degenerates into a statistical mixture. Until now, experiments used free-space components. These macroscopic configurations present a restrictive mechanical instability. These limits oppose the miniaturization required by modern quantum processors. Tomography requires electronic acquisition chains with very high bandwidth. These technological barriers completely prevent the physical scaling of computation architectures. This manuscript describes the development of guided-wave non-Gaussian state generators. The experimental system operates at the telecommunications wavelength (1560 nm). A continuous-wave laser precisely pumps periodically poled lithium niobate (PPLN) waveguides. These crystalline structures generate squeezed vacuum states. A fiber coupler extracts a minimal fraction of the beam. A Fabry-Pérot cavity filters this spatial mode. A superconducting nanowire single-photon detector (SNSPD) signals the arrival of a single photon. This exact detection heralds the subtraction of a photon from the main optical mode. The operation projects the system into a non-Gaussian state. The architecture is entirely fiber-integrated to minimize physical losses. The setup integrates a distribution protocol over two distant nodes. An active feedback system stabilizes the optical interferometers. The control uses a counter-propagating probe, frequency-shifted by 2.7 GHz. Experimental measurements validate this architecture. The device generated Schrödinger kitten states. Homodyne tomographic reconstruction confirms the non-Gaussian nature of the resource. The experiment measures a negative value of the Wigner function, W(0,0) = -0,085. A numerical study evaluates the criteria for quantum tomography. The simulations directly connect the electronic detection bandwidth to the physical preservation of negativity. Strict compliance with the Nyquist-Shannon theorem prevents the spectral aliasing of quantum noise. The detector bandwidth must exceed five times the cavity linewidth. The setup tested the creation of distant non-Gaussian entangled states. The protocol exploits the temporal erasure of path information. The filtering cavity elongates the wave packet and suppresses the distinguishability due to fiber asymmetry. The use of guided waves eliminates the instabilities of non-Gaussian sources. Photon subtraction provides a generation method compatible with fiber networks. This work validates the experimental production of highly sensitive quantum states. These results constitute a solid hardware basis for designing optical processors. The transposition to lithium niobate on insulator (LNOI) integrated photonics will allow the development of continuous-variable quantum computers.
Continuous-variable quantum computing exploits the modes of electromagnetic fields to encode information. Multiplexing generates massive entangled states, called cluster states. These structures support measurement-based quantum computing. Purely Gaussian operations restrict computational power. The Gottesman-Knill theorem demonstrates that a classical algorithm efficiently simulates a Gaussian optical circuit. Quantum advantage requires the introduction of non-Gaussian processes. The generation of Schrödinger kitten states answers this need. These non-classical states authorize the realization of non-linear logic gates. They provide the fundamental resources for Gottesman-Kitaev-Preskill (GKP) error correction codes. The production of these resources constitutes a physical problem. Pure states undergo rapid decoherence under the effect of optical losses. Attenuation alters the quantum properties. The Wigner function loses its negativity. The state degenerates into a statistical mixture. Until now, experiments used free-space components. These macroscopic configurations present a restrictive mechanical instability. These limits oppose the miniaturization required by modern quantum processors. Tomography requires electronic acquisition chains with very high bandwidth. These technological barriers completely prevent the physical scaling of computation architectures. This manuscript describes the development of guided-wave non-Gaussian state generators. The experimental system operates at the telecommunications wavelength (1560 nm). A continuous-wave laser precisely pumps periodically poled lithium niobate (PPLN) waveguides. These crystalline structures generate squeezed vacuum states. A fiber coupler extracts a minimal fraction of the beam. A Fabry-Pérot cavity filters this spatial mode. A superconducting nanowire single-photon detector (SNSPD) signals the arrival of a single photon. This exact detection heralds the subtraction of a photon from the main optical mode. The operation projects the system into a non-Gaussian state. The architecture is entirely fiber-integrated to minimize physical losses. The setup integrates a distribution protocol over two distant nodes. An active feedback system stabilizes the optical interferometers. The control uses a counter-propagating probe, frequency-shifted by 2.7 GHz. Experimental measurements validate this architecture. The device generated Schrödinger kitten states. Homodyne tomographic reconstruction confirms the non-Gaussian nature of the resource. The experiment measures a negative value of the Wigner function, W(0,0) = -0,085. A numerical study evaluates the criteria for quantum tomography. The simulations directly connect the electronic detection bandwidth to the physical preservation of negativity. Strict compliance with the Nyquist-Shannon theorem prevents the spectral aliasing of quantum noise. The detector bandwidth must exceed five times the cavity linewidth. The setup tested the creation of distant non-Gaussian entangled states. The protocol exploits the temporal erasure of path information. The filtering cavity elongates the wave packet and suppresses the distinguishability due to fiber asymmetry. The use of guided waves eliminates the instabilities of non-Gaussian sources. Photon subtraction provides a generation method compatible with fiber networks. This work validates the experimental production of highly sensitive quantum states. These results constitute a solid hardware basis for designing optical processors. The transposition to lithium niobate on insulator (LNOI) integrated photonics will allow the development of continuous-variable quantum computers.
Quantum information processors benefit from high clock frequencies to fully harness quantum advantages before they are lost to decoherence. All-optical systems offer unique benefits due to their inherent 100-THz carrier frequency, enabling the development of THz-clock frequency processors. However, the bandwidth of quantum light sources and measurement devices has been limited to the MHz range, with nonclassical state generation rates in the kHz range. In this study, we demonstrated broadband generation and quantum tomography of non-Gaussian states using an optical parametric amplifier (OPA) as a squeezed light source and an optical phase-sensitive amplifier (PSA). Our system includes a 6-THz squeezed-light source, a 6-THz PSA, and a 66-GHz homodyne detector. We successfully generated non-Gaussian states at a 0.9 MHz rate with sub-nanosecond wave packets using a continuous-wave laser. The performance is currently limited by the jitter of superconducting detectors, restricting the usable bandwidth to 1 GHz. Our technique extends the bandwidth to GHz, potentially increasing non-Gaussian state generation rates for practical optical quantum processors using OPAs. The use of optical phase sensitive amplifiers (PSA) has been shown to allow fast quantum tomography of Gaussian states, but non-Gaussian states are a key component for quantum computation using optical continuous-variable systems. Here, the authors extend PSA-enhanced high-bandwidth homodyne detection to non-Gaussian states.
Quantum information processors greatly benefit from high clock frequency to fully harnessing the quantum advantages before they get washed out by the decoherence. In this pursuit, all-optical systems offer unique advantages due to their inherent 100 THz carrier frequency, permitting one to develop THz clock frequency processors. In practice, the bandwidth of the quantum light sources and the measurement devices has been limited to the MHz range and the generation rate of nonclassical states to kHz order -- a tiny fraction of what can be achieved. In this work, we go beyond this limitation by utilizing optical parametric amplifier (OPA) as a squeezed-light source and optical phase-sensitive amplifiers (PSA) to realize high-rate generation of broadband non-Gaussian states and their quantum tomography. Our state generation and measurement system consists of a 6-THz squeezed-light source, a 6-THz PSA, and a 66-GHz homodyne detector. With this system, we have successfully demonstrated non-Gaussian state generation at a 0.9 MHz rate -- almost three orders of magnitude higher than the current state-of-the-art experiments -- with a sub-nanosecond wave packet using continuous-wave laser. The performance is constrained only by the superconducting detector's jitter which currently limits the usable bandwidth of the squeezed light to 1 GHz, rather than the optical and electronic systems. Therefore, if we can overcome the limitation of the timing jitter of superconducting detector, non-Gaussian state generation and detection at GHz rate, or even THz rate, for optical quantum processors might be possible with OPAs.
We demonstrate the possibility of generating non-Gaussian states of light by exploiting a setup fully based on plug-and-play guided-wave components from classical telecom and non-linear optical technologies. Our scheme relies on heralded single photon subtraction from single mode squeezed states generated in a single-pass configuration in nonlinear optical waveguides and allows generating Schrodinger kitten quantum optical states. We discuss the different parameters affecting the shape of obtained states by comparing the theory and the numerical simulations.
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.
Characterizing non-Gaussian quantum states is of paramount importance for continuous-variable quantum information processing, yet conventional homodyne-measurement-based state tomography remains limited by optical loss, detector efficiency, and measurement bandwidth. Here, we introduce an integrated framework for loss-tolerant characterization and certification using high-gain phase-sensitive optical parametric amplification and power measurements. Our computationally efficient semidefinite programming approach enables faithful reconstruction of parity-symmetric quantum states from amplified quadrature measurements while substantially relaxing detector-efficiency requirements and increasing the measurement bandwidth. We further develop a certification framework that directly quantifies non-Gaussianity via stellar-rank witnesses and Wigner negativity, using the same quadrature-power measurements. We demonstrate the efficacy of the proposed framework through both simulated and experimental data for representative quantum states, including single-photon, Schrodinger cat, and Gottesman-Kitaev-Preskill (GKP) states. By unifying loss-tolerant measurements, state tomography, and nonclassical-state certification within a single experimentally accessible framework, our approach provides a practical pathway toward verifying increasingly complex states and can be readily implemented with current quantum photonic technologies.
Continuous-variable quantum networks constitute a valuable resource for a wide range of quantum communication and information protocols. While frequency- and fast time-multiplexing enable the implementation of scalable quantum architectures, a precise characterization of multimode squeezed-light sources is essential. In this work, we increase both the spectral resolution and the measured squeezing values. Both parameters are relevant for the improved generation of so-called cluster states, a key resource for measurement-based quantum computing. Our source is based on parametric down-conversion in a periodically poled KTP waveguide pumped by a pulsed laser. The source produces squeezed states over a specific set of optical spectra. Increased spectral resolution in spectrally resolved homodyne detection allowed us to enhance the squeezing per spectral mode, increasing it from 1 dB to more than 2.4 dB in the first mode when measuring with a homodyne detector with a bandwidth of a few MHz. Secondly, we implemented a pulse-by-pulse characterization of the resource using a fast homodyne detector with an electronic bandwidth on the order of the laser repetition rate (100 MHz). Using this approach, the fast measurements revealed squeezing values of up to 1.5 dB in the first few spectral modes. To fully access the multimode quantum resource and ultimately improve the amount of squeezing, a high quantum efficiency is required. To avoid limitations imposed by poor spatial mode matching between the quantum resource and the local oscillator in homodyne detection, we implemented a wavefront-shaping scheme to characterize and control both the intensity profile and phase of the local oscillator. In preliminary tests this approach increased the mode-matching between the reference beam and the quantum signal by more than 8% while simultaneously providing a detailed characterization of the beam shape and of the influence of waveguide transmission on the beam profile. To achieve a genuine advantage in quantum information processing over classical protocols, the generation of non-Gaussian states is necessary. We therefore present a complete scheme for the creation of non-Gaussian states in a multimode system via mode-selective single-photon addition and subtraction, implemented through nonlinear optical processes in second-order nonlinear materials. We introduce a simulation framework applied to our squeezing source, which can be adapted to other regimes as well. Based on this framework, we are able to predict specific waveguide configurations that enable full control over the generated quantum states while maintaining high purity, with each process exhibiting a Schmidt number close to K=1, corresponding to effective single-mode addressing. generated quantum states while maintaining high purity, with each process exhibiting a Schmidt number close to K=1, corresponding to effective single-mode addressing.