Entanglement is a fundamental resource for various optical quantum information processing (QIP) applications. To achieve high-speed QIP systems, entanglement should be encoded in short wavepackets. Here we report the real-time observation of ultrafast optical Einstein–Podolsky–Rosen correlation at a picosecond timescale in a continuous-wave system. Optical phase-sensitive amplification using a 6-THz-bandwidth waveguide-based optical parametric amplifier enhances the effective efficiency of 70-GHz-bandwidth homodyne detectors, mainly used in 5G telecommunication, enabling its use in real-time quantum state measurement. Although power measurement using frequency scanning, such as an optical spectrum analyser, is not performed in real time, our observation is demonstrated through the real-time amplitude measurement and can be directly used in QIP applications. The observed Einstein–Podolsky–Rosen states show quantum correlation of 4.5 dB below the shot-noise level encoded in wavepackets with 40 ps period, equivalent to 25 GHz repetition—103 times faster than previous entanglement observation in continuous-wave systems. The quantum correlation of 4.5 dB is already sufficient for several QIP applications, and our system can be readily extended to large-scale entanglement. Moreover, our scheme has high compatibility with optical communication technology such as wavelength-division multiplexing, and femtosecond-timescale observation is also feasible. Our demonstration is a paradigm shift in accelerating accessible quantum correlation—the foundational resource of all quantum applications—from the nanosecond to picosecond timescales, enabling ultrafast optical QIP. Optical Einstein–Podolsky–Rosen correlation is observed on a picosecond timescale in a continuous-wave system. By introducing waveguide optical parametric amplifiers and balanced detectors, the quantum correlation 4.5 dB below the shot-noise level is observed.

We generate and perform real-time measurement of optical entangled states with 60-GHz bandwidth using waveguide optical parametric amplifiers and homodyne measurement with phase sensitive amplification, foreseeing quantum information processing with ultrafast clock frequency.

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.

Optical quantum information processing relies critically on Bell-state measurement, a ubiquitous operation for quantum communication and computing. Its practical realization involves the interference of optical modes and the detection of a single photon in an indistinguishable manner. Yet, in the absence of efficient photon-number-resolution capabilities, errors arise from multiphoton components, decreasing the overall process fidelity. Here, we introduce a hybrid detection scheme for Bell-state measurement, leveraging both on-off single-photon detection and quadrature conditioning via homodyne detection. We derive explicit fidelities for quantum teleportation and entanglement-swapping processes employing this strategy, demonstrating its efficacy. We also compare with photon-number-resolving detectors and find a strong advantage of the hybrid scheme in a wide range of parameters. This work provides a new tool for linear-optics schemes, with applications to quantum state engineering and quantum interconnects.

Optical system is an appealing system for quantum computation as it has tremendous scalability over typical matter-based qubit. This is thanks to its rich degree of freedom that allows multiplexing. A particularly promising approach is the time-domain multiplexing approach where large-scale entangled resources and their usages have been demonstrated. To achieve quantum computation, these resources have to be combined with a type of states called non-Gaussian states. Non-Gaussian state generation requires strong nonlinearity which is challenging in optical system, compared to the matter-based system. In this work, we explain our recent work in the generation of the non-Gaussian states for optical quantum computer. In the first half, we discuss the demonstration of cat-breeding protocol for the generation of Gottesman-Kitaev-Preskill (GKP) qubit. In the second half, we show the demonstration of generation of cat states from broadband light source. In the future, by combining these two techniques, we can achieve high-rate high-quality GKP states crucial for optical quantum computer.

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 generate and observe optical non-Gaussian states defined in wavepackets of sub-nanosecond time width—O(103) faster than previous research—using waveguide optical parametric amplifier made of PPLN crystal, enabling ultrafast quantum information processing.

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.

We formulate a method to efficiently extract the non-classicality o f photon number measurements into optical traveling wave and show that practical logical qubits can be generated over 10% probability in a realistic system.

Enabling large-scale and high-speed quantum computation is a key to practical quantum computation. Continuous-variable approach in optical systems offer advantages in scalability and speed by leveraging their temporal degree of freedom and inherent large carrier frequency. In this paper, we investigate the generation and manipulation of quantum entanglement through a time-domain multiplexing approach. By employing time-domain multiplexing, we generate a two-dimensional cluster state—a universal resource for large-scale quantum computation—and perform quantum operations in the time domain with cluster state. Additionally, our ongoing research focuses on the generation and measurement of broadband optical quantum entanglement through an optical parametric oscillator, which holds potential as a foundation for high-speed quantum computing surpassing limitations of existing systems. By further engineering the quantum entanglement, we have also theoretically formulated a practical teleportation-based architectures for quantum computation in time domain. These advancements form the groundwork for the development of practical optical quantum computation.

Practical quantum computing requires robust encoding of logical qubits in physical systems to protect fragile quantum information. Currently, the lack of scalability limits the logical encoding in most physical systems, and thus the high scalability of propagating light can be a game changer. However, propagating light also has difficulty in logical encoding due to weak nonlinearity. Here, we propose a synthesizer that encodes Gottesman-Kitaev-Preskill (GKP) qubits in propagating light by exploiting the nonlinearity of photon detectors. This synthesizer is based on an approach what we call Gaussian breeding, leading to the following four advantages: (i) systematic and rigorous synthesis of arbitrary GKP qubits, (ii) use of minimal resources, (iii) high fidelity and high success probability, and (iv) robustness against loss. There has been no protocol that incorporates all these advantages, and thus the proposed synthesizer excels in both performance and feasibility. By employing our method, one can generate GKP qubits using a few to several squeezed light sources, beam splitters and photon detectors.

Toward the realization of quantum information processing (QIP), continuous-variable (CV) system is a promising candidate, where scalable quantum information processors are already demonstrated [1]. The next milestone is the demonstration of universality and fault-tolerance on these processors by injecting highly non-classical quantum states, non-Gaussian states (NGSs), with a proper temporal waveform [2]. NGSs are very fragile to optical losses, however, and engineering temporal waveform of NGSs is a quite nontrivial task that requires completely different methodology from conventional optical wave shaping.

We present an architecture of optical quantum computation that does not require optical switches. Our method utilizes quantum teleportation with multimode entanglement making it a scalable architecture that is compatible with the time-domain technology.

Controlling the temporal waveform of light is the key to a versatile light source in classical and quantum electronics. Although pulse shaping of classical light is mature and has been used in various fields, more advanced applications would be realized by a light source that generates arbitrary quantum light with arbitrary temporal waveforms. We call such a device a quantum arbitrary waveform generator (Q-AWG). The Q-AWG must be able to handle various quantum states of light, which are fragile. Thus, the Q-AWG requires a radically different methodology from classical pulse shaping. Here, we invent an architecture of Q-AWGs that can operate semi-deterministically at a repetition rate over gigahertz in principle. We demonstrate its core technology via generating highly nonclassical states with temporal waveforms that have never been realized before. This result would lead to powerful quantum technologies based on Q-AWGs such as practical optical quantum computing.

Continuous-wave (CW) squeezed light is used in the generation of various optical quantum states, and thus is a fundamental resource of fault-tolerant universal quantum computation using optical continuous variables. To realize a practical quantum computer, a waveguide optical parametric amplifier (OPA) is an attractive CW squeezed light source in terms of its THz-order bandwidth and suitability for modularization. The usages of a waveguide OPA in quantum applications thus far, however, are limited due to the difficulty of the generation of the squeezed light with a high purity. In this paper, we report the first observation of Wigner negativity of the states generated by a heralding method using a waveguide OPA. We generate Schrödinger cat states at the wavelength of 1545 nm with Wigner negativity using a quasi-single-mode ZnO-doped periodically poled LiNbO3 waveguide module we developed. Wigner negativity is regarded as an important indicator of the usefulness of the quantum states as it is essential in the fault-tolerant universal quantum computation. Our result shows that our waveguide OPA can be used in wide range of quantum applications leading to a THz-clock optical quantum computer.

We generate highly-pure single-photon states at the telecommunication wave-length with the best record of the minimum Wigner negativity, paving the way toward the integration of universal quantum computers and constructing quantum networks.

We propose the first all-optical method for generating arbitrary Gottesman-Kitaev-Preskill states, which fills in the largest missing piece of practical optical quantum computing.

Telecommunication wavelength with well-developed optical communication technologies and low losses in the waveguide are advantageous for quantum applications. However, an experimental generation of non-classical states called non-Gaussian states at the telecommunication wavelength is still underdeveloped. Here, we generate highly-pure-single-photon states, one of the most primitive non-Gaussian states, by using a heralding scheme with an optical parametric oscillator and a superconducting nano-strip photon detector. The Wigner negativity, the indicator of non-classicality, of the generated single photon state is -0.228 ± 0.004, corresponded to 85.1 ± 0.7% of single photon and the best record of the minimum value at all wavelengths. The quantum-optics-technology we establish can be easily applied to the generation of various types of quantum states, opening up the possibility of continuous-variable-quantum-information processing at the telecommunication wavelength.

Practical quantum computing requires robust encoding of logical qubits in physical systems to protect fragile quantum information. Currently, the lack of scalability limits the logical encoding in most physical systems, and thus the high scalability of propagating light can be a game changer for realizing a practical quantum computer. However, propagating light also has a drawback: the difficulty of logical encoding due to weak nonlinearity. Here, we propose Gaussian breeding that encodes arbitrary Gottesman-Kitaev-Preskill (GKP) qubits in propagating light. The key idea is the efficient and iterable generation of quantum superpositions by photon detectors, which is the most widely used nonlinear element in quantum propagating light. This formulation makes it possible to systematically create the desired qubits with minimal resources. Our simulations show that GKP qubits above a fault-tolerant threshold, including ``magic states'', can be generated with a high success probability and with a high fidelity exceeding 0.99. This result fills an important missing piece toward practical quantum computing.

We observed Wigner negativity of Schrödinger cat states generated with a low-loss continuous-wave optical parametric amplifier, paving the way toward fault-tolerant and universal quantum computation with terahertz clock frequency.