arXiv subjects

Akira Furusawa

Publications and source records attributed to Akira Furusawa.

At least 55 records · Page 3Linked to original sources

arXiv2022

Can the phase of radiation pressure fluctuations be flipped in a single path for laser interferometric gravitational wave detectors?

Radiation pressure (RP) noise, one component of quantum noise, can limit the sensitivity of laser interferometric gravitational wave (GW) detectors at lower frequencies. We conceived a possible RP noise cancellation method, using phase flipped ponderomotive-squeezed light (FPSL) incident on free-mass mirrors in interferometers' arms. This possibility is investigated under the constraint that the method is for space-based GW detectors in a broad frequency band lower than 1 Hz without using a long optical cavity. Considering various patterns in a single path small-scale case to generate the FPSL, we proved that no configuration exists in the single path case.

gr-qc↗

arXiv2022

Efficient backcasting search for optical quantum state synthesis

Non-Gaussian states are essential for many optical quantum technologies. The so-called optical quantum state synthesizer (OQSS), consisting of Gaussian input states, linear optics, and photon-number resolving detectors, is a promising method for non-Gaussian state preparation. However, an inevitable and crucial problem is the complexity of the numerical simulation of the state preparation on a classical computer. This problem makes it very challenging to generate important non-Gaussian states required for advanced quantum information processing. Thus, an efficient method to design OQSS circuits is highly desirable. To circumvent the problem, we offer a scheme employing a backcasting approach, where the circuit of OQSS is divided into some sublayers, and we simulate the OQSS backwards from final to first layers. Moreover, our results show that the detected photon number by each detector is at most 2, which can significantly reduce the requirements for the photon-number resolving detector. By virtue of the potential for the preparation of a wide variety of non-Gaussian states, the proposed OQSS can be a key ingredient in general optical quantum information processing.

quant-ph↗

arXiv2022

Generation of Highly Pure Single-Photon State at Telecommunication Wavelength

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 $-0.228\pm0.004$, corresponded to $85.1\pm0.7\%$ 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 telecommunication wavelengths.

quant-ph↗

arXiv2022

43-GHz bandwidth real-time amplitude measurement of 5-dB squeezed light using modularized optical parametric amplifier with 5G technology

Continuous-variable optical quantum information processing (CVOQIP), where quantum information is encoded in a traveling wave of light called a flying qubit, is a candidate for a practical quantum computer with high clock frequencies. Homodyne detectors for quadrature-phase amplitude measurements have been the major factor limiting the clock frequency. Here, we developed a real-time amplitude measurement method using a modular optical parametric amplifier (OPA) and a broadband balanced photodiode that is commercially used for coherent wavelength-division multiplexing telecommunication of the fifth-generation mobile communication systems (5G). The OPA amplifies one quadrature-phase component of the quantum-level signal to a loss-tolerant macroscopic level, and acts as a "magic wand," which suppresses the loss after the OPA from 92.4\% to only 0.4\%. When the method was applied to a broadband squeezed vacuum with a center wavelength of 1545.32 nm, we observed 5.2 $\pm$ 0.5 dB of squeezing from DC to 43 GHz without any loss correction. The marriage of CVOQIP and 5G technology arranged by the modular OPA will lead to a paradigm shift from the conventional method of using stationary qubits, where the information is encoded in a standing wave system, to a method using flying qubits for ultra-fast practical quantum computation. This means that quantum computer research will move from the stage of developing machines that execute only specific quantum algorithms to a stage of developing machines that can outperform classical computers in running any algorithm.

quant-ph↗

arXiv2022

Analysis of optical quantum state preparation using photon detectors in the finite-temporal-resolution regime

Quantum state preparation is important for quantum information processing. In particular, in optical quantum computing with continuous variables, non-Gaussian states are needed for universal operation and error correction. Optical non-Gaussian states are usually generated by heralding schemes using photon detectors. In previous experiments, the temporal resolution of the photon detectors was sufficiently high relative to the time width of the quantum state, so that the conventional theory of non-Gaussian state preparation treated the detector's temporal resolution as negligible. However, when using various photon detectors including photon-number-resolving detectors, the temporal resolution is non-negligible. In this paper, we extend the conventional theory of quantum state preparation using photon detectors to the finite temporal resolution regime, analyze the cases of single-photon and two-photon preparation as examples, and find that the generated states are characterized by the dimensionless parameter $B$, defined as the product of the temporal resolution of the detectors $Δt$ and the bandwidth of the light source $Δf$. Based on the results, $B\sim0.1$ is required to keep the purity and fidelity of the generated quantum states high.

quant-ph↗

arXiv2022

Quantum arbitrary waveform generator

Controlling the waveform of light is the key for a versatile light source in classical and quantum electronics. Although pulse shaping of classical light is a mature technique 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 waveform. We call such a device a quantum arbitrary waveform generator (Q-AWG). The Q-AWG must be able to handle versatile quantum states of light, which are fragile. Thus, the Q-AWG requires a radically different methodology from classical pulse shaping. In this paper, we invent an architecture of Q-AWGs that can operate semi-deterministically at a repetition rate over GHz in principal. We demonstrate its core technology via generating highly non-classical states with 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.

quant-ph↗

arXiv2022

Switching-free time-domain optical quantum computation with quantum teleportation

Optical switches and rerouting network are main obstacles to realize optical quantum computer. In particular, both components have been considered as essential components to the measurement-based time-domain optical quantum computation, which has seen promising developments regarding scalability in the recent years. Realizing optical switches and rerouting network with sufficient performance is, however, experimentally challenging as they must have extremely low loss, small switching time, high repetition rate, and minimum optical nonlinearity. In this work, we present an optical quantum computation platform that does not require such optical switches. Our method is based on continuous-variable measurement-based quantum computation, where instead of the typical cluster states, we modify the structure of the quantum entanglements, so that quantum teleportation protocol can be employed instead of the optical switching and rerouting. We also show that when combined with Gottesman-Kitaev-Preskill encoding, our architecture can outperform the architecture with optical switches when the optical losses of the switches are not low.

quant-ph↗

arXiv2022

Generating Gottesman-Kitaev-Preskill qubit using a cross-Kerr interaction between a squeezed light and Fock states in optics

Gottesman-Kitaev-Preskill (GKP) qubit is a promising ingredient for fault-tolerant quantum computation (FTQC) in optical continuous variables due to its advantage of noise tolerance and scalability. However, one of the main problems in the preparation of the optical GKP qubit is the difficulty in obtaining the nonlinearity. Cross-Kerr interaction is one of the promising candidates for this nonlinearity. There is no existing scheme to use the cross-Kerr interaction to generate the optical GKP qubit for FTQC. In this work, we propose a generation method of the GKP qubit by using a cross-Kerr interaction between a squeezed light and a superposition of Fock states. We numerically show that the GKP qubit with the 10 dB can be generated with a mean fidelities of 99.99 and 99.9% at the success probabilities of 2.7 and 4.8%, respectively. Therefore, our method has potential method to generate the optical GKP qubit with a quality required for FTQC when we obtain the sufficient technologies for the preparation of ancillary Fock states and a cross-Kerr interaction.

quant-ph↗

arXiv2022

Generation of Schrödinger cat states with Wigner negativity using continuous-wave low-loss waveguide optical parametric amplifier

Continuous-wave (CW) squeezed light is used in generation of various optical quantum states 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 ${\rm LiNbO_3}$ 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.

quant-ph↗

arXiv2022

Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer

A continuous-wave (CW) broadband high-level optical quadrature squeezer is essential for high-speed large-scale fault-tolerant quantum computing on a time-domain-multiplexed continuous-variable optical cluster state. CW THz-bandwidth squeezed light can be obtained with a waveguide optical parametric amplifier (OPA); however, the squeezing level have been insufficient for applications of fault-tolerant quantum computation because of degradation of the squeezing level due to their optical losses caused by the structural perturbation and pump-induced phenomena. Here, by using mechanical polishing processes, we fabricated a low-loss quasi-single-mode periodically-poled LiNbO3 (PPLN) waveguide, which shows 7% optical propagation loss with a waveguide length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed OPA module with a total insertion loss of 21%. Thanks to its directly bonded core on a LiTaO3 substrate, the waveguide does not show pump-induced optical loss even under a condition of hundreds of milliwatts pumping. Furthermore, the quasi-single-mode structure prohibits excitation of higher-order spatial modes, and enables us to obtain larger squeezing level. Even with including optical coupling loss of the modularization, we observe 6.3-dB squeezed light from the DC component up to a 6.0-THz sideband in a fully fiber-closed optical system. By excluding the losses due to imperfections of the modularization and detection, the squeezing level at the output of the PPLN waveguide is estimated to be over 10 dB. Our waveguide squeezer is a promising quantum light source for high-speed large-scale fault-tolerant quantum computing.

quant-ph↗

arXiv2021

Estimation of Gaussian random displacement using non-Gaussian states

In continuous-variable quantum information processing, quantum error correction of Gaussian errors requires simultaneous estimation of both quadrature components of displacements on phase space. However, quadrature operators $x$ and $p$ are non-commutative conjugate observables, whose simultaneous measurement is prohibited by the uncertainty principle. Gottesman-Kitaev-Preskill (GKP) error correction deals with this problem using complex non-Gaussian states called GKP states. On the other hand, simultaneous estimation of displacement using experimentally feasible non-Gaussian states has not been well studied. In this paper, we consider a multi-parameter estimation problem of displacements assuming an isotropic Gaussian prior distribution and allowing post-selection of measurement outcomes. We derive a lower bound for the estimation error when only Gaussian operations are used, and show that even simple non-Gaussian states such as single-photon states can beat this bound. Based on Ghosh's bound, we also obtain a lower bound for the estimation error when the maximum photon number of the input state is given. Our results reveal the role of non-Gaussianity in the estimation of displacements, and pave the way toward the error correction of Gaussian errors using experimentally feasible non-Gaussian states.

quant-ph↗

arXiv2021

Non-Clifford gate on optical qubits by nonlinear feedforward

In a continuous-variable optical system, the Gottesman-Kitaev-Preskill (GKP) qubit is a promising candidate for fault-tolerant quantum computation. To implement non-Clifford operations on GKP qubits, non-Gaussian operations are required. In this context, the implementation of a cubic phase gate by combining nonlinear feedforward with ancillary states has been widely researched. Recently, however, it is pointed out that the cubic phase gate is not the most suitable for non-Clifford operations on GKP qubits. In this work, we show that we can achieve linear optical implementation of non-Clifford operations on GKP qubit with high fidelity by applying the nonlinear feedforward originally developed for the cubic phase gate and using a GKP-encoded ancillary state. Our work shows the versatility of nonlinear feedforward technique important for optical implementation of the fault-tolerant continuous-variable quantum computation.

quant-ph↗

arXiv2021

Quantum detector tomography of superconducting nanostrip photon-number-resolving detector

Superconducting nanostrip photon detectors have been used as single photon detectors, which can discriminate only photons' presence or absence. It has recently been found that they can discriminate the number of photons by analyzing the output signal waveform, and they are expected to be used in various fields, especially in optical quantum information processing. Here, we improve the photon-number-resolving performance for light with a high-average photon number by pattern matching of the output signal waveform. Furthermore, we estimate the positive-operator-valued measure of the detector by a quantum detector tomography. The result shows that the device has photon-number-resolving performance up to five photons without any multiplexing or arraying, indicating that it is useful as a photon-number-resolving detector.

quant-ph↗

arXiv2021

Wave-function engineering via conditional quantum teleportation with non-Gaussian entanglement resource

We propose and analyze a setup to tailor the wave functions of the quantum states. Our setup is based on the quantum teleportation circuit, but instead of the usual two-mode squeezed state, two-mode non-Gaussian entangled state is used. Using this setup, we can generate various classes of quantum states such as Schrödinger cat states, four-component cat states, superpositions of Fock states, and cubic phase states. These results demonstrate the versatility of our system as a state generator and suggest that conditioning using homodyne measurements is an important tool in the generations of the non-Gaussian states in complementary to the photon number detection.

quant-ph↗

arXiv2020

Optimization of quantum noise by completing the square of multiple interferometer outputs in quantum locking for gravitational wave detectors

The quantum locking technique, which uses additional short low-loss sub-cavities, is effective in reducing quantum noise in space gravitational wave antenna DECIGO. However, the quantum noise of the main interferometer depends on the control systems in the sub-cavities. Here we demonstrate a new method to optimize the quantum noise independently of the feedback gain by completing the square in the quantum locking system. We successfully demonstrate in simulations that this method is effective in optimizing the homodyne angle to the best quantum-noise-limited sensitivity.

gr-qc↗

arXiv2020

Reduction of quantum noise using the quantum locking with an optical spring for gravitational wave detectors

In our previous research, simulation showed that a quantum locking scheme with homodyne detection in sub-cavities is effective in surpassing the quantum noise limit for Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) in a limited frequency range. This time we have simulated an optical spring effect in the sub-cavities of the quantum locking scheme. We found that the optimized total quantum noise is reduced in a broader frequency band, compared to the case without the optical spring effect significantly improving the sensitivity of DECIGO to the primordial gravitational waves.

gr-qc↗

arXiv2020

Nonlinear squeezing for measurement-based non-Gaussian operations in time domain

Quantum non-Gaussian gate is a missing piece to the realization of continuous-variable universal quantum operations in the optical system. In a measurement-based implementation of the cubic phase gate, a lowest-order non-Gaussian gate, non-Gaussian ancillary states that has a property we call nonlinear squeezing are required. This property, however, has never been experimentally verified. In this paper, we generate a superposition between a vacuum state and a single photon state whose nonlinear squeezing are maximized by the optimization of the superposition coefficients. The nonlinear squeezing is observed via real-time quadrature measurements, meaning that the generated states are compatible with the real-time feedforward and are suitable as the ancillary states for the cubic phase gate in time domain. Moreover, by increasing the number of the photons, it is expected that nonlinear squeezing can be further improved. The idea presented here can be readily extended to the higher-order phase gates [P. Marek et al., Phys. Rev. A 97, 022329 (2018)]. As such, this work presents an important step to extend the CV quantum information processing from Gaussian regime to non-Gaussian regime.

quant-ph↗

arXiv2020

Generation of optical Schrödinger's cat states by generalized photon subtraction

We propose a high-rate generation method of optical Schrödinger's cat states. Thus far, photon subtraction from squeezed vacuum states has been a standard method in cat-state generation, but its constraints on experimental parameters limit the generation rate. In this paper, we consider the state generation by photon number measurement in one mode of arbitrary two-mode Gaussian states, which is a generalization of conventional photon subtraction, and derive the conditions to generate high-fidelity and large-amplitude cat states. Our method relaxes the constraints on experimental parameters, allowing us to optimize them and attain a high generation rate. Supposing realistic experimental conditions, the generation rate of cat states with large amplitudes ($|α| \ge 2)$ can exceed megacounts per second, about $10^3$ to $10^6$ times better than typical rates of conventional photon subtraction. This rate would be improved further by the progress of related technologies. Ability to generate non-Gaussian states at a high rate is important in quantum computing using optical continuous variables, where scalable computing platforms have been demonstrated but preparation of non-Gaussian states of light remains as a challenging task. Our proposal reduces the difficulty of the state preparation and open a way for practical applications in quantum optics.

quant-ph↗