Complete temporal mode characterization of non-Gaussian states by dual homodyne measurement

Optical quantum states defined in temporal modes, especially non-Gaussian states like photon-number states, play an important role in quantum computing schemes. In general, the temporal-mode structures of these states are characterized by one or more complex functions called temporal-mode functions (TMFs). Although we can calculate TMF theoretically in some cases, experimental estimation of TMF is more advantageous to utilize the states with high purity. In this paper, we propose a method to estimate complex TMFs. This method can be applied not only to arbitrary single-temporal-mode non-Gaussian states but also to two-temporal-mode states containing two photons. This method is implemented by continuous-wave (CW) dual homodyne measurement and doesn't need prior information of the target states nor state reconstruction procedure. We demonstrate this method by analyzing several experimentally created non-Gaussian states

Generation of optical Schr\"{o}dinger's cat states by generalized photon subtraction

We propose a high-rate generation method of optical Schr\"{o}dinger'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 ($|\alpha| \ge 2)$ 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.Comment: 8 pages, 5 figure

Gaussian breeding for encoding a qubit in propagating light

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.Comment: 19 pages, 2 figure

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

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.Comment: 10 pages, 6 figure

24-hour measurement of squeezed light using automated stable fiber system

In order to provide a cloud service of optical quantum computing, it is inevitable to stabilize the optical system for many hours. It is advantageous to construct a fiber-based system, which does not require spatial alignment. However, fiber-based systems are instead subject to fiber-specific instabilities. For instance, there are phase drifts due to ambient temperature changes and external disturbances, and polarization fluctuations due to the finite polarization extinction ratio of fiber components. Here, we report the success of measuring squeezed light with a fiber system for 24 hours. To do this, we introduce stabilization mechanics to suppress fluctuations in the fiber system, and integrated controller to automatically align the entire system. The squeezed light at the wavelength of 1545.3 nm is measured every 2 minutes, where automated alignments are inserted every 30 minutes. The squeezing levels with the average of -4.42 dB are recorded with an extremely small standard deviation of 0.08 dB over 24 hours

Timing jitter removers of photon detectors

Among various performances of photon detectors, the timing jitter is difficult to improve because of its trade-offs with other important performances such as detection efficiency. Such trade-offs have been an issue in applications, especially for high-purity non-Gaussian-state generation necessary in optical quantum computation. Here, we introduce a method using an external fast optical switch -- Timing Jitter Remover (TJR) -- whose time window limits the photon-detectable time of photon detectors and improve the timing jitter without sacrificing other performances. By using a TJR, we experimentally improve the timing jitter of a photon-number-resolving detector based on a transition edge sensor, from 50 ns to 10 ns. Using this improved detector, we generate one of important non-Gaussian states, a Schr\"{o}dinger cat state with Wigner negativity of -0.01, which cannot be observed without TJRs. TJRs would be the key technology for the realization of ultra-fast, fault-tolerant, universal optical quantum computer.Comment: 26 pages, 6 figure

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.Comment: 24 pages, 5 figure