arXiv subjects

Toshiki Kobayashi

Publications and source records attributed to Toshiki Kobayashi.

18 recordsLinked to original sources

arXiv2026

Logical entanglement distribution between distant 2D array qubits

Sharing logical entangled pairs between distant quantum nodes is a key process to achieve fault tolerant quantum computation and communication. However, there is a gap between current experimental specifications and theoretical requirements for sharing logical entangled states while improving experimental techniques. Here, we propose an efficient logical entanglement distribution protocol based on surface codes for two distant 2D qubit array with nearest-neighbor interaction. A notable feature of our protocol is that it allows post-selection according to error estimations, which provides the tunability between the infidelity of logical entanglements and the success probability of the protocol. With this feature, the fidelity of encoded logical entangled states can be improved by sacrificing success rates. We numerically evaluated the performance of our protocol and the trade-off relationship, and found that our protocol enables us to prepare logical entangled states while improving fidelity in feasible experimental parameters. We also discuss a possible physical implementation using neutral atom arrays to show the feasibility of our protocol.

quant-ph↗

arXiv2025

Waveguide-array-based multiplexed photonic interface for atom array

The growing demand for high-capacity quantum communication and large-scale quantum computing underscores the importance of networking quantum processing units via multiplexed photonic channels. A neutral atom array with multiplexed atom-photon entanglement is a promising platform for its realization. Here, we demonstrate a key multiplexed photonic interface guiding the photons from an atom array to a single-mode waveguide array fabricated on a glass-based photonic integrated circuit. Remarkable 10 channels out of the 32-channel waveguide array with 25 $μ$m pitch couple to photons from 10 sites of the atom array with Rydberg gate-enabled separation. Based on the observed correlation between the atomic states and the polarization of the photon with a visibility of 0.87, we anticipate its applicability to a large-scale multiplexed atom-photon entanglement generation for networking quantum processing units.

quant-ph↗

arXiv2025

Channel-selective frequency up-conversion for frequency-multiplexed quantum network

We demonstrate channel-selective frequency up-conversion from telecom wavelengths around 1540 nm for optical fiber communication to visible wavelengths around 780 nm, based on second-order optical nonlinearity in a cavity of the converted modes. In our experiment, we selectively convert a light from any frequency mode within frequency-multiplexed telecom signals to a desired output mode, determined by the cavity resonances. Based on the experimental results of the frequency up-conversion, we derive the signal-to-noise ratio of the process at the single-photon level, and discuss its applicability to channel-selective quantum frequency conversion (CS-QFC) in the context of frequency-multiplexed quantum networks. Finally, we describe specific use cases of the CS-QFC, which show its utility as a reconfigurable switching element in frequency-multiplexed networks, particularly for selectively performing Bell-state measurements between two photons originating from different frequencies.

quant-ph↗

arXiv2024

1xN DWDM channel selective quantum frequency conversion

Dense Wavelength Division Multiplexing (DWDM) is a key technology for realizing high-capacity and flexible quantum communication networks. In addition, to realize the emerging quantum internet, quantum frequency conversion is also essential for bridging different quantum systems over optical fiber networks. In this work, we demonstrate a channel-selective quantum frequency conversion (CS-QFC), which allows active selection of the frequency of the converted photon from multiple DWDM channels. The 2.5 THz bandwidth of our CS-QFC system shows the ability to establish a 100-ch DWDM dynamic link from a single quantum system. It promises to increase the diversity of the quantum network.

quant-ph↗

arXiv2024

Low-noise quantum frequency conversion with cavity enhancement of converted mode

Quantum frequency conversion (QFC) which converts the frequencies of photons while preserving the quantum state is an essential technology for realizing the quantum internet and quantum interconnect. For the QFC based on the frequency downconversion from visible to the telecom wavelengths around 1500 nm, it is widely known that noise photons produced by the strong pump light used for QFC contaminate the frequency-converted photon, which degrades the quality of the quantum property of the photon after QFC. In conventional QFC experiments, noise photons are removed using external narrowband frequency filter systems. In contrast, in this study, we implement a compact QFC device integrating the cavity structure only for the converted mode. While the cavity structure can enhance not only the desired QFC efficiency but also the noise photon generation rate, we show that the cavity-enhanced QFC followed by a relatively wide bandpass filter achieves the signal-to-noise ratio comparable to the QFCs with external narrowband filters. We experimentally demonstrate the cavity-enhanced QFC using a single photon at 780 nm to 1540 nm, in which the non-classical photon statistics is clearly observed after QFC.

quant-ph↗

arXiv2023

Linear Optical Quantum Computation with Frequency-Comb Qubits and Passive Devices

We propose a linear optical quantum computation scheme using time-frequency degree of freedom. In this scheme, a qubit is encoded in single-photon frequency combs, and manipulation of the qubits is performed using time-resolving detectors, beam splitters, and optical interleavers. This scheme does not require active devices such as high-speed switches and electro-optic modulators and is robust against temporal and spectral errors, which are mainly caused by the detectors' finite resolution. We show that current technologies almost meet the requirements for fault-tolerant quantum computation.

quant-ph↗

arXiv2022

Massive-mode polarization entangled biphoton frequency comb

A frequency-multiplexed entangled photon pair and a high-dimensional hyperentangled photon pair are useful to realize a high-capacity quantum communication. A biphoton frequency comb (BFC) with entanglement can be used to prepare both states. We demonstrate polarization entangled BFCs with over 1400 frequency modes, which is approximately two orders of magnitude larger than those of earlier entangled BFCs, by placing a singly resonant periodically poled LiNbO3 waveguide resonator within a Sagnac loop. The BFCs are demonstrated by measuring the joint spectral intensity, cross-correlation, and autocorrelation. Moreover, the polarization entanglement at representative groups of frequency modes is verified by quantum state tomography, where each fidelity is over 0.7. The efficient generation of a massive-mode entangled BFC is expected to accelerate the increase of capacity in quantum communication.

quant-ph↗

arXiv2022

Quantum state tomography of qudits via Hong-Ou-Mandel interference

We propose a method to perform the quantum state tomography (QST) of an $\mathit{n}$-partite qudit state embedded in single photons using the Hong-Ou-Mandel (HOM) interference between the target state and probe state. This method requires only passive beam splitters for the HOM interference and removes all active optical devices in the target modes to control the measurement bases needed in conventional QST. Hence, it is applicable to various degree of freedom of the target state without altering the measurement setup. Moreover, a faithful estimation is realized even with classical probe light such as laser and thermal light. As a proof-of-principle, we performed the experimental demonstration using a polarization qubit. Regardless of the photon statistics of the probe light, the estimated results of state reconstruction are as accurate as those verified by conventional QST.

quant-ph↗

arXiv2022

Optical frequency tweezers

We show a concept of optical frequency tweezers for tweezing light in the optical frequency domain with a high resolution, which is the frequency version of the optical tweezers for spatial manipulation of microscopic objects. We report the proof-of-principle experiment via frequency conversion inside a cavity only for the converted light. Thanks to the atypical configuration, the experimental result successfully achieves the tweezing operation in the frequency domain, which picks a light at a target frequency from the frequency-multiplexed input light and converts to a different frequency, without touching any other light sitting in different frequency positions and shaking frequency by the pump light.

physics.optics↗

arXiv2020

A cavity-enhanced broadband photonic Rabi oscillation

A coherent coupling among different energy photons provided by nonlinear optical interaction is regarded as a photonic version of the Rabi oscillation. Cavity enhancement of the nonlinearity reduces energy requirement significantly and pushes the scalability of the frequency-encoded photonic circuit based on the photonic Rabi oscillation. However, confinement of the photons in the cavity severely limits the number of interactable frequency modes. Here we demonstrate a wide-bandwidth and efficient photonic Rabi oscillation achieving full-cycle oscillation based on a cavity-enhanced nonlinear optical interaction with a monolithic integration. We also show its versatile manipulation beyond the frequency degree of freedom such as an all-optical control for polarizing photons with geometric phase. Our results will open up full control accessible to synthetic dimensional photonic systems over wide frequency modes as well as a large-scale photonic quantum information processing.

physics.optics↗

arXiv2017

Polarization insensitive frequency conversion for an atom-photon entanglement distribution via a telecom network

Quantum network with a current telecom photonic infrastructure is deficient in quantum storages that keep arbitrary quantum state in sufficient time duration for a long-distance quantum communication with quantum repeater algorithms. Atomic quantum storages have achieved subsecond storage time corresponding to 1000 km transmission time for a telecom photon through a quantum repeater algorithm. However, the telecom photon is not directly accessible to typical atomic storages. Solid state quantum frequency conversions fill this wavelength gap and add more abilities, for example, a frequency multiplex. Here we report on the experimental demonstration of a polarization-insensitive solid-state quantum frequency conversion to a telecom photon from a short-wavelength photon entangled with an atomic ensemble. Atom-photon entanglement has been generated with a Rb atomic ensemble and the photon has been translated to telecom range while retaining the entanglement by our nonlinear-crystal-based frequency converter in a Sagnac interferometer.

quant-ph↗

arXiv2017

Mach-Zehnder interferometer using frequency-domain beamsplitter

We demonstrated the first-order interference between coherent light at 1580 nm and 795 nm by using frequency-domain Mach-Zehnder interferometer (MZI). The MZI is implemented by two frequency-domain BSs based on a second-order nonlinear optical effect in a periodically-poled lithium niobate waveguide with a strong pump light. We achieved the visibility of over 0.99 at 50% conversion efficiencies of the BSs. Toward photonic quantum information processing, sufficiently small background photon rate is necessary. From the measurement results with a superconducting single photon detector, we discuss the feasibility of the frequency-domain MZI in a quantum regime. Our estimation shows that single photon interference with the visibility above 0.9 is feasible with practical settings.

quant-ph↗

arXiv2016

Heralded single excitation of atomic ensemble via solid-state-based telecom photon detection

Telecom photonic quantum networks with matter quantum systems enable a rich variety of applications, such as a long distance quantum cryptography and one-way quantum computing. Preparation of a heralded single excitation (HSE) in an atomic ensemble by detecting a telecom wavelength photon having a correlation with the atomic excitation is an important step. Such a system has been demonstrated with a quantum frequency conversion (QFC) to telecom wavelength employing a Rb atomic cloud. However the limited wavelength selection prevents the next step. Here we for the first time demonstrate HSE with a solid-state-based QFC and a detector for a telecom wavelength that will have a great advantage of the utility of mature telecom technologies. We unambiguously show that the demonstrated HSE indicates a non-classical statistics by the direct measurement of the autocorrelation function.

quant-ph↗

arXiv2016

Frequency-domain Hong-Ou-Mandel interference

Hong-Ou-Mandel (HOM) interference unveils a distinct behavior of identical particles which cannot be distinguished from each other. Especially for bosons, two separated identical particles passing through a beamsplitter always go together into one of the output ports, but that is not the case with other particles including fermions or classical ones. So far many elemental properties of quantum physics and information have been discovered through the concatenated HOM effects, which has been demonstrated in photons and recently in plasmons, atoms and phonons. However, all demonstrations in optical region employed two particles in different spatial modes. Here we first report the HOM interference between two photons in a single spatial mode with different frequencies (energies) by using a partial frequency conversion. The demonstrated frequency-domain interferometer allows us to replace spatial optical paths by optical frequency multiplexing, which opens up a distinct architecture of the quantum interferometry.

quant-ph↗

arXiv2014

A low-noise frequency down-conversion to the telecommunication band for a quantum communication based on NV centers in diamond

We demonstrate a low-noise frequency down-conversion of photons at 637 nm to the telecommunication band at 1587 nm by the difference frequency generation in a periodically-poled lithium niobate. An internal conversion efficiency of the converter is estimated to be 0.44 at the maximum which is achieved by a pump power of 430 mW, whereas a rate of internal background photons caused by the strong cw pump laser is estimated to be 9 kHz/mW within a bandwidth of about 1 nm. By using the experimental values related to the intrinsic property of the converter, and using the intensity correlation and the average photon number of a 637-nm input light pulse, we derive the intensity correlation of a converted telecom light pulse. Then we discuss feasibility of a single-photon frequency conversion to the telecommunication band for a long-distance quantum communication based on NV centers in diamond.

quant-ph↗

arXiv2014

Universal gates for transforming multipartite entangled Dicke states

We determine the minimal number of qubits that it is necessary to have access to in order to transform Dicke states into other Dicke states. In general, the number of qubits in Dicke states cannot be increased via transformation gates by accessing only a single qubit, in direct contrast to other multipartite entangled states such as GHZ, W and cluster states. We construct a universal optimal gate which adds spin-up qubits or spin-down qubits to any Dicke state by minimal access. We also show the existence of a universal gate which transforms any size of Dicke state as long as it has access to at least the required number of qubits. Our results have important consequences for the generation of Dicke states in physical systems such as ion traps, all-optical setups and cavity-QED settings where they can be used for a variety of quantum information processing tasks.

quant-ph↗

arXiv2013

Observation of two output light pulses from a partial wavelength converter preserving phase of an input light at a single-photon level

We experimentally demonstrate that both a converted and an unconverted light pulses after wavelength conversion with various conversion efficiencies preserve phase information of an input light at a single-photon level. In our experiment, we converted temporally-separated two coherent light pulses with average photon numbers of $\sim$ 0.1 at 780 nm to light pulses at 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate waveguide. We observed a single-photon interference between temporally-separated two modes for both the converted and the unconverted light pulses at various values of the conversion efficiency. We observed interference visibilities greater than 0.88 without suppressing the background noises for any value of the conversion efficiency the wavelength converter achieves. At a conversion efficiency of $\sim$ 0.5, the observed visibilities are 0.98 for the unconverted light and 0.99 for the converted light.

quant-ph↗

arXiv2013

Non-classical two-photon interference between independent telecom light pulses converted by difference-frequency generation

We experimentally demonstrated the Hong-Ou-Mandel (HOM) interference between two photons after visible-to-telecommunication wavelength conversion. In the experiment, we prepared a heralded single photon by using spontaneous parametric down-conversion and the other photon from a weak laser source at 780 nm. We converted the wavelength of both photons to the telecommunication wavelength of 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate, and then observed the HOM interference between the photons. The observed visibility is $0.76\pm 0.12$ which clearly shows the non-classical interference of the two photons. The high-visibility interference is an important step for fiber-based quantum communications of photons generated from visible photon emitters.

quant-ph↗