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The Japan Society of Applied Physics

Demonstration of Frequency-Domain Linear Optical Quantum Circuits Using Spectrally Shaped Ultrafast Single Photons (II)

Kazuki Takahashi, Aruto Hosaka, Yoshiaki Tsujimoto, Yuta Yamagishi, Akihito Omi, Kentaro Wakui, Fujiwara Mikio, Yutaka Shikano, Masahiro Takeoka, Fumihiko Kannari

Published Jan 25, 2019• 0 citations
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Summary form only given. Optical linear circuit is one of the most important parts for optical quantum computing. The optical circuit, which consists of mode mixers and phase shifters, has been used in various quantum protocols: cluster state generation, Boson sampling, and quantum random walk. To realize a large-scale linear circuit, waveguide-based quantum circuits have experimentally been demonstrated by some groups [1, 2]. However, such a waveguide-based circuit cannot overcome issues of flexibility and loss.On the other hand, recently, a quantum pulse gate (QPG), which enables mode extraction from multimode quantum fields in the frequency domain, has been proposed and experimentally demonstrated [3]. For utilizing the frequency resource of photon field, parallel generations of single photons or multimode squeezed states in the frequency domain, namely time-frequency Schmidt modes, have been demonstrated. In such a wavelengthdivision-multiplexed quantum circuit, this mode extraction corresponds to one row of a linear unitary rotation of the Schmidt modes. It is achieved by sum frequency generation between a spectrally-shaped classical pulse and a quantum weak pulse. The arbitrary mode extraction can be designed by the pulse shaping with a high efficiency. In this report, we propose a multistage QPG which is a simple, flexible and highly efficient quantum linear optical circuit. We devised a novel multipass 4-f system to map the frequency multimode quantum states into the spatiotemporal domain with a single nonlinear crystal (Fig. 1(a)). Furthermore, we show its application to a quantum simulator for calculating molecular vibronic absorption spectra [4] (Fig. 1(b)).

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Quantum correlations of ultrafast quantum optical states in the time–frequency domain manifest prominently on femtosecond time scales, an elusive challenging regime. We develop a new method to probe the time–frequency mode structure of ultrafast two-photon states utilizing dual-comb-based asynchronously overlapping pulses with single-photon frequency upconversion through femtosecond optical sampling. This method enables quick, highly precise joint temporal intensity measurements over a broad temporal dynamic range with only a single linear scanning stage. We demonstrate full characterization of time–frequency entangled two-photon states in both time and frequency domains. We quantified the time–frequency mode structure in both time and frequency domains through Schmidt decomposition, resulting in temporal and spectral purities of 87.7% and 76.9%, respectively, for heralded single-photons without any spectral filtering. Furthermore, through the simple control of pump spectral bandwidth, we demonstrate the ability to tailor time–frequency entanglement, verifying the two-dimensional Fourier duality. This method paves the way for a substantial upgrade to ultrafast quantum optical techniques, enabling a rapid, detailed, and multi-dimensional characterization of entangled states with potential applications in quantum metrology and photon-deficient fluorescence imaging.

APL Photonics•May 1, 2025•2 citations
Summary form only given. Optical linear circuit is one of the most important parts for optical quantum computing. The optical circuit, which consists of mode mixers and phase shifters, has been used in various quantum protocols: cluster state generation, Boson sampling, and quantum random walk. To realize a large-scale linear circuit, waveguide-based quantum circuits have experimentally been demonstrated by some groups [1, 2]. However, such a waveguide-based circuit cannot overcome issues of flexibility and loss.On the other hand, recently, a quantum pulse gate (QPG), which enables mode extraction from multimode quantum fields in the frequency domain, has been proposed and experimentally demonstrated [3]. For utilizing the frequency resource of photon field, parallel generations of single photons or multimode squeezed states in the frequency domain, namely time-frequency Schmidt modes, have been demonstrated. In such a wavelengthdivision-multiplexed quantum circuit, this mode extraction corresponds to one row of a linear unitary rotation of the Schmidt modes. It is achieved by sum frequency generation between a spectrally-shaped classical pulse and a quantum weak pulse. The arbitrary mode extraction can be designed by the pulse shaping with a high efficiency. In this report, we propose a multistage QPG which is a simple, flexible and highly efficient quantum linear optical circuit. We devised a novel multipass 4-f system to map the frequency multimode quantum states into the spatiotemporal domain with a single nonlinear crystal (Fig. 1(a)). Furthermore, we show its application to a quantum simulator for calculating molecular vibronic absorption spectra [4] (Fig. 1(b)).
During this thesis, we have built and characterized a single-photon subtractor to engineer the quantum states of light. The aim is to perform a pure and spectrally selective photon subtraction on a multimode resource in the spectral domain. This subtractor consists in a parametric sum-frequency interaction between a signal beam and a control beam within a non-linear medium. The optical spectrum of the control beam is shaped using ultrafast pulse shaping. The up-converted photon is filtered and detected by a single-photon detector. The subtractor is therefore the combination of all these successive elements. We have developed a theoretical framework to describe the multimode subtraction of a single photon and showed that it can be summed up by a subtraction matrix in a modal basis independently of the physical implementation. Thanks to this formalism, we have shown that the process can be characterized without measuring the transformed signal beam but simply by using a probe beam whose pulses are also shaped. We carried out a process tomography at the single-photon level for a wide range of different control beams. Our results are correctly described by the theory we developed and demonstrates the purity of the process as well as the agility of the technique we used.
Quantum correlations of ultrafast quantum optical states in the time–frequency domain manifest prominently on femtosecond time scales, an elusive challenging regime. We develop a new method to probe the time–frequency mode structure of ultrafast two-photon states utilizing dual-comb-based asynchronously overlapping pulses with single-photon frequency upconversion through femtosecond optical sampling. This method enables quick, highly precise joint temporal intensity measurements over a broad temporal dynamic range with only a single linear scanning stage. We demonstrate full characterization of time–frequency entangled two-photon states in both time and frequency domains. We quantified the time–frequency mode structure in both time and frequency domains through Schmidt decomposition, resulting in temporal and spectral purities of 87.7% and 76.9%, respectively, for heralded single-photons without any spectral filtering. Furthermore, through the simple control of pump spectral bandwidth, we demonstrate the ability to tailor time–frequency entanglement, verifying the two-dimensional Fourier duality. This method paves the way for a substantial upgrade to ultrafast quantum optical techniques, enabling a rapid, detailed, and multi-dimensional characterization of entangled states with potential applications in quantum metrology and photon-deficient fluorescence imaging.