Four-Photon Quantum Interferometry at a Telecom Wavelength

We report the experimental demonstration of four-photon quantum interference using telecom-wavelength photons. Realization of multi-photon quantum interference is essential to linear optics quantum information processing and measurement-based quantum computing. We have developed a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom wavelength region, and have demonstrated the quantum interference exhibiting the reduced fringe intervals that correspond to the reduced de Broglie wavelength of up to the four photon `NOON' state. Our result should open a path to practical quantum information processing using telecom-wavelength photons.Comment: 4 pages, 4 figure

Enhanced optics for time-resolved singlet oxygen luminescence detection

Singlet oxygen luminescence dosimetry (SOLD) is a highly promising direct monitoring method for photodynamic therapy (PDT) in the treatment of cancer. Early SOLD systems have been hampered by inefficient excitation, poor optical collection and immature infrared single photon detection technology. We report carefully engineered improvements addressing all of these deficiencies. We use a supercontinuum source with a tunable filter to precisely target the peak absorption wavelength of the chosen photosensitizer; we have designed a compact and versatile optical package for precise alignment; we have successfully employed state-of-the-art superconducting photon counting technologies. Through these improvements, we can achieve real-time histogram acquisition from a photosensitizer in solution test sample. This setup opens the pathway to physiological SOLD studies for PDT dosimetry

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.Comment: 11 pages, 5 figure

Timing jitter characterization of the SFQ coincidence circuit by optically time-controlled signals from SSPDs

We report on the timing jitter characterization of the superconducting single flux quantum (SFQ) coincidence circuit, which is an essential component of the superconducting coincidence photon counter. Two superconducting nanowire single photon detectors (SSPDs), each of which is irradiated with optically time-controlled photons, are connected to the SFQ coincidence circuit, and the timing jitter of the SFQ circuit is evaluated by changing the relative time delay between two input ports for the SFQ comparator unit. We successfully observe the transition curve of the probability of obtaining the signal from the SFQ coincidence circuit by sweeping photon arrival time to each SSPD and confirm that this curve shifts temporally upon changing the bias current to the Josephson transmission line (JTL) in the SFQ circuit. A systematic investigation reveals that the relation between time delay and the bias current to JTL can be estimated. The full width half maximum timing jitter of the SFQ circuit is 1.1 ps, which is sufficiently low so that it does not influence the entire timing jitter of the coincidence photon counter

Single-shot single-mode optical two-parameter displacement estimation beyond classical limit

Uncertainty principle prohibits the precise measurement of both components of displacement parameters in phase space. We have theoretically shown that this limit can be beaten using single-photon states, in a single-shot and single-mode setting [F. Hanamura et al., Phys. Rev. A 104, 062601 (2021)]. In this paper, we validate this by experimentally beating the classical limit. In optics, this is the first experiment to estimate both parameters of displacement using non-Gaussian states. This result is related to many important applications, such as quantum error correction.Comment: 5 pages, 4 figure