Although near-infrared photons in telecommunication bands are required for long-distance quantum communication, various quantum information tasks have been performed by using visible photons for the past two decades. Recently, such visible photons from diverse media including atomic quantum memories have also been studied. Optical frequency downconversion from visible to telecommunication bands while keeping the quantum states is thus required for bridging such wavelength gaps. Here we report demonstration of a quantum interface of frequency down-conversion from visible to telecommunication bands by using a nonlinear crystal, which has a potential to work over wide bandwidths, leading to a highspeed interface of frequency conversion. We achieved the conversion of a picosecond visible photon at 780 nm to a 1,522-nm photon, and observed that the conversion process retained entanglement between the down-converted photon and another photon.

Hong-Ou-Mandel (HOM) interference[1] 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 [2] have been discovered through the concatenated HOM effects, which has been demonstrated in photons [1,3,4,5,6,7,8] and recently in plasmons [9,10], atoms [11] and phonons [12]. 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.In the past three decades since the HOM interference has been proposed and demonstrated with two photons from spontaneous parametric down-conversion (SPDC) process [1], a huge varieties of experiments based on the HOM interference revealed fundamental properties in quantum physics, especially in quantum optics [2], and its applications are widely spreading over quantum information processing, such as quantum computation [13,14,15,16], quantum key distribution [17,18], quantum repeater [19,20,21] and quantumoptical coherence tomography [22]. HOM interference has been observed with photons generated not only from nonlinear optical phenomenon but also from quantum dots [3,4], trapped neutral atoms[5], trapped ions[6], NV centers[7] and SiV centers[8] in diamond. Furthermore not only photons but also other bosonic particles, e.g., surface plasmons [9,10], Helium 4 atoms[11] and phonons [12] show the HOM interference. In spite of such demonstrations using various kinds of physical systems, to the best of our knowledge, all of them essentially used the spatial or polarization degree of freedom for the HOM interference, including the use of polarization modes of photons that are easily converted to and from spatial modes. The demonstrations use the beamsplitter (BS) which mixes the two particles in different spatial/polarization modes.In this letter, we report the first observation of the HOM interference between two photons with different frequencies in optical region. In contrast to the spatial interferometer, the frequency-domain HOM interferometer is implemented in a single spatial mode with a nonlinear optical frequency conversion [23,24,25]. In the experiment, we input a 780 nm photon and a 1522 nm photon to the frequency converter that partially converts the wavelengths of the photons between 780 nm and 1522 nm as shown in Fig. 1a. We measured coincidence counts between the output photons at 780 nm and those at 1522 nm from the frequency converter. The observed HOM inter...

An all-optical network is identified as a promising infrastructure for fast and energy-efficient communication. Recently, it has been shown that its quantum version based on ‘all-photonic quantum repeaters’—inheriting, at least, the same advantages—expands its possibility to the quantum realm, that is, a global quantum internet with applications far beyond the conventional Internet. Here we report a proof-of-principle experiment for a key component for the all-photonic repeaters—called all-photonic time-reversed adaptive (TRA) Bell measurement, with a proposal for the implementation. In particular, our TRA measurement—based only on optical devices without any quantum memories and any quantum error correction—passively but selectively performs the Bell measurement only on single photons that have successfully survived their lossy travel over optical channels. In fact, our experiment shows that only the survived single-photon state is faithfully teleported without the disturbance from the other lost photons, as the theory predicts.

Quantum weak measurements, wavepacket shifts and optical vortices are universal wave phenomena, which originate from fine interference of multiple plane waves. These effects have attracted considerable attention in both classical and quantum wave systems. Here we report on a phenomenon that brings together all the above topics in a simple one-dimensional scalar wave system. We consider inelastic scattering of Gaussian wave packets with parameters close to a zero of the complex scattering coefficient. We demonstrate that the scattered wave packets experience anomalously large time and frequency shifts in such near-zero scattering. These shifts reveal close analogies with the Goos–Hänchen beam shifts and quantum weak measurements of the momentum in a vortex wavefunction. We verify our general theory by an optical experiment using the near-zero transmission (near-critical coupling) of Gaussian pulses propagating through a nano-fibre with a side-coupled toroidal micro-resonator. Measurements demonstrate the amplification of the time delays from the typical inverse-resonator-linewidth scale to the pulse-duration scale.

Trapped atomic ions are ideal single photon emitters with long-lived internal states which can be entangled with emitted photons. Coupling the ion to an optical cavity enables the efficient emission of single photons into a single spatial mode and grants control over their temporal shape. These features are key for quantum information processing and quantum communication. However, the photons emitted by these systems are unsuitable for long-distance transmission due to their wavelengths. Here we report the transmission of single photons from a single ^{40}Ca^{+} ion coupled to an optical cavity over a 10 km optical fiber via frequency conversion from 866 nm to the telecom C band at 1530 nm. We observe nonclassical photon statistics of the direct cavity emission, the converted photons, and the 10 km transmitted photons, as well as the preservation of the photons' temporal shape throughout. This telecommunication-ready system can be a key component for long-distance quantum communication as well as future cloud quantum computation.