Professor Imoto’s research group at Osaka University built a distinct nonlinear optical element working as a photon splitter in optical frequency domain and observed, for the first time, Hong-Ou-Mandel (HOM) interference by two photons of different colors in collaboration with Professor Koashi of The University of Tokyo and Senior Researcher Miki's group at NICT.
The original HOM interferometer was proposed and demonstrated by Hong, Ou, and Mandel in 1987. Here, a device known as a ‘beam splitter’ is commonly used. A beam splitter has two input ports (port 1,2) and two output ports (port 3, 4). When one photon from port 1 and another photon from port 2 are simultaneously injected into the beam splitter, surprisingly, the two photons never exit in both output ports 3 and 4, but rather always exit in the same output port, either 3 or 4. This phenomenon reflects the bosonic nature of photons, and is a crucial phenomenon for creating quantum correlations at a distance, which has contributed to numerous experiments such as quantum teleportation, quantum computation, quantum repeaters, boson sampling, loop-hole free Bell test, and so on.
The use of this type of conventional HOM interference, however, has a problem with upscaling since a spatial beam splitter allows only the two-input-two-output configuration. The newly developed frequency splitter, on the other hand, allows us to manipulate a large number of photon pairs by a single spatial mode via frequency-division multiplexing. This will certainly open a new path towards the upscaling of quantum information processing, such as quantum computers.
A frequency splitter is realized by a nonlinear optical effect, known as sum frequency generation and difference frequency generation. This splitter has a single input port and a single output port. Each port allows frequency division multiplexed photons at the same time. Using this frequency splitter, a brand-new HOM interferometer becomes possible. When two photons with different colors (frequencies) are input along a single port of the frequency splitter simultaneously, two photons with the same color always appear in the output. In the experiment, PPLN waveguide has been used as a nonlinear optical medium and its highly efficient conversion with low noise contributed to the high visibility HOM interference. The groups clearly show the quantum interference between the two photons in different colors with the high-quality superconducting single-photon detectors built in NICT. Combined with wavelength-division multiplexing, this result will pave the way towards frequency multiplexed quantum computation, and a dramatic improvement of information processing capacity is expected.
Left: Conventional beam splitter. Right: Frequency splitter developed in this research.
This work was published in Nature Photonics on 19th April 2016.
Title: Frequency-domain Hong-Ou-Mandel interference
Journal: Nature Photonics
DOI number: 10.1038/nphoton.2016.74
Authors: Toshiki Kobayashi, Rikizo Ikuta, Shuto Yasui, Shigehito Miki, Taro Yamashita, Hirotaka Terai, Takashi Yamamoto, Masato Koashi, Nobuyuki Imoto