KUGA LABORATORY AT THE UNIVERSITY OF TOKYO

The University of Tokyo

Graduate School of Arts and Sciences,

Department of Basic Science,


Kuga laboratory

Multiplexing of single photon sources

Single photon is useful for quantum cryptography and quantum computing. In our lab oratory, we developed single photon sources with semiconductor nanocrystals. We are now trying to scalable multiplexing of them by spatial light modulator.

Multiplexing of single photon sources

Single photon is photon flux which has a repulsive time distribution, so-called antibunching, which means two photons cannot be found simultaneously in certain time duration. When the power of a laser beam is attenuated, the probability of finding two photons is decreased. At the same time, however, the probability of finding a photon is also decreased. As a result, there is no gain in terms of the signal to noise ratio.

Single photons enable us more secure quantum cryptography and quantum computing. The quantum cryptography is a method to generate a common password between two persons, which any third parties cannot eavesdrop including communication service providers. The quantum computing is more powerful than the existing computers which utilize the entanglement accomplished by interfering two photons.

In our laboratory, we treat semiconductor nanocrystals embedded in a polymer thin film as single photon sources (Figure 2-1). They are wavelength tunable, low-power operation and room-temperature operation, which are the superior features to the other single photon sources.

Figure 2-1 : The system of the single photon source with semiconductor nanocrystal.

Figure 2-1 : The system of the single photon source with semiconductor nanocrystal. The sample is mounted on a right-angle prism, and illuminated by the orange light (wavelength is 593 nm).

A semiconductor nanocrystal emits single photons, which can be observed by a CCD camera easily (Figure 2-2). The intensity correlation of the fluorescence from a blinking point shows the antibunching behavior (Figure 2-3).

  • Figure 2-2: The picture of semiconductor nanocrystals taken by a CCD camera.
    Figure 2-2: The picture of semiconductor nanocrystals taken by a CCD camera. Each blight spot is expected to be a single nanocrystal since the blinking phenomena are observed.
  • Figure 2-3: The intensity correlation of the fluorescence from a semiconductor naocrystal.
    Figure 2-3: The intensity correlation of the fluorescence from a semiconductor naocrystal. The antibunching behavior is obtained which is the typical feature of single photons.

There are many single photon sources in a small area as shown in Figure 2-2. It is fascinating. Many single photon sources can be prepared by slightly manipulating the optical path. A spatial light modulator is used for the manipulation, which is a kind of a liquid crystal display. This scenario can avoid the scale increase of the system. Applications to wavelength-multiplexing in quantum cryptography and quantum computing will be expected.


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