Wavelength (frequency, color) is an important physical parameter of light. Wavelength conversion of single photons is crucial for quantum networking, which requires an interface for photon wavelength. We have developed a lossless scheme for the wavelength conversion of photons using a nonlinear-optical effect called cross-phase modulation (XPM) .
The experimental scheme is illustrated in Fig. 1(a). XPM enables us to control the phase of light (signal pulses) via a change in the refractive index of a medium induced by another light (control pulses). An instantaneous frequency shift is added to the signal pulses when the phase shift is dynamic. Since XPM always occurs regardless of the intensity of the control pulses, by using single photons as the signal pulses we can convert the wavelength of the photons without a photon loss.
Wavelength conversion using XPM has been widely demonstrated for classical optical pulses; however, it has not yet been demonstrated for single photons. This is because it was difficult to add a wavelength shift to single photons while mitigating noise photons induced by the control pulses via other optical processes. We solved this problem by using a photonic crystal fiber (PCF) whose dispersion property was properly designed for the experiment. As a result, we were able to successfully convert the wavelength of telecommunication-band single photons with a wavelength shift as large as 3 nm (0.4 THz in frequency). The amount of the wavelength shift can be easily tuned by adjusting the intensity of the control pulses. Photon loss due to the conversion was not observed.
Using the scheme, we further controlled the quantum correlation of pairs of photons. The demonstrations include the modulations of non-classical frequency correlation [Fig. 1(b)], inter-packet interference, and entanglement between distant photons. These results demonstrate our scheme’s applicability to a wide range of quantum information science and technologies, including computation  and metrology.
This work was supported by KAKENHI.
|Fig. 1. (a) The illustration of the scheme. (b) Reshaping the non-classical frequency correlation between photons. In the experiment, first, photon pairs were created via an optical process called parametric down conversion (left). Then the wavelength of "photon A" was blue shifted with our scheme. As a result, the overall spectral correlation was also blue shifted (right).|