Narrow-band single-photon source and QKD system using same

1. A system for generating narrow-band single photons, comprising:

a pump light source that generates pump light;

a non-linear medium arranged to receive the pump light and generate time-energy entangled signal and idler photons having respective first and second wavelengths and corresponding first and second spectral bandwidths;

a wavelength-selective member arranged to receive the signal and idler photons and direct the photons along respective first and second optical paths;

a narrow-band optical filter having a select spectral bandwidth and arranged along the second optical path so as to filter the idler photons so that only narrow-band idler photons pass therethrough;

a single-photon detector arranged downstream of the optical filter and adapted to detect each narrow-band idler photon that passes through the optical filter; and

wherein said filtering and detecting of the narrow-band idler photons causes a corresponding amount of non-local filtering of corresponding signal photons via said time-energy entanglement, so as to create narrow-band signal photons.

2. The system of claim 1 , wherein the narrow-band optical filter is either formed integral with or is incorporated into the wavelength-selective member.

3. The system of claim 1 , wherein the narrow-band optical filter is centered on a wavelength of about 810 nm and has a bandwidth of about 0.1 nm.

4. The system of claim 3 , wherein the signal photons have a central wavelength of about 1550 nm.

5. A method of forming narrow-band single photons, comprising:

generating pairs of first and second entangled photons via spontaneous parametric downconversion;

separating the first and second photons to travel along different optical paths;

narrow-band-filtering the second photons by passing the second photons through a narrow-band filter having a center wavelength and a spectral bandwidth between about 0.2 nm and about 0.8 nm;

detecting the narrow-band-filtered second photons so as to establish the first photons as a narrow-band single photons; and

generating trigger signals from the detected narrow-band-filtered second photons for processing the narrow-band signal photons.

6. The method of claim 5 , wherein the narrow-band-filter center wavelength is about 810 nm and the narrow-band-filter bandwidth is about 0.1 nm.

7. The method of claim 5 , wherein the narrow-band filtering of the second photons is carried out so as to form first single photons having a wavelength of about 1550 nm and a first-single-photon spectral bandwidth of about 0.4 nm.

8. A QKD system comprising:

a) first and second QKD stations optically coupled by an optical fiber link, the first QKD station having:

i) single-photon source means for generating narrow-band first photons and a corresponding trigger signal based on detecting idler photons;

ii) a first modulator optically coupled to the single-photon source means and the optical fiber link and adapted to randomly modulate the narrow-band first single photons based on select basis modulations;

b) wherein the second QKD station includes:

i) a second modulator timed by the trigger signal and optically coupled to the optical fiber link, the second modulator adapted to receive and modulate the once-modulated first single-photons to form twice-modulated first single photons; and

ii) a single-photon detector unit adapted to detect the twice modulated first single photons and generate clicks representative of modulation states of the twice-modulated first single photons.

9. A method of modulating single-photons in a quantum key distribution (QKD) system, comprising:

a) generating narrow-band single-photon quantum signals by:

i) generating pairs of first and second entangled photons via spontaneous parametric downconversion;

ii) narrow-band-filtering the second photons; and

ii) detecting the narrow-band-filtered second photons so as to establish the first photons as a narrow-band single-photon quantum signals;

b) generating a trigger signal for each detected second photon;

c) selectively randomly modulating the quantum signals at a first QKD station; and

d) transmitting the once-modulated quantum signals to a second QKD station and selectively randomly modulating the once-modulated quantum signals therein using the trigger signal to time the modulation at the second QKD station.