Quantum communication apparatus and quantum communication method

A quantum communication apparatus, comprising:

• n numbers of ensembles denoted as A(i),where i=1, 2, 3, . . . , n, n being 2r, and r is an integer of 2 or more, each ensemble comprising a plurality of physical systems comprising:
• at least one physical system having at least three energy levels of a first level, a second level and a third level in ascending order, in which an angular frequency corresponding to an energy difference between the first and second levels is denoted as ω12;
• a physical system in a state of the first level capable of generating a first photon of an angular frequency ω1−ω12 when irradiated with coherent pulsed light of an angular frequency ω1 having a first detuning with respect to a transition between the first level and the third level;
• and a physical system in a state of the second level capable of generating a second photon of an angular frequency ω2+ω12 when irradiated with coherent pulsed light of an angular frequency ω2 having a second detuning with respect to a transition between the second level and the third level, where the first detuning is larger than the second detuning;
• an optical system for the ensemble A(1), comprising:
• an optical shutter S1(1) disposed in an optical path for pulsed light to A(1);
• and an optical filter F1(1), disposed in an optical path for a photon generated from A(1), which selectively transmits the first photon;
• an optical system for the ensemble A(n), comprising:
• an optical shutter S1(n) disposed in an optical path for pulsed light to A(n);
• and
• an optical filter F1(n), disposed in an optical path for a photon generated from A(n), which selectively transmits the first photon;
• an optical system for the ensemble A(j), where j=2, 3, 4, . . . , n−1comprising:
• an optical shutter S1(j) disposed in an optical path for pulsed light to A(j);
• a polarization beam splitter T(j), disposed in an optical path for a photon generated from A(j), which is set to a direction to transmit the first photon and the second photon;
• a polarizer P1(j), disposed in an optical path for the photon transmitted through the polarization beam splitter T(j), which transmits the first and second photons in which a polarization direction of the second photon is rotated;
• an optical filter F1(j), disposed in an optical path for the photon transmitted through the polarizer P1(j), which transmits the first photon and reflects the second photon;
• a polarizer P2(j), disposed in an optical path for the photon transmitted through the optical filter F1(j), which adjusts polarization of the first photon;
• and an optical filter F2(j), disposed in an optical path, branched from the polarization beam splitter T(j), for the photon reflected by the optical filter F1(j), re-transmitted through the polarizer P1(j), and reflected by the polarization beam splitter T(j) to the branched optical path, which selectively transmits the second photon;
• a control circuit configured to selectively open optical shutters corresponding to a particular set of ensembles to be irradiated with pulsed light;
• beam splitters B1(k), where k=1, 2, 3, . . . , n/2 configured to superpose the photon generated from the ensemble A(2×k−1) and transmitted through the optical filter F(2×k−1), and the photon generated from the ensemble A(2×k) and transmitted through the optical filter F(2×k);
• pairs of photon detectors D1(2×k−1) and D1(2×k) detecting a photon output from the beam splitter B1(k) in two directions, respectively;
• beam splitters B2(l) (l=1, 2, 3, . . . , (n/2)−1) configured to superpose the second photon generated from the ensemble A(2×l), transmitted through the polarization beam splitter T(2×l), transmitted through the polarizer P1(2×l), reflected by the optical filter F1(2×l), re-transmitted through the polarizer P1(2×l), reflected by the polarization beam splitter T(2×l) to a branched optical path and transmitted through the optical filter F1(2×l), the second photon generated from the ensemble A(2×l+1), transmitted through the polarization beam splitter T(2×l⇄1), transmitted through the polarizer P1(2×l+1), reflected by the optical filter F1(2×l+1), re-transmitted through the polarizer P1(2×l+1), reflected by the polarization beam splitter T(2×l+1) to a branched optical path and transmitted through the optical filter F1(2×l+1);
• pairs of photon detectors D1(2×l) and D1(2×l+1) detecting a photon output from the beam splitter B2(l) in two directions, respectively;
• and
• signal processing circuits connected to the respective pairs of photon detectors and configured to generate signals to close the two optical shutters corresponding to the two ensembles participated in photon detection with either one of a particular pair of photon detectors.