Quantum communication apparatus and quantum communication method

Abstract

A quantum communication apparatus is used in operations to generate entanglement between the two physical system ensembles and operations to extend the distance between entangled ensembles by connecting pairs of entangled ensembles. The apparatus uses only passive elements without any actuating parts, active devices having no mechanically actuating parts, such as an electro-optical device capable of ultrahigh-speed operation, and a laser source capable of generating high-speed pulse trains or a continuous wave laser source. The apparatus can rapidly execute light irradiation necessary for entanglement generation and connection, and detection of a generated photon at the needed sites within a decoherence time of the physical system.

1 . A quantum communication apparatus, comprising:
 n numbers of ensembles denoted as A(i) (i=1, 2, 3, . . . , n, n being 2 r , where r is an integer of 2 or more), each ensemble comprising a plurality of physical systems, one of the plurality of physical systems having at least three energy levels of a first level, a second level and a third level viewed from the lower level, in which an angular frequency corresponding to an energy difference between the first and second levels is denoted as ω 12 , a physical system in a state of the first level capable of generating a first photon of an angular frequency ω 1 −ω 12  when irradiated with coherent pulsed light of an angular frequency ω 1  and a physical system in a state of the second level capable of generating a second photon of an angular frequency ω 2 +ω 12  when irradiated with coherent pulsed light of an angular frequency ω 2 ;   an optical system for the ensemble A( 1 ), comprising: an optical shutter S 1 ( 1 ) disposed in an optical path for pulsed light to A( 1 ); and an optical filter F 1 ( 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 S 1 ( n ) disposed in an optical path for pulsed light to A(n); and an optical filter F 1 ( 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) (j=2, 3, 4, . . . , n−1) comprising: an optical shutter S 1 ( 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 P 1 ( 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 F 1 ( j ), disposed in an optical path for the photon transmitted through the polarizer P 1 ( j ), which transmits the first photon and reflects the second photon; a polarizer P 1 ( j ), disposed in an optical path for the photon transmitted through the optical filter F 1 ( j ), which adjusts polarization of the first photon; and an optical filter F 2 ( j ), disposed in an optical path, branched from the polarization beam splitter T(j), for the photon reflected by the optical filter F 1 ( j ), re-transmitted through the polarizer P 1 ( 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 B 1 ( k ) (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 D 1 (2×k−1) and D 1 (2×k) detecting a photon output from the beam splitter B 1 ( k ) in two directions, respectively;   beam splitters B 2 ( 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 P 1 (2×l), reflected by the optical filter F 1 (2×l), re-transmitted through the polarizer P 1 (2×l), reflected by the polarization beam splitter T(2×l) to a branched optical path and transmitted through the optical filter F 1 (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 P 1 (2×l+1), reflected by the optical filter F 1 (2×l+1), re-transmitted through the polarizer P 1 (2×l+1), reflected by the polarization beam splitter T(2×l+1) to a branched optical path and transmitted through the optical filter F 1 (2×l+1);   pairs of photon detectors D 1 (2×l) and D 1 (2×l+1) detecting a photon output from the beam splitter B 2 (I) 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.   
   
   
       2 - 13 . (canceled)