Patent Number: 
Section: claims

1. A method of generating a single photon, comprising:preparing an optical resonator including a resonator mode of a resonance angular frequency ωc;preparing a material contained in the optical resonator, including a low energy state |g> and a high energy state |e>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;applying, to the material, light of an angular frequency ωl different from the resonance angular frequency ωc; andapplying a first external field to the material to vary the transition angular frequency ωa to resonate with the angular frequency ωl, such that a state of the material is changed to the high energy state |e>, and then applying a second external field to the material to vary the transition angular frequency ωa to resonate with the resonance angular frequency ωc, such that the state of the material is restored to the low energy state |g>. 2. The method according to claim 1, wherein the first external field is applied to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then the second external field is applied to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system. 3. The method according to claim 1, wherein the first external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then the second external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 4. The method according to claim 1, wherein the first external field is applied to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then the second external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 5. The method according to claim 1, wherein the first external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then the second external field is applied to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 6. The method according to claim 1, wherein the optical resonator is a one-sided Fabry-Perot resonator. 7. The method according to claim 1, wherein the material is a rare-earth ion contained in crystal, a transition between |g>−|e> corresponds to an f-f transition of the rare-earth ion, and the external field is an electric field or a magnetic field. 8. A method of reading a quantum bit, comprising:preparing an optical resonator including a resonator mode of a resonance angular frequency ωc;preparing a material contained in the optical resonator, including a low energy state |g>, a high energy state |e>, and two states |0> and |1>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;generating a first pulse beam and a second pulse beam that resonate a transition between |g>−|e> and a transition between |l>−|e>, respectively;controlling the first pulse beam and the second pulse beam to temporally overlap each other to shift a first state in which a first intensity of the first pulse beam is higher than a second intensity of the second pulse beam, to a second state in which the second intensity is higher than the first intensity, to generate a third pulse beam;applying the third pulse beam to the material; andapplying a first external field to the material after applying the third pulse beam thereto, to vary the transition angular frequency ωa to resonate with the angular frequency ωl, then applying a second external field to the material to vary the transition angular frequency ωa to resonate with the resonance angular frequency ωc, and reading a quantum bit depending upon whether a photon ejected from the optical resonator is detected. 9. A single-photon generation apparatus comprising:an optical resonator including a resonator mode of a resonance angular frequency ωc;a material contained in the optical resonator, including a low energy state |g> and a high energy state |e>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;a light source configured to apply, to the material, light of an angular frequency ωl different from the resonance angular frequency ωc;an external-field generation unit configured to apply external fields to the material to vary the transition angular frequency ωa to resonate with one of the angular frequency ωl and the resonance angular frequency ωc; anda controller configured to control the light source to apply the light of the angular frequency ωl to the material, and to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc to change a state of the material to the high energy state |e>, and then to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc to restore the state of the material to the low energy state |g>. 10. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system. 11. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 12. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the material to the high energy state |e>, and then controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 13. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl−Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 14. The apparatus according to claim 9, wherein the optical resonator is a one-sided Fabry-Perot resonator. 15. The apparatus according to claim 9, wherein the material is a rare-earth ion contained in crystal, a transition between |g>−|e> corresponds to an f-f transition of the rare-earth ion, and the external-field generation unit applies an external field, such as an electric field or a magnetic field, to the material. 16. A quantum-bit-reading apparatus, comprising:the single-photon generation apparatus as claimed in claim 9, which employs a material including two states |0> and |1>, as well as the low energy state |g> and the high energy state |e>;a generation unit configured to generate a first pulse beam and a second pulse beam that resonate a transition between |g>−|e> and a transition between |1>−|e>, respectively;a controller configured to control the first pulse beam and second pulse beam to temporally overlap each other to shift a first state in which a first intensity of the first pulse beam is higher than a second intensity of the second pulse beam, to a second state in which the second intensity is higher than the first intensity, to generate a third pulse beam;an applying unit configured to apply the third pulse beam to the material; anda controller configured to control the external-field generation unit, after the applying unit applies the third pulse beam, to make the transition angular frequency ωa resonate with the angular frequency ωl, then to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc, and to read a quantum bit depending upon whether a photon ejected from the optical resonator is detected.