An optical system comprising a charged quantum dot having, a charged carrier, first and second ground state levels and a plurality of excited state levels, the first and second ground state energy levels having different spin states such that the said charged carrier cannot transfer between the first and second ground state energy levels without changing its spin state, the system further comprising a controller adapted to control a first radiating beam with energy not more than 100 micro-eV from a first transition within said quantum dot from a first ground state level to a selected excited state level from the plurality of excited state levels to, the system being adapted to enhance the decay rate of a second transition within said quantum dot from the selected excited state level to a second ground state level, but not a first transition, such that a photon is produced due to scattering of a photon from the first radiating beam, wherein the controller is adapted to irradiate the quantum dot with the first radiating beam for a time longer than the radiative lifetime of the selected excited state to produce just one photon, and wherein the first radiating beam comprises at least one pulse.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. An optical system comprising a charged quantum dot having, a charged carrier, first and second ground state levels and a plurality of excited state levels, the first and second ground state energy levels having different spin states such that the said charged carrier cannot transfer between the first and second ground state energy levels without changing its spin state, the system further comprising a controller adapted to control a first radiating beam with energy not more than 100 micro-eV from a first transition energy within said quantum dot from a first ground state level to a selected excited state level from the plurality of excited state levels, the system being adapted to enhance the decay rate of a second transition within said quantum dot from the selected excited state level to a second ground state level, but not a first transition, such that a photon is produced due to scattering of a photon from the first radiating beam, wherein the controller is adapted to irradiate the quantum dot with the first radiating beam for a time longer than the radiative lifetime of the selected excited state to produce just one photon, and wherein the first radiating beam comprises at least one pulse. 2. An optical system according to claim 1, wherein the first radiating beam is a pulsed beam with each pulse having a single intensity maximum, a pulse length greater than the selected excited state lifetime and the controller is adapted to control the pulse area to transfer all spin in the first ground state to the second ground state. 3. An optical system according to claim 1, wherein the controller is further adapted to control a second radiating beam that radiates the quantum dot at an energy within 100 micro-eV of the energy of said second transition, the said second radiating beam being a control beam, the control beam being applied to the quantum dot between two or more pulses of the first radiating beam, the controller being configured to control the first radiating beam such that the charged carrier has an energy state which is a superposition of the first and second ground state energy levels prior to the application of the control beam. 4. An optical system according to claim 1, configured as a source of entangled photons, wherein the controller is configured to control the first radiating beam such that the charged carrier has a superposed energy state which is a superposition of the first and second ground state energy levels, the controller being further adapted to control a second radiating beam to scatter at least one further photon to produce an entangled photon pair using said superposed energy state. 5. An optical system according to claim 4, configured as a source of n entangled photons, where n is an integer of at least 2, the controller being configured to control the second radiating beam to scatter at a plurality of photons to produce entangled photons using said superposed energy state. 6. An optical system according to claim 1, wherein the controller is configured to prepare the charged carrier in the first ground state level prior to irradiating with the first beam. 7. An optical system according to claim 1, wherein the first and second ground state levels are produced by an in-plane magnetic field. 8. An optical system according to claim 1, wherein the system is adapted to enhance the decay rate of a second transition by a cavity tuned to the frequency of the desired transition. 9. An optical system according to claim 1, wherein the system is adapted to enhance the decay rate of a second transition by a waveguide. 10. An optical system according to claim 1, wherein the system is adapted to selectively enhance the decay rate of second transition, but not a first transition, by only supporting a light polarisation that couples to the second transition and not first. 11. An optical system according to claim 1, wherein the first radiation beam is a narrow band beam having a linewidth less than the radiative linewidth of the transition. 12. An optical system according to claim 1, further comprising a filter adapted to pass Raman scattered photons out of the system. 13. An optical system according to claim 1, wherein the carrier in the ground state of the quantum dot is an electron or hole. 14. An optical system according to claim 1, wherein the system changes the probability of decay from the selected excited state to the second ground state to be greater than the probability of decay from the selected excited state to the first ground state. 15. An optical system according to claim 1, wherein a photon created from irradiation the first radiating beam has a length less than the lifetime of the spin in the ground state. 16. An optical system according to claim 1, wherein a laser used to provide the first radiating beam is not detuned from the transitions. 17. A method of operating an optical system, wherein the optical system comprises a quantum dot comprising a charged carrier, first and second ground state levels and a plurality of excited state levels, the first and second ground state energy levels having different spin states such that the said charged carrier cannot transfer between the first and second ground state energy levels without changing its spin state, the method comprising: irradiating the quantum dot with a first radiating beam with an energy not more than 100 micro-eV from a first transition energy within said quantum dot from a first ground state level to a selected excited state level from the plurality of excited state levels, wherein the system is adapted to enhance the decay rate of a second transition within said quantum dot from the selected excited state level to a second ground state level, but not a first transition, such that a photon is produced due to scattering of a photon from the first radiating beam, and wherein the quantum dot is irradiated with the first radiating beam for a time longer than the radiative lifetime of the selected excited state to produce just one photon, and wherein the first radiating beam comprises at least one pulse. 18. A method according to claim 17, wherein the first radiating beam is a pulsed beam with each pulse having a single intensity maximum, a pulse length greater than the selected excited state lifetime, and the pulse area is controlled to transfer all spin in the first ground state to the second ground state. 19. A method according to claim 17, wherein a second radiating beam radiates the quantum dot at an energy within 100 micro-eV of the second transition energy, the said second radiating beam being a control beam, the control beam being applied to the quantum dot between two or more pulses of the first radiating beam, the first radiating beam being controlled such that the charged carrier has an energy state which is a superposition of the first and second ground state energy levels prior to the application of the control beam. 20. A method according to claim 17, adapted to produce entangled photons, wherein the first radiating beam radiates the quantum dot such that the charged carrier a superposed energy state which is a superposition of the first and second ground state energy levels, and wherein a second radiating beam radiates the quantum dot at an energy within 100 micro-eV of the second transition energy, the said second radiating beam being a control beam, the controller being further adapted to control a second radiating beam to scatter at least one further photon to produce an entangled photon pair using said superposed energy state.