This invention concerns a method to switch on and off the exchange interaction J between electron spins bound to donor atoms. The electron spins have the role of ‘qubits’ to carry quantum information, and the exchange interaction J has the role of mediator for two-qubit quantum logic operations. The invention aims at exploiting the existence of a further magnetic interaction, the hyperfine interaction A, between each electron spin and the nuclear spin of the donor atom (301, 302) that binds the electron. The hyperfine interaction A, together with the ability to read out (504) and control the state of the nuclear spins, is used to suppress the effect of the exchange interaction J at all times, except while a quantum logic operation is being performed. In this way, the result of the quantum logic operation is not distorted after the operation has taken place. In a further aspect, the invention concerns an electronic device where the method can be practically implemented, and a large scale device made up of many of the devices.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A method for controlling exchange oscillations between a pair of electron spin states in a quantum computation device comprising a pair of donor atoms incorporated in crystalline silicon, wherein each donor atom has a nucleus and at least one bound electron; wherein, quantum information is encoded in a spin state of the bound electron of each donor atom of the pair of donor atoms, and the spin state of the nucleus of each donor atom is coupled to the spin state of its respective bound electron via the hyperfine interaction (A); and wherein an exchange interaction (J) between the spin state of each of the two electrons results in exchange oscillations between them; the method comprising the steps of: preparing the nuclear spins of the two donors in opposite states; tuning the exchange interaction (J) by the application of a switchable voltage to selectively change the relative strength of the exchange interaction with respect to the hyperfine interaction; and thereby selectively controlling the exchange oscillations between the two bound electrons. 2. The method according to claim 1, comprising the step of tuning the exchange interaction (I) between the donor electron spins, by modifying the relative potential between the two donor atoms. 3. The method according to claim 1, comprising the step of controlling the exchange interaction (I) between the donor electron spins, by modifying the potential barrier between the two donor atoms. 4. The method according to claim 1, wherein the exchange oscillations between the two bound electrons are turned on or off; depending on the tuning of the exchange interaction (J). 5. The method according to claim 1, wherein the amplitude of the exchange oscillations between the two bound electrons is made higher or lower; depending on the tuning of the exchange interaction (J). 6. The method according to claim 1, wherein the method is performed in the context of two qubit exchange gate operations. 7. The method according to claim 6, for performing SWAP operations between the electron spins of two donors in silicon. 8. The method according to claim 6, for performing VSWAP operations between the electron spins of two donors in silicon. 9. The method according to claim 1, wherein an electron reservoir is provided to facilitate initialization. 10. The method according to claim 1, wherein an electrometer is provided to determine a charge state of a donor atom. 11. The method according to claim 1, wherein a Single Electron Transistor (SET) is provided for readout. 12. The method according to claim 11, wherein the Single Electron Transistor (SET) is tunnel-coupled to the donor atoms. 13. The method according to claim 1, wherein preparing the nuclear spins of the two donors in opposite states comprises performing read-out and control of the state of the nuclear spins of the donor atoms. 14. The method according to claim 1, for performing a quantum logic operation between two electron spins by tuning the exchange interaction (J) relative to the hyperfine interaction (A), and preparing the nuclear spins in opposite states, such that quantum logic operations take place while J>>A, whereas the quantum logic operations are stopped to allow the readout of the results while J<<A. 15. The method according to claim 13 wherein read-out involves spin dependent quantum mechanical tunnelling of a donor electron in the ‘spin-up’ state to a charge reservoir upon application of a magnetic field. 16. The method for operating a quantum device according to claim 1, comprising the steps of: initialization of the nuclear spins in opposite states; initialization of the electron spins; exchange operation between the electrons; and, readout of the final electron spins states; wherein, the exchange operation is tuned by the application of a switchable electric field between the pair of ‘J’ gates to selectively modify the relative energy of the two bound electrons. 17. The method for operating a quantum device according to claim 1, comprising the step of detuning to protect against unwanted exchange oscillations during a two qubit exchange operations. 18. A quantum computing device for controlling exchange oscillations between a pair of electron spin states in a quantum computation device comprising: a pair of donor atoms incorporated in crystalline silicon, wherein each donor atom has a nucleus and at least one bound electron; wherein, quantum information is encoded in a spin state of the nucleus or the bound electron, or both, of the donor atoms, and the spin state of the nucleus of each donor atom is coupled to the spin state of its respective bound electron via the hyperfine interaction (A); and wherein an exchange interaction (J) between the spin state of each of the two electrons results in exchange oscillations between them; and wherein the nuclear spins of the two donors are prepared in opposite states; and further wherein tuning the exchange interaction (J) by the application of a switchable voltage selectively changes the relative strength of the exchange interaction with respect to the hyperfine interaction; and thereby selectively controls the exchange oscillations between the two bound electrons. 19. A large scale quantum device comprising plural devices according to claim 18 fabricated on a common silicon wafer.