A quantum memory component including a quantum dot molecule having first and second quantum dots provided in respective first and second layers separated by a barrier layer; an exciton comprising an electron and hole bound state in said quantum dot molecule, the spin state of said exciton forming a
A quantum memory component including a quantum dot molecule having first and second quantum dots provided in respective first and second layers separated by a barrier layer; an exciton comprising an electron and hole bound state in said quantum dot molecule, the spin state of said exciton forming a qubit; first and second electrical contacts respectively provided below the first quantum dot and above the second quantum dot; a voltage source to apply an electric field across said quantum dot molecule; a controller to modulate the electric field across the quantum dot molecule, including an information acquiring circuit to acquire information concerning the relationship between fine structure splitting of the exciton and the applied electric field and a timing circuit to allow switching of the exciton from an indirect configuration to a direct configuration at predetermined times derived from the fine structure splitting.
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1. A component of a memory configured to store and manipulate a qubit state, said component comprising: a quantum dot molecule, said molecule comprising a first quantum dot provided in a first layer and a second quantum dot provided in a second layer, the first and second layers being separated by a
1. A component of a memory configured to store and manipulate a qubit state, said component comprising: a quantum dot molecule, said molecule comprising a first quantum dot provided in a first layer and a second quantum dot provided in a second layer, the first and second layers being separated by a barrier layer, the second layer being arranged above the first layer;an exciton comprising an electron and hole bound state in said quantum dot molecule, the spin state of said exciton forming said qubit;a first electrical contact being provided below the first quantum dot and a second electrical contact being provided above the second quantum dot to allow an electric field to be provided across said quantum dot molecule;a voltage source connected to the electrical contacts configured to apply an electric field across said quantum dot molecule,a voltage source controller configured to modulate the electric field across the quantum dot molecule such that an exciton in the quantum dot molecule is switched between a direct configuration where the electron and hole of the exciton are in the same quantum dot and an indirect configuration wherein the electron and hole are located in different quantum dots of the quantum dot molecule, the voltage source controller comprising an information acquiring circuit and a timing circuit, the information acquiring circuit being configured to acquire information concerning the relationship between the fine structure splitting of said exciton and the applied electric field, the timing circuit being configured to allow switching of the exciton from an indirect configuration to a direct configuration at predetermined times derived from the fine structure splitting. 2. A component of a memory according to claim 1, wherein the first quantum dot, the barrier layer and the second quantum dot have been fabricated by a semiconductor growth technique such that the second quantum dot is formed after the first quantum dot and wherein the first contact is an n-type contact. 3. A component of a memory according to claim 2, wherein the second contact is a p-type contact. 4. A component of a memory according to claim 2, wherein the second contact is a Schottky contact. 5. A component according to claim 1, wherein the acquiring circuit is connected to a memory which contains data relating the fine structure splitting of said exciton to the applied electric field. 6. A component according to claim 1, further comprising an optical source, said optical source being configured to optically excite an exciton of known spin state in said quantum dot molecule. 7. A component according to claim 6, wherein said optical source is configured to have a photon energy which is an integer number of phonon energies greater than the lowest energy exciton state at the electric field adapted for qubit initialisation. 8. A component according to claim 6, wherein said optical source is configured to have a photon energy which is equal to the lowest energy exciton state at the electric field adapted for qubit initialisation. 9. A component according to claim 6, wherein said quantum dot molecule is located in an optical cavity so that; the optical mode of the cavity matches the light pulse from the optical source both in terms of frequency and spatial shape,the quantum dot molecule is positioned in an anti-node of the cavity. 10. A component according to claim 1, further comprising an optical detector having a polarisation detector, said optical detector being configured to measure the emission and polarisation of photons output from said component. 11. A component of a memory according to claim 1, wherein the exciton energy is different in the two quantum dots. 12. A component according to claim 1, wherein the voltage controller is configured to maintain the fine structure splitting below 10 μeV. 13. A component of a memory according to claim 1, wherein said voltage controller is configured to move said exciton from the indirect configuration to the direct configuration once the qubit state has completed an integer number of revolutions around the eigenstate axis such that the voltage controller allows the qubit state as initialised to be read-out, the timing circuit calculating the time to move the exciton to the direct configuration from the fine structure splitting. 14. A component according to claim 1, wherein said voltage controller is configured to move said exciton from the indirect configuration to the direct configuration once the qubit state has completed an integer number of revolutions around the eigenstate axis in addition to a predefined rotation from its original state, such that said voltage controller performs a quantum operation, the timing circuit calculating the time to move the exciton to the direct configuration from the fine structure splitting. 15. A component according to claim 1, wherein the information acquiring circuit being configured to acquire information concerning the relationship between the fine structure splitting of said exciton and the applied electric field as well as the eigenstate orientation on the Bloch sphere. 16. A component according to claim 1, wherein the voltage controller is configured to apply a modulated electric field adapted to change the eigenstates of the qubit for a finite time period at given points in time during qubit storage, thereby allowing the qubit to perform predefined rotations around two different eigenstate axes during storage. 17. A component according to claim 1, wherein the voltage controller is configured to apply a modulated electric field adapted to alternate between two orthogonal eigenstate axes during qubit storage, thereby enabling the action of a “spin echo” to rephase the stored state. 18. A component according to claim 1, wherein the voltage controller is configured to apply the same field to read out the state as to initialise the state. 19. A method of storing a qubit state, said qubit state being stored as an exciton spin state in a quantum dot molecule said exciton comprising an electron and hole bound state, said molecule comprising a first quantum dot provided in a first layer and a second quantum dot provided in a second layer, the first and second layers being separated by a barrier layer, the second layer being arranged above the first layer; the method comprising:applying a first electric field adapted to allow an exciton in said quantum dot molecule to be moved from a direct configuration to an indirect configuration, wherein in the direct configuration the electron and hole are located in one of the quantum dots and in the indirect configuration, the electron of said exciton is located in one quantum dot and the hole of said exciton in the other quantum dot,acquiring information concerning the relationship between the fine structure splitting of an exciton in said quantum dot molecule and the applied electric field,applying a further electric field configured to move the exciton from the indirect configuration to the direct configuration at a predetermined time, wherein the predetermined time is derived from the fine structure splitting. 20. A method according to claim 19, wherein the voltage source controller is adapted to apply: a first electric field across the quantum dot molecule when an exciton is initialised in the quantum dot molecule in the direct configuration, wherein during initialisation the exciton is created in the quantum dot;a second electric field across the quantum dot molecule, wherein at this field, the lowest energy exciton state corresponds to the indirect exciton configuration; anda third electric field across the quantum dot molecule, configured to allow readout of the quantum information stored in the spin state of the exciton, wherein at said third electric field the lowest energy exciton state corresponds to the direct exciton configuration.
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이 특허에 인용된 특허 (6)
Taira Kenichi,JPX ; Suzuki Toshikazu,JPX ; Ono Hideki,JPX, Memory device.
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