Theory of Nondestructive Optical Measurements of Two Electron Spins in a Quantum Dot

Recently, a quantum media converter from a photon qubit to an electron spin qubit was proposed for quantum repeaters. Quantum information can take several different forms and it is preferable to be able to convert among different forms. One form is the photon polarization and another is the electron spin polarization. Photons are the most convenient medium for sharing quantum information between distant locations. However, it is necessary to realize a quantum repeater in order to send the information securely over a very long distance overcoming the photon loss. A quantum repeater requires quantum information storage and the electron spin is a promising candidate of such a quantum memory because of its long decoherence time. The most important ingredient of the quantum repeater is the entanglement swapping through the Bell (correlation) measurement. We propose an optical means to carry out the correlation measurement. The advantage of this method is the nondestructive character which keeps the quantum state unchanged.