Quantum Dots and Spintronics for Quantum Computing and Communication

It is proposed that an entirely new form of information processing, namely quantum computing, could be possible[1] if the states of electron spins in a given solid can be synthesised (or created), manipulated and measured at the single-quantum level. A spin-quantum dot architecture for a quantum computer, thereby indicating a variety of first generation nanostructures, is reviewed. A spin filter and spin detection mechanism [3] at the single-spin level which can be used for read-in and read-out in conventional as well as in quantum computer gates is discussed. Addressing the feasibilty of quantum communication with entangled electrons [4,5], Einstein-Podolsky-Rosen pairs are discussed.

A way of using Quantum Dots (QD) would be to look at it as a producer of electric charge and use the same electric charge as a qubit. Semiconductor QD's can serve as 3-Dimensional boxes with electrostatic potentials which confine charge quanta. Unfortunately, uncontrolled distant charged motion leads to dephasing. Also scattering reactions such as those triggered by phonon interactions cause coherence times to be relatively short for charge states.

Obtaining of Entangled Photons from Quantum Dots in a cavity for usage in Quantum Computation & Communications is steadily growing in popularity. Some progress has been done in this area [ e.g. Bensaon et. al, 2000; Stace et. al,2003; benycouf et. al., 2004; Kumar et. al., 2004]. It is also expected that there will be other such sources soon. A very demanding but also very promising area, while implementing Quantum Communication, is generating fluorescence photons by manipulating trapped atoms or ions. This is closely connected to Quantum Computation with trapped ion systems and cavity QED systems. A reference source for these techniques is, for example, "Focus On Single Photons On Demand " by Grangier et. al. 2004.

Teleportation of single qubits have already been achieved by multiple groups in laboratories using entangled photons ( for e.g., see ref. Galindo et. al below).

[1] D. Loss, D.P. DiVincenzo, Phys. Rev. A 57 (1998) 120; cond-mat/9701055.
[2] G. Burkard, H.A. Engel, D. Loss, cond-mat/0004182 (Review).
[3] P. Recher, E. V. Sukhorukov, D. Loss, cond-mat/0003089.
[4] D. Loss, E. Sukhorukov, Phys. Rev. Lett. 84, 1035 (2000).
[5] G. Burkard, D. Loss, E. Sukhorukov, to appear in Phys. Rev. B RC, cond-mat/9906071.
[6] A. Galindo and M.A. Martin-Delgado: Infomration and Computation: Classical and Quantum Aspects, 74:347-423, 2002