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Condensed Matter Physics Seminar

Wednesday, September 15, 1999, 4 p.m.

Physics Building, Room 1219
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*Quantum Computing and Quantum Communication with Electrons in Quantum
Dots*

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Daniel Loss

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(Department of Physics, University of Basel, Switzerland)

**Abstract:** I review our proposed quantum dot architecture for
a quantum computer, which is based on electron spins in coupled semiconductor
quantum dots[1-3]. The fundamental XOR quantum gate is operated by dynamically
coupling the spins of neighboring dots. We have determined the exchange
coupling in the effective Heisenberg model as a function of magnetic and
electric fields within the Heitler-London and Hund-Mulliken approximation.
The influence of nuclear spins on the dephasing of the electron spin in
the quantum dot will be discussed[2]. Addressing the feasibilty of quantum
communication with entangled electrons[3] we consider electronic EPR pairs
and show that the entanglement of two electrons in a double-dot can be
detected in mesoscopic transport and noise measurements[4,5]. In the Coulomb
blockade and cotunneling regime of a double-dot (attached to leads) the
singlet and triplet states of the double-dot lead to phase-coherent current
and noise contributions of opposite signs and to Aharonov-Bohm and Berry
phase oscillations in response to magnetic fields[5]. These oscillations
are a genuine two-particle Aharonov-Bohm effect and provide a direct measure
of non-locality in entangled states. The ratio of zero-frequency noise
to current (Fano factor) is universal and equal to the electron charge.
[1] D. Loss and D.P. DiVincenzo, Phys. Rev. A 57 (1998) 120; cond-mat/9701055.

[2] G. Burkard, D. Loss, and D.P. DiVincenzo, Phys. Rev. B 59 (1999)
2070; condmat/9808026.

[3] D.P. DiVincenzo and D. Loss, cond-mat/99011137.

[4] G. Burkard, D. Loss, and E. Sukhorukov, cond-mat/9906071.

[5] D. Loss and E. Sukhorukov, cond-mat/9907129..

**Host: **Xuedong Hu

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