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92 J.M. Elzerman et al.
pulses to control the inter-dot coupling, in order to demonstrate superposition
and entanglement of spin states.
For such experiments, the QPC is an invaluable tool. It allows us to probe a
dot that is nearly isolated from the reservoirs, which is a regime not accessible
to conventional transport experiments. Most importantly, it enables us to
study a single spin or charge, rather than measuring average properties of a
large ensemble. The QPC charge and spin detector is therefore essential to
achieve the kind of single-particle control that is required for creating a qubit –
transport experiments are no longer necessary.
The techniques we have developed are not only suitable for quantum computation.
Now that the spin orientation of a single electroncan be measured,
we can think of using the spin as a local probe to explore the semiconductor
environment. For instance, measuring the spin relaxation time in various situations
could reveal details of different mechanisms for spin-orbit coupling.
We could vary the orientation of the magnetic field with respect to the crystal
axes, or investigate the effect of static or time-varying electric fields. Once
we can measure the electron spin resonance frequency, this would allow us to
study the polarization of the nuclear spin ensemble via the Overhauser effect.
In all these cases, the fact that dot parameters such as the Zeeman splitting
or the tunnel coupling to reservoirs can be controlled in situ, makes a lateral
quantum dot filled with a single spin a system of great versatility and
fundamental importance.
Acknowledgments
We thank T. Fujisawa, T. Hayashi, T. Saku and Y. Hirayama for help with
device fabrication, M. Blaauboer, D.P. DiVincenzo, H.A. Engel, C.J.P.M. Harmans,
V. Golovach, D. Loss, R. Schoelkopf, K. Schwab and W.G. van der Wiel
for helpful discussions, and B. van der Enden and R. Schouten for technical
support. We acknowledge financial support from the Specially Promoted
Research Grant-in-Aid for Scientific Research from the Japanese Ministry
of Education, Culture, Sports, Science and Technology, the DARPA-QUIST
program (DAAD19-01-1-0659), the EU-RTN network on spintronics, and the
Dutch Organisation for Fundamental Research on Matter (FOM).
References
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2. D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998). 25, 29, 30, 31, 48, 58, 59, 72, 82, 90
3. R.P. Feynman, The Feynman Lectures on Physics, Vol. 3 (Addison Wesley,
1970). 26, 27
4. M. Riebe et al., Nature 429, 734 (2004). 26