Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

26 J.M. Elzerman et al.
combining fast QPC charge detection with “spin-to-charge conversion”. This
fully electrical technique to read out a spin qubit is then used to determine
the relaxation time of the single spin, giving a value of 0.85 ms at a magnetic
field of 8 Tesla.
Finally, Sect. 6 puts the results in perspective, arriving at a realistic path
towards the experimental demonstration of single- and two-qubit gates and
the creation of entanglement of spins in quantum dot systems.
1 Introduction
This section gives a brief introduction into quantum computing, continuing
with a description of semiconductor quantum dots that covers their fabrication
as well as their electronic behavior. We also describe our experimental setup
for performing low-temperature transport experiments to probe such quantum
dots.
1.1 Quantum Computing
More than three quarters of a century after its birth, quantum mechanics remains
in many ways a peculiar theory [3]. It describes many physical effects
and properties with great accuracy, but uses unfamiliar concepts like superposition,
entanglement and projection, that seem to have no relation with the
everyday world around us. The interpretation of these concepts can still cause
controversy.
The inherent strangeness of quantum mechanics already emerges in the
simplest case: a quantum two-level system. Unlike a classical two-level system,
which is always either in state 0 or in state 1, a quantum two-level system can
just as well be in a superposition of states |0〉 and |1〉. It is, in some sense, in
both states at the same time.
Even more exotic states can occur when two such quantum two-level systems
interact: the two systems can become entangled. Evenifweknowthe
complete state of the system as a whole, for example (|01〉−|10〉)/ √ 2, which
tells us all there is to know about it, we cannot know the state of the two
subsystems individually. In fact, the subsystems do not even have a definite
state! Due to this strong connection between the two systems, a measurement
made on one influences the state of the other, even though it may be arbitrarily
far away. Such spooky non-local correlations enable effects like “quantum
teleportation” [4, 5].
Finally, the concept of measurement in quantum mechanics is rather special.
The evolution of an isolated quantum system is deterministic, as it is
governed by a first order differential equation – the Schrödinger equation.
However, coupling the quantum system to a measurement apparatus forces
it into one of the possible measurement eigenstates in an apparently nondeterministic
way: the particular measurement outcome is random, only the