Electron one-way street In this dual channel, electrons (blue) move on defined, parallel paths. Only one single electron fits through at a time. By means of tunnel coupling, the electron can switch back and forth between the channels, thus occupying two different states, which are denoted by “arrow up” and “arrow down”. The electron virtually flies in both tracks at the same time, its two states overlap. Image: Andreas Wieck

The
alphabet of data processing could include more elements than the “0”
and “1” in future. An international research team headed by scientists
at Ruhr-Universität Bochum has achieved a new kind of bit with single
electrons, called quantum bits, or qubits. With them, considerably more
than two states can be defined. So far, quantum bits have only existed
in relatively large vacuum chambers. The team has now generated them in
semiconductors. They have put an effect in practice, which the RUB
physicist Prof. Dr. Andreas Wieck had already theoretically predicted 22
years ago. This represents another step along the path to quantum
computing. Together with colleagues from Grenoble and Tokyo, Wieck from
the Chair of Applied Solid State Physics reports on the results in the
journal Nature Nanotechnology.

Conventional bits

The
basic units of today’s data processing are the bit states “0” and “1”,
which differ in their electrical voltage. To encode these states, only
the charge of the electrons is crucial. “Electrons also have other
properties though” says Wieck, and these are exactly what you need for
quantum bits. “The extension from bits to quantum bits can dramatically
increase the computational power of computers” says the physicist.

The new bit generation

A
quantum bit corresponds to a single electron in a particular state.
Together with his colleagues, Wieck used the trajectories of an electron
through two closely spaced channels for encoding. In principle, two
different states are possible: the electron either moves in the upper
channel or in the lower channel – which would then only form a binary
system again. According to quantum theory, however, a particle can be in
several states simultaneously, that is, it can quasi fly through both
channels at the same time. These overlapping states can form an
extensive alphabet of data processing.

A recipe for qubits

In
order to generate quantum bits with different states, the researchers
allowed individual electrons to interfere with each other. This works
with the so-called Aharonov-Bohm effect: powered by an external voltage,
the electrons fly through a semiconducting solid. Within this solid,
their trajectory is first forked and then reunited. Thus, each electron
flies simultaneously on both possible paths. When the two paths come
together again, there is interference, i.e., the two electron waves
overlap and quantum bits with different overlapping states are
generated.

Controlling electrons on defined paths

Normally,
an electron wave moves through a solid body on many different paths at
the same time. Due to impurities in the material, it loses its phase
information and thus its ability to encode a particular state. To
maintain the phase information, the researchers at the RUB grew a
high-purity gallium arsenide crystal and used a dual channel proposed by
Wieck more than 20 years ago.

How the dual channel works   

An
electron reaches the fork via two closely spaced channels. These are
coupled with each other (tunnel-coupling), so that the electron flies
simultaneously on two different paths. The phases of the electron waves
are maintained by the coupling. The same dual channel was also used by
the team after the electron waves were reunited at the end of the fork.
In this way, they produced quantum bits with clear states which are
suitable for encoding information. “Unfortunately, not all the electrons
take part in this process, so far it’s only a few percent” commented
Wieck. “Some students in my department are, however, already working on
growing crystals with higher electron densities”.

Electrical control of a solid-state flying qubit

Source:  Ruhr-Universität Bochum