One single atom as data memory: Researchers at the Max Planck Institute of Quantum Optics wrote quantum information into a rubidium atom between two mirrors and read it out again after a certain storage time. Image: Andreas Neuzner |
Quantum
computers will one day be able to cope with computational tasks in no time
where current computers would take years. They will take their enormous
computing power from their ability to simultaneously process the diverse pieces
of information which are stored in the quantum state of microscopic physical
systems, such as single atoms and photons. In order to be able to operate, the
quantum computers must exchange these pieces of information between their
individual components. Photons are particularly suitable for this, as no matter
needs to be transported with them. Particles of matter however will be used for
the information storage and processing. Researchers are therefore looking for
methods whereby quantum information can be exchanged between photons and
matter. Although this has already been done with ensembles of many thousands of
atoms, physicists at the Max Planck Institute of Quantum Optics in Garching
have now proved that quantum information can also be exchanged between single
atoms and photons in a controlled way.
Using a
single atom as a storage unit has several advantages—the extreme
miniaturization being only one, says Holger Specht from the Garching-based Max
Planck Institute, who was involved in the experiment. The stored information
can be processed by direct manipulation on the atom, which is important for the
execution of logical operations in a quantum computer. “In addition, it offers
the chance to check whether the quantum information stored in the photon has
been successfully written into the atom without destroying the quantum state,”
says Specht. It is thus possible to ascertain at an early stage that a
computing process must be repeated because of a storage error.
The fact
that no one had succeeded until very recently in exchanging quantum information
between photons and single atoms was because the interaction between the
particles of light and the atoms is very weak. Atom and photon do not take much
notice of each other, as it were, like two party guests who hardly talk to each
other, and can therefore exchange only a little information. The researchers in
Garching have enhanced the interaction with a trick. They placed a rubidium
atom between the mirrors of an optical resonator, and then used very weak laser
pulses to introduce single photons into the resonator. The mirrors of the
resonator reflected the photons to and fro several times, which strongly
enhanced the interaction between photons and atom. Figuratively speaking, the
party guests thus meet more often and the chance that they talk to each other
increases.
The
photons carried the quantum information in the form of their polarization. This
can be left-handed (the direction of rotation of the electric field is
anti-clockwise) or right-handed (clock-wise). The quantum state of the photon
can contain both polarizations simultaneously as a so-called superposition
state. In the interaction with the photon the rubidium atom is usually excited
and then loses the excitation again by means of the probabilistic emission of a
further photon. The Garching-based researchers did not want this to happen. On
the contrary, the absorption of the photon was to bring the rubidium atom into
a definite, stable quantum state. The researchers achieved this with the aid of
a further laser beam, the so-called control laser, which they directed onto the
rubidium atom at the same time as it interacted with the photon.
The spin
orientation of the atom contributes decisively to the stable quantum state
generated by control laser and photon. Spin gives the atom a magnetic moment.
The stable quantum state, which the researchers use for the storage, is thus
determined by the orientation of the magnetic moment. The state is
characterized by the fact that it reflects the photon’s polarization state: the
direction of the magnetic moment corresponds to the rotational direction of the
photon’s polarization, a mixture of both rotational directions being stored by
a corresponding mixture of the magnetic moments.
This
state is read out by the reverse process: irradiating the rubidium atom with
the control laser again causes it to re-emit the photon which was originally
incident. In the vast majority of cases, the quantum information in the
read-out photon agrees with the information originally stored, as the
physicists in Garching discovered. The quantity that describes this
relationship, the so-called fidelity, was more than 90%. This is significantly
higher than the 67% fidelity that can be achieved with classical methods, i.e.
those not based on quantum effects. The method developed in Garching is
therefore a real quantum memory.
The
physicists measured the storage time, i.e. the time the quantum information in
the rubidium can be retained, as around 180 microseconds. “This is comparable
with the storage times of all previous quantum memories based on ensembles of
atoms,” says Stephan Ritter, another researcher involved in the experiment.
Nevertheless, a significantly longer storage time is necessary for the method
to be used in a quantum computer or a quantum network. There is also a further
quality characteristic of the single-atom quantum memory from Garching which could
be improved: the so-called efficiency. It is a measure of how many of the
irradiated photons are stored and then read out again. This was just under 10%.
The
storage time is mainly limited by magnetic field fluctuations from the
laboratory surroundings, says Ritter. “It can therefore be increased by storing
the quantum information in quantum states of the atoms which are insensitive to
magnetic fields.” The efficiency is limited by the fact that the atom does not
sit still in the centre of the resonator, but moves. This causes the strength
of the interaction between atom and photon to decrease. The researchers can
thus also improve the efficiency: by greater cooling of the atom, i.e. by
further reducing its kinetic energy.
The
researchers at the Max Planck Institute in Garching now want to work on these
two improvements. “If this is successful, the prospects for the single-atom
quantum memory would be excellent,” says Stephan Ritter. The interface between
light and individual atoms would make it possible to network more atoms in a
quantum computer with each other than would be possible without such an
interface; a fact that would make such a computer more powerful. Moreover, the
exchange of photons would make it possible to quantum mechanically entangle
atoms across large distances. The entanglement is a kind of quantum mechanical
link between particles which is necessary to transport quantum information
across large distances. The technique now being developed at the Max Planck
Institute of Quantum Optics could some day thus become an essential component
of a future “quantum Internet”.