Swinging on the quantum level
Researchers propose a new way of generating single photons

Date:
December 21, 2021
Source:
University of Mu"nster
Summary:
For many applications making use of quantum effects, the light has
to be in a certain state -- namely a single photon state. But what
is the best way of generating such single photon states? Researchers
have now proposed an entirely new way.



FULL STORY ========================================================================== After the "first quantum revolution" -- the development of devices such
as lasers and the atomic clock -- the "second quantum revolution" is
currently in full swing. Experts from all over the world are developing fundamentally new technologies based on quantum physics. One key
application is quantum communication, where information is written and
sent in light. For many applications making use of quantum effects, the
light has to be in a certain state -- namely a single photon state. But
what is the best way of generating such single photon states? In the
PRX Quantum journal, researchers from Mu"nster, Bayreuth and Berlin
(Germany) have now proposed an entirely new way of preparing quantum
systems in order to develop components for quantum technology.


==========================================================================
In the experts' view it is highly promising to use quantum systems
for generating single photon states. One well-known example of such
a quantum system is a quantum dot. This is a semiconductor structure,
just a few nanometres in size. Quantum dots can be controlled using laser pulses. Although quantum dots have properties similar to those of atoms,
they are embedded in a crystal matrix, which is often more practical for applications. "Quantum dots are excellent for generating single photons,
and that is something we are already doing in our labs almost every
day," says Dr. Tobias Heindel, who runs an experimental lab for quantum communication at the Technical University of Berlin. "But there is still
much room for improvement, especially in transferring this technology
from the lab to real applications," he adds.

One difficulty that has to be overcome is to separate the generated single photons from the exciting laser pulse. In their work, the researchers
propose an entirely new method of solving this problem. "The excitation exploits a swing-up process in the quantum system," explains Mu"nster University's Thomas Bracht, the lead author of the study. "For this,
we use one or more laser pulses which have frequencies which differ
greatly from those in the system.

This makes spectral filtering very easy." Scientists define the "swing-up process" as a particular behaviour of the particles excited by the laser
light in the quantum system -- the electrons or, to be more precise, electron-hole pairs (excitons). Here, laser light from two lasers is
used which emit light pulses almost simultaneously. As a result of the interaction of the pulses with one another, a rapid modulation occurs,
and in each modulation cycle, the particle is always excited a little,
but then dips towards the ground state again. In this process, however,
it does not fall back to its previous level, but is excited more strongly
with each swing up until it reaches the maximum state. The advantage
of this method is that the laser light does not have the same frequency
as the light emitted by the excited particles. This means that photons generated from the quantum dot can be clearly assigned.

The team simulated this process in the quantum system, thus providing guidelines for experimental implementation. "We also explain the physics
of the swing-up process, which helps us to gain a better understanding of
the dynamics in the quantum system," says associate professor Dr. Doris
Reiter, who led the study.

In order to be able to use the photons in quantum communication, they
have to possess certain properties. In addition, any preparation of the
quantum system should not be negatively influenced by environmental
processes or disruptive influences. In quantum dots, especially the
interaction with the surrounding semiconductor material is often a big
problem for such preparation schemes.

"Our numerical simulations show that the properties of the photons
generated after the swing-up process are comparable with the results
of established methods for generating single photons, which are less practical," adds Prof.

Martin Axt, who heads the team of researchers from Bayreuth.

The study constitutes theoretical work. As a result of the collaboration between theoretical and experimental groups, however, the proposal is
very close to realistic experimental laboratory conditions, and the
authors are confident that an experimental implementation of the scheme
will soon be possible. With their results, the researchers are taking
a further step towards developing the quantum technologies of tomorrow.

========================================================================== Story Source: Materials provided by University_of_Mu"nster. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Thomas K. Bracht, Michael Cosacchi, Tim Seidelmann, Moritz Cygorek,
Alexei Vagov, V. Martin Axt, Tobias Heindel, Doris
E. Reiter. Swing-Up of Quantum Emitter Population Using Detuned
Pulses. PRX Quantum, 2021; 2 (4) DOI: 10.1103/PRXQuantum.2.040354 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211221102742.htm

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