Qubits: Developing long-distance quantum telecommunications networks


Date:
March 22, 2022
Source:
Universite' de Gene`ve
Summary:
Computers, smartphones, GPS: quantum physics has enabled many
technological advances. It is now opening up new fields of research
in cryptography (the art of coding messages) with the aim of
developing ultra-secure telecommunications networks. There is one
obstacle, however: after a few hundred kilometers within an optical
fiber, the photons that carry the qubits or 'quantum bits' (the
information) disappear. They therefore need 'repeaters', a kind of
'relay', which are partly based on a quantum memory. By managing
to store a qubit in a crystal (a 'memory') for 20 milliseconds,
a team has now taken a major step towards the development of
long-distance quantum telecommunications networks.



FULL STORY ========================================================================== Computers, smartphones, GPS: quantum physics has enabled many
technological advances. It is now opening up new fields of research in cryptography (the art of coding messages) with the aim of developing ultra-secure telecommunications networks. There is one obstacle, however:
after a few hundred kilometers within an optical fiber, the photons that
carry the qubits or 'quantum bits' (the information) disappear. They
therefore need 'repeaters', a kind of 'relay', which are partly based on
a quantum memory. By managing to store a qubit in a crystal (a "memory")
for 20 milliseconds, a team from the University of Geneva (UNIGE) has
set a world record and taken a major step towards the development of long-distance quantum telecommunications networks. This research can be
found in the journal npj Quantum Information.


========================================================================== Developed during the 20th century, quantum physics has enabled scientists
to describe the behavior of atoms and particles as well as certain
properties of electromagnetic radiation. By breaking with classical
physics, these theories generated a real revolution and introduced
notions without equivalent in the macroscopic world such as superposition, which describes the possibility for a particle to be in several places
at once, or entanglement, which describes the ability of two particles
to affect each other instantaneously even at a distance ('spooky action
at a distance').

Quantum theories are now at the heart of much research in cryptography, a discipline that brings together techniques for encoding a message. Quantum theories make it possible to guarantee perfect authenticity and
confidentiality for information (a qubit) when it is transmitted between
two interlocutors by a particle of light (a photon) within an optical
fiber. The phenomenon of superposition let the sender know immediately
whether the photon conveying the message has been intercepted.

Memorizing the signal However, there is a major obstacle to the
development of long-distance quantum telecommunication systems:
beyond a few hundred kilometers, the photons are lost and the signal disappears. Since the signal cannot be copied or amplified -- it
would lose the quantum state that guarantees its confidentiality --
the challenge is to find a way of repeating it without altering it by
creating 'repeaters' based, in particular, on a quantum memory.

In 2015, the team led by Mikael Afzelius, a senior lecturer in the
Department of Applied Physics at the Faculty of Science of the University
of Geneva (UNIGE), succeeded in storing a qubit carried by a photon for
0.5 milliseconds in a crystal (a 'memory'). This process allowed the
photon to transfer its quantum state to the atoms of the crystal before disappearing. However, the phenomenon did not last long enough to allow
the construction of a larger network of memories, a prerequisite for
the development of long-distance quantum telecommunications.

Storage record Today, within the framework of the European Quantum
Flagship program, Mikael Afzelius' team has managed to increase this
duration significantly by storing a qubit for 20 milliseconds. "This is a
world record for a quantum memory based on a solid-state system, in this
case a crystal. We have even managed to reach the 100 millisecond mark
with a small loss of fidelity," enthuses the researcher. As in their
previous work, the UNIGE scientists used crystals doped with certain
metals called 'rare earths' (europium in this case), capable of absorbing
light and then re-emitting it. These crystals were kept at - 273,15DEGC (absolute zero), because beyond 10DEGC above this temperature, the
thermal agitation of the crystal destroys the entanglement of the atoms.

"We applied a small magnetic field of one thousandth of a Tesla to the
crystal and used dynamic decoupling methods, which consist in sending
intense radio frequencies to the crystal. The effect of these techniques
is to decouple the rare-earth ions from perturbations of the environment
and increase the storage performance we have known until now by almost
a factor of 40," explains Antonio Ortu, a post-doctoral fellow in the Department of Applied Physics at UNIGE. The results of this research
constitute a major advance for the development of long-distance quantum telecommunications networks. They also bring the storage of a quantum
state carried by a photon to a time scale that can be estimated by humans.

An efficient system in ten years However, there are still several
challenges to be met. "The challenge now is to extend the storage time
further. In theory, it would be enough to increase the duration of
exposure of the crystal to radio frequencies, but for the time being,
technical obstacles to their implementation over a longer period of time prevent us from going beyond 100 milliseconds. However, it is certain
that these technical difficulties can be resolved," says Mikael Afzelius.

The scientists will also have to find ways of designing memories capable
of storing more than a single photon at a time, and thus of having
'entangled' photons which will guarantee confidentiality. "The aim is
to develop a system that performs well on all these points and that can
be marketed within ten years," concludes the researcher.


========================================================================== Story Source: Materials provided by Universite'_de_Gene`ve. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Antonio Ortu, Adrian Holza"pfel, Jean Etesse, Mikael
Afzelius. Storage of
photonic time-bin qubits for up to 20 ms in a rare-earth
doped crystal. npj Quantum Information, 2022; 8 (1) DOI:
10.1038/s41534-022- 00541-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220322111258.htm

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