Physicists at the Max Planck Institute of Quantum Optics have managed to
entangle more than a dozen photons efficiently and in a defined way. They
are thus creating a basis for a new type of quantum computer. Their study is
published in Nature.
The phenomena of the quantum world, which often seem bizarre from the
perspective of the common everyday world, have long since found their way
into technology. For example, entanglement: a quantum-physical connection
between particles that links them in a strange way over arbitrarily long
distances. It can be used, for example, in a quantum computer—a computing
machine that, unlike a conventional computer, can perform numerous
mathematical operations simultaneously. However, in order to use a quantum
computer profitably, a large number of entangled particles must work
together. They are the basic elements for calculations, so-called qubits.
"Photons, the particles of light, are particularly well suited for this
because they are robust by nature and easy to manipulate," says Philip
Thomas, a doctoral student at the Max Planck Institute of Quantum Optics
(MPQ) in Garching near Munich. Together with colleagues from the Quantum
Dynamics Division led by Prof. Gerhard Rempe, he has now succeeded in taking
an important step towards making photons usable for technological
applications such as quantum computing: For the first time, the team
generated up to 14 entangled photons in a defined way and with high
efficiency.
One atom as a photon source
"The trick to this experiment was that we used a single atom to emit the
photons and interweave them in a very specific way," says Thomas. To do
this, the Max Planck researchers placed a rubidium atom at the center of an
optical cavity—a kind of echo chamber for electromagnetic waves. With laser
light of a certain frequency, the state of the atom could be precisely
addressed. Using an additional control pulse, the researchers also
specifically triggered the emission of a photon that is entangled with the
quantum state of the atom.
"We repeated this process several times and in a previously determined
manner," Thomas reports. In between, the atom was manipulated in a certain
way—in technical jargon: rotated. In this way, it was possible to create a
chain of up to 14 light particles that were entangled with each other by the
atomic rotations and brought into a desired state. "To the best of our
knowledge, the 14 interconnected light particles are the largest number of
entangled photons that have been generated in the laboratory so far," Thomas
says.
Deterministic generation process
But it is not only the quantity of entangled photons that marks a major step
towards the development of powerful quantum computers—the way they are
generated is also very different from conventional methods. "Because the
chain of photons emerged from a single atom, it could be produced in a
deterministic way," Thomas explains. This means: in principle, each control
pulse actually delivers a photon with the desired properties. Until now, the
entanglement of photons usually took place in special, non-linear crystals.
The shortcoming: there, the light particles are essentially created randomly
and in a way that cannot be controlled. This also limits the number of
particles that can be bundled into a collective state.
The method used by the Garching team, on the other hand, allows basically
any number of entangled photons to be generated. In addition, the method is
particularly efficient—another important measure for possible future
technical applications: "By measuring the photon chain produced, we were
able to prove an efficiency of almost 50%," says Philip Thomas. This means
almost every second "push of a button" on the rubidium atom delivered a
usable light particle—far more than has been achieved in previous
experiments. "All in all, our work removes a long-standing obstacle on the
path to scalable, measurement-based quantum computing," says department
Director Gerhard Rempe..
More space for quantum communication
The scientists at the MPQ want to remove yet another hurdle. Complex
computing operations for instance would require at least two atoms as photon
sources in the resonator. The quantum physicists speak of a two-dimensional
cluster state. "We are already working on tackling this task," says Philip
Thomas.
The Max Planck researcher also emphasizes that possible technical
applications extend far beyond quantum computing: "Another application
example is quantum communication"—the tap-proof transmission of information,
for example by light in an optical fiber. There, the light experiences
unavoidable losses during its propagation due to optical effects such as
scattering and absorption—which limits the distance over which data can be
transported. Using the method developed in Garching, quantum information
could be packaged in entangled photons and could also survive a certain
amount of light loss—and enable secure communication over greater distances.
Reference:
Philip Thomas et al, Efficient generation of entangled multiphoton graph
states from a single atom, Nature (2022).
DOI: 10.1038/s41586-022-04987-5
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Physics