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Light
propagates through the atomic cloud shown in the center and then falls onto the
SiN membrane shown on the left. As a result of interaction with light the
precession of atomic spins and vibration of the membrane become quantum
correlated. This is the essence of entanglement between the atoms and the
membrane. Credit: Niels Bohr Institute |
A team of researchers from the University of Copenhagen's Niels Bohr Institute has successfully entangled two very distinct quantum particles. The findings, which were reported in Nature Physics, have various possible applications in ultra-precise sensing and quantum communication.
Quantum
communication and quantum sensing are both based on entanglement. It's a
quantum link between two items that allows them to act as if they're one
quantum object.
Researchers
were able to create entanglement between a mechanical oscillator—a vibrating
dielectric membrane—and a cloud of atoms, each serving as a small magnet, or
"spin," according to physicists. By joining these disparate entities
with photons, or light particles, they were able to entangle. The membrane—or
mechanical quantum systems in general—can be used to process quantum
information, and the membrane—or mechanical quantum systems in general—can be
used to store quantum information.
Professor Eugene Polzik, the project's leader, says: "We're on our way to pushing the boundaries of entanglement's capabilities with this new technique. The larger the objects, the further away they are, and the more different they are, the more intriguing entanglement becomes from both a basic and an applied standpoint. Entanglement between highly diverse things is now conceivable thanks to the new result."
Imagine
the position of the vibrating membrane and the tilt of the total spin of all
atoms, similar to a spinning top, to explain entanglement using the example of
spins entangled with a mechanical membrane. A correlation occurs when both
items move randomly yet are observed travelling right or left at the same
moment. The so-called zero-point motion—the residual, uncorrelated motion of
all matter that occurs even at absolute zero temperature—is generally the limit
of such correlated motion. This limits our understanding of any of the systems.
Eugene
Polzik's team entangled the systems in their experiment, which means they moved
in a correlated way with more precision than zero-point motion. "Quantum
mechanics is a double-edged sword—it gives us amazing new technology, but it
also restricts the precision of measurements that would appear simple from a
classical standpoint," explains Micha Parniak, a team member. Even if they
are separated by a large distance, entangled systems can maintain perfect
correlation, a fact that has perplexed academics since quantum physics'
inception more than a century ago.
Christoffer stfeldt, a Ph.D. student, elaborates: "Consider the many methods for manifesting quantum states as a zoo of diverse realities or circumstances, each with its own set of features and potentials. If, for example, we want to construct a gadget that can take advantage of the many attributes they all have and perform different functions and accomplish different tasks, we'll need to invent a language that they can all understand. For us to fully utilise the device's capabilities, the quantum states must be able to communicate. This entanglement of two zoo elements has demonstrated what we are presently capable of."
Quantum
sensing is an example of distinct perspectives on entangling different quantum
things. Different objects have different levels of sensitivity to external
pressures. Mechanical oscillators, for example, are employed in accelerometers
and force sensors, while atomic spins are used in magnetometers. Entanglement
permits only one of the two entangled objects to be measured with a sensitivity
not restricted by the object's zero-point fluctuations when only one of the two
is subject to external perturbation.
The
approach has the potential to be used in sensing for both small and large
oscillators in the near future. The first detection of gravity waves, performed
by the Laser Interferometer Gravitational-wave Observatory, was one of the most
significant scientific breakthroughs in recent years (LIGO). LIGO detects and
monitors extremely faint waves produced by deep-space astronomical events such
as black hole mergers and neutron star mergers. The waves can be seen because
they shake the interferometer's mirrors. However, quantum physics limits LIGO's
sensitivity since the laser interferometer's mirrors are likewise disturbed by
zero-point fluctuations. These variations produce noise, which makes it
impossible to see the tiny movements of the mirrors induced by gravitational
waves.
It is
theoretically possible to entangle the LIGO mirrors with an atomic cloud and so
cancel the reflectors' zero-point noise in the same manner that the membrane
noise is cancelled in the current experiment. Due to their entanglement, the
mirrors and atomic spins have a perfect correlation that can be used in such
sensors to almost eliminate uncertainty. It's as simple as taking data from one
system and applying what you've learned to the other. In this method, one may
simultaneously learn about the position and momentum of LIGO's mirrors,
entering a so-called quantum-mechanics-free subspace and moving closer to
unlimited precision in motion measurements. A model experiment demonstrating
this principle is on the way at Eugene Polzik's laboratory.
References:
Rodrigo A.
Thomas et al. Entanglement between distant macroscopic mechanical and spin
systems, Nature Physics . DOI: 10.1038/s41567-020-1031-5
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