|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.