Entanglement is an essential property of multipartite quantum systems, characterized by the inseparability of quantum states of objects regardless of their spatial separation. The generation of entanglement between increasingly macroscopic and disparate systems is an ongoing effort in quantum science. It enables hybrid quantum networks, quantum-enhanced sensing, and probing of the fundamental limits of quantum theory.
Scientists from the Niels Bohr Institute, University of Copenhagen, have successfully connected two large objects in quantum entanglement. One is a mechanical oscillator, a vibrating dielectric membrane, and the other is a cloud of atoms, each acting as a tiny magnet – what physicists call spin.
Scientists have entangled both entities by connecting them with photons, particles of light. Those photons came courtesy of a thin fog of a billion caesium atoms spinning inside the confines of a small, cold cell.
Professor Eugene Polzik, who led the effort, states that: “With this new technique, we are on route to pushing the boundaries of the possibilities of entanglement. The bigger the objects, the further apart they are, the more desperate they are, the more interesting entanglement becomes from both fundamental and applied perspectives. With the new result, entanglement between very different objects has become possible.”
A team member, Michał Parniak said, “Quantum mechanics is like a double-edged sword – it gives us wonderful new technologies, but also limits the precision of measurements which would seem just easy from a classical point of view. Entangled systems can remain perfectly correlated even if they are at a distance from each other. This feature has puzzled researchers from the very birth of quantum mechanics more than 100 years ago.”
Ph.D. student Christoffer Østfeldt explains further: “Imagine the different ways of realizing quantum states as a kind of zoo of different realities or situations with very different qualities and potentials. If, for example, we wish to build a device of some sort, exploit the different qualities they all possess, and perform different functions and solve a different task, it will be necessary to invent a language they are all able to speak. The quantum states need to be able to communicate to use the full potential of the device. That’s what this entanglement between two elements in the zoo has shown we are now capable of”.
A specific example of perspectives of entangling distinctive quantum objects is quantum sensing. Various articles have a sensitivity to various external forces. For instance, mechanical oscillators are utilized as accelerometers and force sensors, while atomic spins are utilized in magnetometers. When only one of the two diverse entangled objects is dependent upon perturbation, entanglement allows it to be estimated with a sensitivity not restricted by the object’s zero-point fluctuations.
The outlook for the future applications of the new technique
There is a fairly immediate possibility for the application of the technique in sensing both for tiny oscillators and big ones. One of the biggest scientific pieces of news in recent years was the first detection of gravity waves, made by the Laser Interferometer Gravitational-wave Observatory (LIGO). LIGO senses and measures extremely faint waves caused by astronomical events in deep space, such as black hole mergers or neutron star mergers.
The waves can be observed because they shake the mirrors of the interferometer. But even LIGO’s sensitivity is limited by quantum mechanics because the zero-point fluctuations also shake the mirrors of the laser interferometer. Those fluctuations lead to noise, preventing observation of the mirrors’ tiny motion caused by gravitational waves.
- Thomas, R.A., Parniak, M., Østfeldt, C. et al. Entanglement between distant macroscopic mechanical and spin systems. Nat. Phys. (2020). DOI: 10.1038/s41567-020-1031-5