The experimental demonstration of entanglement between mechanical and spin systems

Quantum entanglement is the underlying phenomenon behind the functioning of a variety of quantum systems, including quantum communication, quantum sensing, and quantum computing tools. This phenomenon arises from an interaction (i.e. entanglement) between particles. However, achieving entanglement between distant and very different objects has so far proved extremely challenging.

Researchers at the University of Copenhagen recently generated an entanglement between a mechanical oscillator and a collective atomic spin oscillator. Their work, outlined in a paper published in Physics of nature, introduces a strategy to generate entanglement between these two distinct systems.

“About a decade ago, we proposed a way to generate entanglement between a mechanical oscillator and a spin oscillator using photons, using the principle that was later called ‘free subspaces of quantum mechanics’ or ‘trajectories without quantum uncertainties’. “said Eugene S. Polzik, who led the team that conducted the study. “In our new document, we report the experimental implementation of these proposals”.

To generate entanglement between a mechanical and a spin system, Polzik and his colleagues exploited a key feature of spin oscillators, namely that they can have effective negative mass. When excited, the energy of a spin oscillator is reduced, which allows it to tangle with a more conventional mechanical oscillator that has positive mass. The researchers experimentally generated this entanglement by performing a joint measurement on both oscillators.

“The entanglement between the mechanical and spin systems is generated by sending light through both systems, a positive-mass mechanical oscillator and a spin oscillator with an effective negative mass,” Polzik said. “Taking a measurement on the transmitted light projects the two systems into a state of entanglement. The subsequent repeated measurement verifies entanglement by demonstrating that the quantum fluctuations of the two systems are strongly correlated.”

The experiment conducted by Polzik and his colleagues shows that mechanical movement can, at least in principle, be measured with arbitrary precision by identifying and applying an appropriate reference system. These measurements exceed the so-called “standard quantum measurement limit” which derives from the Heisenberg uncertainty principle, which is applicable to measurements in a standard classical reference framework.

“The essence of the uncertainty principle is the balance between the inaccuracy of the measurement and the disturbance caused by the measurement, the quantum feedback action,” Polzik said. “With a measurement in the reference system of the negative mass, the backaction disturbances imposed on the object and on the reference system distractively interfere and cancel each other out, thus leading to potentially unlimited measurement accuracy.”

This team of researchers was the first to experimentally demonstrate the entanglement between a mechanical system and a spin. In the future, their work could contribute to the development of new quantum technologies and protocols based on entanglement between different types of oscillators. In their forthcoming studies, Polzik and his colleagues intend to evaluate the effectiveness of their approach to perform quantum teleportation and develop other quantum communication tools.

“With the recent observation of quantum back action by the LIGO and VIRGO gravitational wave detector teams, ways to overcome the limitations of quantum back action become particularly relevant for those extremely challenging tools,” Polzik said. “We are building an experiment in which we intend to demonstrate the potential applicability of our approach to the best sensitivity of gravitational wave detectors.”

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