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Hypothetical Unruh Effect

The Unruh effect was described by Stephen Fulling in 1973, Paul Davies in 1975, and William Unruh in 1976. Sometimes called the Fulling-Davies-Unruh effect, it suggests that if you fly through a quantum vacuum with extreme acceleration, the vacuum no longer looks like a vacuum: rather, it looks like a warm bath full of particles. So far it has not been possible to measure or observe it, but now physicists say that instead of studying the empty space in which particles suddenly become visible when accelerating, they can create a Bose-Einstein condensate in which sound particles, or phonons, become audible to an accelerated observer in the silent phonon vacuum; the sound is not created by the detector, rather it is hearing what is there just because of the acceleration.

The Unruh effect predicts a thermal response for an accelerated detector moving through the vacuum. Gooding et al. propose an interferometric scheme to observe an analogue of the circular Unruh effect using a localized laser coupled to a Bose-Einstein condensate. Image credit: University of Nottingham.

The Unruh effect predicts a thermal response for an accelerated detector moving through the vacuum. Gooding et al. propose an interferometric scheme to observe an analogue of the circular Unruh effect using a localized laser coupled to a Bose-Einstein condensate. Image credit: University of Nottingham.

“To observe the Unruh effect directly, as William Unruh — who is a co-author of the current study — described it, is completely impossible for us today,” said co-author Dr. Sebastian Erne, a researcher in the School of Mathematical Sciences at the University of Nottingham, the Vienna Center for Quantum Science and Technology at TU Wien, and the Wolfgang Pauli Institut at Universität Wien.

“You would need a measuring device accelerated to almost the speed of light within a microsecond to see even a tiny Unruh-effect — we can’t do that.”

“However, there is another way to learn about this strange effect: using so-called quantum simulators.”

“Many laws of quantum physics are universal,” said co-author Dr. Jörg Schmiedmayer, a researcher in the Vienna Center for Quantum Science and Technology at TU Wien.

“They can be shown to occur in very different systems. One can use the same formulas to explain completely different quantum systems.”

“This means that you can often learn something important about a particular quantum system by studying a different quantum system.”

“Simulating one system with another has been especially useful for understanding black holes, since real black holes are effectively inaccessible,” said first author Dr. Cisco Gooding, a researcher in the School of Mathematical Sciences at the University of Nottingham.

“In contrast, analogue black holes can be readily produced right here in the lab.”

“This is also true for the Unruh effect: if the original version cannot be demonstrated for practical reasons, then another quantum system can be created and examined in order to see the effect there.”

Just as a particle is a ‘disturbance’ in empty space, there are disturbances in the cold Bose-Einstein condensate — small irregularities (sound waves) that spread out in waves. And these irregularities should be detectable with special laser beams.

Using special tricks, the Bose-Einstein condensate is minimally disturbed by the measurement, despite the interaction with the laser light.

“If you move the laser beam, so that the point of illumination moves over the Bose-Einstein condensate, that corresponds to the observer moving through the empty space,” Dr. Schmiedmayer said.

“If you guide the laser beam in accelerated motion over the atomic cloud, then you should be able to detect disturbances that are not seen in the stationary case — just like an accelerated observer in a vacuum would perceive a heat bath that is not there for the stationary observer.”

“Until now, the Unruh effect was an abstract idea,” said senior author Professor Silke Weinfurtner, a researcher in the School of Mathematical Sciences and the Centre for the Mathematics and Theoretical Physics of Quantum Non-Equilibrium Systems at the University of Nottingham.

“Many had given up hope of experimental verification. The possibility of incorporating a particle detector in a quantum simulation will give us new insights into theoretical models that are otherwise not experimentally accessible.”

The study was published in the journal Physical Review Letters.

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Cisco Gooding et al. 2020. Interferometric Unruh Detectors for Bose-Einstein Condensates. Phys. Rev. Lett 125 (21): 213603; doi: 10.1103/PhysRevLett.125.213603

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