The strange properties of quantum substances are being explored in the hope that they will lead to motors that are more powerful and efficient than their classical counterparts. There’s still a lot we don’t understand about how different aspects of quantum physics can help and hinder machines. A new exploration of one of the most distinctive features of quantum physics, entanglement, shows that it can help increase the usable energy produced by an engine, but not the efficiency of energy conversion.
Like many aspects of quantum mechanics, entanglement makes little sense to people who grew up in a classical world. Einstein famously derided it as “spooky action at a distance” – but eventually the evidence for its existence became too powerful to ignore. Over the past two decades, physicists have had increasing success in entangling larger numbers of subatomic particles over greater and greater distances.
Most plans to harness quantum entanglement for practical purposes involve information processing and transfer, but quantum engines and quantum batteries could also have a place.
Last year, the first quantum engine was demonstrated, using the transformation of a fermion gas to a boson condensate and back again, instead of the differences in heat that engines have used since Watt’s invention. Fermions and bosons are particles that are distinguished by their spins. More relevant is that bosons can clump together much more strongly than fermions, because Pauli’s exclusion principle, which prevents two fermions from occupying the same quantum state at the same time, does not apply to them. Switching back and forth between an expansive fermion gas and a condensed Bose-Einstein condensate of bosons was used to drive small pistons.
That original engine had an efficiency of 25 percent – an astonishing feat for a first attempt, but far less than the engines that power our world today. So the race is on to create something better. Several papers have proposed using quantum entanglement, where the quantum state of each particle is inextricably linked to that of the others.
Dr. Zhou Fei was part of a team that created a quantum engine based on two calcium ions in a trap, where the degree of quantum entanglement could be varied to measure its effects. The engine operates on a four-stroke cycle, starting with the absorption of photons from a red laser, an expansion phase, a sideband transition to couple the system to a quantum load and finally compression.
“The highlight of our study is the first experimental realization of a quantum engine with entangled properties. [It] quantitatively proved that entanglement can serve as a kind of ‘fuel’,” Fei told the South China Morning Post.
“We chose the entangled states of two spinning ions as the working substance [their] vibration modes that act as loads. Through precise adjustments of the laser frequency, amplitude and duration, the ions were transferred from their original pure state to highly entangled states,” Zhou added.
The conversion efficiency, measured by the number of vibrations the engine created for each photon applied, did not improve with entanglement. However, mechanical efficiency was higher with entanglement, meaning more usable energy was produced for the same input.
Even with more energy, quantum engines will still have quite limited applications. So far they only work at temperatures around absolute zero. However, because quantum computers already require these temperatures to operate, quantum engines can fill the related roles, although they will need to expand significantly from this proof-of-concept.
The work has been published in the journal Physical Review Letters.