Quantum effects boost engine performance

20 Feb 2019

Power boost: a quantum heat engine in synthetic diamond (Courtesy: Jonas Becker)

Physicists have in recent years built a number of microscopic heat engines to investigate how the laws of thermodynamics might change on the atomic scale. To date, however, no such machine has demonstrated quantum-mechanical effects. Now, researchers in the UK and Israel have created minuscule engines within a block of synthetic diamond, and have shown that electronic superposition can boost their power beyond that of classical devices.

A heat engine is any device that does work by exploiting a flow of heat between hot and cold baths. Usually, it contains a physical piston that moves up and down like gas or other fluid expands and contracts. But its performance does not depend on any quantum-mechanical property of the gas.

That would not be true of a so-called quantum heat engine. In 2015 Ronnie Kosloff and colleagues at the Hebrew University of Jerusalem in Israel theoretically analyzed the workings of an engine that exploits quantum coherence via a superposition of energy states. They found that although such a machine could not exceed the Carnot efficiency – which sets a performance limit for any reversible heat engine – over short cycles it should generate more power than any equivalent classical device operating between the same thermal baths.

In the latest work, James Klatzow of Oxford University, Raam Uzdin of the Hebrew University of Jerusalem, Eilon Poem of the Weizmann Institute of Science, also in Israel, and co-workers at Oxford and Bath University, have built such an engine in the laboratory. As they report in a paper recently accepted for publication in Physical Review Letters, the device exploits what are known as nitrogen-vacancy centers, gaps within a diamond lattice created by nitrogen impurities that act as if they were atoms containing a set of discrete energy levels. The diamond in question is a slab about 5 mm by 5 mm, which is exposed both to microwaves and green laser light.

Two-stroke engine

The engine cycle consists of two strokes, each lasting just a few tens of nanoseconds, although these do not involve the movement of a piston as would, say, a combustion engine. The first stroke is thermal, in which electrons are boosted to a higher energy level by the laser light before dropping back down to an intermediate level and fluorescing in the red portion of the spectrum. Then comes the power stroke, during which microwave photons of just the right frequency stimulate the electrons to drop back down to their ground state. The net result is that two photons are emitted for every one absorbed.

As well as transferring electrons between the ground and intermediate states, the microwave interaction creates a quantum superposition between these states. The engine is rendered quantum-mechanical by making use of this superposition to increase the production rate of stimulated photons – an effect that comes into play only when the strokes are very brief and the quantum superposition remains coherent. This doesn’t boost the engine’s overall energy output – meaning there is no contravention of thermodynamic laws – but it does lead to a speed-up. In other words, it raises the device’s power compared to an engine without quantum superposition.

As Klatzow points out, four years ago a group in Germany built a heat engine using just a single ion of calcium, which the researchers forced back and forth along a small funnel by turning electrodes on and off at a certain rate. He describes that work as “very impressive” but says they did not show that quantum coherence affects the engine’s performance, even though a single ion is unquestionably a quantum object. Klatzow and colleagues demonstrated this performance boost by measuring how much work the engine could do in each cycle as they varied the duration of the thermal stroke. The idea was to find out what happened as the stroke duration approached the decoherence time (about 75 ns) – which is when the engine becomes less quantum-like. And indeed they found that the work done per cycle dropped as the stroke got longer.

Quantum power

In contrast, he says, the latest device is susceptible to quantum effects because it can be operated using minuscule amounts of heat – thanks to extremely sensitive measurements they carry out via laser fluorescence. “We are the first to have shown quantum coherence effects in the operation of heat engines,” he says.

Klatzow reckons that practical applications of the research remain some way off, particularly those relying on high efficiencies – with the current performance, he says, being “certainly nowhere close to Carnot”. On the other hand, he believes it may help improve our understanding of photosynthesis since plants work in effect as a heat engine by converting sunlight into stored electrical energy. “People suspect that there might be some sort of quantum coherent processes, which would be fantastic if we could mimic it,“ he says. “That might potentially be useful for very efficient solar cells.”

Kosloff congratulates the experimental group for its “very important contribution” to quantum thermodynamics, and is quite bullish about applications. The latest research, he argues, “paves the way to asking about quantum supremacy” in designs of heat engines and refrigerators. “In the near future quantum refrigerators will become a crucial enabler in quantum technology,” he says.

PHYSICSWORLD.COM   20/2/2019