Maxwell’s lesser demon helps new quantum heat engine

CQT’s Stella Seah and Valerio Scarani (pictured) and their collaborator CQT alumnus Stefan Nimmrichter describe a new model of a quantum heat engine in the journal Physical Review Letters.

CQT researchers and their collaborator have proposed a new model for a quantum heat engine. Their design, employing a ‘lesser Maxwell’s demon’, has the advantage of being more stable to fluctuations in the temperature of the environment.

“We didn't expect to see an advantage, so it was surprising and interesting,” says Stella Seah, first author on the finding published 10 March in Physical Review Letters. Studies of quantum engines can both clarify fundamental physics and, in the long term, suggest a route to power other quantum technologies.

An engine needs a few standard components: a working medium, a hot reservoir and a cold reservoir. In a quantum heat engine, the working medium can be a qubit coupled to a hot reservoir. When the qubit gets knocked into an excited state by thermal energy from the hot reservoir, a measurement can extract this energy. This brings the qubit back to the ground state and the process is repeated.

Previous quantum engine models have used a measurement pointer, usually imagined to be another qubit, to measure the internal state of the working qubit. The team’s new model uses a macroscopic pointer instead.

The role of Maxwell’s demon in quantum engines is to acquire information about the system and perform the appropriate action – that is, to take energy from the qubit when it is in the excited state. The new paper describes its demon as ‘lesser’ because it only interacts with the macroscopic pointer, rather than the entire system as in other engine models.

The paper’s authors are Stella, a PhD student at the Department of Physics at NUS, her supervisor CQT Principal Investigator Valerio Scarani and CQT alumnus Stefan Nimmrichter, who was then at the Max Planck Institute for the Science of Light in Erlangen, Germany. Stefan is now at the University of Siegen.

Unexpected advantages

The team’s pointer is a harmonic oscillator that interacts with the spin of a qubit, pointing to the left or right depending on the qubit state. The pointer also interacts with the engine’s cold reservoir, which will bring it back to its equilibrium point.

The team found that the macroscopic pointer is more stable to noise than a microscopic qubit pointer. As long as the temperature of the cold bath is low enough, there is a clear spatial separation of the pointer states that persists even in the presence of noise. In other words, the demon can still tell if the pointer is pointing left or right.

This is unlike the qubit pointer. In the presence of noise, the qubit pointer might not accurately reflect the state of the qubit working medium. When this happens, errors in information feedback occur.

This meant that the engine could operate in temperature regions beyond that of engines which use a qubit pointer, giving a wider regime of operation.

Additionally, the engine has the advantage that measurements of the system can be made at random times, without having to sync to any kind of pattern of engine strokes as is the case for some models. This is known as allowing ‘incoherent measurements’.

“Everything can run continuously. As and when I need energy, I can send a probe to extract energy or set up a laser to continuously extract energy from the system,” says Stella.

The devil is in the details

The original incarnation of Maxwell’s demon, in a thought experiment first proposed by physicist James Clerk Maxwell in 1867, challenged the laws of thermodynamics.

Maxwell imagined a demon that created a temperature gradient in a system at thermal equilibrium. The demon monitored the motion of atoms or molecules in a container of gas with two chambers connected by a door, opening the door to allow hotter particles to collect on one side and colder particles on the other. This was a problem for the second law of thermodynamics which states that the entropy, or disorder, of a system always increases. The temperature gradient could then have been used to do work.

Researchers since have explored energy and entropy flows in various models of the demon. Taking a global view typically shows how the demon still respects thermodynamics – for example, after considering the energy costs to the demon of storing and erasing information.

In the case of this quantum engine, the macroscopic pointer uses energy as it continuously measures and reflects the changing qubit state and gets reset. These measurement and erasure costs contribute to the overall increase in entropy of the system.