Inspired by Maxwell’s demon, heat flow acts as a witness to quantum properties

In a new study published in Physical Review Letters, scientists have discovered a novel approach to detecting the quantum properties of a system by simply using heat as a witness, requiring no direct measurement of the quantum system itself.

The study proposes connecting thermodynamics with quantum information theory, drawing inspiration from the concept of Maxwell’s demon.

The concept proposed in the 19th century involves an imaginary entity, Maxwell’s demon, that can sort gas molecules by their velocities inside a closed box, seemingly violating the second law of thermodynamics.

Phys.org spoke to the authors of the study, Dr. Alexssandre de Oliveira Junior and Prof. Jonatan Bohr Brask from the Technical University of Denmark, and Prof. Patryk Lipka-Bartosik from Jagiellonian University in Poland.

“Inspired by Maxwell’s demon, a famous thought experiment in physics, we explored what happens when classical memory is replaced with a quantum one,” explained Dr. Oliveira Junior.

“This led us to uncover a fundamental connection between heat flow and uniquely quantum properties. We found that quantum systems reveal their special traits simply by exchanging heat with their surroundings.”

Measuring quantum properties

Traditional methods for observing the quantum properties of a system, like entanglement and coherence, rely on specific measurements that collapse the quantum state of the system.

This method destroys the quantum information stored in the system. Traditional methods also introduce noise into the system since they require external input to make the measurements.

The new approach offers an alternative.

Prof. Brask explains, “What makes our approach exciting is that it doesn’t rely on any specific model or system. “Instead, we can explore fundamental constraints on heat exchange in a quantum process simply by measuring a thermal ancilla, or heat bath, acting as the environment.”

The fundamental idea is that coherence influences energy transfer in ways that classical systems cannot replicate, allowing for indirect detection.

Therefore, the quantum properties of the system can be detected by observing the heat signatures when it interacts with the environment or heat bath.

How it works

The researchers developed a theoretical framework where a main quantum system interacts with a thermal environment.

A quantum memory acts as a catalyst for this interaction without exchanging energy. Instead, it affects the dynamics of how heat flows between the system and environment by holding quantum information.

The researchers explained the process using an intuitive example.

Alice and Bob want to determine if their shared quantum state is entangled. They use two extra systems—one acting as a heat bath and another as a quantum memory, with no restrictions on their size.

First, they measure the thermal ancilla to check its initial energy. Then, they apply a unitary operation that connects all three systems and lets them evolve.

The quantum memory plays a crucial role here. Because it’s quantum in nature, it can form quantum connections with the system, which a classical memory can’t.

These quantum connections allow heat to flow between the system and bath in unique ways that depend on whether the original system has quantum properties like entanglement. In other words, it opens pathways for heat flow that only exist when quantum properties are present.

After the interaction, they measure the thermal ancilla again to see how much its energy has changed. By comparing this energy difference to our theoretical bounds, they can check if the result falls outside the expected range. If it does, that’s a clear sign that their state is entangled.

This range was derived using a mathematical framework developed by the researchers based on optimal heat exchange.

Applications and the broader impact

The researchers demonstrated their heat-based witness approach with two examples—entanglement detection and coherence certification.

They also highlight its implementation in current experimental platforms.

Prof. Lipka-Bartosik said, “Our approach can be implemented in state-of-the-art experimental setups. For instance, nuclear magnetic resonance (NMR) and cavity-QED with superconducting qubits have been used to realize energy-preserving unitary processes, a fundamental building block of our framework. Additionally, both techniques can implement a quantum memory.”

The team also suggests that other platforms, such as single-electron devices and trapped ions, could verify their results, having previously been used to test related ideas.

By establishing a relationship between thermodynamics and quantum information theory, the research opens new avenues for studying quantum properties.

Speaking of future directions, the researchers highlight generalizing the approach to certify and quantify multipartite quantum correlations.

“It is well known that measuring such correlations typically comes with an exponential cost. This naturally raises the question of whether it is possible to construct witnesses that enable a practical measure of multipartite quantum correlations based on differences in heat exchange,” explained Dr. Oliveira Junior.

With advances in quantum technologies, this approach could play a crucial role in developing practical quantum applications.

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