Researchers Describe Extended Quantum Maxwell’s Demon

A research team from the Moscow Institute of Physics and Technology, Argonne National Laboratory, U.S., and ETH Zurich has elucidated an extended quantum Maxwell’s demon—a device that locally violates the second law of thermodynamics in a system situated 1–5 m away from the demon.

The device can possibly be used in quantum computers and tiny refrigerators that cool down minute objects with excellent precision. The study has been published in Physical Review B on December 4^th, 2018.

According to the second law of thermodynamics, the entropy—that is, the degree of randomness or disorder—of an isolated system never reduces.

Our demon causes a device called a qubit to transition into a more orderly state. Importantly, the demon does not alter the qubit’s energy and acts over a distance that is huge for quantum mechanics.

Andrey Lebedev, Study Lead Author, MIPT and ETH Zurich

So far, all quantum Maxwell’s demons created or described by the authors or by other investigators had an extremely limited range of action: They sat right beside the object they worked on.

Since the demon has to be prepared, or “initialized,” before every interaction with the qubit, some amount of energy is inexorably spent at the demon’s location. This implies that the second law still persists at the global level.

Demonic “purity”

According to the study, the qubit can be implemented as a superconducting artificial atom, a tiny device similar to the one proposed earlier by the scientists for use as a quantum magnetometer. That kind of qubit would be composed of thin aluminum films deposited on a silicon chip. This system is known as an artificial atom because at near absolute zero temperatures, it behaves similarly to an atom with two basis states—the excited state and the ground state.

A qubit can have “impure,” “pure,” or mixed states. In case a qubit exists in one of the two basis states and it is not known for certain in which, then its state is known as “impure.” In such cases, a traditional probability for finding the synthetic atom in one of the two states can possibly be measured. Conversely, similar to a real atom, the qubit might be in a quantum superposition of the excited and ground states. In the field of quantum physics, this denotes to a unique state that can be decreased to neither of the basis states. Moreover, this so-called pure state, which challenges the traditional concept of probability, is related to more order, and thus less entropy. It degenerates back into an impure state after existing only for a fraction of a second.

The demon, which has been elucidated in the paper, is another qubit linked to the first one through a coaxial cable that carries microwave signals. An outcome of the Heisenberg uncertainty principle is that as soon as the qubits are joined by a transmission line, they begin to exchange virtual photons, which are portions of microwave radiation. It is this photon exchange that allows the qubits to swap their states.

If the demon is artificially induced with a pure state, this state will subsequently swap states with the target qubit and endow it with “purity” in exchange for an impure state of the same amount of energy. Purifying the target qubit will reduce its entropy but will not affect its energy. This causes the demon to channel the entropy away from a system separated in terms of energy—that is, the target qubit. As a result, the second law is obviously violated if the target qubit is regarded locally.

Quantum nanorefrigerator

From a practical viewpoint, it is important to purify a target qubit over a macroscopic distance. It is possible to switch the pure state into the excited or the ground state in a relatively simple and predictable manner with the help of an electromagnetic field; however, this is not possible in the impure state. Such an operation may prove handy in a quantum computer, in which the qubits have to be switched into the ground state upon launch. It is important to do this from a distance because if a demon is present near the quantum computer it would impact the latter in extreme ways.

Another potential use of the demon has to do with the following: when the target qubit is switched into the pure state and then into the ground state, its immediate environment would become slightly colder. This converts the recommended system into a nanosized refrigerator that has the ability to cool parts of molecules with excellent precision.

A conventional refrigerator cools its entire volume, while the qubit ‘nanofridge’ would target a particular spot. This might well be more effective in some case. For example, you could implement what’s known as algorithmic cooling. This would involve supplying the code of a primary, ‘quantum’ program with a subprogram designed to target-cool specifically the hottest qubits. A further twist is that with any ‘heat machine,’ you can run it in reverse, turning a heat engine into a refrigerator or vice versa. This lands us with a highly selective heater as well. To turn it on, we would switch the target qubit into the excited rather than the ground state, making the qubit’s whereabouts hotter.

Gordey Lesovik, Study Co-Author, Head, Laboratory of Quantum Information Theory, MIPT

This heating or cooling cycle can be run constantly because the target qubit is able to retain its pure state for a short time, and then enters the impure state, emitting or consuming the environment’s thermal energy. With each iteration, the qubit’s location becomes increasingly cooler or warmer.

In addition to the demon’s range, the authors were also able to estimate the highest temperature of the coaxial cable that runs between the qubits. Over this temperature, the system’s quantum properties are lost and the demon does not work anymore. Even though the temperature of the cable may not go beyond a few degrees above the absolute zero temperature, this is still around 100 times hotter than the qubits’ working temperature. This makes it much easier to apply the recommended setup experimentally. The researchers are already working to implement the experiment.

The study authors are Andrey Lebedev, and Gordey Lesovik from MIPT; Gianni Blatter from ETH Zurich; and Valerii Vinokur from Argonne National Laboratory. This research was financed by the Swiss National Science Foundation, the Russian Foundation for Basic Research, the Foundation for the Advancement of Theoretical Physics BASIS, the Ministry of Education and Science of the Russian Federation, and the Government of the Russian Federation. The work of Valerii Vinokur was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.