University of Science and Technology of China demonstrates the advantages of quantum heat engine

Du Jiangfeng and Wang Ya, Key Laboratory of micromagnetic resonance, Chinese Academy of Sciences, University of science and technology of China, demonstrated the quantum superiority caused by quantum correlation on the quantum Hillard heat engine constructed by diamond nitrogen-vacancy color center system. This research achievement is entitled "spin quantum heat engine quantified by quantum steering" and published in the recent physics review express [Phys. Rev. Lett. 128, 090602 (2022)]. The work was selected by the PRL editor as the article recommended by the editor and reported in the Journal of Physics under the title of "steering toward a quantum advantage".

Heat engine plays an important role in the development process and life of human society. How to improve the efficiency of heat engines has always been the core scientific problem of thermodynamics. With the development of single-molecule and single-atom manipulation technology by quantum technology, the intersection of thermodynamics and quantum technology is expected to build the smallest quantum heat engine on the microscale and improve the efficiency of the heat engine by using quantum characteristics. So far, people usually pay attention to the quantum coherence of the working medium mass subsystem in the quantum heat engine and think it is the most critical quantum resource to improve efficiency. However, the research shows that its role is still controversial and there is no clear conclusion. This work adopts a new idea, focuses on the role of quantum correlation in quantum heat engines, and finds that a special kind of quantum correlation called "quantum guidance"

Hillard heat engine is an ideal experiment proposed by physicist Leo Hillard in 1926. It is a heat engine that uses the information to extract work. Imagine that there is a single molecule in a box, a piston is placed in the middle of the box, and the molecules in the initial state can be randomly in the left or right half of the box. Without knowing the molecular position, if a heavy object is hung on one side of the piston, half of the molecules will do positive work by colliding with the piston and lifting the heavy object, while the other half will do negative work. Due to the randomness of the initial state, on average, molecules do no external work. Hillard proposed that if a Maxwell Demon measures the position of the molecule and selects the position of the weight according to the measured information, as long as the weight is hung on the side of the molecule every time, it can ensure that the molecule can always do positive work. In this way, the information obtained from the measurement can be used to make a system in a random initial state do work externally. This transformation from information to work inspires people to connect quantum information with thermodynamics.

Figure 1: schematic diagram of quantum Hillard heat engine. On the left is the Maxwell Demon, and on the right is the quantum Hillard heat engine, which can distinguish between classical and quantum by the amount of work done, that is, whether a quantum correlation exists or not.

In this work, researchers in the Key Laboratory of micromagnetic resonance of the Chinese Academy of Sciences cooperated with Ren Changliang of Hunan Normal University to build a quantum version of the Hillard heat engine. In the quantum version, the working medium changes from a single molecule into a quantum bit composed of a single nuclear spin, the left and right states of the molecular position become the 0 and 1 quantum states of the quantum bit, and the process of hanging weight to make the molecule do work becomes a unitary operation applied to the nuclear spin quantum state. An important difference between the quantum system and the classical system is that the measurement of the quantum state will generally change the quantum state. Therefore, the researchers added an operation that does not exist in the classical version to the quantum version: using a single electron spin quantum bit as the environmental bit to establish a quantum correlation between the environmental bit and the working medium bit when preparing the initial state, Thus, instead of directly measuring the working medium bits, the state information of the working medium bits can be obtained by measuring the environment bits. By applying the controlled unitary operation to the working medium bit according to the state of the environment bit, the working medium bit can always do positive work externally. Through such a design, people can study the role of quantum correlation in a quantum heat engine.

Based on the experiment of the diamond nitrogen-vacancy color center system, the researchers realized the above quantum Hillard heat engine. By studying the relationship between the working medium and the environment, it is found that there is a close relationship between quantum guidance and the work done by the heat engine. Quantum guidance is one of the basic concepts of quantum correlation first proposed by Erwin Schrodinger in 1935. Quantum guidance is a special kind of quantum correlation between quantum entanglement and Bell nonlocality, but different from the other two concepts, quantum guidance has natural asymmetry. The strength of this correlation is usually described quantitatively by the destruction of quantum guidance inequality, which is similar to the description of bell nonlocality by the destruction of Bell inequality. The experimental results show that the existence of quantum guidance plays an important role in the work of quantum Hillard heat engine: the Hillard heat engine with quantum guidance (quantum) has more work than the Hillard heat engine without quantum guidance (classical), and the greater the destruction of quantum guidance inequality (the stronger the quantum correlation), The greater the damage of the work extraction inequality of the quantum Hillard heat engine (used to measure the size of work beyond the classical limit), the more it reflects the quantum superiority. Conversely, we can also use the magnitude of work to distinguish between classical and quantum, that is, whether a quantum correlation exists or not.

Figure 2: experimental results of quantum Hillard heat engine work extraction. (a) According to whether the work extraction exceeds the classical limit, the parameter space of the heat engine can be divided into the quantum region and classical region. (b) Taking the region of q = 1 as an example, it can be seen that the average work extraction of the quantum Hillard heat engine exceeds the upper bound of classical work extraction. (c) There is a positive correlation between the destruction of quantum guidance inequality and the destruction of work extraction inequality, which shows that the size of quantum correlation plays an important role in work extraction in the quantum Hillard heat engine.

Taking Hillard heat engine as an example, this work shows the unique role of quantum correlation in a quantum heat engine and provides a new idea for establishing the bridge between quantum information and quantum thermodynamics.

Ji Wentao, a postdoctoral researcher in the Key Laboratory of micromagnetic resonance of the Chinese Academy of Sciences, Chai Zihua, a doctoral student, and Wang Mengqi, a special associate researcher, are the co-first authors of this work, and academician Du Jiangfeng, Professor Wang Ya and Professor Ren Changliang are the co-corresponding authors. The research was supported by the Ministry of science and technology, the Chinese Academy of Sciences, the National Natural Science Foundation of China, Anhui Province, and Hunan Province.

Link:

[1]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.090602

[2] https://physics.aps.org/articles/v15/s28