Xue Peng's group's first experimental demonstration of a quantum engine powered by measurements alone
Professor Peng Xue's group at the Beijing Research Center for Computing Sciences and his collaborators experimentally demonstrated a two-qubit quantum engine powered by entanglement and local measurements; it was extended and applied to a multi-qubit system by extending the spatial pattern of single photons. This series of experiments provides insight into entanglement and local measurements as the driving force of quantum engines.
The related results are published in the journal Physical Review Research under the title "Experimental Demonstration of Quantum Engines Driven by Entanglement and Local Measurements" [1].
01The combination of quantum information and thermodynamics
A common task of engines is to extract mechanical work from a heat bath (hot bath) in a cyclic repetitive manner. Although the goal is the same, the source of randomness can be different when the working matter of the engine is quantum. Today, the analysis of quantum engines is mainly devoted to the study of quantum effects in thermodynamics, quantum analogues of classical engines. The growing enthusiasm for quantum engines will make it possible to overcome the traditional classical efficiency limits, to better understand the thermalization of the quantum domain, and to control the nonequilibrium dynamics of microscopic systems.
To understand the possible impact of quantum thermodynamic effects, quantum measurements will play a major role. Both selective (or readout measurements that project the system under test to a selected eigenstate) and non-selective (or unreadout measurements that average one over all possible outcomes) measurements have been used as a "fuel" to drive the measurement engine. However, previous studies have used classical measurement devices, where the "fuel" is identified as the energy counterpart of the measurement theorem.
Understanding entanglement and quantum measurements from a thermodynamic perspective is a fundamental and challenging task. In this experiment, Prof. Peng Xue's team proposed a two-qubit quantum engine as a suitable platform to address these challenges: the experiment simulates a quantum measurement dynamical engine composed of two single-photon encoded quantum bits with different degrees of freedom, which become entangled through a quantum-inspired coherent exchange realized by an interference network. The team then implemented a local measurement, which was modeled as an entanglement between a quantum bit and a gauge, and then determined the "fuel" to eliminate the quantum-related energy cost.
(a) Illustration of the two-qubit (A and B) engine cycle: (i) entanglement evolution, (ii) measurement, (iii) feedback, and (iv) erasure. (b) Experimental setup. A photon pair is generated by spontaneous parametric down-conversion in a periodic potassium phosphate titanate crystal (PPKTP), one of which acts as a trigger and the other as a heralded single photon injected into the interferometric network. The initial state of the two quantum bits is |RH⟩, and subsequently, the interferometric network consisting of the beam displacer (BD), half-wave piece (HWP) and quarter-wave piece (QWP) realizes the unitary operation U(t). To achieve local measurements of the quantum bit B, another BD is used to extend the spatial modes that are used to encode the instrumental quantum bits. Measurements of B are realized by two BDs and HWPs placed in different spatial modes, projection measurements and state tomography are used to measure the energy and entropy of the engine, respectively; photons are detected by a silicon avalanche photodiode (APD).
Finally, feedback is applied through the information extracted from the measurements, and the results show that the power efficiency increases with the information extracted from the measurements; classical mutual information can be extracted to drive the quantum measurement engine during local measurements. By measuring the energy change, the team found that the energy gain comes from the measurement channel and corresponds to the cost of erasing the quantum correlation between quantum bits.
(a) Evolution of the local energy EL (red solid line), binding energy EC (blue solid line) and total energy ET (gray dashed line) as a function of τ for fixed ωA=1 (ωA is the transition frequency of the quantum bit A) and δ=g=0.8. (b) Average energy Emeas and (c) entropy Smeas of the input during the measurement and its ratio Emeas/Smeas as a function of detuning δ for different coupling strengths g and fixed ωA = 1. (e) Emeas, (f) Smeas, and (g) Emeas/Smeas as a function of g for different δ and fixed ωA = 1. The black dotted dashed line indicates the limit δg. The experimental data are indicated by colored symbols and the error bars are due to the statistical uncertainty of the photon counts.
(a) Theoretical prediction of the work extraction rate η (color scale) as a function of detuning δ and classical mutual information I (AB:M). The parameters ωA = 1 and g = 0.8 are fixed. The gray area corresponds to η ≤ 0, where no energy can be extracted during the feedback process. Experimental results for (b) η and (c) I(AB : M) as a function of δ with different error probabilities P. The theoretical predictions are represented by colored curves and the experimental data are represented by colored symbols. The error bars are due to the statistical uncertainty in photon counting.
All these processes, including the evolution of the interaction between two quantum bits, local measurements and feedback, are implemented with single photons and linear optics.
02Further extension: N -quantum bit engine system
Subsequently, the scheme was further extended to an N-quantum bit engine system: where the low energy of the first quantum bit A can be upconverted to an arbitrarily high energy of the last quantum bit B.
(a) Energy upconversion based on entanglement and local measurements by extending the fuel mechanism to a chain of N quantum bits. (b) Measurement success probability Psucc for energy upconversion as a function of g for different chain lengths N, fixed ωA = 1 and δ = 0.8.
(a) Circuit for energy upconversion in a chain of N -quantum bits. (b) Above: simplified circuit of (a), achieved by converting measurements on different quantum bits to measurements on only two quantum bits at different times. Lower panel: experimental setup. By initializing the states of the photons in |LH⟩, the interactions are achieved by only two BDs and wave slices.
By extending the energy upconversion to the N -quantum bit chain, the experimental results provide a thorough understanding of this quantum engine with entanglement and local measurements as new fuel, as well as a general platform for exploring quantum engines.
Thus, this experimental work provides a versatile platform for systematic experimental studies of quantum engines, and will stimulate further studies of entangled quantum engines on a variety of physical platforms. This research elucidates fuel mechanisms that will help future engines to start using quantum measurements as fuel.
Regarding Professor Xue Peng [2].
Peng Xue, female, is a professor and doctoral supervisor at Beijing Research Center for Computational Sciences. She graduated from the Department of Physics, University of Science and Technology of China, and entered the Key Laboratory of Quantum Information, Chinese Academy of Sciences, for her PhD without examination, under the supervision of academician Guangcan Guo. He then joined the Department of Physics, University of Innsbruck, Austria, the Institute of Quantum Optics and Quantum Information, Austrian Academy of Sciences, and the Department of Physics, University of Calgary, Canada, as a postdoctoral fellow to work on the physical implementation of quantum information and quantum optics, and to work on the implementation of a pervasive quantum information processing platform using quantum walks. He was transferred to the Research Center for Computational Science in Beijing in 2018.
[1]https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.4.L032042
[2]http://www.csrc.ac.cn/groups/xuepeng/members.html
