Experimental demonstration of a quantum engine driven by entanglement and local measurements

Letter
Open Access

Experimental demonstration of a quantum engine driven by entanglement and local measurements

Kunkun Wang, Ruqiao Xia, Léa Bresque, and Peng Xue

Phys. Rev. Research 4, L032042 – Published 22 September 2022

Abstract

Understanding entanglement and quantum measurements from a thermodynamics point of view is a fundamental and challenging task. Recently, a two-qubit engine was put forward as an appropriate platform to tackle these challenges. Here we achieve an experimental simulation and provide the direct experimental proof of these findings using single photons and linear optics. Encoding the qubits by polarization and transverse spatial modes of single photons, entanglement is created through the interaction between them. We show that, upon local measurement, classical mutual information can be extracted in order to fuel a quantum measurement engine. By measuring the energy changes, we identify that the energy gain comes from the measurement channel and corresponds to the cost of erasing the quantum correlations between qubits. The scheme is further generalized to an $N$ -qubit chain for energy upconversion. Our experimental results provide a thorough understanding of this quantum engine with entanglement and local measurements as a new kind of fuel, as well as a general platform for exploration of quantum engines.

Received 30 April 2022
Accepted 12 September 2022

DOI:https://doi.org/10.1103/PhysRevResearch.4.L032042

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Optical tests of quantum theory Quantum entanglement Quantum information architectures & platforms Quantum measurements Quantum simulation

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Kunkun Wang ^1,2, Ruqiao Xia^1,3, Léa Bresque ⁴, and Peng Xue ^1,*

¹Beijing Computational Science Research Center, Beijing 100084, China
²School of Physics and Optoelectronic Engineering, Anhui University, Hefei 230601, China
³Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
⁴Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France

^*gnep.eux@gmail.com

Article Text

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Issue

Vol. 4, Iss. 3 — September - November 2022

Subject Areas

Quantum Physics

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Images

Figure 1
(a) Illustration of a two-qubit engine cycle: (i) entangling evolution, (ii) measurement, (iii) feedback, and (iv) erasure. (b) Experimental setup. A photon pair is generated by the spontaneous parametric downconversion in the periodically polled potassium titanyl phosphate crystal (PPKTP), with one serving as the trigger and the other injected into the interferometric network as a heralded single photon. The initial state of the two qubits is prepared in $| R H 〉$ . Subsequently, a unitary operation $U (t)$ is realized by an interferometric network consisting of beam displacers (BDs), half-wave plates (HWPs), and quarter-wave plates (QWPs). To realize the local measurement on $B$ , another BD is applied to extend the spatial modes, which are used to encode the meter qubit. The feedback is realized by two BDs and the HWPs placed in different spatial modes due to the outcomes of the measurement on $B$ . Projective measurements and state tomography are used to measure the energy and entropy of the engine, respectively. Photons are detected by coincidence using silicon avalanche photodiodes (APDs).
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Figure 2
(a) Evolution of the local energy $E_{L}$ (red solid line), binding energy $E_{C}$ (blue solid line), and total energy $E_{T}$ (gray dashed line) as a function of $τ$ with fixed $ω_{A} = 1$ and $δ = g = 0.8$ . (b) The mean energy $E_{meas}$ and (c) entropy $S_{meas}$ input by the measurement process and their ratio $E_{meas} / S_{meas}$ as a function of the detuning $δ$ with various coupling strength $g$ and fixed $ω_{A} = 1$ . The black dashed lines indicate the limit $g ≫ δ$ . (e) $E_{meas}$ , (f) $S_{meas}$ , and (g) $E_{meas} / S_{meas}$ as a function of $g$ with various $δ$ and fixed $ω_{A} = 1$ . The black dot-dashed lines indicate the limit $δ ≫ g$ . Experimental data are represented by color symbols. Error bars are due to the statistical uncertainty in photon-number counting.
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Figure 3
(a) Theoretical predictions of the work extraction ratio $η$ (color scale) as a function of the detuning $δ$ and the classical mutual information $I (A B : M)$ . The parameters $ω_{A} = 1$ and $g = 0.8$ are fixed. The gray area corresponds to $η \leq 0$ , where energy cannot be extracted during feedback. Experimental results of (b) $η$ and (c) $I (A B : M)$ as a function of $δ$ with various error probabilities $P$ . Theoretical predictions are shown in colored curves and experimental data are represented by color symbols. Error bars are due to the statistical uncertainty in photon-number counting.
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Figure 4
(a) Entanglement and local measurements based energy upconversion by extending the fueling mechanism to a chain of $N$ qubits. (b) Measured successful probability $P_{succ}$ of energy upconversion as a function of $g$ with various chain length $N$ , fixed $ω_{A} = 1$ , and $δ = 0.8$ . Theoretical predictions are shown in colored curves and experimental data are represented by color symbols. Error bars are due to the statistical uncertainty in photon-number counting.
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Figure 5
Energy upconversion. (a) Circuit to achieve the energy upconversion in an $N$ -qubit chain. (b) Upper panel: the simplified circuit of (a), which is achieved by translating measurements on different qubits into those on only two qubits at different times. Lower panel: experiment setup. By initializing the state of photons in $| L H 〉$ , the interaction is simply realized by two BDs and wave plates.
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