Field Demonstration of Distributed Quantum Sensing without Post-Selection

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Field Demonstration of Distributed Quantum Sensing without Post-Selection

Si-Ran Zhao, Yu-Zhe Zhang, Wen-Zhao Liu, Jian-Yu Guan, Weijun Zhang, Cheng-Long Li, Bing Bai, Ming-Han Li, Yang Liu, Lixing You, Jun Zhang, Jingyun Fan, Feihu Xu, Qiang Zhang, and Jian-Wei Pan

Phys. Rev. X 11, 031009 – Published 14 July 2021

Abstract

Distributed quantum sensing can provide quantum-enhanced sensitivity beyond the shot-noise limit (SNL) for sensing spatially distributed parameters. To date, distributed quantum sensing experiments have mostly been accomplished in laboratory environments without a real space separation for the sensors. In addition, the post-selection is normally assumed to demonstrate the sensitivity advantage over the SNL. Here, we demonstrate distributed quantum sensing with discrete variables in field and show the unconditional violation (without post-selection) of SNL up to 0.916 dB for the field distance of 240 m. The achievement is based on a loophole-free Bell test setup with entangled photon pairs at the averaged heralding efficiency of 73.88%. Moreover, to test quantum sensing in real life, we demonstrate the experiment for long distances (with 10-km fiber), together with the sensing of a completely random parameter. The results represent an important step towards a practical quantum sensing network for widespread applications.

Received 7 November 2020
Revised 30 March 2021
Accepted 18 May 2021

DOI:https://doi.org/10.1103/PhysRevX.11.031009

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)

Quantum metrology Quantum sensing

Quantum Information, Science & Technology

Authors & Affiliations

Si-Ran Zhao^1,2,3,*, Yu-Zhe Zhang ^1,2,3,*, Wen-Zhao Liu^1,2,3, Jian-Yu Guan^1,2,3, Weijun Zhang⁴, Cheng-Long Li^1,2,3, Bing Bai^1,2,3, Ming-Han Li^1,2,3, Yang Liu^1,2,3, Lixing You⁴, Jun Zhang ^1,2,3, Jingyun Fan ^1,2,3,5, Feihu Xu ^1,2,3,6, Qiang Zhang ^1,2,3,6, and Jian-Wei Pan ^1,2,3

¹Hefei National Laboratory for Physical Sciences at the Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China
²Shanghai Branch, CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai 201315, People’s Republic of China
³Shanghai Research Center for Quantum Sciences, Shanghai 201315, People’s Republic of China
⁴State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
⁵Shenzhen Institute for Quantum Science and Engineering and Department of Physics, Southern University of Science and Technology, Shenzhen, 518055, People’s Republic of China
⁶Key Laboratory of Space Active Opto-electronics Technology, Chinese Academy of Sciences, Shanghai 200083, People’s Republic of China

^*These authors contributed equally to this work.

Popular Summary

Distributed quantum sensing networks play a significant role in quantum-enhanced metrology, an active area of research because of its many possible applications ranging from atomic clocks to biological imaging. With a global distribution of sensors, one can determine global properties of the network by precisely estimating all of its parameters. Many studies suggest that by using entanglement among sensors, the sensitivity can surpass the shot-noise limit (SNL), a fundamental sensitivity threshold that cannot be surpassed with classical methods. However, this has not yet been shown in a real-world distributed quantum sensing network. We demonstrate, for the first time, a field test of distributed quantum sensing that unconditionally beats the SNL.

In our experiment, we attempt to measure the phases of two widely separated entangled photons. In the field, we build a distributed quantum sensing apparatus, which includes a source of entangled photon pairs and two sensors—one for each photon in the pair—240 m apart. Our experiment achieves a phase precision of 0.916 dB below the SNL. It also demonstrates a state-of-the-art heralding efficiency—the probability that one photon is detected once the other has been detected—of 73.88%, surpassing the theoretical 59.1% limit that we calculate is needed to violate the SNL. In addition, we simulate the actual sensing working circumstances and show that the unknown phases can be precisely measured for a sensor separation of 10 km.

Our work not only boosts the development of a quantum sensing network with more nodes and larger scale but also transitions the quantum sensing network toward practical applications.

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Vol. 11, Iss. 3 — July - September 2021

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Figure 1
Schematics of the experiment. (a) Bird’s-eye view of the experimental layout. Alice and Bob are on different sides of the entanglement source, and the linear distance between Alice (Bob) and the source is $93 \pm 1 (90 \pm 1) m$ . (b) Creation of pairs of entangled photons: By injecting a 1560-nm-wavelength laser into the periodically poled lithium niobate (PPLN) waveguide, the wavelength of generated photons is up-converted to 780 nm. The pump photons of 780 nm are injected into the periodically poled potassium titanyl phosphate (PPKTP) crystal in the Sagnac loop to generate polarization-entangled photon pairs of 1560-nm wavelength. Then, the photons are sent by fibers in opposite directions to Alice and Bob for different phase shifts and measurements. (c) Realization of different phase shifts: With the combination of QWP, HWP, and QWP, the phase shift is implemented between two optical axes. HWP can be controlled by a quantum random number generator (QRNG) (see the main text for more details). (d) Single-photon polarization measurement: Photons are projected into one of the $σ_{x}$ bases and are then detected by superconducting nanowire single-photon detectors (SNSPDs). The time-digital convertor (TDC) is applied to record the single-photon detection and random number generation events. DM: dichroic mirror; RM: reflection mirror; PC: polarization controller; WDM: Wavelength Division Multiplexer.
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Figure 2
Experimental results for a distance of 240 m. (a) Experimental interference fringes of four detection events versus different phase shifts. The horizontal axis represents $\hat{θ} = (θ_{A} - 2 θ_{B}) / 3$ , $\hat{θ} \in [0, (2 π / 3)]$ . (b) Experimental phase standard deviation (purple line) per trial versus different phase shifts. The shaded areas (blue narrow bands) correspond to the 99.7% confidence regions, which are calculated from the uncertainty of the fit parameters. Orange dashed line: theoretical limit for SNL. Red dashed line: theoretical bound for HL. The error bars of the standard deviations are discussed in the main text.
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Figure 3
Experimentally measured phase estimate of the random detection events and phase standard deviations for a distance of 10 km. (a) Experimental measurement of the random detection events, as shown by the markers. Lines represent the four probabilities from Appendix pp6. (b) Experimental phase standard deviation per trial of random detection events. Purple line: expected phase standard deviation calculated from the Fisher information from Appendix pp6. The shaded areas (blue bands) correspond to the 99.7% confidence regions, which are calculated from the uncertainty of the fit parameters. Orange dashed line: theoretical limit for SNL. Red dashed line: theoretical bound for HL. The error bars of the standard deviations are discussed in the main text.
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Figure 4
Tomography of the produced two-photon state. The real and imaginary components are demonstrated in panels (a) and (b), respectively.
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Figure 5
Experimental results with a linear function of $θ_{A} / 2 - θ_{B} / 2$ for a distance of 240 m. (a) Experimental interference fringes of four detection events versus different phase shifts. The horizontal axis represents $\hat{θ} = (θ_{A} - θ_{B}) / 2$ , $\hat{θ} \in [0, π]$ . (b) Experimental phase standard deviation (purple line) per trial versus different phase shifts. The shaded areas (blue narrow bands) correspond to the 99.7% confidence regions, which are calculated from the uncertainty of the fit parameters. Orange dashed line: theoretical limit for SNL. Red dashed line: theoretical bound for HL. The error bars of the standard deviations are discussed in the main text.
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Figure 6
Experimental interference fringes of four detection events versus different phase shifts for a distance of 10 km. The horizontal axis represents $\hat{θ} = (θ_{A} - 2 θ_{B}) / 3$ , $\hat{θ} \in [0, (2 π / 3)]$ .
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