Cited by 351Quantum Information and CryptographyOrbital Angular Momentum in OpticsQuantum Mechanics and Applications

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Photonic polarization gears for ultra-sensitive angular measurements

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Abstract

Quantum metrology bears a great promise in enhancing measurement precision, but is unlikely to become practical in the near future. Its concepts can nevertheless inspire classical or hybrid methods of immediate value. Here we demonstrate NOON-like photonic states of m quanta of angular momentum up to m=100, in a setup that acts as a ‘photonic gear’, converting, for each photon, a mechanical rotation of an angle θ into an amplified rotation of the optical polarization by mθ, corresponding to a ‘super-resolving’ Malus’ law. We show that this effect leads to single-photon angular measurements with the same precision of polarization-only quantum strategies with m photons, but robust to photon losses. Moreover, we combine the gear effect with the quantum enhancement due to entanglement, thus exploiting the advantages of both approaches. The high ‘gear ratio’ m boosts the current state of the art of optical non-contact angular measurements by almost two orders of magnitude. Beating the standard measurement limits is a goal of metrology, as it would allow for more precise estimation of physical quantities. Borrowing concepts from NOON-state quantum metrology, this work presents a single-photon scheme to measure rotation angles of light with super-resolution precision.

Overview

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TL;DR

The paper demonstrates a photonic gear using q-plates that converts a mechanical rotation θ into an amplified polarization rotation mθ, enabling super-resolving Malus-law angular measurements with SAM–OAM states up to m≈100. It reports single-photon precision matching polarization-only quantum strategies with m photons while being loss-robust, and combines the gear with two-photon entanglement for hybrid enhancement.

Key findings

  • A q-plate generates SAM–OAM superpositions with m=2q±1 quanta of total angular momentum; q-plates with q up to 50 produced m as large as 101 (abstract states up to m=100), so a relative rotation θ becomes a polarization rotation mθ.
  • In the single-photon regime, the maximum enhancement over the polarization-only strategy was ≈55; matching this precision would require 55²=3,025 more trials or polarization-only NOON states with N≈55 photons. The precision was attained for total photon numbers νN≈10² to 10⁴, and total losses with transmissivity η simply rescale the photon number to νNη, unlike NOON states where one lost photon destroys phase information.
  • Two-photon entangled states with total angular momentum up to m₁+m₂=18 showed HH correlations with hybrid enhancement factor mN=mA+mB=2qA+2qB+2; a quantum enhancement factor of 2 was superimposed on the classical photonic gear effect.
  • The gear effect also appeared with intense classical laser light; combining it with existing polarization-based roll-angle methods is predicted to improve sensitivity to 10⁻⁶ degrees (≈0.01 arcsec), from typical values of about 10⁻² degrees for 30–360° dynamic range or 10⁻⁴ degrees for ~1° range.

Methods

  • q-plates were fabricated from nematic liquid crystals using photoalignment, with optical-axis orientation θ=qφ+α; q-plates map circular polarization to OAM changes of ±2q, and a half-wave plate yields m=2q±1 states.
  • Single photons came from a Ti:sapphire mode-locked laser (120 fs pulses, 76 MHz, 795 nm), frequency-doubled to 397.5 nm at 700 mW, then type-II spontaneous parametric down-conversion in a 1.5 mm beta-barium-borate crystal, followed by filtering and single-mode fiber coupling.
  • Experiments tested classical intense laser light, single-photon states, and two-photon entangled states; measurements used polarization fringes/super-resolving Malus law and HH correlations, with statistical errors compared to shot-noise and Heisenberg limits.

Limitations

  • Stated: experimental imperfections reduce fringe visibility; all curves showed visibility greater than 0.73, and loss of visibility increases the statistical error. The hybrid entangled strategy bears the same loss-sensitivity problems as polarization-only quantum strategies for large N, though these can be dealt with for small N.
  • Stated/evident: the entangled demonstration is a proof-of-principle with N=2 and total angular momentum up to 18, not large N. High-q q-plate fabrication required finer rotation steps (up to 0.1° for q≥25) and careful control of the central defect, and the observed maximum enhancement ≈55 is below the full gear ratio m because of visibility and detection efficiency.

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Crossref · 10.1038/ncomms3432

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Figures

Figure 1 Photonic gear concept. (a) A sender Alice prepares and sends to a receiver Bob photonic probes to measure the relative angle θ between their reference frames. (b–d) Equivalent interferometric scheme. The action of the physical rotation can be schematically represented as an interferometer, where the two arms correspond to the right- and left-circular components of the photon. (b) Polarization-only states are used. The physical rotation introduces a relative phase between the right- and left-circular components of the photon, corresponding to a rotation of the final photon polarization by the same angle θ. The measurement is repeated ν times, and polarization fringes are recorded (with π=H,V), from which the angle θ is retrieved with a statistical error Δθ (represented as a blurred arrow pointing to a goniometer). (c–d) Hybrid SAM−OAM photon states are generated by exploiting q-plates [(c) q=1, (d) q=10] and are used to estimate the angle θ. The physical rotation introduces a relative phase between the two components which varies m=2q+1 times faster than the polarization-only case, so that the output photon polarization rotates m times faster (photonic gear effect). The recorded polarization fringes after decoding with a second q-plate now present a periodicity ∝ 1/m, leading to an improved angular sensitivity Δθ/m. The intensity (squared blue contour plots) and phase patterns (squared contour plots in false colours) of the linear and circular polarization components of the employed SAM-OAM states are also shown. (e) Experimental setup. In the single-photon regime, Alice uses photons generated by a parametric down-conversion heralded source. In the classical regime, Alice uses coherent laser pulses. The quantum regime, in turn, uses entangled photons and is described in Fig. 4. Bob’s detection apparatus is mounted in a compact and robust stage which can be freely rotated around the light propagation axis28. QWP, quarter-wave plate; PBS, polarizing beam-splitter; APD, fibre-coupled single-photon detector.
Figure 1 Photonic gear concept. (a) A sender Alice prepares and sends to a receiver Bob photonic probes to measure the relative angle θ between their reference frames. (b–d) Equivalent interferometric scheme. The action of the physical rotation can be schematically represented as an interferometer, where the two arms correspond to the right- and left-circular components of the photon. (b) Polarization-only states are used. The physical rotation introduces a relative phase between the right- and left-circular components of the photon, corresponding to a rotation of the final photon polarization by the same angle θ. The measurement is repeated ν times, and polarization fringes are recorded (with π=H,V), from which the angle θ is retrieved with a statistical error Δθ (represented as a blurred arrow pointing to a goniometer). (c–d) Hybrid SAM−OAM photon states are generated by exploiting q-plates [(c) q=1, (d) q=10] and are used to estimate the angle θ. The physical rotation introduces a relative phase between the two components which varies m=2q+1 times faster than the polarization-only case, so that the output photon polarization rotates m times faster (photonic gear effect). The recorded polarization fringes after decoding with a second q-plate now present a periodicity ∝ 1/m, leading to an improved angular sensitivity Δθ/m. The intensity (squared blue contour plots) and phase patterns (squared contour plots in false colours) of the linear and circular polarization components of the employed SAM-OAM states are also shown. (e) Experimental setup. In the single-photon regime, Alice uses photons generated by a parametric down-conversion heralded source. In the classical regime, Alice uses coherent laser pulses. The quantum regime, in turn, uses entangled photons and is described in Fig. 4. Bob’s detection apparatus is mounted in a compact and robust stage which can be freely rotated around the light propagation axis28. QWP, quarter-wave plate; PBS, polarizing beam-splitter; APD, fibre-coupled single-photon detector.CC BY-NC-ND · PubMed Central
Figure 2 Single-photon experimental fringes. Experimental results for single photons and q-plate charges (a) q=1/2 (l=1), (b) q=3 (l=6), (c) q=5 (l=10), (d) q=25 (l=50) and (e) q=50 (l=100). For each case (a−e), we report: the q-plate axis pattern, corresponding to the distribution of the liquid crystal molecular director (top row, left panels); the calculated intensity and phase profiles of the generated OAM fields (top row, middle and right panels); the theoretical and experimental intensities after projection on the H-polarization state (middle panels); the measured fringe patterns (blue dots) as a function of the mechanical rotation angle θ, accompanied by sinusoidal best-fit curves (blue lines) and by the polarization-only case (red dots and lines) (bottom panels). The fringe patterns reported for cases (a,d,e) correspond to m=2q+1 [(a) m=2, (d) m=51 and (e) m=101]. Fringe patterns with m=2q–1 (without the HWPs, left plots) and m=2q+1 (with the HWPs, right plots) are shown in (b) (m=5 and 7) and (c) (m=9 and 11). In (d,e), an inset with a zoomed-in region of the fringes is also shown. Error bars in the polarization fringes are due to the Poissonian statistics of the recorded events, whereas error bars in the set value of the angle θ are due to the mechanical resolution of the rotation stage.
Figure 2 Single-photon experimental fringes. Experimental results for single photons and q-plate charges (a) q=1/2 (l=1), (b) q=3 (l=6), (c) q=5 (l=10), (d) q=25 (l=50) and (e) q=50 (l=100). For each case (a−e), we report: the q-plate axis pattern, corresponding to the distribution of the liquid crystal molecular director (top row, left panels); the calculated intensity and phase profiles of the generated OAM fields (top row, middle and right panels); the theoretical and experimental intensities after projection on the H-polarization state (middle panels); the measured fringe patterns (blue dots) as a function of the mechanical rotation angle θ, accompanied by sinusoidal best-fit curves (blue lines) and by the polarization-only case (red dots and lines) (bottom panels). The fringe patterns reported for cases (a,d,e) correspond to m=2q+1 [(a) m=2, (d) m=51 and (e) m=101]. Fringe patterns with m=2q–1 (without the HWPs, left plots) and m=2q+1 (with the HWPs, right plots) are shown in (b) (m=5 and 7) and (c) (m=9 and 11). In (d,e), an inset with a zoomed-in region of the fringes is also shown. Error bars in the polarization fringes are due to the Poissonian statistics of the recorded events, whereas error bars in the set value of the angle θ are due to the mechanical resolution of the rotation stage.CC BY-NC-ND · PubMed Central
Figure 3 Estimation of a rotation angle with photonic gears in the single-photon regime. Ratio between the statistical errors Δθ0/Δθm for the polarization-only strategy versus the gears strategy in the single-photon regime for q<10 (a) and for all the implemented values of q (b). In (a) for each point the pattern of the corresponding q-plate is also shown. We obtain a maximum precision enhancement of ≈55 for q=50, corresponding to the generation of optical states with an OAM component with l=100. Red dashed line: theoretical prediction for the ideal case. Blue solid line: model taking into account experimental imperfections (see equation (9) and Supplementary Note 1 for more details). Black horizontal dashed line: experimental maximum enhancement. (c,d) Convergence of the angle estimation procedure as a function of the number of repeated experiments ν for m=7 (q=3). (c) Measured angle versus the number of incident single photons ν (the red area corresponds to the true angle set in the apparatus, up to mechanical resolution). Error bars correspond to the statistical error in the estimation process. (d) Ratio showing the convergence to the Cramér–Rao bound (see Supplementary Note 1).
Figure 3 Estimation of a rotation angle with photonic gears in the single-photon regime. Ratio between the statistical errors Δθ0/Δθm for the polarization-only strategy versus the gears strategy in the single-photon regime for q<10 (a) and for all the implemented values of q (b). In (a) for each point the pattern of the corresponding q-plate is also shown. We obtain a maximum precision enhancement of ≈55 for q=50, corresponding to the generation of optical states with an OAM component with l=100. Red dashed line: theoretical prediction for the ideal case. Blue solid line: model taking into account experimental imperfections (see equation (9) and Supplementary Note 1 for more details). Black horizontal dashed line: experimental maximum enhancement. (c,d) Convergence of the angle estimation procedure as a function of the number of repeated experiments ν for m=7 (q=3). (c) Measured angle versus the number of incident single photons ν (the red area corresponds to the true angle set in the apparatus, up to mechanical resolution). Error bars correspond to the statistical error in the estimation process. (d) Ratio showing the convergence to the Cramér–Rao bound (see Supplementary Note 1).CC BY-NC-ND · PubMed Central
Figure 4 Entangled photonic gears. (a) Experimental setup. An entangled photon pair in the polarization state |ψ−› is generated by type-II spontaneous parametric down conversion. The state can be converted to the |−› state by inserting a HWP in the path of Bob’s photon. Each photon is then converted into SAM−OAM hybrid states by the q-plates qA and qB and a HWP, as before, and is sent to a different rotation stage for the analysis. (b) Normalized experimental correlations (blue points), with ππ=HH, obtained with the |ψ−› state by measuring the two-fold coincidences in the H-polarization bases on both modes for different values of the rotation angles θA and θB. We observe the gear enhancement with respect to the polarization-only case (red surface, theory) in the oscillation frequencies in both directions θA (with mA=2qA−1=2) and θB (with mB=2qB+1=11). (c) Normalized experimental correlations again with |ψ−› (blue points) but for mA=2qA+1=7 and mB=2qB+1=11. (d) Normalized experimental correlations obtained with the |ψ−› and |−› states when rotating the two stages by the same angle θA=θB=θ, for mA=2qA+1=7 and mB=2qB+1=11. The polarization correlations (blue points: data for |ψ−›, red points: data for |−›) now present an oscillation pattern with a periodicity enhancement of (mA+mB) for |−› and mA–mB for |ψ−›, due to quantum entanglement combined with the gear effect. The theoretical polarization-only HH correlation (without the gear enhancement) is also shown, for reference, as a red solid curve in the |−› state case, oscillating as 2θ, and as a blue solid curve in the |ψ−› state case, which is constant and vanishing. Yellow points: experimental data for single-photon gear with m=(mA+mB)/2=9, oscillating at half the frequency of . Dashed curves: best fit of the experimental data. The visibility of the pattern for |−› state is . In all plots, error bars in the correlations are due to the Poissonian statistics of the recorded events, whereas error bars in the set value of the angle θ are due to the mechanical resolution of the rotation stage.
Figure 4 Entangled photonic gears. (a) Experimental setup. An entangled photon pair in the polarization state |ψ−› is generated by type-II spontaneous parametric down conversion. The state can be converted to the |−› state by inserting a HWP in the path of Bob’s photon. Each photon is then converted into SAM−OAM hybrid states by the q-plates qA and qB and a HWP, as before, and is sent to a different rotation stage for the analysis. (b) Normalized experimental correlations (blue points), with ππ=HH, obtained with the |ψ−› state by measuring the two-fold coincidences in the H-polarization bases on both modes for different values of the rotation angles θA and θB. We observe the gear enhancement with respect to the polarization-only case (red surface, theory) in the oscillation frequencies in both directions θA (with mA=2qA−1=2) and θB (with mB=2qB+1=11). (c) Normalized experimental correlations again with |ψ−› (blue points) but for mA=2qA+1=7 and mB=2qB+1=11. (d) Normalized experimental correlations obtained with the |ψ−› and |−› states when rotating the two stages by the same angle θA=θB=θ, for mA=2qA+1=7 and mB=2qB+1=11. The polarization correlations (blue points: data for |ψ−›, red points: data for |−›) now present an oscillation pattern with a periodicity enhancement of (mA+mB) for |−› and mA–mB for |ψ−›, due to quantum entanglement combined with the gear effect. The theoretical polarization-only HH correlation (without the gear enhancement) is also shown, for reference, as a red solid curve in the |−› state case, oscillating as 2θ, and as a blue solid curve in the |ψ−› state case, which is constant and vanishing. Yellow points: experimental data for single-photon gear with m=(mA+mB)/2=9, oscillating at half the frequency of . Dashed curves: best fit of the experimental data. The visibility of the pattern for |−› state is . In all plots, error bars in the correlations are due to the Poissonian statistics of the recorded events, whereas error bars in the set value of the angle θ are due to the mechanical resolution of the rotation stage.CC BY-NC-ND · PubMed Central

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    The photons are generated in the maximally entangled polarization Bell state.

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Metadata

Authors
D'Ambrosio, V., Spagnolo, N., Del Re, L., Slussarenko, S., Li, Y., Kwek, L. C., Marrucci, L., Walborn, S. P., Aolita, L., & Sciarrino, F.
Affiliations
  • Sapienza University of Rome (IT)
  • University of Naples Federico II (IT)
  • Centre for Quantum Technologies (SG)
  • Universidade Federal do Rio de Janeiro (BR)
  • Institute of Photonic Sciences (ES)
  • National Institute of Optics (IT)
Corresponding author
Sciarrino, Fabio
Venue
Nature Communications
Published
2013-09-18
Volume
4
Issue
1
Pages
2432-2432
ISSN
2041-1723 (ISSN-L)
OpenAlex
W2104690772
Cited by
351
References
44
Citations by year
2026: 28 · 2025: 31 · 2024: 26 · 2023: 33 · 2022: 24 · 2021: 37 · 2020: 30 · 2019: 29 · 2018: 17 · 2017: 27 · 2016: 21 · 2015: 28 · 2014: 18 · 2013: 2
FWCI
11.14
Type
journal article
Language
en
Open access
Locations
  • Nature Communications (publishedVersion, cc-by-nc-nd)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by-nc-nd)
  • PubMed Central (publishedVersion, cc-by-nc-nd)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • National University of Singapore (submittedVersion)
  • RePEc: Research Papers in Economics (submittedVersion)
  • IRIS Research product catalog (Sapienza University of Rome) (submittedVersion)
  • Refubium (Universitätsbibliothek der Freien Universität Berlin) (submittedVersion, cc-by-nc-nd)
  • Griffith Research Online (submittedVersion, cc-by-nc-nd)
  • arXiv (Cornell University)
  • Universitätsbibliothek der FU Berlin Hochschulschriftenstelle u. Dokumentenserver
Funders
  • European Commission (255914, 299141)
  • National Research Foundation
  • Conselho Nacional de Desenvolvimento Científico e Tecnológico
  • Sapienza Università di Roma
  • Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro
Topics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Information and Cryptography
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Orbital Angular Momentum in Optics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
Keywords
angular momentum of light, NOON states, super-resolution, Malus's law, quantum metrology, entanglement, photon loss robustness

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