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In Situ Coherence Measurements of Scattered Light in Magnetically Trapped Cold Atomic Clouds: Probe-Driven Atomic Dynamics

T0 review · 4 major / 5 minor · reviewed 2026-08-01 · deepseek-v4-flash

Pith's one-line read The discrepancy between in-situ coherence spectra and time-of-flight temperatures in a magnetically trapped cold cloud is fully explained by radiation pressure and heating from the probe itself, establishing coherence spectroscopy as a quan

desk verdict First coherence measurement in a purely magnetic trap, with a plausible radiation-pressure/heating model, but the quantitative case rests on difference data, so the 'fully explained' claim is not yet closed. read the letter →

arxiv 2607.18808 v1 pith:44BKD4QS submitted 2026-07-21 physics.atom-ph

classification physics.atom-ph
keywords temporalcoherencespectroscopyfirst-ordercorrelationfunctioncoldatomsmagneticquadrupoletrapradiation-pressureaccelerationrecoilheatingin-situthermometrytime-of-flight
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper demonstrates that temporal coherence spectroscopy—measuring the first-order correlation function g^(1)(τ) of light scattered by atoms—can probe atomic motion inside a purely magnetic quadrupole trap, a configuration where such measurements had not previously been reported. The measured scattered-light spectrum is broader and shifted relative to the prediction based on the independently measured time-of-flight (TOF) temperature of 85 µK. The authors show that this mismatch does not come from the magnetic field: varying the trap gradient leaves the spectrum unchanged. Instead, a simple dynamical model in which each scattered photon accelerates the atom by one recoil (shifting subsequent emitted frequencies) and heats it by T_r/3 per event reproduces the observed frequency shift and linewidth broadening for two probe durations across the full range of saturation parameters. This establishes coherence spectroscopy as a quantitative in-situ probe of velocity distributions in trapped ensembles, complementing destructive TOF imaging and opening the way to time-resolved studies of nonequilibrium dynamics.

What carries the argument

The central object is the first-order temporal coherence function g^(1)(τ) of the scattered light, measured through a heterodyne Hanbury Brown–Twiss setup in which a local oscillator derived from the probe is injected into the second port of a fiber beam splitter; the beat-note correlation g^(2)_BN(τ) contains g^(1) as a fringe at the beat frequency f_BN and g^(2) near zero frequency. In the low-saturation single-scattering regime the g^(1) spectrum is Gaussian, with HWHM Δf_T = (k_L/π)√(ln2 k_B T/m)(1 - cosθ), linking linewidth directly to temperature. The load-bearing mechanism is the probe-back-action model: each photon scattered by an atom adds one recoil velocity v_r (shifting the next

What would settle it

Measure the g^(1) spectrum with the probe switched to the red side of resonance: radiation pressure then pushes atoms opposite the probe direction, so the accumulated Doppler shift should change sign with the same magnitude predicted by Eq. (7), while the heating-induced broadening stays the same. If the sign or magnitude of the shift does not follow N_ex k_L (1 - cosθ) v_r, the acceleration mechanism is not the full explanation. Alternatively, invert the model for several probe durations and saturation values and check that the recovered starting temperature is identical every time; any syste

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Extended reading notes

Core claim

The central discovery is that the apparent disagreement between coherence spectra and TOF-based temperatures for a cloud of 87Rb atoms in a quadrupole magnetic trap originates not from the trap but from the measurement itself. While the probe is on, radiation pressure accelerates the cloud in the probe direction—each of the N_ex scattered photons adds a recoil velocity v_r, so the Nth scattered photon is Doppler-shifted by -N_ex k_L (1 - cosθ) v_r—and each scattering event also heats the atoms by T_r/3, broadening their velocity distribution. Summing the per-event Gaussian spectra over the pulse reproduces the measured shift of the g^(1) center frequency and the broadening of its width as fu

Load-bearing premise

The whole analysis rests on the assumption that the probe's only back-action is one recoil Doppler shift and a fixed T_r/3 heating increment per scattering event, and that the cloud's initial temperature is exactly the TOF value—if either rate is wrong or another broadening mechanism contributes, the agreement between model and data could be fortuitous.

Editorial extensions

If this is right

  • Coherence spectroscopy now works as a quantitative in-situ thermometer inside a purely magnetic trap, without needing destructive time-of-flight expansion.
  • Apparent discrepancies between in-situ and TOF temperatures can be fully accounted for by probe-induced acceleration and heating; the magnetic field itself does not perturb the temperature measurement.
  • The dynamical model provides a calibration rule: for a given saturation parameter and probe duration, the frequency shift and broadening are predictable, so the initial atom temperature can in principle be extracted.
  • Since the method is minimally destructive and works inside the trap, it enables time-resolved studies of thermalization and nonequilibrium dynamics in confined cold gases, complementing TOF imaging.
  • Because g^(2) is measured without a local oscillator, it is largely immune to radiation-pressure acceleration and offers a cleaner (though noisier) channel for studying heating alone.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural inversion the paper does not perform: treating the measured frequency shift itself as a readout of scattered-photon number would let the probe serve as its own calibration for scattering rate, since the shift is predicted to grow linearly with N_ex.
  • The model's heating rate T_r/3 is assumed rather than derived; a decisive test would be to invert the model across several probe durations and saturation values and check that the recovered initial temperature is constant—if not, one of the two back-action rates is mis-specified.
  • In the quantum-degenerate regime the single-scattering and dilute assumptions will break down; using g^(2), which is insensitive to radiation-pressure acceleration, could isolate heating and avoid the Doppler-ramp complication.
  • A red-detuned probe should reverse the sign of the accumulated Doppler shift while keeping broadening unchanged; that asymmetry is a signature a future experiment could check to confirm the mechanism.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper reports in-situ temporal coherence measurements of light scattered by a 87Rb cloud in a quadrupole magnetic trap. A Gaussian fit to the heterodyne g^(1) spectrum yields a width (332±5 kHz) and a central-frequency shift that deviate from the simple thermal expectation of Eq. (5) (192±11 kHz) for the independently measured TOF temperature of 85±10 μK. The authors attribute this discrepancy to radiation-pressure-induced acceleration and heating during the probe pulse. They support this by comparing, for two acquisition windows (100 μs and 300 μs), the window-difference frequency shift and linewidth broadening against a simple model that adds a Doppler shift and a T_r/3 heating per scattering event. They also report that these quantities show no dependence on the magnetic-field gradient. The paper claims that, once probe-induced dynamics are accounted for, coherence spectroscopy is a quantitative in-situ probe of velocity distributions in trapped ensembles.

Significance. If the claimed agreement is robust, the result would be valuable: it would demonstrate that light-scattering correlation spectroscopy can be applied to atoms in a conservative magnetic trap, and that probe back-action can be modeled and subtracted. The work extends prior diffusing-wave spectroscopy and cold-atom coherence studies to a new trapping geometry. The paper has concrete strengths: the use of two integration windows within the same experimental run and the variation of the magnetic-field gradient are good experimental controls, and the model is simple and parameter-free apart from the assumed heating rate and the externally supplied initial temperature. Machine-checkable code is not included, but the model is sufficiently transparent that it can be reproduced. The main limitation is that the quantitative validation is performed only on differences between windows, while the central claim concerns the absolute linewidth and the capability to extract velocity distributions in situ.

major comments (4)
  1. [Probe-Driven Atomic Dynamics, Fig. 3] The quantitative comparison in Fig. 3 uses only the differences Δf0 and Δf1 between the 100 μs and 300 μs acquisition windows. Any time-independent contribution to the measured linewidth (residual magnetic inhomogeneity, multiple scattering, an incorrect initial temperature, or an unmodeled static broadening) cancels in these differences. The central claim that probe-induced acceleration and heating 'fully explain' the measured absolute width is therefore not actually tested. The authors should plot the measured and simulated absolute spectra or absolute fit parameters (Δf1 and f0 for each window) side by side, or at least provide a table of absolute values for the same conditions as Fig. 3.
  2. [Eq. (7) and subsequent model] The model assumes a heating rate of T_r/3 per scattering event, but no derivation or reference is given. The standard result for momentum diffusion from random recoil kicks would need to be stated explicitly, and the factor 1/3 (rather than, say, 2/3 or a full recoil temperature) must be justified. Since the broadening in the model depends directly on this parameter, an unsupported choice weakens the claim of quantitative agreement. Please derive T_r/3 from the scattering process or cite the relevant treatment, and show the sensitivity of the predicted Δf1 to this parameter.
  3. [Probe-Driven Atomic Dynamics, magnetic-field-gradient test] The paper states that measurements were made for four different magnetic-field gradients and after 2 ms of time of flight, and that 'no clear dependence' was observed, but no data or plot for this null test is shown. This claim is used to exclude magnetic-trap-related broadening, which is a necessary part of the argument that the discrepancy arises from probe back-action. The authors should show these results (e.g., Δf1 and f0 versus ΔB∥) or include a representative comparison in the main text or supplement.
  4. [Perspectives and Conclusion] The paper concludes that coherence spectroscopy is a 'reliable in situ probe of velocity distributions,' but the initial temperature T=85±10 μK is taken from TOF measurements and used as an input to the model, rather than being extracted from the coherence data. The current analysis therefore demonstrates that the model can reproduce the observed trends when T is known, but not that the measurement alone can determine T. To support the claimed inversion capability, the authors should either perform an inversion test (treating T as a free parameter and showing that the fit returns the TOF value) or explicitly state this limitation in the conclusion.
minor comments (5)
  1. [Experimental setup] The text says the dominant contribution is the |F=2,m_F=2> to |F'=3,m_F'=2> transition, but the saturation intensity is computed for the stretched-state σ± transition with a Clebsch-Gordan factor 1/3. Clarify which transition is used for the saturation parameter and how the polarization selection affects the Clebsch-Gordan coefficients.
  2. [Eq. (3)] The expression for g_BN^(2)(τ) has a term proportional to |g_sc^(1)(τ)| cos(2π f_BN τ). The sign convention and the relative phase of the local oscillator are not discussed. Please state the phase convention or note that only the amplitude around f_BN is used.
  3. [Fig. 2 caption and text] The figure shows a 'clear frequency shift and a reduced HWHM' for the shorter probe duration, but the text refers to a broadening with increasing probe duration. Consider rewording the caption to be consistent: the 100 μs window has a narrower line than the 300 μs window.
  4. [Probe-Driven Atomic Dynamics] The model sums Gaussian spectra after each scattering event, but the description does not specify whether the initial velocity distribution is Maxwell-Boltzmann at T=85 μK, whether the Doppler shift is applied to the whole spectrum or only to the scattered photon, and how the random direction of spontaneous emission is treated. A short equation or algorithm outline would make the model reproducible.
  5. [General] The paper would benefit from a statement of the number of experimental repetitions and how the error bars in Fig. 3 were obtained (e.g., fit uncertainty only or shot-to-shot scatter). This is important for judging the claimed 'agreement within error bars.'

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the forward model uses independently fixed parameters and is not fitted to the coherence data it explains.

full rationale

The derivation chain is not circular. The temperature T=85±10 µK entering Eq. (5) and the simulation is an external TOF measurement, the saturation parameter s is set by the laser intensity, and N_ex is computed from the standard scattering rate (Eq. (6)); none of these is adjusted to the g^(1) data. The model spectra are generated event-by-event from Doppler shifts and the standard T_r/3 recoil-heating increment, then processed with the same Gaussian fitting used on data; the comparison in Fig. 3 is between the 100 µs and 300 µs windows, so no fitted constant is being renamed as a prediction. Self-citations to prior work ([9], [12]) supply standard Doppler-broadening and heterodyne-HBT formulas that are independent of the present fitted values and are not used as a self-contained uniqueness or ansatz constraint. The main limitation—that absolute spectra are not shown side by side, so static contributions to the linewidth cancel in the window differences—is a validation/scope gap, not a circular reduction. Hence no circular step is present.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central comparison is anchored by an external TOF temperature and independently measured laser parameters; the only hand-chosen physical constant is the T_r/3 heating rate. No new physical entities are introduced. The main assumptions are the single-scattering/elastic regime, the heterodyne HBT separation, and the semi-classical per-photon Doppler/heating model.

free parameters (1)
  • Heating per scattering event = T_r/3 ≈ 120.65 nK
    Hand-chosen heating rate in the dynamical model. Standard recoil heating is often quoted as (2/3)T_r in 3D, so this value is plausible but not derived or cross-checked here.
assumptions (5)
  • domain assumption The cloud remains in the single-scattering, predominantly elastic regime (b(δ)=0.012, s<0.01), so Eq. (5) applies.
    Invoked in the setup; if multiple scattering or inelastic scattering contributed significantly, the coherence width would not map simply to the Doppler-broadened thermal spectrum.
  • standard math The heterodyne HBT relation (Eq. 3) correctly separates g1 and g2, assuming chaotic scattered light and a stable local oscillator.
    Taken from the authors' prior work (Ferreira et al., ref [12]); no independent verification is provided in this paper.
  • domain assumption Radiation pressure accumulates as a Doppler shift of -N_ex k_L(1−cosθ)v_r per photon, with independent recoil heating T_r/3 per event.
    Semiclassical single-atom scattering model underlying Eq. (7) and the simulated spectra; the heating rate is chosen without derivation.
  • domain assumption Zeeman shifts up to 0.7Γ and trap gradients do not significantly modify the measured spectrum.
    Supported by the absence of observed gradient dependence in 200–280 G/cm, but only tested over a limited range and for one cloud size.
  • domain assumption The time-of-flight temperature 85±10 μK is the correct unperturbed temperature of the trapped cloud.
    Used as the initial temperature in the model; if the TOF value were biased, the agreement between model and coherence data could be coincidental.

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Cite this review

Pith. "Pith review of In Situ Coherence Measurements of Scattered Light in Magnetically Trapped Cold Atomic Clouds: Probe-Driven Atomic Dynamics." pith.science (2026). https://pith.science/paper/44BKD4QS

@misc{pith2026260718808,
  author       = {Pith},
  title        = {Pith review of: In Situ Coherence Measurements of Scattered Light in Magnetically Trapped Cold Atomic Clouds: Probe-Driven Atomic Dynamics},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/44BKD4QS}},
  note         = {Machine review of arXiv:2607.18808}
}
abstract

The use of temporal correlations in scattered photons to probe the microscopic dynamics of ultracold quantum gases has emerged as a powerful, minimally destructive approach for in situ analysis. Here, we demonstrate that temporal coherence spectroscopy can quantitatively characterize atomic motion in a magnetic trap, despite the perturbative effects of the probing light. By measuring the first-order correlation function g (1) ($\tau$ ) of light scattered by a 87 Rb cloud confined in a quadrupole trap, we identify radiation-pressure-induced acceleration and heating as the origin of the apparent discrepancy between coherence spectra and temperatures inferred from time-of-flight measurements. A simple dynamical model incorporating these effects restores agreement between theory and experiment, establishing coherence spectroscopy as a reliable in situ probe of velocity distributions in trapped atomic ensembles. Our results pave the way for time-resolved studies of nonequilibrium dynamics and thermalization processes in confined cold gases, complementing conventional destructive imaging techniques.

Figures

Figures reproduced from arXiv: 2607.18808 by the authors.

Figure 1
Figure 1. FIG. 1: Experimental setup. A cold atomic cloud (CA) [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2: Grey solid curve: Fourier transform of [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3: (a) Frequency shift of ˜g [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: FIG. 4: Dashed curve: simulated spectrum for [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]

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Reference graph

Works this paper leans on

30 extracted references

  1. [1]

    M. A. Moreno-Armijos, A. R. Fritsch, A. D. Garc ´ ıa- Orozco, S. Sab, G. Telles, Y. Zhu, L. Madeira, S. Nazarenko, V. I. Yukalov, and V. S. Bagnato, Ob- servation of relaxation stages in a nonequilibrium closed quantum system: Decaying turbulence in a trapped su- perfluid, Phys. Rev. Lett.134, 023401 (2025)

  2. [2]

    D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Her- bolzheimer, Diffusing wave spectroscopy, Phys. Rev. Lett.60, 1134 (1988)

  3. [3]

    D. Pine, D. Weitz, J. Zhu, and E. Herbolzheimer, Diffusing-wave spectroscopy: dynamic light scattering in the multiple scattering limit, Journal de Physique51, 2101 (1990)

  4. [4]

    D. A. Weitz, D. J. Pine, P. N. Pusey, and R. J. A. Tough, Nondiffusive brownian motion studied by diffusing-wave spectroscopy, Phys. Rev. Lett.63, 1747 (1989). 7

  5. [5]

    Fraden and G

    S. Fraden and G. Maret, Multiple light scattering from concentrated, interacting suspensions, Phys. Rev. Lett. 65, 512 (1990)

  6. [6]

    H´ ebraud, F

    P. H´ ebraud, F. Lequeux, J. P. Munch, and D. J. Pine, Yielding and rearrangements in disordered emulsions, Phys. Rev. Lett.78, 4657 (1997)

  7. [7]

    Menon and D

    N. Menon and D. J. Durian, Diffusing-wave spectroscopy of dynamics in a three-dimensional granular, Science 275, 1920 (1997)

  8. [8]

    Bicout and G

    D. Bicout and G. Maret, Multiple light scattering in taylor-couette flow, Physica A: Statistical Mechanics and its Applications210, 87 (1994)

Show all 30 references
  1. [9]

    A. Eloy, Z. Yao, R. Bachelard, W. Guerin, M. Fouch´ e, and R. Kaiser, Diffusing-wave spectroscopy of cold atoms in ballistic motion, Phys. Rev. A97, 013810 (2018)

  2. [10]

    Ortiz, R

    L. Ortiz, R. C. Teixeira, A. Eloy, D. Ferreira, R. Kaiser, R. Bachelard, and M. Fouch´ e, Mollow triplet in cold atoms, New Journal of Physics21, 093019 (2019)

  3. [11]

    Lass` egues, M

    P. Lass` egues, M. A. F. Biscassi, M. Morisse, A. Cidrim, P. G. S. Dias, H. Eneriz, R. C. Teixeira, R. Kaiser, R. Bachelard, and M. Hugbart, Transition from classical to quantum loss of light coherence, Phys. Rev. A108, 042214 (2023)

  4. [12]

    Ferreira, R

    D. Ferreira, R. Bachelard, W. Guerin, R. Kaiser, and M. Fouch´ e, Connecting field and intensity correlations: The siegert relation and how to test it, American Journal of Physics88, 831 (2020)

  5. [13]

    P. D. Lett, R. N. Watts, C. I. Westbrook, W. D. Phillips, P. L. Gould, and H. J. Metcalf, Observation of atoms laser cooled below the doppler limit, Phys. Rev. Lett. 61, 169 (1988)

  6. [14]

    Gerbier, S

    F. Gerbier, S. Trotzky, S. F¨ olling, U. Schnorrberger, J. D. Thompson, A. Widera, I. Bloch, L. Pollet, M. Troyer, B. Capogrosso-Sansone, N. V. Prokof’ev, and B. V. Svis- tunov, Expansion of a quantum gas released from an op- tical lattice, Phys. Rev. Lett.101, 155303 (2008)

  7. [15]

    J. N. Kupferschmidt and E. J. Mueller, Role of interac- tions in time-of-flight expansion of atomic clouds from optical lattices, Phys. Rev. A82, 023618 (2010)

  8. [16]

    Tenart, C

    A. Tenart, C. Carcy, H. Cayla, T. Bourdel, M. Mancini, and D. Cl´ ement, Two-body collisions in the time-of-flight dynamics of lattice bose superfluids, Phys. Rev. Res.2, 013017 (2020)

  9. [17]

    E. A. L. Henn, J. A. Seman, G. Roati, K. M. F. Mag- alh˜ aes, and V. S. Bagnato, Emergence of turbulence in an oscillating bose-einstein condensate, Phys. Rev. Lett. 103, 045301 (2009)

  10. [18]

    T. W. Neely, A. S. Bradley, E. C. Samson, S. J. Rooney, E. M. Wright, K. J. H. Law, R. Carretero-Gonz´ alez, P. G. Kevrekidis, M. J. Davis, and B. P. Anderson, Character- istics of two-dimensional quantum turbulence in a com- pressible superfluid, Phys. Rev. Lett.111, 235301 (2013)

  11. [19]

    Griffin, M

    A. Griffin, M. Gaudesius, R. Kaiser, S. Nazarenko, and G. Labeyrie, Analysing spatiotemporal instabilities in magneto-optical traps with the tools of turbulence the- ory, Europhysics Letters141, 25002 (2023)

  12. [20]

    Muhammed Shafi, D

    K. Muhammed Shafi, D. Pandey, B. Suryabrahmam, B. S. Girish, and H. Ramachandran, Time-delayed in- tensity–interferometry of the emission from ultracold atoms in a steady-state magneto-optical trap, Journal of Physics B: Atomic, Molecular and Optical Physics49, 025301 (2015)

  13. [21]

    S. Bali, D. Hoffmann, J. Sim´ an, and T. Walker, Mea- surements of intensity correlations of scattered light from laser-cooled atoms, Phys. Rev. A53, 3469 (1996)

  14. [22]

    Stites, M

    R. Stites, M. Beeler, L. Feeney, S. Kim, and S. Bali, Sensitive measurement of radiation trapping in cold-atom clouds by intensity correlation detection, Opt. Lett.29, 2713 (2004)

  15. [23]

    Nakayama, Y

    K. Nakayama, Y. Yoshikawa, H. Matsumoto, Y. Torii, and T. Kuga, Precise intensity correlation measurement for atomic resonance fluorescence from optical molasses, Opt. Express18, 6604 (2010)

  16. [24]

    J. A. Grover, P. Solano, L. A. Orozco, and S. L. Rolston, Photon-correlation measurements of atomic-cloud tem- perature using an optical nanofiber, Phys. Rev. A92, 013850 (2015)

  17. [25]

    Cipris, M

    A. Cipris, M. A. F. Biscassi, J. C. C. Capella, M. Morisse, H. Naim, H. Sedlacek, A. S. Deo, S. Asselie, R. Kaiser, R. C. Teixeira, R. Bachelard, and M. Hugbart, Dimen- sional control of the coherence time of scattered light in cold atom clouds, Phys. Rev. A113, 022208 (2026)

  18. [26]

    Hanbury Brown and R

    R. Hanbury Brown and R. Q. Twiss, Correlation between photons in two coherent beams of light, Nature177, 27 (1956)

  19. [27]

    Kagan, E

    Y. Kagan, E. L. Surkov, and G. V. Shlyapnikov, Evo- lution of a bose-condensed gas under variations of the confining potential, Phys. Rev. A54, R1753(R) (1996)

  20. [28]

    Castin and R

    Y. Castin and R. Dum, Bose-einstein condensates in time dependent traps, Phys. Rev. Lett.77, 5315 (1996)

  21. [29]

    Y. Lu, Y. Margalit, and K. W, Bosonic stimulation of atom-light scattering in an ultracold gas, Nat. Phys.19, 210 (2023)

  22. [30]

    Konstantinou, Y

    K. Konstantinou, Y. Zhang, P. H. C. Wong, F. Wang, Y.-K. Lu, N. Dogra, C. Eigen, T. Satoor, W. Ket- terle, and Z. Hadzibabic, Suppression and enhancement of bosonic stimulation by atomic interactions, Nat. Phys. 22, 362–366 (2026)

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