REVIEW 5 major objections 5 minor 49 references
Realizing exceptional points by Floquet dissipative couplings in thermal atoms
T0 review · 5 major / 5 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read Periodic magnetic driving of a thermal atomic vapor realizes a tunable Floquet exceptional point far from the static phase boundary.
desk verdict Experiment looks real, but the Floquet-EP mechanism is unproven as written because the common Zeeman drive can be gauged away; the missing Supplement is load-bearing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the effective two-level non-Hermitian Hamiltonian of Eq. (3), whose off-diagonal imaginary coupling is the Floquet dissipative coupling. The periodic magnetic field dresses each channel's spin wave into sidebands with Bessel weights $J_m(\delta_B/\omega_B)$; atomic motion transports the coherence between the two channels, giving $\Gamma_{\mathrm{eff}}=|J_{n_1}(\delta_B/\omega_B)J_{n_2}(\delta_B/\omega_B)|\Gamma_c$. This identity sets the exceptional-point condition $|\Delta_0-n\omega_B|=2\Gamma_{\mathrm{eff}}$ and is the quantitative prediction that the measurements of EP location test.
What would settle it
Fix $\omega_B$ and sweep the modulation amplitude $\delta_B$ while watching the EIT splitting between the coupled sidebands; the law $\Gamma_{\mathrm{eff}}=|J_0(\delta_B/\omega_B)J_1(\delta_B/\omega_B)|\Gamma_c$ predicts that the coupling and the exceptional-point splitting vanish at the zeros of the Bessel factors. If a nonzero splitting persists at those zeros, the two-level truncation or the Bessel-product coupling law is wrong.
Extended reading notes
Core claim
The discovery is that dissipative coupling between two atomic spin waves can be created and tuned by Floquet engineering rather than by static parameters. In the experiment, two laser channels in a paraffin-coated $^{87}\mathrm{Rb}$ cell create collective ground-state coherences via $\Lambda$-type electromagnetically induced transparency, and a time-varying magnetic field $B_1\cos(\omega_B t)$ periodically shakes the Zeeman splitting, dressing each spin wave with sidebands labeled by integers $n$. Choosing the single-photon detuning so that the carrier in one channel is nearly degenerate with the first sideband in the other ($n_1-n_2=1$) makes the atomic motion mediate an effective imaginary coupling $\Gamma_{\mathrm{eff}}$ between the two sidebands. The system is then governed by the two-level anti-PT Hamiltonian of Eq. (3), whose eigenvalues give an exceptional point at $|\Delta_0-n\omega_B|=2\Gamma_{\mathrm{eff}}$. The paper reports that the measured splitting and the extracted EP locations match the predicted $\Gamma_{\mathrm{eff}}=|J_0(\delta_B/\omega_B)J_1(\delta_B/\omega_B)|\Gamma_c$, demonstrating an anti-PT transition in a regime where the static system is deep in the broken phase.
Load-bearing premise
The result stands or falls on the reduction of the full periodically driven multi-sideband system to the two-level Hamiltonian with the Bessel-product coupling rate; if higher sidebands or extra coherences contribute, the observed transition would not be the claimed Floquet dissipative coupling.
Editorial extensions
If this is right
- The exceptional point can be placed at essentially arbitrary detunings by choosing the modulation frequency and depth, not just at the static threshold $2\Gamma_c$.
- By setting $\omega_B=|\Delta_0|/n$, higher-order Floquet transitions with $n_1-n_2=2,3$ can be activated, as demonstrated for the second- and third-order couplings.
- The tunable imaginary coupling between sidebands in a synthetic frequency dimension and a real spatial dimension can serve as a building block for Floquet dissipative band structures and ladders with a tunable number of legs.
- The hertz-level resolution of the phase-transition threshold turns the vapor cell into a precision platform for testing Floquet non-Hermitian spectral predictions.
- The experiment demonstrates controllable non-local dissipation in a quantum-accessible atomic medium, a step toward dissipative Floquet phases without static analogues.
Reading between the lines
- A further test this claim suggests is to sweep the modulation depth through the zeros of $J_0$ or $J_1$; if the Bessel-product law is exact, the coupling and the EP splitting should vanish there.
- A natural next step the paper does not take is to extend the two-channel setup to three or more channels, forming dissipatively coupled Floquet lattices in the combined frequency-space dimension.
- Since the medium is a quantum atomic ensemble, one could probe whether the Floquet dissipative coupling produces or preserves quantum correlations between the channels, extending the experiment beyond classical-field observations.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports an experiment in a warm 87Rb vapor cell in which two spatially separated EIT channels are driven by a common oscillating magnetic field. The authors claim that the periodic drive creates Floquet sidebands of the atomic spin-wave coherences and that atomic-motion-mediated dissipative coupling between sidebands with different Floquet indices realizes a tunable Floquet dissipative coupling. They infer an effective Hamiltonian (Eq. 3) with a Floquet-dependent coupling, predict an anti-PT-symmetry exceptional point at a detuning far from the static threshold, and report EIT spectra, peak-separation data, beat-frequency data, and an extracted coupling rate (Eq. 5) consistent with a product of Bessel functions. The central claims rest on Eqs. (3)-(5), whose derivations are not presented in the main text and whose supporting Supplemental Material reference [42] is a placeholder.
Significance. If the mechanism is correct, this would be a noteworthy advance: it would provide a first atomic-vapor realization of Floquet dissipative coupling, demonstrate an anti-PT transition induced by periodic driving far from the static threshold, and open a route to Floquet non-Hermitian topological phases in a platform with both real and synthetic dimensions. The measurements are potentially high-quality: the beat-frequency readout in Fig. 3(b) and the sideband-resolved EIT spectra in Fig. 2 give direct, non-circular evidence that something resembling a Floquet-avoided-level picture is at work. The authors also explicitly expose the Bessel-function sideband structure in Fig. 1(d). However, the paper's quantitative conclusion depends on model equations that are asserted, not derived here, and on fits whose parameters are extracted from the same data used for verification. The result is therefore an interesting but insufficiently supported claim in the present form.
major comments (5)
- [Eq. (3) and text preceding it] Equation (3) is introduced as the Floquet effective Hamiltonian with no derivation, and the only reference to its derivation, Ref. [42], is a placeholder with URL 'http://link.aps.org/supplemental/XXX'. This is load-bearing: the drive is described as a periodic Zeeman shift δB(t) = δ0 + δB cos(ωB t) that affects both channels in the same way, so a naive Markovian two-level reduction would produce a common scalar term δB cos(ωB t) I. Such a term commutes with the static Hamiltonian and can be removed by a gauge transformation, implying no new exceptional point. The off-diagonal phases e^{±i(Δ0−nωB)t} in Eq. (3) therefore require a derivation that makes clear how the finite-time, non-Markovian nature of the dissipative coupling produces this structure. Without that derivation, or an available Supplemental Material, the central claim is not yet substantiated.
- [Eq. (5) and Fig. 4(a)] The quantitative verification of Eq. (5) is partially circular: the data points for Γeff are obtained from the EP locations using the EP condition derived from Eq. (3), and then Eq. (5) is fitted to those same points with Γc and δB as free parameters. The reported 'Hz-level agreement' is therefore not a parameter-free prediction. The authors should provide an independent calibration of δB (for example from the sideband-height fits in Fig. 1(d)) and an independent measurement of Γc (for example from the static EP at B1=0), and then compare the Eq. (5) curve with no refitted parameters, or at least report the fit uncertainties and confidence intervals for Γc and δB.
- [Fig. 4(a) and Fig. 1(d)] The apparent values of the modulation depth differ between calibrations: Fig. 1(d) is fitted with δB ≈ 3 kHz, while Fig. 4(a) is fitted with δB ≈ 4.3 kHz. These may correspond to different driving strengths in different runs, but the manuscript does not state this explicitly. If both are from the same apparatus with nominally the same driving conditions, the discrepancy must be explained. The reader needs to know how the driving voltage is converted to δB, and whether δB is stable across the measurements.
- [Eq. (3) and multi-sideband truncation] The effective Hamiltonian in Eq. (3) truncates the infinite Floquet sideband ladder to two selected bands (n1 in CH1 and n2 in CH2) without a quantitative argument that all other sidebands can be neglected. Since the observed spectra in Fig. 1(c) show multiple sidebands, the neglected bands could in principle modify the level repulsion or the apparent EP. The authors should justify the two-band truncation explicitly, for instance by estimating the relative Bessel weights of the neglected channels and showing that their contributions are small over the parameter range of Figs. 2-4.
- [Ref. [42] and Supplemental Material availability] The manuscript repeatedly refers to 'the supplementary material[42]' for derivations, parameter estimates, and experimental details, but Ref. [42] is incomplete and not accessible on arXiv. This is not a formatting detail: a reader cannot verify the central derivation of Eq. (3), the derivation of Eq. (5), the relation between control power and Γc, the estimation of δB, or the fitting procedures. The authors should either include the derivation in the main text or make the Supplemental Material available in a complete, citable form.
minor comments (5)
- [Abstract] The abstract contains the typo 'exception point' instead of 'exceptional point'.
- [Fig. 2 caption] The labels '53Hz' and '6Hz' and the legend entries 'uncoupled'/'coupled' in Fig. 2 are not fully explained; the reader should be told which frequency splittings these numbers denote and how they were extracted.
- [Eq. (3)] The index n is defined only as n = n1 − n2 after Eq. (3); this notation would be clearer if defined before the equation, and the symbols n1 and n2 used in Eq. (3) should be explicitly tied to the Floquet bands in CH1 and CH2.
- [Fig. 4 caption] The caption of Fig. 4 mixes three different descriptions of driving strength ('≈ 5.5 kHz', '= 2.31 Vpp', '≈ 4 kHz') without specifying which run each applies to; the authors should give the magnetic-field amplitude (or a single calibrated quantity) for each panel.
- [Ref. [42] and Ref. [41]] Reference [42] is incomplete, and Ref. [41] lacks the article number or page range; a complete reference list with accessible links is needed.
Circularity Check
The anomalous anti-PT transition is directly observed, but the claimed confirmation of the Floquet coupling rate (Eq. 5) is a two-parameter fit to model-extracted data, not an independent prediction.
-
fitted input called prediction
[Section 'Realizing the highly controlled Floquet dissipative coupling', Eq. (5) and Fig. 4(a)]
"Under a fixed modulation depth δB, EPs locations are recorded at various modulation frequencies ωB. Fig. 4(a) plots the deduced effective coupling rate Γ eff as a function of ωB, in a good agreement with the expected function (blue solid line) Γeff = |J0(δB/ωB)J1(δB/ωB)| Γc. ... Γc=93 Hz is extracted from the numerical fit."
The data points shown as validating Eq. (5) are not independent measurements of Γeff: they are inferred by imposing Eq. (3)'s EP condition |−Δ0−nωB| = 2Γeff on the observed transition detunings. The 'expected' curve is then obtained by fitting the same dataset with Γc (scale) and δB (modulation depth) as free parameters (δB ≈ 4.3 kHz, Γc = 93 Hz). The agreement therefore tests only whether a two-parameter Bessel-product curve can represent the model-extracted points; it does not constitute a parameter-free prediction of the Floquet dissipative coupling rate.
full rationale
The central observation—an anti-PT phase transition at |Δ0| ≈ ωB ≫ 2Γc in the driven system while the static system is deep in the broken phase—is a direct experimental result and does not reduce to the fitted parameters, so the paper is not wholly circular. Equation (3)'s EP condition is a model prediction that is at least qualitatively tested by the observed threshold location. The main circularity is confined to the quantitative validation of Eq. (5): Γeff is extracted through the same effective-Hamiltonian relation used to state the prediction, and Γc and δB are fitted to those extracted values, making the 'good agreement' a consistency check rather than an independent confirmation. The derivation of the effective Hamiltonian is deferred to Supplemental Material [42], which is a completeness/support issue rather than a circularity; static Hamiltonian Eq. (1) rests on independent prior work [41], and the self-citations [33,43] are not load-bearing for the central claim. Overall: partial, not pervasive, circularity.
Assumptions & free parameters
free parameters (2)
- Γc =
93 Hz
- δB =
≈4.3 kHz (Fig. 4a), ≈3 kHz (Fig. 1d)
assumptions (4)
- standard math Floquet theorem: a time-periodic Hamiltonian generates sidebands with Bessel-function weights.
- domain assumption Two-level truncation: the system can be described by only two Floquet sidebands (n1 and n2), with all other sidebands neglected.
- domain assumption Dissipative coupling mechanism: ballistic atomic motion and wall bouncing couple spin waves in different spatial channels at rate Γc, as established in prior work.
- ad hoc to paper Floquet dissipative coupling rate: Γeff = |J_{n1}(δB/ωB) J_{n2}(δB/ωB)| Γc.
Cite this review
Pith. "Pith review of Realizing exceptional points by Floquet dissipative couplings in thermal atoms." pith.science (2026). https://pith.science/paper/7YXDAXYP
@misc{pith2026250413616,
author = {Pith},
title = {Pith review of: Realizing exceptional points by Floquet dissipative couplings in thermal atoms},
year = {2026},
howpublished = {\url{https://pith.science/paper/7YXDAXYP}},
note = {Machine review of arXiv:2504.13616}
}
read the original abstract
Exceptional degeneracies and generically complex spectra of non-Hermitian systems are at the heart of numerous phenomena absent in the Hermitian realm. Recently, it was suggested that Floquet dissipative coupling in the space-time domain may provide a novel mechanism to drive intriguing spectral topology with no static analogues, though its experimental investigation in quantum systems remains elusive. We demonstrate such Floquet dissipative coupling in an ensemble of thermal atoms interacting with two spatially separated optical beams, and observe an anomalous anti-parity-time symmetry phase transition at an exception point far from the phase-transition threshold of the static counterpart. Our protocol sets the stage for Floquet engineering of non-Hermitian topological spectra, and for engineering new quantum phases that cannot exist in static systems.
Figures
Reference graph
Works this paper leans on
-
[42]
Peng, P. et al. Anti-parity-time symmetry with flying atoms. Nat. Phys. 12, 1139-1145 (2016)
work page 2016
-
[1]
C. M. Bender and S. Boettcher, Real spectra in non-Hermitian Hamiltonians having PT symmetry.Phys. Rev. Lett. 80, 5243-5246 (1998)
work page 1998
-
[2]
Z. Gong, Y. Ashida, K. Kawabata, K. Takasan, S. Higashikawa, and M. Ueda, Topological Phases of Non-Hermitian Systems. Phys. Rev. X 8, 031079 (2018)
work page 2018
-
[3]
K. Kawabata, K. Shiozaki, M. Ueda, and M. Sato, Symmetry and Topology in Non-Hermitian Physics. Phys. Rev. X 9, 041015 (2019)
work page 2019
-
[4]
C. Coulais, R. Fleury, and J. van Wezel, Topology and broken Hermiticity. Nat. Phys. 17, 9-13 (2021)
work page 2021
- [5]
-
[6]
K. Wang, A. Dutt, C. C. Wojcik, et al. Topological complex-energy braiding of non-Hermitian bands. Nature 598, 59-64 (2021)
work page 2021
-
[7]
W. Wang, X. Wang, and G. Ma. Non-Hermitian morphing of topological modes. Nature 608, 50-55 (2022)
work page 2022
Show all 49 references
-
[8]
E. J. Bergholtz, J. C. Budich, F. K. Kunst, Exceptional topology of non-Hermitian systems. Rev. Mod. Phys 93, 015005 (2021)
2021
-
[9]
K. Ding, C. Fang, and G. Ma, Non-Hermitian topology and exceptional-point geometries. Nat. Rev. Phys. 4, 745-760 (2022)
2022
-
[10]
T. E. Lee, Anomalous Edge State in a Non-Hermitian Lattice. Phys. Rev. Lett. 116, 133903 (2016)
2016
-
[11]
F. K. Kunst, E. Edvardsson, J. C. Budich, and E. J. Bergholtz, Biorthogonal Bulk-Boundary Correspondence in Non-Hermitian Systems. Phys. Rev. Lett. 121, 026808 (2018)
2018
-
[12]
V. M. Martinez Alvarez, J. E. Barrios Vargas, and L. E. F. Foa Torres, Non-Hermitian robust edge states in one dimension: Anomalous localization and eigenspace condensation at exceptional points. Phys. Rev. B 97, 121401 (2018). 6
2018
-
[13]
Xiong, Why does bulk boundary correspondence fail in some non-Hermitian topological models
Y. Xiong, Why does bulk boundary correspondence fail in some non-Hermitian topological models. J. Phys. Commun. 2, 035043 (2018)
2018
-
[14]
Yao, and Z
S. Yao, and Z. Wang, Edge States and Topological Invariants of Non-Hermitian Systems. Phys. Rev. Lett. 121, 086803 (2018)
2018
-
[15]
L. Xiao, T. Deng, K. Wang, G. Zhu, Z. Wang, W. Yi, and P. Xue. Non-Hermitian bulk–boundary correspondence in quantum dynamics. Nat. Phys. 16, 761-766 (2020)
2020
-
[16]
Carlstr¨ om and E
J. Carlstr¨ om and E. J. Bergholtz, Exceptional links and twisted Fermi ribbons in non-Hermitian systems. Phys. Rev. A 98, 042114 (2018)
2018
-
[17]
C. H. Lee, G. Li, Y. Liu, T. Tai, R. Thomale, and X. Zhang, Tidal surface states as fingerprints of non-Hermitian nodal knot metals. arXiv:1812.02011
-
[18]
Carlstr¨ om, M
J. Carlstr¨ om, M. St˚ alhammar, J. C. Budich, and E. J. Bergholtz, Knotted non-Hermitian metals,” Phys. Rev. B 99, 161115 (2019)
2019
-
[19]
H. Xu, D. Mason, L. Jiang, and J. G. E. Harris, Topological energy transfer in an optomechanical system with exceptional points. Nature 537, 80-83 (2016)
2016
-
[20]
Andr˙ e Eckardt, Atomic quantum gases in periodically driven optical lattices. Rev. Mod. Phys. 89, 011004 (2017)
2017
-
[21]
Goldman and J
N. Goldman and J. Dalibard, Periodically Driven Quantum Systems: Effective Hamiltonians and Engineered Gauge Fields. Phys. Rev. X 4, 031027 (2014)
2014
-
[22]
Goldman, J
N. Goldman, J. C. Budich, and P. Zoller, Topological quantum matter with ultracold gases in optical lattices. Nat. Phys. 12, 639-645 (2016)
2016
-
[23]
M. S. Rudner, N. H. Lindner, E. Berg, and M. Levin, Anomalous Edge States and the Bulk-Edge Correspondence for Periodically Driven Two-Dimensional Systems. Phys. Rev. X 3, 031005 (2013)
2013
-
[24]
N. H. Lindner, G. Refae, abd V. Galitski, Floquet topological insulator in semiconductor quantum wells. Nat. Phys. 7, 490-495 (2011)
2011
-
[25]
Carl Budich, Y
J. Carl Budich, Y. Hu, and P. Zoller, Helical Floquet Channels in 1D Lattices. Phys. Rev. Lett. 118, 105302 (2017)
2017
-
[26]
D. A. Anderson, S. A. Miller, G. Raithel, J. Gordon, M. Butler, and C. Holloway, Optical Measurements of Strong Microwave Fields with Rydberg Atoms in a Vapor Cell, Phys. Rev. Appl. 5, 034003 (2016)
2016
-
[27]
S. A. Miller, D. A. Anderson, and G. Raithel, Radio frequency-modulated Rydberg states in a vapor cell, New J. Phys. 18, 053017 (2016)
2016
-
[28]
Jotzu, M
G. Jotzu, M. Messer, R. Desbuquois, M. Lebrat, T. Uehlinger, D. Greif, and T. Esslinger, Experimental realization of the topological Haldane model with ultracold fermions. Nature 515, 237-240 (2014)
2014
-
[29]
Aidelsburger, M
M. Aidelsburger, M. Lohse, C. Schweizer, M. Atala, J. T. Barreiro, S. Nascimb˙ ene, N. R. Cooper, I. Bloch and N. Goldman, Measuring the Chern number of Hofstadter bands with ultracold bosonic atoms. Nat. Phys. 11, 162-166 (2015)
2015
-
[30]
and other intriguing phenomena. On the theoretical side, recent studies [31, 32] conjectured that Floquet non-Hermitian systems featuring dissipative couplings in combined space-time (Floquet) domain can host unique spectral topology and anomalous skin modes that cannot exist ...
2025 arXiv
-
[31]
Weidemann, M
S. Weidemann, M. Kremer, S. Longhi, and A. Szameitand, Topological triple phase transition in non-Hermitian Floquet quasicrystals. Nature 601, 354-359 (2022)
2022
-
[32]
Zhang and J
X. Zhang and J. Gong, Non-Hermitian Floquet topological phases: Exceptional points, coalescent edge modes, and the skin effect. Phys. Rev. B 101, 045415 (2020)
2020
-
[33]
Wu and J.-H
H. Wu and J.-H. An, Floquet topological phases of non-Hermitian systems. Phys. Rev. B 102, 041119(R) (2020)
2020
-
[34]
W. Cao, X. Lu, X. Meng, J. Sun, H. Shen, and Y. Xiao, Reservoir-Mediated Quantum Correlations in Non-Hermitian Optical System. Phys. Rev. Lett. 124, 030401 (2020)
2020
-
[35]
J. Sun, X. Zhang, W. Qu, E. E. Mikhailov, I. Novikova, H. Shen, and Y. Xiao, Spatial Multiplexing of Squeezed Light by Coherence Diffusion. Phys. Rev. Lett. 123, 030401 (2019)
2019
-
[36]
Boller, A
K.-J. Boller, A. Imamoˇ glu, and S. E. Harris, Observation of electromagnetically induced transparency. Phys. Rev. Lett. 66, 2593 (1991)
1991
-
[37]
Fleischhauer, A
M. Fleischhauer, A. Imamoglu, and J. P. Marangos, Electromagnetically induced transparency: Optics in coherent media. Rev. Mod. Phys. 77, 633 (2005)
2005
-
[38]
Novikova, R
I. Novikova, R. L. Walsworth, and Y. Xiao, Electromagnetically induced transparency-based slow and stored light in warm atoms. Laser & Photonics Review 6, 333-353 (2012)
2012
-
[39]
Ge and H
L. Ge and H. E. T¨ ureci, Antisymmetric PT-photonic structures with balanced positive- and negative-index materials. Phys. Rev. A 88, 053810 (2013)
2013
-
[40]
Li et al
Y. Li et al. , Anti-parity-time symmetry in diffusive systems. Science 364, 170 (2019)
2019
-
[41]
Zhang, R
H. Zhang, R. Huang, S.-D. Zhang, Y. Li, C.-W. Qiu, F. Nori, and H. Jing, Breaking Anti-PT Symmetry by Spinning a Resonator. Nano Lett. 20 7594 (2020)
2020
-
[43]
See Supplemental Material at http://link.aps.org/supplemental/XXX
-
[44]
D. Hao, L. Wang, X. Lu, X. Cao, S. Jia, Y. Hu, and Y. Xiao, Topological Atomic Spin Wave Lattices by Dissipative Couplings. Phys. Rev. Lett. 130, 153602 (2023)
2023
-
[45]
L. Ding, K. Shi, Q. Zhang, D. Shen, X. Zhang, and W. Zhang, Experimental Determination of PT -Symmetric Exceptional Points in a Single Trapped Ion. Phys. Rev. Lett. 126, 083604 (2021)
2021
-
[46]
Bardyn, et al
C.-E. Bardyn, et al. Topology by dissipation. N. J. Phys. 15, 085001 (2013)
2013
-
[47]
Leefmans, et al
C. Leefmans, et al. Topological dissipation in a time-multiplexed photonic resonator network. Nat. Phys. 18, 442-449 (2022)
2022
-
[48]
Yoshida, and Y
T. Yoshida, and Y. Hatsugai, Bulk edge correspondence of classical difusion phenomena. Sci. Rep. 11, 888 (2021)
2021
-
[49]
Parto, C
M. Parto, C. Leefmans, J. Williams, F. Nori, and A. Marandi, Non-Abelian effects in dissipative photonic topological lattices. Nat. Commun. 14, 1440 (2023)
2023
Reviewed August 16, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.