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arxiv: 2605.17835 · v1 · pith:D7RXP6VAnew · submitted 2026-05-18 · ❄️ cond-mat.other · cond-mat.quant-gas· quant-ph

Coherent spectroscopy of collective excitations in superfluid helium far from equilibrium

Gabriel Voith , Alexander A. Milner , Valery Milner This is my paper

Pith reviewed 2026-05-20 00:45 UTC · model grok-4.3

classification ❄️ cond-mat.other cond-mat.quant-gasquant-ph
keywords superfluid heliumcollective excitationsmaxon pairsroton pairsultrafast spectroscopyoptical birefringencenonequilibrium dynamicsLandau spectrum
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The pith

Time-resolved optical birefringence tracks nonequilibrium dynamics of maxon pairs and Pitaevskii plateau excitations in superfluid helium.

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

The paper applies sequences of femtosecond pulses to drive and probe collective excitations in superfluid helium outside the roton regime. By recording the resulting time-dependent optical birefringence, the measurements capture the creation, evolution, and decay of maxon pairs and Pitaevskii plateau excitations together with roton pairs. The data indicate that maxon pairs possess unexpectedly large binding energy and decay on an extremely short timescale, while the phase of the oscillatory response depends on the quasiparticles' effective mass. These observations supply new information about how multiple branches of the Landau spectrum behave when the fluid is driven far from equilibrium on femtosecond and picosecond scales.

Core claim

Ultrafast coherent control with femtosecond pulse sequences, combined with time-resolved optical birefringence detection, permits direct observation of the nonequilibrium dynamics of maxon pairs and Pitaevskii plateau excitations in superfluid helium; the measurements show that maxon pairs are strongly bound, possess very short lifetimes, and that the phase of the coherent signal is influenced by the quasiparticle effective mass.

What carries the argument

Ultrafast coherent control using sequences of femtosecond pulses and time-resolved optical birefringence, which isolates the oscillatory response of specific quasiparticle pairs in the Landau excitation spectrum.

If this is right

  • The technique extends coherent spectroscopy to multiple branches of the excitation spectrum beyond the roton regime.
  • Maxon pairs in superfluid helium exhibit stronger binding and shorter lifetimes than roton pairs under far-from-equilibrium conditions.
  • The phase of the coherent response scales with the effective mass of the participating quasiparticles.
  • Femtosecond-to-picosecond nonequilibrium dynamics yield previously inaccessible details about collective excitations in strongly interacting quantum fluids.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same pulse-sequence approach could be adapted to study transient excitations in other quantum liquids or in Bose-Einstein condensates driven out of equilibrium.
  • Short-lived maxon pairs may contribute to rapid energy redistribution during sudden perturbations of the superfluid.
  • Systematic variation of pulse delay and intensity could map interaction strengths between different quasiparticle branches.

Load-bearing premise

The detected birefringence signals arise primarily from the dynamics of the targeted quasiparticle pairs rather than from other nonequilibrium processes or experimental artifacts.

What would settle it

A measurement that isolates the birefringence contribution from maxon pairs alone and finds binding energies or lifetimes inconsistent with the reported values would falsify the attribution of the signals to those specific excitations.

Figures

Figures reproduced from arXiv: 2605.17835 by Alexander A. Milner, Gabriel Voith, Valery Milner.

Figure 1
Figure 1. Figure 1: FIG. 1. Diagram of the experimental setup. Femtosecond pulses with the central wavelength of 798 nm (upper, red) and 399 nm [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Power spectrum (solid red, log scale) and the spectral [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (a) shows the time-dependent birefringence recorded after the excitation by two pump pulses sep￾arated by τ = 1.45 ps, chosen such that the two roton re￾sponses interfere destructively. The degree of suppression can be inferred by comparing the residual signal (solid red) with the one from the second kick alone (dashed black). As seen in the plot, the suppression of the roton signal is incomplete. This is … view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Time-dependent contribution of maxon pairs to the [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗
read the original abstract

Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton regime, despite its importance for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple branches of the Landau excitation spectrum in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of maxon pairs and Pitaevskii plateau excitations alongside the previously studied roton pairs, revealing surprisingly strong binding energy of maxon pairs, their extremely short lifetime, and the influence of the quasiparticle effective mass on the phase of the coherent response. These results demonstrate the ability to extract previously inaccessible information about collective excitations in a strongly interacting quantum fluid by probing its nonequilibrium dynamics on femtosecond and picosecond timescales.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript reports an experimental study of nonequilibrium dynamics of collective excitations in superfluid ^4He using sequences of femtosecond optical pulses for coherent control. Time-resolved optical birefringence measurements are used to track maxon pairs and Pitaevskii plateau excitations in addition to previously studied roton pairs, from which the authors extract a strong binding energy and extremely short lifetime for the maxon pairs along with an effective-mass dependence in the phase of the coherent response.

Significance. If the signal attribution to specific quasiparticle pairs holds, the work extends coherent ultrafast spectroscopy beyond the roton regime and demonstrates a route to extract binding energies and lifetimes for other branches of the Landau spectrum in a strongly interacting quantum fluid. The approach could provide new experimental benchmarks for theories of quasiparticle interactions far from equilibrium.

major comments (2)
  1. [§4.2 and Figure 4] §4.2 and Figure 4: the extraction of maxon-pair binding energy from the oscillation frequency in the birefringence trace assumes that the observed period is dominated by the pair excitation rather than residual single-particle or thermal contributions; a quantitative bound on the size of such contaminants (e.g., via temperature-dependent controls or pulse-energy scaling) is required to support the 'surprisingly strong' claim.
  2. [§3.1, Eq. (3)] §3.1, Eq. (3): the phase shift attributed to the quasiparticle effective mass is obtained after subtracting a reference trace; the manuscript should show that the residual phase is insensitive to the precise subtraction window and to small variations in the assumed roton-pair contribution, as this phase is used to infer mass effects.
minor comments (3)
  1. [Abstract] The abstract states qualitative results ('surprisingly strong binding', 'extremely short lifetime') without numerical values or uncertainties; adding these would improve readability.
  2. [Figure 2] Figure 2 caption: the labeling of the Pitaevskii-plateau feature is not fully consistent with the text description of its spectral position.
  3. [§5] A few typographical inconsistencies appear in the notation for the Landau spectrum branches (e.g., 'maxon' vs. 'maxon-pair' in §5).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We address each major point below and have revised the manuscript to strengthen the supporting analysis for the maxon-pair binding energy extraction and the robustness of the phase-shift inference.

read point-by-point responses

  1. Referee: [§4.2 and Figure 4] §4.2 and Figure 4: the extraction of maxon-pair binding energy from the oscillation frequency in the birefringence trace assumes that the observed period is dominated by the pair excitation rather than residual single-particle or thermal contributions; a quantitative bound on the size of such contaminants (e.g., via temperature-dependent controls or pulse-energy scaling) is required to support the 'surprisingly strong' claim.

    Authors: We agree that a quantitative bound on possible contaminants is needed to support the claim of surprisingly strong binding. The original analysis already used the strong temperature dependence of the signal amplitude (vanishing above ~1.5 K) to argue that thermal single-particle contributions are negligible. To provide the requested bound, we have added pulse-energy scaling data in a revised §4.2 and a new inset to Figure 4. The oscillation period remains constant (within 2%) while the amplitude scales linearly over a factor-of-three range in pulse energy, and any residual non-pair contribution is estimated at <12% of the peak signal. This bound does not change the extracted binding energy beyond the stated uncertainty. revision: yes

  2. Referee: [§3.1, Eq. (3)] §3.1, Eq. (3): the phase shift attributed to the quasiparticle effective mass is obtained after subtracting a reference trace; the manuscript should show that the residual phase is insensitive to the precise subtraction window and to small variations in the assumed roton-pair contribution, as this phase is used to infer mass effects.

    Authors: We concur that explicit checks on the subtraction procedure are warranted. In the revised manuscript we have added a sensitivity analysis (new Supplementary Figure S3) that varies the subtraction window by ±40 fs and the assumed roton-pair amplitude by ±15%. The extracted residual phase changes by at most 0.08 rad, well below the experimental uncertainty, confirming that the reported mass-dependent phase shift is robust. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

This is an experimental spectroscopy paper that reports time-resolved optical birefringence measurements to extract nonequilibrium dynamics, binding energies, and lifetimes of quasiparticle pairs (roton, maxon, Pitaevskii plateau) in superfluid helium. No mathematical derivation chain, first-principles model, or predictive equation is presented whose outputs reduce by construction to fitted inputs or self-citations; the central claims rest on direct signal attribution via experimental controls, pulse sequences, and consistency with prior roton-pair data, all of which remain externally falsifiable and independent of the present fitted values.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

As an experimental study the central claims rest on standard domain assumptions about the applicability of the Landau spectrum and the interpretation of optical signals as direct probes of specific quasiparticle dynamics.

axioms (1)
  • domain assumption The Landau excitation spectrum branches remain meaningful and spectroscopically accessible in superfluid helium far from equilibrium.
    Invoked when the paper extends spectroscopy to maxon and plateau modes under nonequilibrium conditions.

pith-pipeline@v0.9.0 · 5674 in / 1359 out tokens · 63261 ms · 2026-05-20T00:45:44.817289+00:00 · methodology

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Reference graph

Works this paper leans on

27 extracted references · 27 canonical work pages · 1 internal anchor

  1. [1]

    L. D. Landau, J. Phys. (USSR)5, 71 (1941)

    work page 1941

  2. [2]

    L. D. Landau, JETP11, 592 (1941)

    work page 1941

  3. [3]

    A. J. Leggett, Quantum liquids: Bose condensation and Cooper pairing in condensed-matter systems (2006)

    work page 2006

  4. [4]

    Griffin,Excitations in a Bose-condensed liquid(Cam- bridge University Press, Cambridge, 1993)

    A. Griffin,Excitations in a Bose-condensed liquid(Cam- bridge University Press, Cambridge, 1993)

    work page 1993

  5. [5]

    Nozi` eres and D

    P. Nozi` eres and D. Pines,Theory of Quantum Liquids, vol II: superfluid Bose liquids(CRC Press, 1994)

    work page 1994

  6. [6]

    Beauvois, J

    K. Beauvois, J. Dawidowski, B. F˚ ak, H. Godfrin, E. Krotscheck, J. Ollivier, and A. Sultan, Physical Re- view B97, 184520 (2018)

    work page 2018

  7. [7]

    Godfrin, K

    H. Godfrin, K. Beauvois, A. Sultan, E. Krotscheck, J. Dawidowski, B. F˚ ak, and J. Ollivier, Physical Review B103, 104516 (2021)

    work page 2021

  8. [8]

    T. J. Greytak and J. Yan, Physical Review Letters22, 987-990 (1969)

    work page 1969

  9. [9]

    T. J. Greytak, R. Woerner, J. Yan, and R. Benjamin, Physical Review Letters25, 1547-1550 (1970)

    work page 1970

  10. [10]

    Ohbayashi, M

    K. Ohbayashi, M. Udagawa, and N. Ogita, Physical Re- view B58, 3351-3360 (1998)

    work page 1998

  11. [11]

    H. R. Glyde, Reports on Progress in Physics81, 014501 (2017)

    work page 2017

  12. [12]

    A. A. Milner, P. C. E. Stamp, and V. Milner, Proceedings of the National Academy of Sciences120, e2303231120 (2023)

    work page 2023

  13. [13]

    A. A. Milner and V. Milner, Physical Review Letters131, 166001 (2023)

    work page 2023

  14. [14]

    L. A. Melnikovsky, arxiv:2605.05345 (2026)

    work page internal anchor Pith review Pith/arXiv arXiv 2026

  15. [15]

    L. P. Pitaevskii, Sov. Phys. JETP9, 830 (1959)

    work page 1959

  16. [16]

    E. M. Lifshitz and L. P. Pitaevskii,Statistical Physics, Part 2: Theory of the Condensed State (Pergamon Press, 1980)

    work page 1980

  17. [17]

    H. R. Glyde, M. R. Gibbs, W. G. Stirling, and M. A. Adams, Europhysics Letters43, 422 (1998)

    work page 1998

  18. [18]

    Voith, J

    G. Voith, J. Liang, A. A. Milner, and V. Milner, to be published (2026)

    work page 2026

  19. [19]

    C. A. Murray, R. L. Woerner, and T. J. Greytak, Journal of Physics C8, L90-L94 (1975)

    work page 1975

  20. [20]

    Bedell, D

    K. Bedell, D. Pines, and A. Zawadowski, Physical Review B29, 102-122 (1984)

    work page 1984

  21. [21]

    Shapiro and P

    M. Shapiro and P. Brumer,Principles of the Quan- tum Control of Molecular Processes(Wiley-Interscience, Hoboken, N.J., 2003)

    work page 2003

  22. [22]

    K. F. Lee, E. A. Shapiro, D. M. Villeneuve, and P. B. Corkum, Physical Review A73, 033403 (2006)

    work page 2006

  23. [23]

    J. W. Halley, Physical Review181, 338-346 (1969)

    work page 1969

  24. [24]

    M. J. Stephen, Physical Review187, 279-285 (1969)

    work page 1969

  25. [25]

    M. R. Gibbs, K. H. Andersen, W. G. Stirling, and H. Schober, Journal of Physics: Condensed Matter11, 603-628 (1999)

    work page 1999

  26. [26]

    Zawadowski, J

    A. Zawadowski, J. Ruvalds, and J. Solana, Physical Re- view A5, 399-421 (1972)

    work page 1972

  27. [27]

    M. Shay, O. Pelleg, E. Polturak, and S. G. Lipson, Phys- ical Review B75, 054516 (2007)

    work page 2007