REVIEW 4 major objections 6 minor 45 references
Post-selection shifts the transition frequency of helium in an atomic beam
T0 review · 4 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Post-selecting atoms shifts helium frequency by 55 kHz
desk verdict A well-observed -55 kHz post-selection shift in helium beam spectroscopy, with a parameter-free model that holds up; the corrected frequency needs a residual-shift control and the paper's own numbers need reconciling before I'd trust the 0.86 kHz error bar. 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 mechanism is the velocity-space entanglement between atomic internal state and transverse momentum: the detector slit plays the role of a post-selector on final velocity, while the excitation probability selects on initial velocity through the detuning $\delta - k v_0$. The load-bearing identity is the PSS formula $\Delta\nu = -\frac{h}{mc^2}\frac{L}{\ell+L}\nu_c^2$, which turns the geometry of probe-to-first-slit distance $\ell$ and probe-to-detector-slit distance $L$ into a frequency shift that is independent of the sign of the probe direction. This is why averaging counter-propagating probe beams removes the ordinary first-order Doppler shift but not the post-selection shift.
What would settle it
Repeat the frequency measurement with the 10 mm slit, then with a 20 mm slit, and then with the slit removed entirely, holding all other conditions fixed; if the center frequencies differ by more than the quoted 0.86 kHz uncertainty, the residual PSS is not negligible and the reported frequency and $\delta r^2$ carry a hidden bias.
Extended reading notes
Core claim
The paper's central claim is that position-based post-selection in the SCTOP beam method induces a systematic red shift, the post-selection shift (PSS), in the measured $2^3S_1$–$2^3P_0$ transition of $^4$He. Only atoms whose transverse velocity after photon absorption and spontaneous emission lets them pass the final narrow slit are detected; because the photon recoil $v_R = \hbar k/m$ must be compensated, the detected atoms carry a nonzero initial transverse velocity $v_0 = -v_R L/(\ell+L)$, producing a first-order Doppler shift. The result is Eq. (5), $\Delta\nu = -\frac{h}{mc^2}\frac{L}{\ell+L}\nu_c^2$, evaluated at about $-55$ kHz for this transition. The shift is observed directly by comparing spectra with and without the narrow slit, and it is reproduced by an analytic velocity-distribution model and by Monte Carlo wave-function simulations. With the slit widened to 10 mm the shift is suppressed, and the paper reports the corrected frequency $276\,764\,094\,712.45 \pm 0.86$ kHz; applying the same reasoning to an earlier narrow-slit measurement brings it into agreement.
Load-bearing premise
The final frequency assumes that the 10 mm wide slit suppresses the post-selection shift to well below the 0.86 kHz total uncertainty, but no direct measurement of the residual post-selection shift in that final configuration is reported.
Editorial extensions
If this is right
- Any Stern-Gerlach-type atomic-beam measurement of this helium transition that post-selects atoms with a narrow detector slit inherits a bias of roughly $-55$ kHz unless corrected.
- Applying the PSS correction to the earlier narrow-slit measurement reconciles its centroid with the new wide-slit result.
- The new $^4$He frequency, combined with the published $^3$He frequency, gives $\delta r^2 = 1.0733 \pm 0.0021$ fm$^2$, matching the updated $2^3S$–$2^1S$ value within uncertainty.
- The same $\delta r^2$ differs from the muonic helium ion value by $2.8\sigma$, sharpening the electron–muon universality question in bound-state QED.
- Shiner's earlier beam result is also identified as PSS-affected, although the isotope dependence prevents a direct numerical correction.
Reading between the lines
- Because the PSS scales as $\nu_c^2$, the same slit-based post-selection would produce larger biases for higher-frequency transitions in comparable beam geometries; a survey of published beam spectroscopies could reveal hidden shifts of tens to hundreds of kHz.
- A direct measurement of residual PSS as a function of slit width in the final wide-slit configuration—for example, comparing 10 mm, 20 mm, and no-slit detection—would test the claim that the suppression is complete at the 0.86 kHz level.
- If PSS is only partially suppressed, the quoted frequency and the derived $\delta r^2$ would move systematically; the sign of the bias suggests that the tension with muonic helium could be partly geometric rather than new physics, a possibility the current paper does not exclude.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports the observation of a post-selection shift (PSS) of approximately -55 kHz in the 2^3S_1-2^3P_0 transition frequency of ^4He in an atomic beam when a narrow slit selects atoms by transverse position. The shift is attributed to the fact that the slit selects atoms whose initial transverse velocity compensates the photon-recoil velocity, producing a residual Doppler shift that is not canceled by the counter-propagating probe beams. A phenomenological model (Eq. 5) and a more detailed velocity-distribution model (Eq. 10) reproduce the observed shift with parameters taken from geometry and beam characterization. Using a 10 mm slit to suppress PSS, the authors obtain a corrected frequency 276,764,094,712.45 ± 0.86 kHz and, combined with the ^3He isotope shift, derive δr^2 = 1.0733 ± 0.0021 fm^2, which they report as a 2.8σ deviation from muonic helium measurements.
Significance. The paper's central observation—that post-selection with a narrow slit systematically shifts the measured frequency by about -55 kHz—is directly demonstrated in Fig. 2 and is reproduced by a model whose parameters are not fitted to the shift. This is an important systematic effect for atomic-beam spectroscopy and potentially for other precision measurements. The derived charge-radius difference, if confirmed, strengthens the e-He versus muonic-He discrepancy and bears on lepton-universality tests. However, the final frequency relies on an extrapolated suppression of PSS for the 10 mm slit, and the manuscript contains mutually inconsistent reported values, so the significance of the corrected frequency is currently tempered.
major comments (4)
- [Abstract, §III, Table I, Appendix A] The manuscript reports two mutually inconsistent sets of values for the central result: the abstract, §III, and Table I give f0 = 276 764 094 712.45 ± 0.86 kHz and centroid 276 736 495 655.21 ± 0.87 kHz, while Appendix A (text after Table I) and Table II give f0 = 276 764 094 712.73(86) kHz and centroid 276 736 495 655.48(87) kHz. Because the corrected frequency is the central claim, the authors must identify which value is final and correct the inconsistent entries.
- [Table I and §III] The final frequency is obtained with a 10 mm slit, for which the authors state that PSS is 'effectively eliminated,' but no residual-PSS term appears in the uncertainty budget (Table I). The model curve in Fig. 5b is only shown up to σ1 = 0.6 m/s, whereas the 10 mm slit at vz = 290 m/s gives σ1 ≈ 0.91 m/s, so the suppression is extrapolated rather than measured. The authors should either measure the PSS as a function of slit width (including the 10 mm case) or provide a quantitative upper limit for the residual PSS from Eq. (10) and include it as a systematic uncertainty in Table I.
- [Appendix C, Fig. 5] The parameters used for the illustrative calculation in Fig. 5 (σ0 = 0.2 m/s, vR = 0.1 m/s, Ω/2π = 0.3 MHz) differ from the actual experimental values quoted later in Appendix C (σ0 = 0.1 m/s, vR = 0.09 m/s, Ω = 2.1 MHz). Since Fig. 5b is the basis for the claim that PSS vanishes for wide slits, the extrapolation should be performed with the experimental parameters, and the resulting residual PSS at σ1 ≈ 0.91 m/s should be reported.
- [§IV and Appendix D] The treatment of Shiner et al. is internally inconsistent: §IV states that their δr² value 'significantly deviates from our results and may be influenced by the PSS effect,' while Appendix D states that their centroid is 'consistent with our result, though with a larger uncertainty and also affected by the post-selection effect, which should be corrected.' Because no PSS correction is applied to Shiner et al., the reader cannot tell whether that measurement supports or contradicts the new result; this should be clarified.
minor comments (6)
- [Fig. 1 caption] The caption contains the typo 'Stern-Glarch' instead of 'Stern-Gerlach'.
- [Table III caption] The caption contains the typo 'Calulation' instead of 'Calculation'.
- [Appendix C] The statement that PSS tends to zero when 'σ1 ≫ kvR' is dimensionally inconsistent; the condition should presumably be 'σ1 ≫ vR' or an equivalent velocity-scale comparison.
- [Eq. (12)] The exponent in Eq. (12) appears as (v0−v0,b)²/σ0², missing the factor 2 in the denominator that is present in Eq. (10); this should be corrected for consistency.
- [Appendix C, text near Fig. 5] The statement that the average center frequency of approximately 54 kHz is 'close to kvR/2π = 84 kHz' is inaccurate; 54 kHz is substantially below 84 kHz, and the authors should rephrase to avoid implying a closer agreement than shown.
- [Appendix E] There is a typo in the sentence 'we consider only he velocity vx in the x-axis direction'—'he' should be 'the'.
Circularity Check
No significant circularity: the PSS model and the wide-slit frequency measurement are independent, though residual-PSS suppression at 10 mm is assumed rather than directly verified.
full rationale
The central derivation chain is not circular. The post-selection shift (PSS) is observed directly as the difference between fitted center frequencies obtained from the same dataset with and without position selection (Fig. 2e/f), giving about -55 kHz. The theoretical model (Eq. 10, Appendix C) predicts this shift from independently specified physical quantities: the recoil velocity v_R = hbar k / m = 0.09 m/s, the initial lateral velocity spread sigma_0 = 0.1 m/s derived from slit geometry, the detection spread sigma_1 from the slit width, and the Rabi frequency Omega from the laser power. No model parameter is fitted to reproduce the -55 kHz value; the predicted scale (roughly 54-84 kHz) brackets the observation, and Fig. 7a compares the prediction with data without adjusting the prediction to the data. The final 2^3S_1-2^3P_0 frequency is a fresh wide-slit/no-slit measurement whose central value does not use Eq. (5) as a correction; the recoil correction in Table I is the standard -42.48 kHz term. Self-citations [31], [39], and [45] are present but not load-bearing: [31] documents the SCTOP apparatus; [45] is cited for the trend that PSS decreases as sigma_1 grows, which the current paper re-derives independently in Appendix C; and [39] is an earlier same-group result that is corrected rather than used as input. One substantive weakness is that Section III asserts 'With the PSS effect effectively eliminated' and Table I contains no residual-PSS uncertainty term, so the suppression of PSS at the 10 mm slit is assumed rather than directly measured. This is a robustness and systematic-error concern, not a reduction of the prediction to its inputs, because the wide-slit measurement is independent of the PSS model. The 2.8 sigma comparison with muonic helium uses external muonic data [22] and external 3He data [26], so it is not self-referential. Score 2 reflects only the presence of ancillary self-citations and the unverified PSS-suppression assumption, neither of which makes the central claim circular.
Assumptions & free parameters
free parameters (3)
- σ0 (initial transverse velocity spread) =
0.1 m/s at vz=290 m/s
- σ1 (detection velocity spread) =
Δx3/[2(t1+t2)]
- v0,b (mean initial transverse velocity) =
0 m/s
assumptions (4)
- domain assumption Atoms undergo at most one excitation and one spontaneous emission during the probe interaction.
- domain assumption Transverse velocity distributions of initial and detected atoms are Gaussian.
- domain assumption Probe 1 and Probe 2 are counter-propagating and aligned to within 1e-5 rad.
- domain assumption The Stern-Gerlach magnet and slit select only MJ=0 atoms with no leakage from other states.
Cite this review
Pith. "Pith review of Post-selection shifts the transition frequency of helium in an atomic beam." pith.science (2026). https://pith.science/paper/A3Q33WXD
@misc{pith2026241109958,
author = {Pith},
title = {Pith review of: Post-selection shifts the transition frequency of helium in an atomic beam},
year = {2026},
howpublished = {\url{https://pith.science/paper/A3Q33WXD}},
note = {Machine review of arXiv:2411.09958}
}
abstract
Post-selecting output states in measurements can effectively amplify weak signals and improve precision. However, post-selection effects may also introduce unintended biases in precision measurements. Here, we investigate the influence of post-selection in the precision spectroscopy of the $2^3S - 2^3P$ transition of helium ($^4$He) using an atomic beam. We directly observe that post-selection based on atomic positions causes a shift in the measured transition frequency, amounting to approximately -55 kHz. After accounting for this post-selection shift, we obtain a corrected frequency of $276,764,094,712.45 \pm 0.86$ kHz for the $2^3S_1 - 2^3P_0$ transition. Combining this result with existing data for $^3$He, we derive a new value for the difference in squared nuclear charge radii, $\delta r^2 [r_{h}^{2} - r_{\alpha}^{2}] = 1.0733 \pm 0.0021$ fm$^2$. This value shows a $2.8\sigma$ deviation from measurements of muonic helium ions, potentially pointing to new physics that challenges lepton universality in quantum electrodynamics.
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Works this paper leans on
-
[1]
Aharonov, D
Y. Aharonov, D. Z. Albert, and L. Vaidman, Phys. Rev. Lett. 60, 1351 (1988)
1988
-
[2]
Dressel, M
J. Dressel, M. Malik, F. M. Miatto, A. N. Jordan, and R. W. Boyd, Rev. Mod. Phys. 86, 307 (2014)
2014
- [3]
-
[4]
T. Denkmayr, H. Geppert, S. Sponar, H. Lemmel, A. Matzkin, J. Tollaksen, and Y. Hasegawa, Nat. Com- mun. 5, 4492 (2014)
work page 2014
- [5]
-
[6]
Hosten and P
O. Hosten and P. Kwiat, Science 319, 787 (2008)
2008
-
[7]
Kocsis, B
S. Kocsis, B. Braverman, S. Ravets, M. J. Stevens, R. P. Mirin, L. K. Shalm, and A. M. Steinberg, Science 332, 1170 (2011)
2011
-
[8]
M. E. Goggin, M. P. Almeida, M. Barbieri, B. P. Lanyon, J. L. O’Brien, A. G. White, and G. J. Pryde, Proc. Na- tion. Acad. Sci. 108, 1256 (2011)
work page 2011
Show all 45 references
-
[9]
J. S. Lundeen, B. Sutherland, A. Patel, C. Stewart, and C. Bamber, Nature 474, 188 (2011)
2011
-
[10]
Feizpour, X
A. Feizpour, X. Xing, and A. M. Steinberg, Phys. Rev. Lett. 107, 133603 (2011)
2011
-
[11]
A. N. Jordan, J. Mart ´ ınez-Rinc´ on, and J. C. Howell, Phys. Rev. X 4, 011031 (2014)
2014
-
[12]
Hallaji, A
M. Hallaji, A. Feizpour, G. Dmochowski, J. Sinclair, and A. M. Steinberg, Nat. Phys. 13, 540 (2017)
2017
-
[13]
Liu, W.-W
Z.-H. Liu, W.-W. Pan, X.-Y. Xu, M. Yang, J. Zhou, Z.- Y. Luo, K. Sun, J.-L. Chen, J.-S. Xu, C.-F. Li, and G.-C. Guo, Nat. Commun. 11, 3006 (2020)
2020
-
[14]
Markowitz, R
W. Markowitz, R. G. Hall, L. Essen, and J. V. L. Parry, Phys. Rev. Lett. 1, 105 (1958)
1958
-
[15]
M. S. Safronova, D. Budker, D. DeMille, D. F. J. Kimball, A. Derevianko, and C. W. Clark, Rev. Mod. Phys. 90, 025008 (2018)
2018
-
[16]
Ficek, D
F. Ficek, D. F. J. Kimball, M. G. Kozlov, N. Leefer, S. Pustelny, and D. Budker, Phys. Rev. A 95, 032505 (2017)
2017
-
[17]
G. S. Adkins, D. B. Cassidy, and J. Perez-Rios, Phys. Rep. 975, 1 (2022)
2022
-
[18]
N. L. Figueroa, J. C. Berengut, V. A. Dzuba, V. V. Flam- baum, D. Budker, and D. Antypas, Phys. Rev. Lett.128, 073001 (2022)
2022
- [19]
-
[20]
R. J. Rengelink, Y. van der Werf, R. P. M. J. W. Noter- mans, R. Jannin, K. S. E. Eikema, M. D. Hoogerland, and W. Vassen, Nat. Phys. 14, 1132 (2018)
2018
-
[21]
van Rooij, J
R. van Rooij, J. S. Borbely, J. Simonet, M. D. Hooger- land, K. S. Eikema, R. A. Rozendaal, and W. Vassen, Science 333, 196 (2011)
2011
-
[22]
Schuhmann, L
K. Schuhmann, L. M. P. Fernandes, F. Nez, M. A. Ahmed, F. D. Amaro, P. Amaro, F. Biraben, T.-L. Chen, D. S. Covita, A. J. Dax, M. Diepold, B. Franke, S. Galtier, A. L. Gouvea, J. G¨ otzfried, T. Graf, T. W. H¨ ansch, M. Hildebrandt, P. Indelicato, L. Julien, K. Kirch, A. Knech...
-
[23]
J. J. Krauth, K. Schuhmann, M. A. Ahmed, F. D. Amaro, P. Amaro, F. Biraben, T. L. Chen, D. S. Covita, A. J. Dax, M. Diepold, L. M. P. Fernandes, B. Franke, S. Galtier, A. L. Gouvea, J. Gotzfried, T. Graf, T. W. Hansch, J. Hartmann, M. Hildebrandt, P. Indelicato, L. Julien, K. ...
2021
-
[24]
Matveev, C
A. Matveev, C. G. Parthey, K. Predehl, J. Alnis, A. Beyer, R. Holzwarth, T. Udem, T. Wilken, N. Ko- lachevsky, M. Abgrall, D. Rovera, C. Salomon, P. Lau- rent, G. Grosche, O. Terra, T. Legero, H. Schnatz, S. Weyers, B. Altschul, and T. W. Hansch, Phys. Rev. Lett. 110, 230801 (2013)
2013
-
[25]
Antognini, F
A. Antognini, F. Nez, K. Schuhmann, F. D. Amaro, F. Biraben, J. M. Cardoso, D. S. Covita, A. Dax, S. Dhawan, M. Diepold, L. M. Fernandes, A. Giesen, A. L. Gouvea, T. Graf, T. W. Hansch, P. Indelicato, L. Julien, C. Y. Kao, P. Knowles, F. Kottmann, E. O. 14 Le Bigot, Y. W. Liu,...
2013
-
[26]
Cancio Pastor, L
P. Cancio Pastor, L. Consolino, G. Giusfredi, P. De Na- tale, M. Inguscio, V. A. Yerokhin, and K. Pachucki, Phys. Rev. Lett. 108, 143001 (2012)
2012
-
[27]
Shiner, R
D. Shiner, R. Dixson, and V. V. Vedantham, Phys. Rev. Lett. 74, 3553 (1995)
1995
-
[28]
Huang, Y.-C
Y.-J. Huang, Y.-C. Guan, J.-L. Peng, J.-T. Shy, and L.- B. Wang, Phys. Rev. A 101, 062507 (2020)
2020
-
[29]
Sick, Phys
I. Sick, Phys. Rev. C 90, 064002 (2014)
2014
-
[30]
N. F. Ramsey, Phys. Rev. 76, 996 (1949)
1949
-
[31]
Wen, J.-D
J.-L. Wen, J.-D. Tang, J.-F. Dong, X.-J. Du, S.-M. Hu, and Y. R. Sun, Phys. Rev. A 107, 042811 (2023)
2023
-
[32]
J. J. Chen, Y. R. Sun, J. L. Wen, and S. M. Hu, Phys. Rev. A 101, 053824 (2020)
2020
-
[33]
Zheng, Y
X. Zheng, Y. R. Sun, J. J. Chen, W. Jiang, K. Pachucki, and S. M. Hu, Phys. Rev. Lett. 118, 063001 (2017)
2017
-
[34]
Shiner, R
D. Shiner, R. Dixson, and P. Zhao, Phys. Rev. Lett. 72, 1802 (1994)
1994
-
[35]
Tiesinga, P
E. Tiesinga, P. J. Mohr, D. B. Newell, and B. N. Taylor, Rev. Mod. Phys. 93, 025010 (2021)
2021
-
[36]
van Wijngaarden, F
A. van Wijngaarden, F. Holuj, and G. W. F. Drake, Phys. Rev. A 63, 012505 (2000)
2000
-
[37]
E. A. Hessels, P. W. Arcuni, F. J. Deck, and S. R. Lun- deen, Phys. Rev. A 46, 2622 (1992)
1992
-
[38]
D. J. Berkeland, E. A. Hinds, and M. G. Boshier, Phys. Rev. Lett. 75, 2470 (1995)
1995
-
[39]
Zheng, Y
X. Zheng, Y. R. Sun, J.-J. Chen, W. Jiang, K. Pachucki, and S.-M. Hu, Phys. Rev. Lett. 119, 263002 (2017)
2017
-
[40]
Marsman, M
A. Marsman, M. Horbatsch, and E. A. Hessels, Phys. Rev. A 86, 040501 (2012)
2012
-
[41]
Cancio Pastor, G
P. Cancio Pastor, G. Giusfredi, P. De Natale, G. Hagel, C. De Mauro, and M. Inguscio, Phys. Rev. Lett. 92, 230011 (2004)
2004
-
[42]
Cancio Pastor, G
P. Cancio Pastor, G. Giusfredi, P. De Natale, G. Hagel, C. de Mauro, and M. Inguscio, Phys. Rev. Lett. 97, 10.1103/PhysRevLett.97.139903 (2006)
2006 doi
-
[43]
Patk´ oˇ s, V
V. Patk´ oˇ s, V. A. Yerokhin, and K. Pachucki, Phys. Rev. A 103, 042809 (2021)
2021
-
[44]
Pachucki, V
K. Pachucki, V. Patk´ oˇ s, and V. A. Yerokhin, Phys. Rev. A 95, 062510 (2017)
2017
-
[45]
Zheng, Y
X. Zheng, Y. R. Sun, J. J. Chen, J. L. Wen, and S. M. Hu, Phys. Rev. A 99, 032506 (2019)
2019
Reviewed August 12, 2026 · model on record in the stance chip above.
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