Recognition: unknown
Sensitive dependence of Poor Man's Majorana modes on the length of the superconductor
Pith reviewed 2026-05-10 14:02 UTC · model grok-4.3
The pith
The number of Poor Man's Majorana modes oscillates between zero and two as the length of the finite superconductor changes by atomic spacings.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Treating the superconductor as a finite-length 1D tight-binding chain on equal footing with the two quantum dots yields conditions for Poor Man's Majorana modes valid at arbitrary length and arbitrary tunneling and Zeeman parameters. These conditions reveal that the number of such modes oscillates between zero and two with a period set by the Fermi wavelength while four modes appear in the long-superconductor limit where the effective coupling between the dots vanishes. Separately localized modes at the two outer ends are absent for finite length, so only nearly localized modes can be identified at sufficiently strong magnetic fields, thereby locating the generalized sweet spot of the device
What carries the argument
The finite-length 1D tight-binding chain representation of the superconductor, solved together with the two quantum dots, which produces length-dependent phase accumulation that controls the presence or absence of zero-energy states.
If this is right
- Poor Man's Majorana modes exist only for specific superconductor lengths spaced by roughly one angstrom.
- In the long-superconductor limit the two quantum dots effectively decouple and four modes appear.
- Fully separated localized modes at both ends of the hybrid structure cannot form at any finite length.
- The practical sweet spot requires strong magnetic fields to produce only nearly localized modes.
- Length must be treated as a tunable parameter rather than an idealized infinite bulk.
Where Pith is reading between the lines
- Device fabrication would need atomic-scale control of superconductor length to reliably produce or suppress the modes.
- Gate-voltage or magnetic-field sweeps alone may not suffice if length is off by even one lattice spacing.
- Topological protection expected from infinite-chain models is reduced or absent in the finite-length regime.
- Similar length sensitivity could appear in other hybrid systems where a normal region sits between two superconducting contacts.
Load-bearing premise
A simple uniform one-dimensional chain with no disorder, interface scattering, or higher-dimensional modes accurately describes the finite superconducting segment contacted by the quantum dots.
What would settle it
Scanning the superconductor length in increments of one angstrom while counting the number of zero-bias peaks or their spatial localization in a two-quantum-dot device, which should show repeated switches between zero and two modes.
Figures
read the original abstract
In a hybrid system where two quantum dots (QDs) are coupled to a conventional $s$-wave superconductor, Poor Man's Majorana modes (PMMs) have been proposed. Existing theories often idealize the superconductor (SC) as a bulk system or an infinitely long chain, or treat it as another quantum dot with proximity-induced superconductivity, while experiments employ superconducting segments of finite length. Here, we model the SC as a finite-length 1D chain and treat the QDs and SC on equal footing. We obtain the conditions for the existence of PMMs, valid for arbitrary SC length and applicable to arbitrary tunneling strengths and magnetic fields. We find that the number of PMMs is highly sensitive to the SC length: it oscillates between zero and two with a period set by the Fermi wavelength ($\sim1\,\text{\AA}$), while four PMMs appear in the long-SC limit where the effective coupling between the two QDs becomes negligible. We further demonstrate that the PMMs that are separately localized at the two ends of the hybrid system do not exist in the finite-length case. Consequently, only nearly localized PMMs can be identified when the magnetic field is strong enough. In this way, the generalized `sweet spot' of the practical system can be found.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents a theoretical study of Poor Man's Majorana modes (PMMs) in a hybrid system of two quantum dots coupled via a finite-length one-dimensional s-wave superconductor. Modeling the superconductor as a 1D tight-binding chain and solving for the conditions of PMM existence at arbitrary lengths, the authors find that the number of PMMs oscillates between zero and two with a period set by the Fermi wavelength of approximately 1 Å. In the limit of long superconductors, four PMMs emerge as the inter-dot coupling becomes negligible. They further argue that fully end-localized PMMs do not exist for finite lengths, and only nearly localized modes are possible at strong magnetic fields, leading to a generalized sweet spot for practical systems.
Significance. If the central results are robust, this work is significant because it reveals a strong dependence of PMM physics on the finite length of the superconductor, which has been overlooked in previous idealizations of infinite chains or bulk systems. This could have direct implications for the design and interpretation of experiments on hybrid superconductor-quantum dot devices aimed at realizing Majorana zero modes. The provision of length-dependent conditions for arbitrary parameters is a positive aspect, offering a more complete theoretical framework.
major comments (2)
- [§2 (Hamiltonian/model)] §2 (Hamiltonian/model): The superconductor segment is represented as a uniform 1D tight-binding chain with constant hopping t and pairing Δ (no interface scattering or disorder terms). This discretization directly produces the reported oscillations with period set by the Fermi wavelength (~1 Å) via phase accumulation across discrete sites. The central claim that this yields a 'generalized sweet spot of the practical system' therefore depends on the untested assumption that the 1D uniform chain is representative; real devices involve 3D segments, interface potentials, and disorder whose coherence lengths greatly exceed the lattice spacing and would generically suppress or average such atomic-scale oscillations.
- [Results section (long-SC limit and PMM counting)] Results section (long-SC limit and PMM counting): The assertion that four PMMs appear once the effective QD-QD coupling becomes negligible is load-bearing for the distinction between finite- and long-SC regimes. The manuscript should explicitly show the zero-mode counting (e.g., via the topological invariant or the number of exact zero-energy eigenvalues after diagonalization) and confirm that residual finite-size effects or numerical tolerances do not alter the count; without this, the transition to four PMMs remains tied to the specific parameter choices rather than a general limit.
minor comments (2)
- The phrase 'generalized sweet spot' appears in the abstract and conclusion but is not given an explicit operational definition (e.g., in terms of a specific condition on parameters or localization) in the main text.
- [Figure captions] Figure captions would benefit from listing the exact parameter values (t, Δ, μ, B, etc.) used in each panel to improve reproducibility.
Simulated Author's Rebuttal
We thank the referee for their thorough review and insightful comments on our manuscript. We address each of the major comments below and have revised the manuscript to incorporate the suggestions where possible.
read point-by-point responses
-
Referee: [§2 (Hamiltonian/model)] §2 (Hamiltonian/model): The superconductor segment is represented as a uniform 1D tight-binding chain with constant hopping t and pairing Δ (no interface scattering or disorder terms). This discretization directly produces the reported oscillations with period set by the Fermi wavelength (~1 Å) via phase accumulation across discrete sites. The central claim that this yields a 'generalized sweet spot of the practical system' therefore depends on the untested assumption that the 1D uniform chain is representative; real devices involve 3D segments, interface potentials, and disorder whose coherence lengths greatly exceed the lattice spacing and would generically suppress or average such atomic-scale oscillations.
Authors: We appreciate the referee pointing out the simplifications inherent in our 1D model. The uniform tight-binding chain allows for an exact analytical treatment of PMM conditions for arbitrary superconductor lengths, which is the core contribution of our work. The oscillations with the Fermi wavelength are a direct consequence of the finite-length phase accumulation in the superconducting pairing, a physical feature that persists beyond simple discretization in models with continuous spectra. While we agree that real experimental devices are three-dimensional with potential interface effects and disorder, these would likely introduce additional averaging; however, our results highlight the importance of considering finite-length effects even in idealized cases. In the revised manuscript, we have added a new paragraph in Section 2 discussing the model's assumptions and their implications for experimental relevance, including a note on how disorder might affect the oscillations. This provides a more balanced view without altering the main calculations. revision: partial
-
Referee: [Results section (long-SC limit and PMM counting)] Results section (long-SC limit and PMM counting): The assertion that four PMMs appear once the effective QD-QD coupling becomes negligible is load-bearing for the distinction between finite- and long-SC regimes. The manuscript should explicitly show the zero-mode counting (e.g., via the topological invariant or the number of exact zero-energy eigenvalues after diagonalization) and confirm that residual finite-size effects or numerical tolerances do not alter the count; without this, the transition to four PMMs remains tied to the specific parameter choices rather than a general limit.
Authors: We agree with the referee that explicit demonstration of the zero-mode counting is necessary to support our claims. In the revised manuscript, we have expanded the Results section to include a detailed analysis of the zero-energy spectrum. Specifically, we now present the number of exact zero-energy eigenvalues obtained from numerical diagonalization of the full Bogoliubov-de Gennes Hamiltonian across a range of superconductor lengths, confirming the transition to four PMMs in the long-SC limit. Additionally, we have incorporated a discussion of the topological invariant, calculated via the Pfaffian of the skew-symmetric matrix or by tracking the parity of negative-energy states, to rigorously establish the mode counting independent of numerical tolerances. This confirms that the four PMMs emerge generally when inter-dot coupling vanishes, strengthening the distinction between regimes. revision: yes
Circularity Check
No circularity: derivation starts from standard Hamiltonian and computes length dependence directly
full rationale
The paper models the system with a conventional 1D tight-binding chain for the finite-length superconductor coupled to two QDs, places all components on equal footing, and solves for the spectrum and zero-mode conditions as a function of SC length. The reported oscillation of PMM number with Fermi-wavelength period and the long-SC limit behavior follow from direct diagonalization or analytic conditions on this Hamiltonian; no parameter is fitted to data and then relabeled as a prediction, no self-citation supplies a uniqueness theorem or ansatz, and no step equates an output to its own input by definition. The modeling assumptions (uniform 1D chain) are stated explicitly and can be falsified externally, keeping the central claim independent of the authors' prior work.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The superconductor segment can be represented as a finite 1D tight-binding chain with uniform hopping and pairing amplitudes.
Forward citations
Cited by 1 Pith paper
-
Poor man's Majorana bound states in quantum dot based Kitaev chain coupled to a photonic cavity
Cavity photons screen attractive or repulsive interactions in a quantum-dot Kitaev chain, allowing the system to reach the sweet spot for poor man's Majorana bound states.
Reference graph
Works this paper leans on
-
[1]
A. Y . Kitaev, Unpaired majorana fermions in quantum wires, Phys. Usp.44, 131 (2001)
2001
-
[2]
J. D. Sau, R. M. Lutchyn, S. Tewari, and S. Das Sarma, Generic new platform for topological quantum computation using semiconductor heterostructures, Phys. Rev. Lett.104, 040502 (2010)
2010
-
[3]
Alicea, New directions in the pursuit of majorana fermions in solid state systems, Rep
J. Alicea, New directions in the pursuit of majorana fermions in solid state systems, Rep. Prog. Phys.75, 076501 (2012)
2012
-
[4]
R. M. Lutchyn, E. P. Bakkers, L. P. Kouwenhoven, P. Krogstrup, C. M. Marcus, and Y . Oreg, Majorana zero modes in superconductor–semiconductor heterostructures, Nat. Rev. Mater.3, 52 (2018)
2018
-
[5]
Prada, P
E. Prada, P. San-Jose, M. W. de Moor, A. Geresdi, E. J. Lee, J. Klinovaja, D. Loss, J. Nygård, R. Aguado, and L. P. Kouwenhoven, From andreev to majorana bound states in hy- brid superconductor–semiconductor nanowires, Nat. Rev. Phys. 2, 575 (2020)
2020
-
[6]
R. M. Lutchyn, J. D. Sau, and S. Das Sarma, Majorana fermions and a topological phase transition in semiconductor- superconductor heterostructures, Phys. Rev. Lett.105, 077001 (2010)
2010
-
[7]
Y . Oreg, G. Refael, and F. von Oppen, Helical liquids and ma- jorana bound states in quantum wires, Phys. Rev. Lett.105, 177002 (2010)
2010
-
[8]
Flensberg, F
K. Flensberg, F. von Oppen, and A. Stern, Engineered platforms for topological superconductivity and majorana zero modes, Nat. Rev. Mater.6, 944 (2021)
2021
-
[9]
Fu and C
L. Fu and C. L. Kane, Superconducting proximity effect and majorana fermions at the surface of a topological insulator, Phys. Rev. Lett.100, 096407 (2008)
2008
-
[10]
X.-L. Qi, T. L. Hughes, and S.-C. Zhang, Chiral topological superconductor from the quantum hall state, Phys. Rev. B82, 184516 (2010)
2010
-
[11]
J. Wang, Q. Zhou, B. Lian, and S.-C. Zhang, Chiral topologi- cal superconductor and half-integer conductance plateau from quantum anomalous hall plateau transition, Phys. Rev. B92, 064520 (2015)
2015
-
[12]
S. B. Chung, X.-L. Qi, J. Maciejko, and S.-C. Zhang, Conduc- tance and noise signatures of majorana backscattering, Phys. Rev. B83, 100512(R) (2011)
2011
-
[13]
A. Uday, G. Lippertz, B. Bhujel, A. A. Taskin, and Y . Ando, Non-majorana origin of the half-integer conductance quan- tization elucidated by multiterminal superconductor–quantum anomalous hall insulator heterostructure, Phys. Rev. B111, 035440 (2025)
2025
-
[14]
Qiao, S.-W
G.-J. Qiao, S.-W. Li, and C. P. Sun, Magnetic field constraint for majorana zero modes in a hybrid nanowire, Phys. Rev. B 106, 104517 (2022)
2022
-
[15]
Yue, G.-J
X. Yue, G.-J. Qiao, and C. P. Sun, Refined majorana phase dia- gram in a topological insulator–superconductor hybrid system, Phys. Rev. B108, 195405 (2023)
2023
-
[16]
G.-J. Qiao, X. Yue, and C. P. Sun, Dressed majorana fermion in a hybrid nanowire, Phys. Rev. Lett.133, 266605 (2024)
2024
-
[17]
Yue, G.-J
X. Yue, G.-J. Qiao, and C. P. Sun, Finite-size effects on met- allization versus chiral majorana fectmions, Phys. Rev. B113, 115416 (2026)
2026
-
[18]
G.-J. Qiao, Z.-L. Zhang, X. Yue, and C. Sun, Size opti- mization for observeing majorana fermions, arXiv preprint arXiv:2511.21423 (2025)
-
[19]
Kells, D
G. Kells, D. Meidan, and P. W. Brouwer, Near-zero-energy end states in topologically trivial spin-orbit coupled superconduct- ing nanowires with a smooth confinement, Phys. Rev. B86, 100503 (2012)
2012
-
[20]
C.-X. Liu, J. D. Sau, T. D. Stanescu, and S. Das Sarma, An- dreev bound states versus majorana bound states in quantum dot-nanowire-superconductor hybrid structures: Trivial ver- sus topological zero-bias conductance peaks, Phys. Rev. B96, 075161 (2017)
2017
-
[21]
S. D. Sarma, In search of majorana, Nature Physics19, 165 (2023)
2023
-
[22]
Leijnse and K
M. Leijnse and K. Flensberg, Parity qubits and poor man’s ma- jorana bound states in double quantum dots, Phys. Rev. B86, 134528 (2012)
2012
-
[23]
Luethi, H
M. Luethi, H. F. Legg, D. Loss, and J. Klinovaja, From perfect to imperfect poor man’s majoranas in minimal kitaev chains, Phys. Rev. B110, 245412 (2024)
2024
-
[24]
C.-X. Liu, A. M. Bozkurt, F. Zatelli, S. L. ten Haaf, T. Dvir, and M. Wimmer, Enhancing the excitation gap of a quantum- dot-based kitaev chain, Commun. Phys.7, 235 (2024)
2024
-
[25]
Z.-L. Zhang, G.-J. Qiao, and C. Sun, Poor man’s majora- non in two quantum dots dressed by superconducting quasi- 11 excitations, arXiv preprint arXiv:2506.10367 (2025)
-
[26]
Optimal Majoranas in Mesoscopic Kitaev Chains
M. Alvarado, R. S. Souto, M. J. Calder ´on, and R. Aguado, Optimal majoranas in mesoscopic kitaev chains, arXiv preprint arXiv:2604.13945 (2026)
work page internal anchor Pith review Pith/arXiv arXiv 2026
-
[27]
Tsintzis, R
A. Tsintzis, R. S. Souto, and M. Leijnse, Creating and detecting poor man’s majorana bound states in interacting quantum dots, Phys. Rev. B106, L201404 (2022)
2022
-
[28]
Luethi, H
M. Luethi, H. F. Legg, D. Loss, and J. Klinovaja, Fate of poor man’s majoranas in the long kitaev chain limit, Phys. Rev. B 111, 115419 (2025)
2025
-
[29]
T. Dvir, G. Wang, N. van Loo, C.-X. Liu, G. P. Mazur, A. Bor- din, S. L. Ten Haaf, J.-Y . Wang, D. van Driel, F. Zatelli,et al., Realization of a minimal kitaev chain in coupled quantum dots, Nat.614, 445 (2023)
2023
-
[30]
S. L. Ten Haaf, Q. Wang, A. M. Bozkurt, C.-X. Liu, I. Kulesh, P. Kim, D. Xiao, C. Thomas, M. J. Manfra, T. Dvir,et al., A two-site kitaev chain in a two-dimensional electron gas, Nat. 630, 329 (2024)
2024
-
[31]
Zatelli, D
F. Zatelli, D. van Driel, D. Xu, G. Wang, C.-X. Liu, A. Bordin, B. Roovers, G. P. Mazur, N. van Loo, J. C. Wolff,et al., Robust poor man’s majorana zero modes using yu-shiba-rusinov states, Nat. Commun.15, 7933 (2024)
2024
-
[32]
Bordin, G
A. Bordin, G. Wang, C.-X. Liu, S. L. D. ten Haaf, N. van Loo, G. P. Mazur, D. Xu, D. van Driel, F. Zatelli, S. Gazibegovic, G. Badawy, E. P. A. M. Bakkers, M. Wimmer, L. P. Kouwen- hoven, and T. Dvir, Tunable crossed andreev reflection and elas- tic cotunneling in hybrid nanowires, Phys. Rev. X13, 031031 (2023)
2023
-
[33]
van Loo, F
N. van Loo, F. Zatelli, G. O. Steffensen, B. Roovers, G. Wang, T. Van Caekenberghe, A. Bordin, D. van Driel, Y . Zhang, W. D. Huisman,et al., Single-shot parity readout of a minimal kitaev chain, Nat.650, 334 (2026)
2026
-
[34]
Leijnse and K
M. Leijnse and K. Flensberg, Coupling spin qubits via super- conductors, Phys. Rev. Lett.111, 060501 (2013)
2013
-
[35]
C.-X. Liu, G. Wang, T. Dvir, and M. Wimmer, Tunable super- conducting coupling of quantum dots via andreev bound states in semiconductor-superconductor nanowires, Phys. Rev. Lett. 129, 267701 (2022)
2022
-
[36]
C. Reeg, D. Loss, and J. Klinovaja, Finite-size effects in a nanowire strongly coupled to a thin superconducting shell, Phys. Rev. B96, 125426 (2017)
2017
-
[37]
R. M. Lutchyn, T. D. Stanescu, and S. Das Sarma, Search for majorana fermions in multiband semiconducting nanowires, Phys. Rev. Lett.106, 127001 (2011)
2011
-
[38]
J. D. Sau and S. D. Sarma, Realizing a robust practical majo- rana chain in a quantum-dot-superconductor linear array, Nat. Commun.3, 964 (2012)
2012
-
[39]
Qiao, Z.-L
G.-J. Qiao, Z.-L. Zhang, S.-W. Li, and C. P. Sun, Controlling a superconducting transistor by coherent light, Communications in Theoretical Physics77, 095103 (2025)
2025
-
[40]
T. D. Stanescu, J. D. Sau, R. M. Lutchyn, and S. Das Sarma, Proximity effect at the superconductor–topological insulator in- terface, Phys. Rev. B81, 241310 (2010)
2010
-
[41]
T. D. Stanescu and S. Das Sarma, Proximity-induced low-energy renormalization in hybrid semiconductor- superconductor majorana structures, Phys. Rev. B96, 014510 (2017)
2017
-
[42]
T. D. Stanescu, R. M. Lutchyn, and S. Das Sarma, Majorana fermions in semiconductor nanowires, Phys. Rev. B84, 144522 (2011)
2011
-
[43]
Li, Z.-Z
S.-W. Li, Z.-Z. Li, C. Cai, and C. P. Sun, Probing zero modes of a defect in a kitaev quantum wire, Phys. Rev. B89, 134505 (2014)
2014
-
[44]
Bordin, C.-X
A. Bordin, C.-X. Liu, T. Dvir, F. Zatelli, S. L. Ten Haaf, D. Van Driel, G. Wang, N. Van Loo, Y . Zhang, J. C. Wolff, et al., Enhanced majorana stability in a three-site kitaev chain, Nat. Nanotechnol.20, 726 (2025)
2025
-
[45]
Bordin, F
A. Bordin, F. J. Bennebroek Evertsz’, B. Roovers, J. D. Tor- res Luna, W. D. Huisman, F. Zatelli, G. P. Mazur, S. L. Ten Haaf, G. Badawy, E. P. Bakkers,et al., Probing majorana localization of a phase-controlled three-site kitaev chain with an additional quantum dot, Nat. Commun.17, 2313 (2026)
2026
-
[46]
S. L. Ten Haaf, Y . Zhang, Q. Wang, A. Bordin, C.-X. Liu, I. Kulesh, V . P. Sietses, C. G. Prosko, D. Xiao, C. Thomas, et al., Observation of edge and bulk states in a three-site kitaev chain, Nature641, 890 (2025)
2025
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.