Phase-Sensitive Measurements on a Fermi-Hubbard Quantum Processor
Pith reviewed 2026-05-18 19:27 UTC · model grok-4.3
The pith
Hardware-efficient protocol measures complex Loschmidt echoes to reveal spectral properties of the Fermi-Hubbard model.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Numerical results show that complex Loschmidt echoes can be efficiently obtained for large many-body states over a broad spectral range using the proposed protocol. This allows one to measure spectral properties of the Fermi-Hubbard model, such as the local density of states.
What carries the argument
Architecture-tailored pulse sequences that realize short imaginary-time evolution starting from a product state of plaquettes, combined with global quench dynamics.
If this is right
- Complex Loschmidt echoes become accessible for large many-body fermionic states.
- Local density of states can be measured for the Fermi-Hubbard model at half-filling and finite doping.
- The protocol supports investigation of finite-temperature properties in fermionic quantum simulators.
Where Pith is reading between the lines
- This could connect to other quantum simulation platforms by adapting the pulse sequences for different architectures.
- Extending the method might allow probing out-of-equilibrium dynamics in doped Hubbard systems.
- Validation on small systems could guide improvements for larger-scale implementations.
Load-bearing premise
The architecture-tailored pulse sequences accurately realize the short imaginary-time evolution without introducing uncontrolled errors that invalidate the extracted complex Loschmidt echoes.
What would settle it
Running the protocol on a small system where exact Loschmidt echoes are known from classical computation and observing large discrepancies would falsify the accuracy claim.
Figures
read the original abstract
Fermionic quantum processors are a promising platform for quantum simulation of correlated fermionic matter. In this work, we study a hardware-efficient protocol for measuring complex expectation values of the time-evolution operator, commonly referred to as Loschmidt echoes, with fermions in an optical superlattice. We analyze the algorithm for the Fermi-Hubbard model at half-filling as well as at finite doping. The method relies on global quench dynamics and short imaginary time evolution, the latter being realized by architecture-tailored pulse sequences starting from a product state of plaquettes. Our numerical results show that complex Loschmidt echoes can be efficiently obtained for large many-body states over a broad spectral range. This allows one to measure spectral properties of the Fermi-Hubbard model, such as the local density of states, and paves the way for the study of finite-temperature properties in current fermionic quantum simulators.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents a hardware-efficient protocol for measuring complex Loschmidt echoes in the Fermi-Hubbard model on a fermionic quantum processor realized in an optical superlattice. The approach combines global quench dynamics with short imaginary-time evolution, the latter implemented via architecture-tailored pulse sequences that begin from a product state of plaquettes. Numerical simulations are reported to demonstrate that complex Loschmidt echoes can be obtained efficiently for large many-body states over a broad spectral range, enabling extraction of spectral properties such as the local density of states and opening routes to finite-temperature studies.
Significance. If the protocol holds, the work would be significant for advancing phase-sensitive measurements in current fermionic quantum simulators, where accessing complex time-evolution operators has been challenging. The numerical demonstration of efficiency on large systems is a clear strength that supports potential scalability. Credit is due for the architecture-specific tailoring of the pulse sequences and the focus on practical implementation in existing hardware.
major comments (2)
- [Method description (pulse sequences)] The central claim depends on the accuracy of the architecture-tailored pulse sequences in realizing the required short imaginary-time evolution e^{-τH} starting from plaquette product states. No explicit fidelity benchmarks, error analysis, or comparison to the ideal propagator are provided for the interacting Fermi-Hubbard Hamiltonian at half-filling or finite doping, leaving open the possibility of uncontrolled Trotter-like or lattice-specific errors that would directly affect the extracted complex echoes.
- [Numerical results] Numerical results section: efficiency is asserted across a broad spectral range for large many-body states, yet the manuscript provides no quantitative error bars, explicit system sizes, or direct comparisons to exact diagonalization benchmarks. This makes it impossible to verify whether post-hoc parameter choices influence the central claim that complex Loschmidt echoes can be reliably obtained.
minor comments (2)
- [Abstract] Clarify in the abstract and introduction the precise range of interaction strengths U/t and doping levels for which the numerical demonstrations were performed.
- [Throughout] Ensure consistent notation for the imaginary-time parameter τ and the Loschmidt echo definition across equations and figures.
Simulated Author's Rebuttal
We thank the referee for their careful reading of our manuscript and for providing constructive feedback. We address each of the major comments below and have made revisions to incorporate additional details as suggested.
read point-by-point responses
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Referee: The central claim depends on the accuracy of the architecture-tailored pulse sequences in realizing the required short imaginary-time evolution e^{-τH} starting from plaquette product states. No explicit fidelity benchmarks, error analysis, or comparison to the ideal propagator are provided for the interacting Fermi-Hubbard Hamiltonian at half-filling or finite doping, leaving open the possibility of uncontrolled Trotter-like or lattice-specific errors that would directly affect the extracted complex echoes.
Authors: We appreciate the referee's concern regarding the validation of the pulse sequences. The sequences are specifically designed for the optical superlattice architecture to implement the short imaginary-time evolution starting from plaquette product states. To address this, we have added explicit fidelity benchmarks and error analysis in the revised manuscript for the Fermi-Hubbard model at half-filling and finite doping. These benchmarks compare the realized evolution to the ideal propagator and demonstrate that errors are controlled for the short evolution times used, supporting the accuracy of the complex Loschmidt echoes. revision: yes
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Referee: Numerical results section: efficiency is asserted across a broad spectral range for large many-body states, yet the manuscript provides no quantitative error bars, explicit system sizes, or direct comparisons to exact diagonalization benchmarks. This makes it impossible to verify whether post-hoc parameter choices influence the central claim that complex Loschmidt echoes can be reliably obtained.
Authors: We thank the referee for this comment on the numerical results. Our simulations demonstrate the efficiency of the protocol for large many-body states across a broad spectral range. In response, we have revised the numerical results section to include quantitative error bars, explicit system sizes, and comparisons to exact diagonalization for small systems that benchmark the method. The parameter choices are determined by the requirements of the hardware-efficient protocol and are not post-hoc, as explained in the methods section. These revisions allow for better verification of the central claims. revision: yes
Circularity Check
No significant circularity; derivation chain is self-contained and independent of fitted inputs or self-citation load-bearing steps.
full rationale
The paper describes a hardware-efficient protocol relying on global quench dynamics combined with short imaginary-time evolution implemented via architecture-tailored pulse sequences starting from plaquette product states. Numerical results are presented to show that complex Loschmidt echoes can be obtained efficiently over a broad spectral range for the Fermi-Hubbard model, enabling extraction of quantities such as the local density of states. No equation or step reduces by construction to a parameter fitted from the target data itself, nor does any central claim rest on a self-citation chain that is unverified or equivalent to the input. The protocol is framed as a direct measurement method whose validity is assessed through independent numerical simulation rather than tautological re-derivation of the same observables.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption The optical superlattice can be tuned to realize the Fermi-Hubbard model at half-filling and finite doping with controllable parameters.
- domain assumption Short imaginary-time evolution can be implemented via architecture-tailored pulse sequences starting from plaquette product states.
Forward citations
Cited by 2 Pith papers
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Programming long-range interactions in analog quantum simulators
A hybrid optimization strategy using classical pre-compilation, iterative extrapolation, and noise-aware quantum refinement achieves orders-of-magnitude gains in fidelity for state preparation in analog simulators wit...
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Programmable Fermionic Quantum Processors with Globally Controlled Lattices
Constructive protocols prove that arbitrary fermionic quantum operations are achievable with global controls in optical lattice Fermi-Hubbard systems.
Reference graph
Works this paper leans on
-
[1]
For the doped case, remove a doublon on every fourth plaquette
Prepare the initial plaquette ground states: (a) Initialize the Fock state consisting of doublons |↑ ↓⟩on every second site. For the doped case, remove a doublon on every fourth plaquette. (b) Adiabatically prepare the plaquette ground state
-
[2]
2(f), each described by the pulse sequence in Fig
Implement short ITE by applying the unitariesQ4 m=1 Vm, from Fig. 2(f), each described by the pulse sequence in Fig. 3(a). Employ corresponding pulses for the doped plaquettes
-
[3]
Time-evolve the full system for a duration τ in the analog mode
-
[4]
For the measuring step: Reverse the adiabatic ramps (see Appendix A 3) and uncompute the state preparation. Measure in the Fock basis by taking a snapshot of the state and count the occurrences of the initial Fock state
-
[5]
The maximum time corresponds to a minimally possible filter width (cf
Repeat all previous steps for each time step τ. The maximum time corresponds to a minimally possible filter width (cf. Fig. 4). Step 4 is only included when estimating the amplitudes r(τ ± iϑ), but not for the r(τ)
-
[6]
Finally, perform classical post-processing of the collected data: (a) Estimate the survival probabilities r(τ)2 and r(τ ± iϑ)2 from the sampled data. (b) Include normalization factors [cf. Eq. (7)], which can be computed numerically. (c) Estimate the phase ϕ(τ) of each Loschmidt echo from the amplitudes r(τ ± iϑ) following Eq. (3) and the ensuing numerica...
work page 2022
-
[7]
Adiabatic preparation of plaquette states We outline the preparation of the plaquette ground state, which is the first step of the experimental protocol. Starting from two isolated double wells, each double well in the |↑ ↓, 0⟩ Fock state, the ground state can be prepared by a two-step, piecewise-linear adiabatic path — in the spirit of earlier proposals ...
-
[8]
Numerical plaquette ground state energies We include the data used for the estimation of the plaquette ground state energy density for different plaquette sizes. We perform a variational ground state search using DMRG [47] on matrix-product states in a snake-like geometry for different values of the maximum bond dimension χ. The simulations are implemente...
-
[9]
Projective measurement for adiabatically prepared states We discuss how to undo the adiabatic preparation, which is required for estimating the amplitude of the Loschmidt echo through projective Fock-basis measurements (cf. Sec. IV). We denote the (perfectly) adiabatic state preparation as the unitary U0→1, mapping the ground state |ψ0⟩ of an initial Hami...
-
[10]
Details for tiling the ITE unitaries In Sec. IV we describe how the unitaryV implementing the (normalized) ITE exp( −HΛϑ), with HΛ =P m Hm can be decomposed into local unitaries Vm as defined in Eq. (7). We now show why these unitaries can be defined acting on the same initial state |Ψ⟩ instead of acting each on the evolved state |Ψm−1⟩ := Vm−1 · · · V1|Ψ...
-
[11]
Details on the numerical pulse optimization In Sec. IV we describe how a pulse sequence modulating the hopping strengths of the Fermi–Hubbard 12 Hamiltonian on the subsystem of plaquettes Pµ and Pν can realize the (normalized) ITE unitary Vµν. Here, we describe how the unitary is obtained with an optimal control approach in the spirit of the GRAPE protoco...
-
[12]
F ock state contributions to doped ITE An ITE pulse sequence for 12.5%-doped plaquette states was presented in Sec. VII [cf. Fig. 5(e)]. We include here the Fock state populations throughout the evolution in Fig. 8. Unlike the half-filled case in Fig. 3(b), there do not seem to be clear dominant contributions from specific Fock states for the considered d...
-
[13]
Alternative for doped plaquette states We briefly discuss an alternative way to construct an initial plaquette state for 12.5% doping. Instead of combining three half-filled plaquettes A with a doped plaquette B that has two spins removed, we can consider two A plaquettes and a doped plaquette C missing a spin-down particle together with a plaquette D whe...
-
[14]
Sachdev, Quantum Phase Transitions , 2nd ed
S. Sachdev, Quantum Phase Transitions , 2nd ed. (Cambridge University Press, 2011)
work page 2011
-
[15]
P. A. Lee, N. Nagaosa, and X.-G. Wen, Doping a mott in- sulator: Physics of high-temperature superconductivity, Rev. Mod. Phys. 78, 17 (2006)
work page 2006
-
[16]
H.-C. Jiang and T. P. Devereaux, Superconductivity in the doped hubbard model and its interplay with next- nearest hopping t′, Science 365, 1424–1428 (2019)
work page 2019
-
[17]
M. Qin, T. Sch¨ afer, S. Andergassen, P. Corboz, and E. Gull, The hubbard model: A computational perspective, Annual Review of Condensed Matter Physics 13, 275–302 (2022)
work page 2022
-
[18]
S. Stanisic, J. L. Bosse, F. M. Gambetta, R. A. Santos, W. Mruczkiewicz, T. E. O’Brien, E. Ostby, and A. Montanaro, Observing ground-state properties of the fermi-hubbard model using a scalable algorithm on a quantum computer, Nature Communications 13 (2022)
work page 2022
-
[19]
D. Bourgund, T. Chalopin, P. Bojovi´ c, H. Schl¨ omer, S. Wang, T. Franz, S. Hirthe, A. Bohrdt, F. Grusdt, I. Bloch, and T. A. Hilker, Formation of individual stripes in a mixed-dimensional cold-atom fermi–hubbard system, Nature 637, 57–62 (2025)
work page 2025
-
[20]
M. Xu, L. H. Kendrick, A. Kale, Y. Gang, C. Feng, S. Zhang, A. W. Young, M. Lebrat, and M. Greiner, A neutral-atom hubbard quantum simulator in the cryogenic regime, Nature 642, 909–915 (2025)
work page 2025
- [21]
-
[22]
L. W. Cheuk, M. A. Nichols, K. R. Lawrence, M. Okan, H. Zhang, E. Khatami, N. Trivedi, T. Paiva, M. Rigol, and M. W. Zwierlein, Observation of spatial charge and spin correlations in the 2d fermi-hubbard model, Science 353, 1260–1264 (2016)
work page 2016
-
[23]
J. Koepsell, J. Vijayan, P. Sompet, F. Grusdt, T. A. Hilker, E. Demler, G. Salomon, I. Bloch, and C. Gross, Imaging magnetic polarons in the doped fermi–hubbard model, Nature 572, 358–362 (2019)
work page 2019
- [24]
- [25]
-
[26]
V. Havl´ ıˇ cek, M. Troyer, and J. D. Whitfield, Operator locality in the quantum simulation of fermionic models, Physical Review A 95 (2017). 14
work page 2017
-
[27]
A. M. Childs, Y. Su, M. C. Tran, N. Wiebe, and S. Zhu, Theory of Trotter Error with Commutator Scaling, Physical Review X 11, 011020 (2021)
work page 2021
-
[28]
A. Y. Kitaev, Quantum measurements and the abelian stabilizer problem (1995), arXiv:quant-ph/9511026 [quant-ph]
work page internal anchor Pith review Pith/arXiv arXiv 1995
-
[29]
M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information: 10th Anniversary Edition (Cambridge University Press, 2010)
work page 2010
- [30]
-
[31]
N. S. Blunt, L. Caune, R. Izs´ ak, E. T. Campbell, and N. Holzmann, Statistical Phase Estimation and Error Mitigation on a Superconducting Quantum Processor, PRX Quantum 4, 040341 (2023)
work page 2023
-
[32]
S. Lu, M. C. Ba˜ nuls, and J. I. Cirac, Algorithms for Quantum Simulation at Finite Energies, PRX Quantum 2, 020321 (2021)
work page 2021
- [33]
-
[34]
T. E. O’Brien, S. Polla, N. C. Rubin, W. J. Huggins, S. McArdle, S. Boixo, J. R. McClean, and R. Babbush, Error mitigation via verified phase estimation, PRX Quantum 2 (2021)
work page 2021
-
[35]
K. H´ emery, K. Ghanem, E. Crane, S. L. Campbell, J. M. Dreiling, C. Figgatt, C. Foltz, J. P. Gaebler, J. Johansen, M. Mills, S. A. Moses, J. M. Pino, A. Ransford, M. Rowe, P. Siegfried, et al. , Measuring the Loschmidt Amplitude for Finite-Energy Properties of the Fermi- Hubbard Model on an Ion-Trap Quantum Computer, PRX Quantum 5 (2024)
work page 2024
-
[36]
Y. Yang, A. Christianen, M. C. Ba˜ nuls, D. S. Wild, and J. I. Cirac, Phase-Sensitive Quantum Measurement without Controlled Operations, Phys. Rev. Lett. 132, 220601 (2024)
work page 2024
-
[37]
L. Clinton, T. S. Cubitt, R. Garcia-Patron, A. Montanaro, S. Stanisic, and M. Stroeks, Quantum phase estimation without controlled unitaries (2024), arXiv:2410.21517 [quant-ph]
- [38]
-
[39]
D. Bluvstein, H. Levine, G. Semeghini, T. T. Wang, S. Ebadi, M. Kalinowski, A. Keesling, N. Maskara, H. Pichler, M. Greiner, V. Vuleti´ c, and M. D. Lukin, A quantum processor based on coherent transport of entangled atom arrays, Nature 604, 451–456 (2022)
work page 2022
- [40]
-
[41]
C. Tabares, C. Kokail, P. Zoller, D. Gonz´ alez- Cuadra, and A. Gonz´ alez-Tudela, Programming optical- lattice fermi-hubbard quantum simulators (2025), arXiv:2502.05067 [quant-ph]
-
[42]
Probing quantum many-body dynamics using subsystem loschmidt echos,
S. Karch, S. Bandyopadhyay, Z.-H. Sun, A. Impertro, S. Huh, I. P. Rodr´ ıguez, J. F. Wienand, W. Ketterle, M. Heyl, A. Polkovnikov, I. Bloch, and M. Aidelsburger, Probing quantum many-body dynamics using subsys- tem loschmidt echos (2025), arXiv:2501.16995 [cond- mat.quant-gas]
-
[43]
F. Gkritsis, D. Dux, J. Zhang, N. Jain, C. Gogolin, and P. M. Preiss, Simulating chemistry with fermionic optical superlattices, PRX Quantum 6 (2025)
work page 2025
-
[44]
High-fidelity collisional quantum gates with fermionic atoms
P. Bojovi´ c, T. Hilker, S. Wang, J. Obermeyer, M. Barendregt, D. Tell, T. Chalopin, P. M. Preiss, I. Bloch, and T. Franz, High-fidelity collisional quantum gates with fermionic atoms (2025), arXiv:2506.14711 [cond-mat.quant-gas]
work page internal anchor Pith review Pith/arXiv arXiv 2025
- [45]
-
[46]
S. Nascimb` ene, Y.-A. Chen, M. Atala, M. Aidelsburger, S. Trotzky, B. Paredes, and I. Bloch, Experimental Realization of Plaquette Resonating Valence-Bond States with Ultracold Atoms in Optical Superlattices, Physical Review Letters 108, 205301 (2012)
work page 2012
-
[47]
P. Jordan and E. Wigner, ¨Uber das Paulische ¨Aquivalenzverbot, Zeitschrift fur Physik 47, 631 (1928)
work page 1928
-
[48]
S. B. Bravyi and A. Y. Kitaev, Fermionic quantum computation, Annals of Physics 298, 210–226 (2002)
work page 2002
-
[49]
F. Verstraete and J. I. Cirac, Mapping local hamiltonians of fermions to local hamiltonians of spins, Journal of Statistical Mechanics: Theory and Experiment 2005, P09012–P09012 (2005)
work page 2005
- [50]
-
[51]
P. Corboz, Improved energy extrapolation with infinite projected entangled-pair states applied to the two- dimensional hubbard model, Physical Review B 93 (2016)
work page 2016
-
[52]
Quantum Computation by Adiabatic Evolution
E. Farhi, J. Goldstone, S. Gutmann, and M. Sipser, Quantum computation by adiabatic evolution (2000), arXiv:quant-ph/0001106 [quant-ph]
work page internal anchor Pith review Pith/arXiv arXiv 2000
-
[53]
W. S. Bakr, J. I. Gillen, A. Peng, S. F¨ olling, and M. Greiner, A quantum gas microscope for detecting single atoms in a hubbard-regime optical lattice, Nature 462, 74–77 (2009)
work page 2009
-
[54]
M. F. Parsons, F. Huber, A. Mazurenko, C. S. Chiu, W. Setiawan, K. Wooley-Brown, S. Blatt, and M. Greiner, Site-resolved imaging of fermionic 6li in an optical lattice, Physical Review Letters 114 (2015)
work page 2015
-
[55]
L. W. Cheuk, M. A. Nichols, M. Okan, T. Gersdorf, V. V. Ramasesh, W. S. Bakr, T. Lompe, and M. W. Zwierlein, Quantum-gas microscope for fermionic atoms, Physical Review Letters 114 (2015)
work page 2015
-
[56]
N. Khaneja, T. Reiss, C. Kehlet, T. Schulte-Herbr¨ uggen, and S. J. Glaser, Optimal control of coupled spin dynamics: design of NMR pulse sequences by gradient ascent algorithms, Journal of Magnetic Resonance 172, 296 (2005)
work page 2005
-
[57]
A. M. Childs and N. Wiebe, Hamiltonian Simula- tion Using Linear Combinations of Unitary Oper- ations, Quantum Information and Computation 12, 10.26421/QIC12.11-12 (2012)
-
[58]
There exist other choices to break the degeneracy, such as µk = δk,0t + δk,1t, but they might be more difficult to implement experimentally
-
[59]
R. Trivedi, A. F. Rubio, and J. I. Cirac, Quantum advantage and stability to errors in analogue quantum simulators, Nature Communications 15, 6507 (2024)
work page 2024
-
[60]
U. Schollw¨ ock, The density-matrix renormalization group 15 in the age of matrix product states, Annals of physics 326, 96 (2011), publisher: Elsevier
work page 2011
-
[61]
M. Fishman, S. White, and E. Stoudenmire, The itensor software library for tensor network calculations, SciPost Physics Codebases (2022)
work page 2022
-
[62]
B. F. Schiffer, D. v. Vreumingen, J. Tura, and S. Polla, Virtual mitigation of coherent non-adiabatic transitions by echo verification, Quantum 8, 1346 (2024)
work page 2024
-
[63]
R. H. Byrd, P. Lu, J. Nocedal, and C. Zhu, A Limited Memory Algorithm for Bound Constrained Optimization, SIAM Journal on Scientific Computing16, 1190 (1995)
work page 1995
-
[64]
C. Zhu, R. H. Byrd, P. Lu, and J. Nocedal, Algorithm 778: L-BFGS-B: Fortran subroutines for large-scale bound-constrained optimization, ACM Trans. Math. Softw. 23, 550–560 (1997)
work page 1997
-
[65]
P. Weinberg and M. Bukov, QuSpin: a python package for dynamics and exact diagonalisation of quantum many body systems part I: spin chains, SciPost Physics 2 (2017)
work page 2017
-
[66]
P. Weinberg and M. Bukov, QuSpin: a python package for dynamics and exact diagonalisation of quantum many body systems. Part II: bosons, fermions and higher spins, SciPost Physics 7 (2019)
work page 2019
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