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Quantum algorithms to simulate many-body physics of correlated fermions

Simulate strongly correlated fermionic systems — notoriously hard for classical computers — on a quantum computer with 2D or linear (1D) nearest-neighbor qubit-qubit couplings, of the kind typical of superconducting transmon qubit arrays, including preparing the relevant quantum states and evolving the system in time, with the Fermi-Hubbard model as a worked example.

Attested & literatureUpdated

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  • Record typeLiterature reference
  • Reference implementationfermi_hubbard_model_1D
  • Reported costThree separate bounds, each attached to the subroutine the abstract attaches it to, not merged into one figure for the algorithm as a whole: preparing an arbitrary fermionic Gaussian state costs O(N²) gates and O(N) circuit depth; the 2D fermionic Fourier transform on a 2D qubit array costs O(N^1.5) gates and O(√N) circuit depth, described as the minimum depth needed for quantum information to cross the array; and simulating each time step of the 2D Fermi-Hubbard model, again on a 2D qubit array, costs O(N) gates and O(√N) circuit depth. None of the three is a total cost for a full simulation, and none is stated by the abstract specifically for the one-dimensional (linear) geometry this record's reference implementation demonstrates: the abstract names linear geometry as within its scope but attaches no gate or depth bound to it.
  • Papers1

Jiang, Sung, Kechedzhi, Smelyanskiy and Boixo discuss quantum simulation of strongly correlated fermionic systems, following Feynman's proposal to use a quantum computer for a problem notoriously hard on classical ones, and focus specifically on 2D and linear geometry with nearest-neighbor qubit-qubit couplings, typical for superconducting transmon qubit arrays. They improve an existing algorithm for preparing an arbitrary Slater determinant by exploiting a unitary symmetry, and present a quantum algorithm to prepare an arbitrary fermionic Gaussian state with O(N²) gates and O(N) circuit depth; both algorithms, the authors say, are optimal in that the number of parameters in the circuit equals the number needed to describe the state. They also propose an algorithm for the 2D fermionic Fourier transform on a 2D qubit array with O(N^1.5) gates and O(√N) circuit depth, which they identify as the minimum depth required for quantum information to travel across the array. Separately, they present methods to simulate each time step of the evolution of the 2D Fermi-Hubbard model, again on a 2D qubit array, with O(N) gates and O(√N) circuit depth. The authors close by discussing how these algorithms can be used to determine the ground-state properties and phase diagrams of strongly correlated quantum systems, with the Hubbard model as their example. Reported cost: Three separate bounds, each attached to the subroutine the abstract attaches it to, not merged into one figure for the algorithm as a whole: preparing an arbitrary fermionic Gaussian state costs O(N²) gates and O(N) circuit depth; the 2D fermionic Fourier transform on a 2D qubit array costs O(N^1.5) gates and O(√N) circuit depth, described as the minimum depth needed for quantum information to cross the array; and simulating each time step of the 2D Fermi-Hubbard model, again on a 2D qubit array, costs O(N) gates and O(√N) circuit depth. None of the three is a total cost for a full simulation, and none is stated by the abstract specifically for the one-dimensional (linear) geometry this record's reference implementation demonstrates: the abstract names linear geometry as within its scope but attaches no gate or depth bound to it..

References

  • Quantum algorithms to simulate many-body physics of correlated fermions

    Zhang Jiang, Kevin J. Sung, Kostyantyn Kechedzhi, Vadim N. Smelyanskiy, Sergio Boixo · 2017

    Primary source: it presents three separate subroutines with the gate-count and circuit-depth bounds quoted above — fermionic Gaussian state preparation, the 2D fermionic Fourier transform, and one time step of 2D Fermi-Hubbard evolution — an improved Slater-determinant preparation algorithm, and a discussion of using these algorithms for ground-state properties and phase diagrams via the Hubbard model. Consult it for what happens in the one-dimensional (linear) geometry the abstract names but does not bound, for the meaning of N, and for the Slater-determinant improvement, none of which the abstract itself gives beyond what is quoted here.

Attested & literature
  • Peer-reviewed paper
Method
Curation only. This record documents the algorithm a published reference implementation demonstrates, not the demonstration itself: nothing in Leona runs Qmod. The primary reference's title, authors and submission year were read from its arXiv abs page, and the complexity claim taken from a clause of that paper's abstract (The abstract of arXiv:1711.05395 states three separate gate-count and depth bounds, each attached by the abstract itself to a different subroutine; this record keeps them attached to those subroutines rather than folding them into one figure for the whole algorithm. On Gaussian state preparation: "We also present a quantum algorithm to prepare an arbitrary fermionic Gaussian state with $O(N^2)$ gates and $O(N)$ circuit depth." On the 2D fermionic Fourier transform: "we propose an algorithm to implement the 2-dimensional (2D) fermionic Fourier transformation on a 2D qubit array with only $O(N^{1.5})$ gates and" — the abstract then gives the depth bound in TeX using a square root, rendered here as O(√N) since it is written outside quotation marks — and resumes: "circuit depth, which is the minimum depth required for quantum information to travel across the qubit array." On the Fermi-Hubbard time step: "We also present methods to simulate each time step in the evolution of the 2D Fermi-Hubbard model---again on a 2D qubit array---with $O(N)$ gates and" — again a TeX square-root depth bound, rendered O(√N) outside quotes — before: "circuit depth." All three bounds are stated for a 2D qubit array. The same abstract separately states, "We focus specifically on 2D and linear geometry with nearest neighbor qubit-qubit couplings, typical for superconducting transmon qubit arrays." That sentence puts linear (one-dimensional) geometry within the paper's scope but attaches no gate or depth bound to it anywhere in the abstract. This record's reference implementation demonstrates the one-dimensional case, so none of the three quoted bounds is claimed here to describe it. The demonstration gives no bound of its own.). No circuit was constructed, simulated or run.
Result
Pass · the record's problem statement and primary reference agree with the sources named above.
Caveat
This is a literature record. No circuit was built, compiled, simulated or run for it, no fermionic state was prepared, and no Fermi-Hubbard evolution was carried out; the record documents the algorithms the paper presents and does not reproduce or verify the demonstration at applications/physical_systems/fermi_hubbard_model_1D. The pinned index read for this record gives only that directory path and the file list fermi_hubbard_1D.ipynb and fermi_hubbard_1D.qmod, nothing about what those files contain. The three gate-count and depth bounds quoted above are each the abstract's own figure for a specific subroutine — Gaussian state preparation, the 2D fermionic Fourier transform, and one Fermi-Hubbard time step — and this record keeps them attached to those subroutines rather than reporting one figure for the whole algorithm. Critically, none of the three is stated by the abstract for the one-dimensional geometry that this record's reference implementation demonstrates: all three are given for a 2D qubit array, and while the abstract does name linear (1D) geometry as within the paper's scope, it attaches no gate or depth bound to that case anywhere in the text this record read. None of the bounds includes a constant factor, an error tolerance, or a qubit count beyond the variable N itself, and the abstract does not say what N counts in physical terms — sites, orbitals, or qubits — beyond calling the parameter counts of the Gaussian-state and Slater-determinant circuits optimal in the sense that they equal the number of parameters needed to describe the corresponding states. The improvement to the existing Slater-determinant algorithm, described as exploiting a unitary symmetry, carries no bound at all in the abstract. The closing sentence about determining ground-state properties and phase diagrams using the Hubbard model as an example is a stated direction of use, not a result, and nothing here quantifies it.
Quantum algorithms to simulate many-body physics of correlated fermions ↗
Kind
curated reference
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License
Citation metadata only; source publication terms apply
correlated fermionsfermionic gaussian statefermionic fourier transformfermi-hubbard model2d qubit array

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