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arxiv: 2506.03435 · v2 · submitted 2025-06-03 · 🪐 quant-ph · cs.CC

Computational Complexity and Simulability of Non-Hermitian Quantum Dynamics

Pith reviewed 2026-05-19 10:32 UTC · model grok-4.3

classification 🪐 quant-ph cs.CC
keywords non-Hermitian quantum dynamicscomputational complexityPostBQPpostselectionquantum circuitsclassical simulabilityquantum trajectories
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0 comments X p. Extension

The pith

Quantum circuits using a fixed non-unitary gate with coherent renormalization after each application can decide any problem in PostBQP.

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

The paper establishes that non-Hermitian quantum dynamics, when modeled as repeated use of one fixed non-unitary gate on a constant number of qubits inside otherwise standard polynomial-size circuits, confer the full power of postselection. This yields NHBQP(U) equal to PostBQP, which equals PP, for any such fixed gate U. A reader would care because PostBQP is widely believed to lie outside efficient classical or quantum computation under standard assumptions, so the result indicates that scalable non-Hermitian advantage cannot be obtained without incurring prohibitive efficiency costs. The work further shows that when the non-Hermitian evolution admits a purification into a classically simulable unitary family, the model stays efficiently simulable provided the relevant postselected outcomes occur with inverse-polynomial probability.

Core claim

We prove that NHBQP(U), the class of problems solvable by poly-size circuits that may apply a fixed non-unitary O(1)-qubit gate U with renormalization after each use, equals PostBQP for every such U. In the uniform circuit-family model the characterization is tight: NHBQP(U) = PostBQP = PP. Unitary circuits augmented with postselection can simulate not only NH Hamiltonian evolution but arbitrary quantum trajectories; consequently any NH system whose purification lies inside a strongly simulable unitary family remains efficiently classically simulable whenever postselected events have probability at least inverse polynomial.

What carries the argument

The NHBQP(U) model: polynomial-size quantum circuits that combine a universal unitary gate set with repeated applications of one fixed non-unitary gate U on O(1) qubits, followed by coherent renormalization of the state after each use of U.

If this is right

  • If coherent normalized non-unitary evolution can be realized with only polynomial overhead, the resulting model implements postselection.
  • Any scalable computational advantage from non-Hermiticity must come with a cost that limits its efficiency.
  • Unitary circuits with postselection can simulate evolution under arbitrary NH Hamiltonians and general quantum trajectories.
  • NH models whose purifications belong to simulable families such as Clifford or matchgate circuits remain efficiently classically simulable when postselected events occur with probability Omega(2^{-poly(n)}).

Where Pith is reading between the lines

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

  • The result suggests that any physical implementation attempting to exploit non-Hermiticity for computation will face exponential suppression or normalization costs that grow with circuit depth.
  • The separation between simulable and powerful regimes is sharpened: non-Hermiticity alone does not create new computational power inside already simulable unitary families.

Load-bearing premise

The model assumes perfect coherent normalization is possible after each application of the fixed non-unitary gate and that the circuit family remains uniform.

What would settle it

An explicit construction of a fixed non-unitary gate U such that NHBQP(U) is contained in BQP, or an efficient classical algorithm deciding all languages in NHBQP(U), would falsify the claimed equality to PostBQP.

Figures

Figures reproduced from arXiv: 2506.03435 by Brian Barch, Daniel Lidar.

Figure 1
Figure 1. Figure 1: FIG. 1. Circuit depicting usage of an [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Geometric representation of two rounds of unitary [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Decomposition of a non-unitary effective channel [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. The circuit describing a single Trotter step of simu [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Two-qubit meter circuit used to implement [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Product nature of [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Effective circuit generating Lindbladian dynamics [PITH_FULL_IMAGE:figures/full_fig_p014_8.png] view at source ↗
read the original abstract

Non-Hermitian (NH) quantum systems demonstrate striking differences from their Hermitian counterparts, leading to claims of NH advantage in areas ranging from metrology to entanglement generation. We show that in the context of quantum computation, any such NH advantage is unlikely to be scalable as an efficient computational resource: if coherent normalized non-unitary evolution could be realized with only polynomial overhead, then the resulting model could implement postselection, implying implausibly strong complexity-theoretic power under standard assumptions. We define NHBQP(U) as the computational power of poly-size quantum circuits that, in addition to a standard universal unitary gate set, may apply a fixed gate U on $O(1)$ qubits that is not proportional to a unitary, with the state renormalized after each use of U. We prove this model is powerful enough to decide PostBQP. In the standard uniform circuit-family model this characterization is tight: for any fixed such U, NHBQP(U)=PostBQP=PP. PostBQP is believed intractable, so this suggests that any scalable NH computational advantage must come with a cost limiting its efficiency. Additionally, we study locality-preserving purifications of restricted classes of non-unitary systems. Using this framework, we show that unitary gates with postselection can simulate not only evolution under NH Hamiltonians but arbitrary quantum trajectories. Any NH model whose purification lies in a strongly simulable unitary family (e.g., Clifford, matchgate, or low-bond-dimension tensor-network circuits) remains efficiently classically simulable, provided the relevant postselected events occur with probability $\Omega(2^{-\text{poly}(n)})$. Thus adding non-Hermiticity to a universal unitary system makes it infeasibly computationally powerful, while adding it to a strongly simulable system adds no computational power in this setting.

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

0 major / 3 minor

Summary. The manuscript defines the NHBQP(U) model consisting of polynomial-size quantum circuits that may apply a fixed non-unitary gate U on O(1) qubits (with coherent renormalization after each use) in addition to a standard universal unitary gate set. It proves that NHBQP(U) = PostBQP = PP for any fixed non-unitary U in the uniform circuit-family model. The paper further develops a framework of locality-preserving purifications of non-unitary evolutions, showing that postselected unitary circuits can simulate both NH Hamiltonian evolution and arbitrary quantum trajectories, and that any NH model whose purification lies in a strongly simulable unitary family (Clifford, matchgate, or low-bond-dimension tensor networks) remains efficiently classically simulable when the relevant postselected events occur with probability Ω(2^{-poly(n)}).

Significance. If the central characterizations hold, the work supplies a complexity-theoretic obstruction to scalable computational advantage from non-Hermitian dynamics: efficient coherent normalization would enable postselection and therefore solve PP-complete problems. The simulability results supply a complementary positive statement, delineating precisely when non-Hermiticity adds no computational power beyond the underlying unitary model. The proofs rely on standard uniform circuit families and explicit reductions that preserve uniformity, which strengthens the applicability of the conclusions.

minor comments (3)
  1. [§2] Definition of NHBQP(U) (likely §2): the precise semantics of the coherent renormalization step after each application of U should be stated formally, including whether the normalization factor is computed exactly or approximated and how it is incorporated into the circuit description.
  2. [§3] Proof of PostBQP ⊆ NHBQP(U) (likely §3): while the amplification argument via repeated applications aligned to the dominant singular direction is sketched, an explicit bound on the number of repetitions in terms of the singular-value gap of the fixed U would make the polynomial overhead fully transparent.
  3. [simulability section] Simulability section: the condition Ω(2^{-poly(n)}) on postselection probability is stated, but the manuscript should clarify whether this bound is uniform across all simulable families or depends on the bond dimension or circuit depth of the purification.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their careful review and for recommending minor revision. The referee's summary accurately captures the definition of the NHBQP(U) model, the equivalence to PostBQP=PP in the uniform circuit model, and the framework of locality-preserving purifications together with the simulability results for restricted non-Hermitian systems. We appreciate the recognition that these results supply both an obstruction to scalable non-Hermitian advantage and a precise delineation of when non-Hermiticity adds no computational power.

Circularity Check

0 steps flagged

No significant circularity; derivation relies on standard complexity inclusions

full rationale

The paper defines the NHBQP(U) model explicitly via repeated application of a fixed non-unitary gate U followed by coherent renormalization, then proves equivalence to PostBQP by two explicit reductions: (i) embedding postselection into repeated applications of U to amplify amplitudes along its dominant singular vector, and (ii) mapping each renormalization step to postselection on the no-jump branch of a unitary dilation. Both directions preserve uniformity and are justified by the known result PostBQP = PP (Aaronson 2005) rather than any self-citation or fitted parameter. No step reduces by construction to its own input; the central claim is an equivalence between independently defined models and does not invoke ansatzes, uniqueness theorems, or renamings from prior author work.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper introduces the NHBQP(U) model but rests on standard complexity facts and the definition of coherent renormalization; no free parameters or new physical entities are introduced.

axioms (1)
  • standard math PostBQP equals PP
    This known equality is invoked to conclude that NHBQP(U)=PP for any fixed U.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Magic Steady State Production: Non-Hermitian, Dissipative, and Stochastic Pathways

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    Non-Hermitian and dissipative dynamics engineer magic steady states in qubits that attract every initial state to high-magic targets.

Reference graph

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76 extracted references · 76 canonical work pages · cited by 1 Pith paper · 5 internal anchors

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    Normalized Evolution In order to consider time evolutionU as mapping states to states, one approach is to normalize its action on pure states |ψt⟩ |ψt⟩ ≡ U |ψ0⟩ ∥U |ψ0⟩∥ (4) or trace-normalize its action on mixed states ρt: ρt ≡ U ρ0U † Tr[U ρ0U †] (5) This is equivalent to dividing by the normalizing factor in the Bayes rule for conditional probability d...

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    Diagonalizing H When H is diagonalizable but NH, it can be diagonal- ized in terms of right |ri⟩ and left ⟨li| eigenvectors as H = X i λi|ri⟩ ⟨li| (8) for λi the (in general complex) eigenvalues. We can always biorthonormalize the eigenvectors such that ⟨li|ri⟩ = δij, and may additionally normalize ⟨ri|ri⟩ = 1, but cannot do the same for {⟨li|}. Given thi...

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    Simulability Not all quantum systems are as hard as BQP. In fact, many have outputs that can be reproduced by algorithms in BPP or even P. These systems are referred to as weakly or strongly simulable [51–53]. Weakly simulable quantum systems admit classical al- gorithms that can sample measurement outcomes with approximately the correct distribution in p...

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    Complexity of NH Systems The computational power of models of computation generated by NH Hamiltonians is a subset of PostBQP, but is otherwise relatively unstudied. Evolution under an NH Hamiltonian even for short times is known to be able to solve certain decision problems outside BQP [21, 24]. This is striking, as NH systems are often used as an ap- pr...

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