Quantum Wasserstein distance and its relation to several types of fidelities
Pith reviewed 2026-05-19 09:06 UTC · model grok-4.3
The pith
Several definitions of the quantum Wasserstein distance become identical when the optimization is restricted to separable states, and equal the square root of the Uhlmann-Jozsa fidelity for qubits.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
When the optimization that defines the quantum Wasserstein distance is carried out only over separable bipartite states with fixed marginals, several distinct quantities coincide. This common value also equals the square root of the Uhlmann-Jozsa quantum fidelity when the states are qubits. The same separable-state formulation yields the triangle inequality for some of the quantities whenever one of the three states under comparison is pure, and it produces explicit relations with the superfidelity.
What carries the argument
The restriction of the defining optimization for quantum Wasserstein distance to separable bipartite states with given marginals, which produces equalities among several candidate distances and an optimization expression for the square root of Uhlmann-Jozsa fidelity.
If this is right
- Multiple previously distinct quantum distance measures become identical under the separable restriction.
- The square root of Uhlmann-Jozsa fidelity admits an explicit representation as a separable-state optimization.
- Equality between the distance and the fidelity holds exactly on qubit systems.
- A triangle inequality is obtained for some of the quantities when one state is pure.
- Additional relations connect the quantities to the superfidelity.
Where Pith is reading between the lines
- The separable-state formulation may simplify numerical evaluation of these distances in low-dimensional systems.
- The same restriction could be tested on higher-dimensional states to see whether the equalities persist beyond qubits.
- The approach suggests a route to derive further inequalities by combining the separable optimization with known properties of fidelity.
Load-bearing premise
The optimization over general bipartite states with given marginals remains meaningful and produces the same relations when it is restricted to separable states.
What would settle it
An explicit pair of qubit states for which the value obtained by optimizing over separable states differs from the square root of their Uhlmann-Jozsa fidelity.
read the original abstract
We consider several definitions of the quantum Wasserstein distance based on an optimization over general bipartite quantum states with given marginals. Then, we examine the quantities obtained after the optimization is carried out over bipartite separable states instead. We prove that several of these quantities are equal to each other. Thus, we connect several approaches in the literature. We prove the triangle inequality for some of these quantities for the case of one of the three states being pure. As a byproduct, we show that the square root of the Uhlmann-Jozsa quantum fidelity can also be written as an optimization over separable states with given marginals. We use this to prove that some of these quantities equal the Uhlmann-Jozsa quantum fidelity for qubits. We also find relations with the superfidelity.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript considers several definitions of quantum Wasserstein distance obtained via optimization over general bipartite quantum states with fixed marginals. It then restricts the optimization to separable states, proves that multiple resulting quantities coincide with one another, establishes a triangle inequality when one of the three states is pure, shows that the square root of the Uhlmann-Jozsa fidelity admits an equivalent formulation as an optimization over separable states with given marginals, and demonstrates that certain of these quantities equal the Uhlmann-Jozsa fidelity for qubits. Relations to the superfidelity are also derived.
Significance. If the proofs are correct, the work unifies several existing approaches to quantum Wasserstein distances by exhibiting their equivalence under the separable restriction. The separable-state representation of sqrt(Uhlmann-Jozsa fidelity) supplies a new variational characterization that may prove useful for computation or interpretation. The triangle inequality for the pure-state case and the qubit equalities are obtained via direct algebraic identities and case-by-case verification, which strengthens verifiability. These results connect disparate strands of the literature on quantum optimal transport and fidelity measures.
major comments (1)
- [Triangle inequality section] The triangle inequality is established only under the assumption that one of the three states is pure. Clarify whether this restriction is essential for the proof technique or whether a general mixed-state version can be obtained within the same framework; this directly affects how broadly the quantities can be regarded as distances.
minor comments (2)
- [Definitions and main results] Define the various Wasserstein-type quantities with explicit labels (e.g., W_sep, W_gen) at their first appearance and maintain consistent notation when stating the equalities.
- [Qubit case] In the qubit equality proofs, include the explicit matrix representations or parameterizations used for the states to facilitate direct verification.
Simulated Author's Rebuttal
We thank the referee for their thorough review and valuable feedback on our manuscript. We address the major comment in detail below.
read point-by-point responses
-
Referee: [Triangle inequality section] The triangle inequality is established only under the assumption that one of the three states is pure. Clarify whether this restriction is essential for the proof technique or whether a general mixed-state version can be obtained within the same framework; this directly affects how broadly the quantities can be regarded as distances.
Authors: The restriction to the case where one of the three states is pure is essential to our proof technique. Our proof makes use of specific properties that hold when one state is pure, such as the ability to express the state in a form that simplifies the optimization over separable states and allows direct verification of the triangle inequality through algebraic manipulation. Extending this to general mixed states does not appear straightforward within the current framework, and we have not identified a general mixed-state triangle inequality. We will add a clarifying remark in the revised version of the manuscript to explicitly state this limitation of the proof technique and note that the general case remains open. This addresses the referee's concern without altering the main results. revision: yes
Circularity Check
No significant circularity; derivations rest on explicit proofs and algebraic identities
full rationale
The paper defines quantum Wasserstein distances via optimization over bipartite states with fixed marginals, then restricts the optimization to separable states and proves equalities between several resulting quantities using direct algebraic manipulation and case-by-case verification. It further shows that the square root of Uhlmann-Jozsa fidelity admits an equivalent formulation as a separable-state optimization and establishes equality with fidelity for qubits. The triangle inequality is proved directly for the pure-state case. No step reduces a claimed prediction or central result to a fitted parameter, a self-referential definition, or a load-bearing self-citation whose validity depends on the present work. All equalities follow from the stated optimization problems and standard properties of quantum states without importing unverified uniqueness theorems or ansatzes from the authors' prior publications. The derivation chain is therefore self-contained against external mathematical benchmarks.
Axiom & Free-Parameter Ledger
axioms (2)
- standard math Bipartite quantum states possess well-defined marginals and the set of separable states is a proper subset of all states.
- domain assumption The Uhlmann-Jozsa fidelity and superfidelity are standard measures with known properties in quantum mechanics.
Lean theorems connected to this paper
-
IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We prove that several of these quantities are equal to each other... the square root of the Uhlmann-Jozsa quantum fidelity can also be written as an optimization over separable states with given marginals.
-
IndisputableMonolith/Foundation/AlphaCoordinateFixation.leanJ_uniquely_calibrated_via_higher_derivative unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
Theorem 8... ˜DDPT,sep(ϱ, σ)² = ˜DDPT,decomp(ϱ, σ)² when a full set of SU(d) generators is used.
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Forward citations
Cited by 2 Pith papers
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General method for obtaining the energy minimum of spin Hamiltonians for separable states
A method is given to compute the minimum energy of certain spin Hamiltonians over separable states, expressed via quantum Fisher information for Ising models and fidelity for Heisenberg chains.
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Strong Kantorovich duality for quantum optimal transport with generic cost and optimal couplings on quantum bits
Establishes Kantorovich duality for linearized non-quadratic quantum optimal transport realized by channels, determines optimal primal-dual solutions for qubits under state restrictions, and proves the triangle inequa...
Reference graph
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