Monochromatic quantum graphs (inherited vertex colorings) #
This file studies the existence of monochromatic quantum graphs: edge-coloured, edge-weighted complete graphs whose perfect matchings induce vertex colourings, with the property that
- every non-monochromatic inherited vertex colouring has total weight
0, while - each of the
Dmonochromatic colourings has total weight1.
In the quantum-optics motivation, such a construction corresponds to generating high-dimensional multipartite GHZ-type states using probabilistic pair sources and linear optics (without additional resources), where interference patterns can be expressed as weighted sums over perfect matchings.
Main questions (informal) #
- For
N = 4andD ≥ 4, does there exist such a graph/weighting? - For even
N ≥ 6andD ≥ 3, does there exist such a graph/weighting?
Formalisation sketch #
A quantum graph with N vertices and D colours can be encoded by a weight function
W : EdgeN N D α → α (for a coefficient domain α).
For each assignment of vertex indices ι : V N → Fin D, we define a perfect-matching sum
pmSumN N D W ι (a sum over perfect matchings, where each matching contributes the product of the
corresponding edge weights determined by ι). The equation system EqSystemN N D W requires
pmSumN N D W ι = 1 iff ι is constant (all entries equal), and 0 otherwise.
The open conjectures in this file ask for non-existence/existence of such W over various
coefficient domains (e.g. ℂ, ℝ, ℤ, and restricted integer weights).
References #
[Krenn2017] M. Krenn, X. Gu, A. Zeilinger, "Quantum Experiments and Graphs: Multiparty States as Coherent Superpositions of Perfect Matchings", Physical Review Letters 119(24), 240403 (2017).
[MO2018] Vertex coloring inherited from perfect matchings (motivated by quantum physics), MathOverflow question 311325.
[Gu2019] X. Gu, M. Erhard, A. Zeilinger, M. Krenn, "Quantum experiments and graphs II: Quantum interference, computation, and state generation", PNAS 116(10), 4147–4155 (2019).
[Krenn2019] Questions on the Structure of Perfect Matchings inspired by Quantum Physics by M. Krenn, X. Gu, U. Soltész, Proc. 2nd Croatian Combinatorial Days, 57–70 (2019).
[Chandran2022] Edge-coloured graphs with only monochromatic perfect matchings and their connection to quantum physics by N. Chandran, S. Gajjala (2022).
[Chandran2024] Krenn–Gu conjecture for sparse graphs by N. Chandran, S. Gajjala, S. Illickan, M. Krenn, MFCS 2024.
Vertices of $K_N$.
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Weights on edges.
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- MonochromaticQuantumGraph.WeightsN N D α = (MonochromaticQuantumGraph.EdgeN N D → α)
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Ordered vertex list $[0, 1, \ldots, N-1]$.
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Chain-equality along a list of vertices.
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- MonochromaticQuantumGraph.allEqualList ι L = List.IsChain (fun (v w : MonochromaticQuantumGraph.V N) => ι v = ι w) L
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Instance: allEqualList ι L is decidable.
Instance: allEqual ι is decidable.
Auxiliary perfect-matching sum on a vertex list, using a fuel parameter n for termination.
When called as pmSumListAux W ι L.length L, this computes the weighted sum over all perfect
matchings on the vertices in L. The recursion pairs the head vertex with each later vertex and
recurses on the remaining vertices.
For lists of odd length, there are no perfect matchings and the value is 0.
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- MonochromaticQuantumGraph.pmSumListAux W ι 0 x✝ = 1
- MonochromaticQuantumGraph.pmSumListAux W ι 1 x✝ = 0
- MonochromaticQuantumGraph.pmSumListAux W ι n.succ.succ [] = 1
- MonochromaticQuantumGraph.pmSumListAux W ι n.succ.succ [head] = 0
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Perfect-matching sum on a list: run pmSumListAux with fuel = L.length.
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The monochromatic quantum graph equation system for $K_N$.
For every index assignment $\iota : V_N \to \mathrm{Fin}\, D$, the perfect-matching sum equals $1$ if $\iota$ is constant (monochromatic inherited vertex colouring), and equals $0$ otherwise.
Equations
- MonochromaticQuantumGraph.EqSystemN N D W = ∀ (ι : MonochromaticQuantumGraph.V N → Fin D), MonochromaticQuantumGraph.pmSumN N D W ι = if MonochromaticQuantumGraph.allEqual ι then 1 else 0
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For $N = 4$ and $D = 4$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 4$ and all $D \geq 4$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 6$ and $D = 3$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 6$ and all $D \geq 3$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 8$ and $D = 3$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 8$ and $D = 10$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 10$ and $D = 3$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For $N = 10$ and $D = 10$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?
For all even $N \geq 6$ and $D \geq 3$, does there exist no solution to the monochromatic quantum graph equation system over $\mathbb{Z}$ with weights in $\{-1, 0, 1\}$?