Open Quantum Problem 35: existence of absolutely maximally entangled pure states #
Problem: For which numbers of parties $n$ and local dimensions $d$ does there exist a pure absolutely maximally entangled state $\psi$?
A pure state $\psi$ on $n$ parties of local dimension $d$ is called absolutely maximally entangled (AME) if, for every subset of at most half of the parties, the corresponding reduced density matrix is maximally mixed.
References:
- Open Quantum Problems, Problem 35: https://oqp.iqoqi.oeaw.ac.at/existence-of-absolutely-maximally-entangled-pure-states
- Formal Conjectures issue #3452: https://github.com/google-deepmind/formal-conjectures/issues/3452
- W. Helwig, W. Cui, A. Riera, J. I. Latorre, and H.-K. Lo, Absolute Maximal Entanglement and Quantum Secret Sharing, Phys. Rev. A 86, 052335 (2012), arXiv:1204.2289.
- D. Goyeneche, D. Alsina, J. I. Latorre, A. Riera, and K. Życzkowski, Absolutely Maximally Entangled states, combinatorial designs and multi-unitary matrices, Phys. Rev. A 92, 032316 (2015), arXiv:1506.08857.
- A. Higuchi and A. Sudbery, How entangled can two couples get?, Phys. Lett. A 273, 213-217 (2000), arXiv:quant-ph/0005013.
- A. J. Scott, Multipartite entanglement, quantum-error-correcting codes, and entangling power of quantum evolutions, Phys. Rev. A 69, 052330 (2004), arXiv:quant-ph/0310137.
- F. Huber, O. Gühne, and J. Siewert, Absolutely maximally entangled states of seven qubits do not exist, Phys. Rev. Lett. 118, 200502 (2017), arXiv:1608.06228.
- F. Huber and M. Grassl, Quantum Codes of Maximal Distance and Highly Entangled Subspaces, Quantum 4, 284 (2020), arXiv:1907.07733.
- S. A. Rather, A. Burchardt, W. Bruzda, G. Rajchel-Mieldzioć, A. Lakshminarayan, and K. Życzkowski, Thirty-six entangled officers of Euler: Quantum solution to a classically impossible problem, Phys. Rev. Lett. 128, 080507 (2022), arXiv:2104.05122.
- G. Rajchel-Mieldzioć, R. Bistroń, A. Rico, A. Lakshminarayan, and K. Życzkowski, Absolutely maximally entangled pure states of multipartite quantum systems, arXiv:2508.04777 (2025).
This file formalizes the problem of determining for which pairs $(n,d)$ there exists an absolutely maximally entangled pure state $\mathrm{AME}(n,d)$.
We represent an $n$-partite state of local dimension $d$ by the finite-dimensional Hilbert space
EuclideanSpace ℂ (Config n d), whose coordinates in the computational basis are amplitudes.
The helper mkStateVector turns an amplitude function into such a state, and normalization is
imposed explicitly via IsNormalized, i.e. via the ambient $L^2$ norm.
The main reusable lemma is reducedDensityFirst_of_completion: if a state is a
uniform superposition over the graph of an injective completion function
completion : Config m d → Config (n - m) d,
then the reduced state on the first $m$ parties is maximally mixed.
As demonstration, we show that the Bell states with $n=2$ and GHZ states with $n=3$ are AME states, and the GHZ state with $n=4$ is not an AME state.
A computational-basis configuration of $n$ parties with local dimension $d$.
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- OpenQuantumProblem35.Config n d = (Fin n → Fin d)
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A state vector in the computational basis, viewed as a finite-dimensional Hilbert space.
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Build a state vector from its computational-basis amplitudes.
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A state vector can be evaluated on a computational-basis configuration to read its amplitude.
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- OpenQuantumProblem35.instCoeFunStateVectorForallConfigComplex = { coe := fun (ψ : OpenQuantumProblem35.StateVector n d) => ψ.ofLp }
A state built from amplitudes has those amplitudes as its coordinates.
A state vector is normalized if it has $L^2$ norm $1$.
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- OpenQuantumProblem35.IsNormalized ψ = (‖ψ‖ = 1)
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A state is normalized iff its squared $L^2$ norm is $1$.
Permute the parties of a configuration.
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- OpenQuantumProblem35.permuteConfig π x i = x (π i)
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The identity permutation leaves a configuration unchanged.
Permute the parties of a state vector.
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- OpenQuantumProblem35.permuteState π ψ = OpenQuantumProblem35.mkStateVector fun (x : OpenQuantumProblem35.Config n d) => ψ.ofLp (OpenQuantumProblem35.permuteConfig π x)
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Evaluating a permuted state vector reads the amplitude at the permuted configuration.
The identity permutation leaves a state vector unchanged.
The embedding of the first $m$ indices into $\mathrm{Fin}\, n$.
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- OpenQuantumProblem35.leftIndex hm i = ⟨↑i, ⋯⟩
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The reduced density matrix obtained by tracing out the last $n-m$ parties.
The subsystem is always the first $m$ parties; different subsystems are handled by first permuting the parties.
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- One or more equations did not get rendered due to their size.
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The maximally mixed state on $m$ parties.
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- OpenQuantumProblem35.maximallyMixed m d = (↑(Fintype.card (OpenQuantumProblem35.Config m d)))⁻¹ • 1
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A state has maximally mixed reduction on the first $m$ parties.
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A state $\psi$ is absolutely maximally entangled.
Standard AME definitions quantify over all subsets $A \subseteq \mathrm{Fin}\, n$ with $|A| \le \lfloor n/2 \rfloor$ and require that the reduction on $A$ be maximally mixed. For pure states it is enough to check subsets of size exactly $\lfloor n/2 \rfloor$; see the references of Helwig--Cui--Riera--Latorre--Lo (2012) and Goyeneche--Alsina--Latorre--Riera--Życzkowski (2015). In this file, a subsystem of that size is encoded by first permuting the chosen parties to the front and then tracing out the remaining parties.
We also require $\psi$ to be normalized explicitly.
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- One or more equations did not get rendered due to their size.
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Existence of an $\mathrm{AME}(n,d)$ state.
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The number of computational-basis configurations on $m$ parties of local dimension $d$ is $d^m$.
The matrix entries of the maximally mixed state are diagonal and equal to the inverse subsystem dimension.
The common amplitude of the Bell and GHZ witnesses.
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A configuration is constant if all coordinates agree.
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- OpenQuantumProblem35.IsConstantConfig x = ∀ (i j : Fin n), x i = x j
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The constant configuration with value $a$.
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Every constant configuration is constant.
A simple binary two-party configuration with different entries is not constant.
The diagonal $n$-party state: the uniform superposition over constant computational-basis strings.
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- One or more equations did not get rendered due to their size.
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Evaluating the diagonal state returns the uniform coefficient on constant strings and 0 otherwise.
The standard $d$-dimensional Bell state.
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The standard $d$-dimensional GHZ state on $3$ parties.
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The standard $d$-dimensional GHZ state on $4$ parties.
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The completion function for constant-support states reduced to one party.
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On a nonempty index type, different constants give different constant configurations.
A configuration on a nonempty index type is constant iff it is equal to some constant configuration.
The squared norm of the uniform coefficient is the inverse local dimension.
The squared norm of the uniform coefficient is the inverse local dimension.
For $n \ge 1$ and $d \ge 1$, the diagonal state is normalized.
Permuting the parties preserves the property of being a constant configuration.
The diagonal state is invariant under permutations of the parties.
The completion map for constant configurations is injective once $n \ge 2$.
The diagonal state on a split configuration is nonzero exactly on the graph of the constant completion map.
A uniform superposition over the graph of an injective completion map has reduced density matrix $(c\overline c) I$ on the first subsystem.
The completion criterion gives a maximally mixed reduced state once the coefficient has the correct squared norm.
The diagonal state has maximally mixed one-party reductions once $n \ge 2$.
If $\lfloor n/2 \rfloor = 1$, then the diagonal state is $\mathrm{AME}(n,d)$ for every $d \ge 2$.
The Bell state witnesses the existence of $\mathrm{AME}(2,d)$ for every local dimension $d \ge 2$.
The $3$-party GHZ state witnesses the existence of $\mathrm{AME}(3,d)$ for every local dimension $d \ge 2$.
On $4$ parties, the diagonal state vanishes on any split configuration whose first two entries are different.
Source-backed benchmark statement: the Bell state witnesses the existence of an $\mathrm{AME}(2,2)$ state.
Source-backed benchmark statement: the three-qubit GHZ state witnesses the existence of an $\mathrm{AME}(3,2)$ state.
Source-backed benchmark statement: an $\mathrm{AME}(5,2)$ state exists. This is one of the four qubit cases $n=2,3,5,6$; see the OQP page and Scott (2004).
Source-backed benchmark statement: an $\mathrm{AME}(6,2)$ state exists. This is one of the four qubit cases $n=2,3,5,6$; see the OQP page and Scott (2004).
Source-backed benchmark statement: no $\mathrm{AME}(4,2)$ state exists; see Higuchi--Sudbery (2000) and the OQP page.
Source-backed benchmark statement: no $\mathrm{AME}(7,2)$ state exists; see Huber--Gühne--Siewert (2017) and the OQP page.
Source-backed benchmark statement: an $\mathrm{AME}(4,3)$ state exists; see Helwig et al. (2012) and Goyeneche et al. (2015).
Source-backed benchmark statement: an $\mathrm{AME}(4,6)$ state exists; see Rather et al. (2022).
Open benchmark statement: does an $\mathrm{AME}(7,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(7,10)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(8,4)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(8,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(8,10)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(9,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(9,10)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(10,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(10,10)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(11,3)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(11,4)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(11,5)$ state exist?
The DeepMind prover agent has shown that such a state exists.
Open benchmark statement: does an $\mathrm{AME}(11,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(11,10)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(12,5)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(12,6)$ state exist?
Open benchmark statement: does an $\mathrm{AME}(12,10)$ state exist?