arXiv subjects

A. Vourdas

Publications and source records attributed to A. Vourdas.

At least 19 recordsLinked to original sources

arXiv2026

The Heisenberg-Weyl-parity group its coherent states and a unified Wigner-Weyl function

The Heisenberg-Weyl group $HW(d)$ related to a $d$-dimensional Hilbert space $H(d)$, is enlarged into the Heisenberg-Weyl-parity group $HWP(d)$ that incorporates parity transformations. It consists of $2d^3$ elements, of which $d^3$ elements belong to the $HW(d)$ subgroup, and extra $d^3$ elements which are related through a Fourier transform with the former ones. It is shown that $HWP(d)$ is a generalised version of the dihedral group. The properties of operators that combine displacements and parity, are discussed. $HWP(d)$ is shown to be a solvable group, and commutators of its elements perform displacement and parity transformations of quantum states, along loops in the discrete phase space.$2d^2$ coherent states related to the $HWP(d)$ group are introduced, which consist of $d^2$ coherent states related to the $HW(d)$ subgroup, and extra $d^2$ coherent states which are related through a Fourier transform with the former ones. In noisy cases, expansion of an arbitrary state in terms of the $2d^2$ coherent states with Bargmann coefficients, is advantageous in comparison to expansion in terms of the $d^2$ coherent states related to $HW(d)$. One of the consequences of the $HWP(d)$ group, is a natural unification of the Wigner and Weyl functions. The properties of the unified Wigner-Weyl function are discussed.

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arXiv2026

Random walks in finite Abelian groups with Birkhoff subpolytopes of doubly stochastic matrices and their physical implementation

Random walks in a finite Abelian group $G$ are studied. They use Markov chains with doubly stochastic transition matrices, in a Birkhoff subpolytope ${\cal B}(G)$ associated with the group $G$. It is shown that all future probability vectors belong to a polytope which does not depend on the transition matrices, and which shrinks during time evolution. Various quantities are used to describe the probability vectors: the majorization preorder, Lorenz values and the Gini index, entropic quantities, and the total variation distance. The general results are applied to the additive group ${\mathbb Z}(d)$, and to the Heisenberg-Weyl group $HW(d)/{\mathbb Z}(d)$. A physical implementation of random walks in ${\mathbb Z}(d)$ that involves a sequence of non-selective projective measurements, is discussed. A physical implementation of random walks in the Heisenberg-Weyl group $HW(d)/{\mathbb Z}(d)$ using a sequence of non-selective POVM measurements with coherent states, is also presented.

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arXiv2025

Effective approach to open systems with probability currents and the Grothendieck formalism

An effective approach to open systems and irreversible phenomena is presented, where an open system $\Sigma(d)$ with $d$-dimensional Hilbert space, is a subsystem of a larger isolated system $\Sigma(2d)$ (the `full universe') with $2d$-dimensional Hilbert space. A family of Bargmann-like representations (called $z$-Bargmann representations) introduces naturally the larger space. The $z$-Bargmann representations are defined through semi-unitary matrices (which are a coherent states formalism in disguise). The `openness' of the system is quantified with the probability current that flows from the system to the external world. The Grothendieck quantity ${\cal Q}$ is shown to be related to the probability current, and is used as a figure of merit for the `openness' of a system. ${\cal Q}$ is expressed in terms of `rescaling transformations' which change not only the phase but also the absolute value of the wavefunction, and are intimately linked to irreversible phenomena (e.g., damping/amplification). It is shown that unitary transformations in the isolated system $\Sigma(2d)$ (full universe), reduce to rescaling transformations when projected to its open subsystem $\Sigma(d)$. The values of the Grothendieck ${\cal Q}$ for various quantum states in an open system, are compared with those for their counterpart states in an isolated system.

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arXiv2025

Partial order and topology of Hermitian matrices and quantum Choquet integrals for density matrices with given expectation values

The set $M$ of $d\times d$ Hermitian matrices (observables) is studied as a partially ordered set with the L\"{o}wner partial order. Upper and lower sets in it, define the concept of cumulativeness (used mainly with scalar quantities) in the context of Hermitian matrices. Partial order and topology are intimately related to each other and the set $M$ of Hermitian matrices is also studied as a topological space, where open and closed sets are the upper and lower sets. It is shown that the set $M$ of Hermitian matrices is a $T_0$ topological space, and its subset ${\mathfrak D}$ of density matrices is Hausdorff totally disconnected topological space. These ideas are a prerequisite for studying quantum Choquet integrals with Hermitian matrices (as opposed to classical Choquet integrals with scalar quantities). Capacities (non-additive probabilities), cumulative quantities that involve Hermitian matrices, and M\"obius transforms that remove the overlaps between non-commuting observables, are used in quantum Choquet integrals. An application of the formalism is to find a density matrix, with given expectation values with respect to $n$ (non-commuting) observables. Examples of calculations of such a density matrix (with quantified errors in its expectation values), are presented.

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arXiv2024

Rescaling transformations and the Grothendieck bound formalism in a single quantum system

The Grothedieck bound formalism is studied using `rescaling transformations', in the context of a single quantum system. The rescaling transformations enlarge the set of unitary transformations (which apply to isolated systems), with transformations that change not only the phase but also the absolute value of the wavefunction, and can be linked to irreversible phenomena (e.g., quantum tunnelling, damping and amplification, etc). A special case of rescaling transformations are the dequantisation transformations, which map a Hilbert space formalism into a formalism of scalars. The Grothendieck formalism considers a `classical' quadratic form ${\cal C}(θ)$ which takes values less than $1$, and the corresponding `quantum' quadratic form ${\cal Q}(θ)$ which takes values greater than $1$, up to the complex Grothendieck constant $k_G$. It is shown that ${\cal Q}(θ)$ can be expressed as the trace of the product of $θ$ with two rescaling matrices, and ${\cal C}(θ)$ can be expressed as the trace of the product of $θ$ with two dequantisation matrices. Values of ${\cal Q}(θ)$ in the `ultra-quantum' region $(1,k_G)$ are very important, because this region is classically forbidden (${\cal C}(θ)$ cannot take values in it). An example with ${\cal Q}(θ)\in (1,k_G)$ is given, which is related to phenomena where classically isolated by high potentials regions of space, communicate through quantum tunnelling. Other examples show that `ultra-quantumness' according to the Grothendieck formalism (${\cal Q}(θ)\in (1,k_G)$), is different from quantumness according to other criteria (like quantum interference or the uncertainty principle).

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arXiv2024

Fast Fourier transforms and fast Wigner and Weyl functions in large quantum systems

Two methods for fast Fourier transforms are used in a quantum context. The first method is for systems with dimension of the Hilbert space $D=d^n$ with $d$ an odd integer, and is inspired by the Cooley-Tukey formalism. The `large Fourier transform' is expressed as a sequence of $n$ `small Fourier transforms' (together with some other transforms) in quantum systems with $d$-dimensional Hilbert space. Limitations of the method are discussed. In some special cases, the $n$ Fourier transforms can be performed in parallel. The second method is for systems with dimension of the Hilbert space $D=d_0...d_{n-1}$ with $d_0,...,d_{n-1}$ odd integers coprime to each other. It is inspired by the Good formalism, which in turn is based on the Chinese reminder theorem. In this case also the `large Fourier transform' is expressed as a sequence of $n$ `small Fourier transforms' (that involve some constants related to the number theory that describes the formalism). The `small Fourier transforms' can be performed in a classical computer or in a quantum computer (in which case we have the additional well known advantages of quantum Fourier transform circuits). In the case that the small Fourier transforms are performed with a classical computer, complexity arguments for both methods show the reduction in computational time from ${\cal O}(D^2)$ to ${\cal O}(D\log D)$. The second method is also used for the fast calculation of Wigner and Weyl functions, in quantum systems with large finite dimension of the Hilbert space.

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arXiv2023

Ultra-quantum coherent states in a single finite quantum system

A set of $n$ coherent states is introduced in a quantum system with $d$-dimensional Hilbert space $H(d)$. It is shown that they resolve the identity, and also have a discrete isotropy property. A finite cyclic group acts on the set of these coherent states, and partitions it into orbits. A $n$-tuple representation of arbitrary states in $H(d)$, analogous to the Bargmann representation, is defined. There are two other important properties of these coherent states which make them `ultra-quantum'. The first property is related to the Grothendieck formalism which studies the `edge' of the Hilbert space and quantum formalisms. Roughly speaking the Grothendieck theorem considers a `classical' quadratic form ${\mathfrak C}$ that uses complex numbers in the unit disc, and a `quantum' quadratic form ${\mathfrak Q}$ that uses vectors in the unit ball of the Hilbert space. It shows that if ${\mathfrak C}\le 1$, the corresponding ${\mathfrak Q}$ might take values greater than $1$, up to the complex Grothendieck constant $k_G$. ${\mathfrak Q}$ related to these coherent states is shown to take values in the `Grothendieck region' $(1,k_G)$, which is classically forbidden in the sense that ${\mathfrak C}$ does not take values in it. The second property complements this, showing that these coherent states violate logical Bell-like inequalities (which for a single quantum system are quantum versions of the Frechet probabilistic inequalities). In this sense also, our coherent states are deep into the quantum region.

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arXiv2023

Unitarily inequivalent local and global Fourier transforms in multipartite quantum systems

A multipartite system comprised of $n$ subsystems, each of which is described with `local variables' in ${\mathbb Z}(d)$ and with a $d$-dimensional Hilbert space $H(d)$, is considered. Local Fourier transforms in each subsystem are defined and related phase space methods are discussed (displacement operators, Wigner and Weyl functions, etc). A holistic view of the same system might be more appropriate in the case of strong interactions, which uses `global variables' in ${\mathbb Z}(d^n)$ and a $d^n$-dimensional Hilbert space $H(d^n)$. A global Fourier transform is then defined and related phase space methods are discussed. The local formalism is compared and contrasted with the global formalism. Depending on the values of $d,n$ the local Fourier transform is unitarily inequivalent or unitarily equivalent to the global Fourier transform. Time evolution of the system in terms of both local and global variables, is discussed. The formalism can be useful in the general area of Fast Fourier transforms.

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arXiv2022

Coherent states with minimum Gini uncertainty for finite quantum systems

Uncertainty relations $Δ(ρ)\ge η_d$ in terms of the Gini index are studied. The `Gini uncertainty constant' $η_d$ is estimated numerically and compared to an upper bound $\tilde η_d\ge η_d$. It is shown that for large $d$ we get $\tilde η_d\approx η_d$. States $\ket{g}$ with minimum Gini uncertainty and displacement transformations are used to define coherent states $\ket{α, β}_g$ (where $α, β\in {\mathbb Z}_d$) with minimum Gini uncertainty ($Δ[\ket{α, β}_g\;_g\bra{α, β}]\approx η_d$). The $\ket{α, β}_g$ resolve the identity, and therefore an arbitrary state can be expanded in terms of them. This expansion is robust in the presence of noise.

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arXiv2022

Grothendieck bound in a single quantum system

Grothendieck's bound is used in the context of a single quantum system, in contrast to previous work which used it for multipartite entangled systems and the violation of Bell-like inequalities. Roughly speaking the Grothendieck theorem considers a `classical' quadratic form ${\cal C}$ that uses complex numbers in the unit disc, and takes values less than $1$. It then proves that if the complex numbers are replaced with vectors in the unit ball of the Hilbert space, then the `quantum' quadratic form ${\cal Q}$ might take values greater than $1$, up to the complex Grothendieck constant $k_G$. The Grothendieck theorem is reformulated here in terms of arbitrary matrices (which are multiplied with appropriate normalisation prefactors), so that it is directly applicable to quantum quantities. The emphasis in the paper is in the `Grothendieck region' $(1,k_G)$, which is a classically forbidden region in the sense that ${\cal C}$ cannot take values in it. Necessary (but not sufficient) conditions for ${\cal Q}$ taking values in the Grothendieck region are given. Two examples that involve physical quantities in systems with $6$ and $12$-dimensional Hilbert space, are shown to lead to ${\cal Q}$ in the Grothendieck region $(1,k_G)$. They involve projectors of the overlaps of novel generalised coherent states that resolve the identity and have a discrete isotropy.

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arXiv2022

Markov chains with doubly stochastic transition matrices and application to a sequence of non-selective quantum measurements

A time-dependent finite-state Markov chain that uses doubly stochastic transition matrices, is considered. Entropic quantities that describe the randomness of the probability vectors, and also the randomness of the discrete paths, are studied. Universal convex polytopes are introduced which contain all future probability vectors, and which are based on the Birkhoff-von Neumann expansion for doubly stochastic matrices. They are universal in the sense that they depend only on the present probability vector, and are independent of the doubly stochastic transition matrices that describe time evolution in the future. It is shown that as the discrete time increases these convex polytopes shrink, and the minimum entropy of the probability vectors in them increases. These ideas are applied to a sequence of non-selective measurements (with different projectors in each step) on a quantum system with $d$-dimensional Hilbert space. The unitary time evolution in the intervals between the measurements, is taken into account. The non-selective measurements destroy stroboscopically the non-diagonal elements in the density matrix. This `hermaphrodite' system is an interesting combination of a classical probabilistic system (immediately after the measurements) and a quantum system (in the intervals between the measurements). Various examples are discussed. In the ergodic example, the system follows asymptotically all discrete paths with the same probability. In the example of rapidly repeated non-selective measurements, we get the well known quantum Zeno effect with `frozen discrete paths' (presented here as a biproduct of our general methodology based on Markov chains with doubly stochastic transition matrices).

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arXiv2021

Multipartite quantum systems: an approach based on Markov matrices and the Gini index

An expansion of row Markov matrices in terms of matrices related to permutations with repetitions, is introduced.It generalises the Birkhoff-von Neumann expansion of doubly stochastic matrices in terms of permutation matrices (without repetitions).An interpretation of the formalism in terms of sequences of integers that open random safes described by the Markov matrices, is presented. Various quantities that describe probabilities and correlations in this context, are discussed. The Gini index is used to quantify the sparsity (certainty) of various probability vectors. The formalism is used in the context of multipartite quantum systems with finite dimensional Hilbert space, which can be viewed as quantum permutations with repetitions or as quantum safes. The scalar product of row Markov matrices, the various Gini indices, etc, are novel probabilistic quantities that describe the statistics of multipartite quantum systems. Local and global Fourier transforms are used to define locally dual and also globally dual statistical quantities. The latter depend on off-diagonal elements that entangle (in general) the various components of the system. Examples which demonstrate these ideas are also presented.

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arXiv2020

Uncertainty relations in terms of the Gini index for finite quantum systems

Lorenz values and the Gini index are popular quantities in Mathematical Economics, and are used here in the context of quantum systems with finite-dimensional Hilbert space. They quantify the uncertainty in the probability distribution related to an orthonormal basis. It is shown that Lorenz values are superadditive functions and the Gini indices are subadditive functions. The supremum over all density matrices of the sum of the two Gini indices with respect to position and momentum states, is used to define an uncertainty coefficient which quantifies the uncertainty in the quantum system. It is shown that the uncertainty coefficient is positive, and an upper bound for it is given. Various examples demonstrate these ideas.

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arXiv2020

Equivalence classes of coherent projectors in a Hilbert space with prime dimension: Q functions and their Gini index

Coherent subspaces spanned by a finite number of coherent states are introduced, in a quantum system with Hilbert space that has odd prime dimension $d$. The set of all coherent subspaces is partitioned into equivalence classes, with $d^2$ subspaces in each class.The corresponding coherent projectors within an equivalence class, have the `closure under displacements property' and also resolve the identity. Different equivalence classes provide different granularisation of the Hilbert space, and they form a partial order `coarser' (and `finer'). In the case of a two-dimensional coherent subspace spanned by two coherent states, the corresponding projector (of rank $2$) is different than the sum of the two projectors to the subspaces related to each of the two coherent states. We quantify this with `non-addditivity operators' which are a measure of quantum interference in phase space, and also of the non-commutativity of the projectors. Generalized $Q$ and $P$ functions of density matrices, which are based on coherent projectors in a given equivalence class, are introduced. Analogues of the Lorenz values and the Gini index (which are popular quantities in Mathematical Economics) are used here to quantify the inequality in the distribution of the $Q$ function of a quantum state, within the granular structure of the Hilbert space....

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arXiv2020

The Choquet integral as an approximation to density matrices with incomplete information

A total set of $n$ states $|i\rangle$ and the corresponding projectors $Π(i)=|i\rangle \langle i|$ are considered, in a quantum system with $d$-dimensional Hilbert space $H(d)$. A partially known density matrix $ρ$ with given $p(i)={\rm Tr}[ρΠ(i)]$ (where $i=1,...,n$ and $d\le n\le d^2-1$) is considered, and its ranking permutation is defined. It is used to calculate the Choquet integral ${\cal C}(ρ)$ which is a positive semi-definite Hermitian matrix. Comonotonicity is an important concept in the formalism, which is used to formalise the vague concept of physically similar density matrices. It is shown that ${\cal C}(ρ)/{\rm Tr}[{\cal C}(ρ)]$ is a density matrix which is a good approximation to the partially known density matrix $ρ$.

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arXiv2019

Random projectors with continuous resolutions of the identity in a finite-dimensional Hilbert space

Random sets are used to get a continuous partition of the cardinality of the union of many overlapping sets. The formalism uses Möbius transforms and adapts Shapley's methodology in cooperative game theory, into the context of set theory. These ideas are subsequently generalized into the context of finite-dimensional Hilbert spaces. Using random projectors into the subspaces spanned by states from a total set, we construct an infinite number of continuous resolutions of the identity, that involve Hermitian positive semi-definite operators. The simplest one is the diagonal continuous resolution of the identity, and it is used to expand an arbitrary vector in terms of a continuum of components. It is also used to define the $F(x_1,x_2)$ function on the `probabilistic quadrant' $[0,\infty) \times [0,\infty)$, which is analogous to the Wigner function for the harmonic oscillator, on the phase-space plane. Systems with finite-dimensional Hilbert space (which are naturally described with discrete variables) are described here with continuous probabilistic variables.

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arXiv2019

Probabilistic inequalities and measurements in bipartite systems

Various inequalities (Boole inequality, Chung-Erdös inequality, Frechet inequality) for Kolmogorov (classical) probabilities are considered. Quantum counterparts of these inequalities are introduced, which have an extra `quantum correction' term, and which hold for all quantum states. When certain sufficient conditions are satisfied, the quantum correction term is zero, and the classical version of these inequalities holds for all states. But in general, the classical version of these inequalities is violated by some of the quantum states. For example in bipartite systems, classical Boole inequalities hold for all rank one (factorizable) states, and are violated by some rank two (entangled) states. A logical approach to CHSH inequalities (which are related to the Frechet inequalities), is studied in this context.It is shown that CHSH inequalities hold for all rank one (factorizable) states, and are violated by some rank two (entangled) states. The reduction of the rank of a pure state by a quantum measurement with both orthogonal and coherent projectors, is studied. Bounds for the average rank reduction are given.

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arXiv2018

Exterior calculus and fermionic quantum computation

Exterior calculus with its three operations meet, join and hodge star complement, is used for the representation of fermion-hole systems and for fermionic analogues of logical gates. Two different schemes that implement fermionic quantum computation, are proposed. The first scheme compares fermionic gates with Boolean gates, and leads to novel electronic devices that simulate fermionic gates. The second scheme usesa well known map between fermionic and multi-qubit systems, to simulate fermionic gates within multi-qubit systems.

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