arXiv subjects

Magdalena Musat

Publications and source records attributed to Magdalena Musat.

13 recordsLinked to original sources

arXiv2025

Entanglement in C$^*$-algebras: tensor products of state spaces

We analyze the Namioka-Phelps minimal and maximal tensor products of compact convex sets which arise as the state spaces of unital C$^*$-algebras. Relatedly, we study entanglement in (infinite dimensional) C$^*$-algebras. While the minimal Namioka-Phelps tensor product of the state spaces of two C$^*$-algebras is the well-known set of separable (= un-entangled) states on the (minimal) tensor product of the C$^*$-algebras, we also describe the more elusive maximal Namioka-Phelps tensor product of state spaces of C$^*$-algebras. We show that the minimal and maximal tensor products of state spaces of C$^*$-algebras agree precisely when one of the two C$^*$-algebras is commutative, which confirms Barker's conjecture in the case where the compact convex sets are state paces of C$^*$-algebras. Further, the Namioka-Phelps tensor product of the trace simplexes of two or more unital C$^*$-algebras is shown to be the trace simplex of the (minimal or maximal) tensor product of the C$^*$-algebras. This enables a systematic way of determining the trace simplex of a tensor product of C$^*$-algebras.

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arXiv2025

The Connes-Kirchberg Problem and infinite-dimensional phenomena in quantum information theory

We give an overview of results tying together a circle of problems connected to the Connes Embedding Problem, Kirchberg's reformulations thereof, Tsirelson's conjecture and its relation to quantum information theory, and a class of quantum channels, called factorizable, introduced by Anantharaman-Delaroche. While parts of the article are more expository, there are new results, including obstructions for channels to being $k$-noisy (admitting a factorization through a full matrix algebra).

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arXiv2020

Extreme Points and Factorizability for New Classes of Unital Quantum Channels

We introduce and study two new classes of unital quantum channels. The first class describes a 2-parameter family of channels given by completely positive (CP) maps $M_3({\bf C}) \mapsto M_3({\bf C})$ which are both unital and trace-preserving. Almost every member of this family is factorizable and extreme in the set of CP maps which are both unital and trace-preserving, but is not extreme in either the set of unital CP maps or the set of trace-preserving CP maps. We also study a large class of maps which generalize the Werner-Holevo channel for $d = 3$ in the sense that they are defined in terms of partial isometries of rank $d-1$. Moreover, we extend this to maps whose Kraus operators have the form $t |e_j \rangle \langle e_j | \oplus V $ with $V \in M_{d-1} ({\bf C}) $ unitary and $t \in (-1,1)$. We show that almost every map in this class is extreme in both the set of unital CP maps and the set of trace-preserving CP maps. We analyze in detail a particularly interesting subclass which is extreme unless $t = -1/(d-1)$. For $d = 3$, this includes a pair of channels which have a dual factorization in the sense that they can be obtained by taking the partial trace over different subspaces after using the same unitary conjugation in $M_3({\bf C}) \otimes M_3({\bf C})$.

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arXiv2019

Factorizable maps and traces on the universal free product of matrix algebras

We relate factorizable quantum channels on $M_n$, for $n \ge 2$, via their Choi matrix, to certain correlation matrices, which, in turn, are shown to be parametrized by traces on the unital free product $M_n * M_n$. Factorizable maps that admit a finite dimensional ancilla are parametrized by finite dimensional traces on $M_n * M_n$, and factorizable maps that approximately factor through finite dimensional C*-algebras are parametrized by traces in the closure of the finite dimensional ones. The latter set is shown to be equal to the set of hyperlinear traces on $M_n * M_n$. We finally show that each metrizable Choquet simplex is a face of the simplex of tracial states on $M_n * M_n$.

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arXiv2019

Non-closure of quantum correlation matrices and factorizable channels that require infinite dimensional ancilla

We show that there exist factorizable quantum channels in each dimension $\ge 11$ which do not admit a factorization through any finite dimensional von Neumann algebra, and do require ancillas of type II$_1$, thus witnessing new infinite-dimensional phenomena in quantum information theory. We show that the set of n by n matrices of correlations arising as second-order moments of projections in finite dimensional von Neumann algebras with a distinguished trace is non-closed, for all $n \ge 5$, and we use this to give a simplified proof of the recent result of Dykema, Paulsen and Prakash that the set of synchronous quantum correlations $C_q^s(5,2)$ is non-closed. Using a trick originating in work of Regev, Slofstra and Vidick, we further show that the set of correlation matrices arising from second-order moments of unitaries in finite dimensional von Neumann algebras with a distinguished trace is non-closed in each dimension $\ge 11$, from which we derive the first result above.

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arXiv2017

Just-infinite C*-algebras

By analogy with the well-established notions of just-infinite groups and just-infinite (abstract) algebras, we initiate a systematic study of just-infinite C*-algebras, i.e., infinite dimensional C*-algebras for which all proper quotients are finite dimensional. We give a classification of such C*-algebras in terms of their primitive ideal space that leads to a trichotomy. We show that just-infinite, residually finite dimensional C*-algebras do exist by giving an explicit example of (the Bratteli diagram of) an AF-algebra with these properties. Further, we discuss when C*-algebras and *-algebras associated with a discrete group are just-infinite. If $G$ is the Burnside-type group of intermediate growth discovered by the first named author, which is known to be just-infinite, then its group algebra $C[G]$ and its group C*-algebra $C^*(G)$ are not just-infinite. Furthermore, we show that the algebra $B = π(C[G])$ under the Koopman representation $π$ of $G$ associated with its canonical action on a binary rooted tree is just-infinite. It remains an open problem whether the residually finite dimensional C*-algebra $C^*_π(G)$ is just-infinite.

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arXiv2014

An asymptotic property of factorizable completely positive maps and the Connes embedding problem

We establish a reformulation of the Connes embedding problem in terms of an asymptotic property of factorizable completely positive maps. We also prove that the Holevo-Werner channels W_n^- are factorizable, for all odd integers n different from 3. Furthermore, we investigate factorizability of convex combinations of W_3^+ and W_3^-, a family of channels studied by Mendl and Wolf, and discuss asymptotic properties for these channels.

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arXiv2010

Factorization and dilation problems for completely positive maps on von Neumann algebras

We study factorization and dilation properties of Markov maps between von Neumann algebras equipped with normal faithful states, i.e., completely positive unital maps which preserve the given states and also intertwine their automorphism groups. The starting point for our investigation has been the question of existence of non-factorizable Markov maps, as formulated by C. Anantharaman-Delaroche. We provide simple examples of non-factorizable Markov maps on M_n(C) for all n\geq 3, as well as an example of a one-parameter semigroup (T(t))_{t\geq 0} of Markov maps on M_4(C) such that T(t) fails to be factorizable for all small values of t > 0. As applications, we solve in the negative an open problem in quantum information theory concerning an asymptotic version of the quantum Birkhoff conjecture, as well as we sharpen the existing lower bound estimate for the best constant in the noncommutative little Grothendieck inequality.

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arXiv2007

The Effros-Ruan conjecture for bilinear forms on C^*-algebras

In 1991 Effros and Ruan conjectured that a certain Grothendieck-type inequality for a bilinear form on C$^*$-algebras holds if (and only if) the bilinear form is jointly completely bounded. In 2002 Pisier and Shlyakhtenko proved that this inequality holds in the more general setting of operator spaces, provided that the operator spaces in question are exact. Moreover, they proved that the conjecture of Effros and Ruan holds for pairs of C$^*$-algebras, of which at least one is exact. In this paper we prove that the Effros-Ruan conjecture holds for general C$^*$-algebras, with constant one. More precisely, we show that for every jointly completely bounded (for short, j.c.b.) bilinear form on a pair of C$^*$-algebras $A$ and $B$, there exist states $f_1$, $f_2$ on $A$ and $g_1$, $g_2$ on $B$ such that for all $a\in A$ and $b\in B$, |u(a, b)| \leq ||u||_{jcb}(f_1(aa^*)^{1/2}g_1(b^*b)^{1/2} + f_2(a^*a)^{1/2}g_2(bb^*)^{1/2}) . While the approach by Pisier and Shlyakhtenko relies on free probability techniques, our proof uses more classical operator algebra theory, namely, Tomita-Takesaki theory and special properties of the Powers factors of type III$_λ$, $0< λ< 1$ .

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arXiv2007

Classification of hyperfinite factors up to completely bounded isomorphism of their preduals

In this paper we consider the following problem: When are the preduals of two hyperfinite (=injective) factors $\M$ and $\N$ (on separable Hilbert spaces) cb-isomorphic (i.e., isomorphic as operator spaces)? We show that if $\M$ is semifinite and $\N$ is type III, then their preduals are not cb-isomorphic. Moreover, we construct a one-parameter family of hyperfinite type III$_0$-factors with mutually non cb-isomorphic preduals, and we give a characterization of those hyperfinite factors $\M$ whose preduals are cb-isomorphic to the predual of the unique hyperfinite type III$_1$-factor. In contrast, Christensen and Sinclair proved in 1989 that all infinite dimensional hyperfinite factors with separable preduals are cb-isomorphic. More recently Rosenthal, Sukochev and the first-named author proved that all hyperfinite type III$_λ$-factors, where $0< λ\leq 1$, have cb-isomorphic preduals.

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arXiv2007

On the best constants in noncommutative Khintchine-type inequalities

We obtain new proofs with improved constants of the Khintchine-type inequality with matrix coefficients in two cases. The first case is the Pisier and Lust-Piquard noncommutative Khintchine inequality for $p=1$, where we obtain the sharp lower bound of $\frac1{\sqrt{2}}$ in the complex Gaussian case and for the sequence of functions $\{e^{i2^nt}\}_{n=1}^\infty$ . The second case is Junge's recent Khintchine-type inequality for subspaces of the operator space $R\oplus C$, which he used to construct a cb-embedding of the operator Hilbert space $OH$ into the predual of a hyperfinite factor. Also in this case, we obtain a sharp lower bound of $\frac1{\sqrt{2}}$ . As a consequence, it follows that any subspace of a quotient of $(R\oplus C)^*$ is cb-isomorphic to a subspace of the predual of the hyperfinite factor of type $III_1$, with cb-isomorphism constant $\leq \sqrt{2}$ . In particular, the operator Hilbert space $OH$ has this property.

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arXiv2006

A noncommutative version of the John-Nirenberg theorem

We prove a noncommutative version of the John-Nirenberg theorem for nontracial filtrations of von Neumann algebras. As an application, we obtain an analogue of the classical large deviation inequality for elements of the associated $BMO$ space.

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arXiv2005

On the operator space UMD property for noncommutative Lp-spaces

We study the operator space UMD property, introduced by Pisier in the context of noncommutative vector-valued Lp-spaces. It is unknown whether the property is independent of p in this setting. We prove that for 1<p,q<\infty, the Schatten q-classes Sq are OUMDp. The proof relies on properties of the Haagerup tensor product and complex interpolation. Using ultraproduct techniques, we extend this result to a large class of noncommutative Lq-spaces. Namely, we show that if M is a QWEP von Neumann algebra (i.e., a quotient of a C^*-algebra with Lance's weak expectation property) equipped with a normal, faithful tracial state τ, then Lq(M,τ) is OUMDp for 1<p,q<\infty.

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