A remark on the paper ``Randomizing quantum states: Constructions and applications''

The concept of \e-randomizing quantum channels has been introduced by Hayden, Leung, Shor and Winter in connection with approximately encrypting quantum states. They proved using a discretization argument that sets of roughly $d \log d$ random unitary operators provide examples of such channels on \C^d. We show that a simple trick improves the efficiency of the argument and reduces the number of unitary operators to roughly $d$

Partial transposition of random states and non-centered semicircular distributions

Let W be a Wishart random matrix of size d^2 times d^2, considered as a block matrix with d times d blocks. Let Y be the matrix obtained by transposing each block of W. We prove that the empirical eigenvalue distribution of Y approaches a non-centered semicircular distribution when d tends to infinity. We also show the convergence of extreme eigenvalues towards the edge of the expected spectrum. The proofs are based on the moments method. This matrix model is relevant to Quantum Information Theory and corresponds to the partial transposition of a random induced state. A natural question is: "When does a random state have a positive partial transpose (PPT)?". We answer this question and exhibit a strong threshold when the parameter from the Wishart distribution equals 4. When d gets large, a random state on C^d tensor C^d obtained after partial tracing a random pure state over some ancilla of dimension alpha.d^2 is typically PPT when alpha>4 and typically non-PPT when alpha<4.Comment: 24 pages. V2 : includes the convergence of extreme eigenvalues. V3 : includes probability estimates showing that p=4d^2 is a sharp threshol

Stochastic domination for iterated convolutions and catalytic majorization

We study how iterated convolutions of probability measures compare under stochastic domination. We give necessary and sufficient conditions for the existence of an integer $n$ such that $\mu^{*n}$ is stochastically dominated by $\nu^{*n}$ for two given probability measures $\mu$ and $\nu$. As a consequence we obtain a similar theorem on the majorization order for vectors in $\R^d$. In particular we prove results about catalysis in quantum information theory

Catalytic majorization and ℓp\ell_p norms

An important problem in quantum information theory is the mathematical characterization of the phenomenon of quantum catalysis: when can the surrounding entanglement be used to perform transformations of a jointly held quantum state under LOCC (local operations and classical communication) ? Mathematically, the question amounts to describe, for a fixed vector $y$, the set $T(y)$ of vectors $x$ such that we have $x \otimes z \prec y \otimes z$ for some $z$, where $\prec$ denotes the standard majorization relation. Our main result is that the closure of $T(y)$ in the $\ell_1$ norm can be fully described by inequalities on the $\ell_p$ norms: $\|x\|_p \leq \|y\|_p$ for all $p \geq 1$. This is a first step towards a complete description of $T(y)$ itself. It can also be seen as a $\ell_p$-norm analogue of Ky Fan dominance theorem about unitarily invariant norms. The proofs exploits links with another quantum phenomenon: the possibiliy of multiple-copy transformations ($x^{\otimes n} \prec y^{\otimes n}$ for given $n$). The main new tool is a variant of Cram\'er$ theorem on large deviations for sums of i.i.d. random variables

Realigning random states

We study how the realignment criterion (also called computable cross-norm criterion) succeeds asymptotically in detecting whether random states are separable or entangled. We consider random states on \C^d \otimes \C^d obtained by partial tracing a Haar-distributed random pure state on \C^d \otimes \C^d \otimes \C^s over an ancilla space \C^s. We show that, for large $d$, the realignment criterion typically detects entanglement if and only if $s \leq (8/3\pi)^2 d^2$. In this sense, the realignment criterion is asymptotically weaker than the partial transposition criterion

Maximal inequality for high-dimensional cubes

We present lower estimates for the best constant appearing in the weak $(1,1)$ maximal inequality in the space (\R^n,\|\cdot\|_{\iy}). We show that this constant grows to infinity faster than $(\log n)^{1-o(1)}$ when $n$ tends to infinity. To this end, we follow and simplify the approach used by J.M. Aldaz. The new part of the argument relies on Donsker's theorem identifying the Brownian bridge as the limit object describing the statistical distribution of the coordinates of a point randomly chosen in the unit cube $[0,1]^n$ ($n$ large).Comment: The overall presentation has been changed. To appear in Confluentes Mathematic

Catalysis in the trace class and weak trace class ideals

Given operators $A,B$ in some ideal $\mathcal{I}$ in the algebra $\mathcal{L}(H)$ of all bounded operators on a separable Hilbert space $H$, can we give conditions guaranteeing the existence of a trace-class operator $C$ such that $B \otimes C$ is submajorized (in the sense of Hardy--Littlewood) by $A \otimes C$ ? In the case when $\mathcal{I} = \mathcal{L}_1$, a necessary and almost sufficient condition is that the inequalities ${\rm Tr} (B^p) \leq {\rm Tr} (A^p)$ hold for every $p \in [1,\infty]$. We show that the analogous statement fails for $\mathcal{I} = \mathcal{L}_{1,\infty}$ by connecting it with the study of Dixmier traces

Maximal exponent of the Lorentz cones

We show that the maximal exponent (i.e., the minimum number of iterations required for a primitive map to become strictly positive) of the n-dimensional Lorentz cone is equal to n. As a byproduct, we show that the optimal exponent in the quantum Wielandt inequality for qubit channels is equal to 3.Comment: 1 figure. v2: simplified the proof, added a section about quantum Wielandt inequalit