An algorithm to explore entanglement in small systems

A quantum state’s entanglement across a bipartite cut can be quantified with entanglement entropy or, more generally, Schmidt norms. Using only Schmidt decompositions, we present a simple iterative algorithm to maximize Schmidt norms. Depending on the choice of norm, the optimizing states maximize or minimize entanglement, possibly across several bipartite cuts at the same time and possibly only among states in a specified subspace. Recognizing that convergence but not success is certain, we use the algorithm to explore topics ranging from fermionic reduced density matrices and varieties of pure quantum states to absolutely maximally entangled states and minimal output entropy of channels.

Multiplicativity of superoperator norms for some entanglement breaking channels

It is known that the minimal output entropy is additive for any product of entanglement breaking (EB) channels. The same is true for the Renyi entropy, where additivity is equivalent to multiplicativity of the \norm{1}{q} norm for all $q \ge 1$. In this paper we consider the related question of multiplicativity of the \norm{2}{q} norm for $q > 2$ for entanglement breaking channels. We prove that multiplicativity holds in this case for certain classes of EB channels, including both the CQ and QC channels.

ENTROPIC CHARACTERIZATION OF QUANTUM OPERATIONS

We investigate decoherence induced by a quantum channel in terms of minimal output entropy and map entropy. The latter is the von Neumann entropy of the Jamiołkowski state of the channel. Both quantities admit q-Renyi versions. We prove additivity of the map entropy for all q. For the case q = 2, we show that the depolarizing channel has the smallest map entropy among all channels with a given minimal output Renyi entropy of order two. This allows us to characterize pairs of channels such that the output entropy of their tensor product acting on a maximally entangled input state is larger than the sum of the minimal output entropies of the individual channels. We conjecture that for any channel Φ1 acting on a finite dimensional system, there exists a class of channels Φ2 sufficiently close to a unitary map such that additivity of minimal output entropy for Ψ1 ⊗ Ψ2 holds.

ADDITIVITY FOR TRANSPOSE DEPOLARIZING CHANNELS

Additivity of the minimal output entropy for the family of transpose depolarizing channels introduced by Fannes et al.5 is considered. It is shown that using the method of Ref. 3 allows us to prove the additivity for the range of the parameter values for which the problem was left open in Ref. 5. Together with the result of Ref. 5, this covers the whole family of transpose depolarizing channels. In addition, the multiplicativity of the maximal p-norm for this family of channels is proved for all 1 ≤ p ≤ 2.

ADDITIVITY CONJECTURE AND COVARIANT CHANNELS

In this survey paper we discuss the relation between the minimal output entropy and the χ-capacity for irreducibly covariant quantum channels implying equivalence of the additivity property for both quantities for such channels. The structure of the Weyl-covariant channels is described in detail.