Device-independent tests of quantum channels

TL;DR

This paper introduces a device-independent method for testing quantum channels by characterizing input-output correlations, providing a complete framework for binary correlations and specific classes of channels.

Contribution

It develops a comprehensive, device-independent framework for testing quantum channels based solely on observed correlations, including full characterizations for certain classes of channels.

Findings

Extremal correlations achieved by orthogonal encodings and measurements.

Full characterization of binary correlations for dihedrally-covariant qubit channels.

Complete description of correlations for universal covariant channels in arbitrary dimensions.

Abstract

We develop a device-independent framework for testing quantum channels. That is, we falsify a hypothesis about a quantum channel based only on an observed set of input-output correlations. Formally, the problem consists of characterizing the set of input-output correlations compatible with any arbitrary given quantum channel. For binary (i.e., two input symbols, two output symbols) correlations, we show that extremal correlations are always achieved by orthogonal encodings and measurements, irrespective of whether or not the channel preserves commutativity. We further provide a full, closed-form characterization of the sets of binary correlations in the case of: i) any dihedrally-covariant qubit channel (such as any Pauli and amplitude-damping channels), and ii) any universally-covariant commutativity-preserving channel in an arbitrary dimension (such as any erasure, depolarizing, universal cloning, and universal transposition channels).