Prefix-free quantum Kolmogorov complexity

TL;DR

This paper introduces a quantum analogue of Kolmogorov complexity, called $QK$, and explores its properties and connections to quantum randomness notions, providing characterizations similar to classical results.

Contribution

It defines $QK$, a quantum prefix-free Kolmogorov complexity, and establishes its properties and relationships with quantum randomness concepts, extending classical complexity and randomness characterizations.

Findings

$QK$ measures the complexity of quantum states using classical prefix-free machines.

Initial segments of certain quantum random states are incompressible in terms of $QK$.

Quantum Schnorr randomness admits Levin-Schnorr and Chaitin characterizations via $QK_C$.

Abstract

We introduce quantum-K ($QK$), a measure of the descriptive complexity of density matrices using classical prefix-free Turing machines and show that the initial segments of weak Solovay random and quantum Schnorr random states are incompressible in the sense of $QK$. Many properties enjoyed by prefix-free Kolmogorov complexity ($K$) have analogous versions for $QK$; notably a counting condition. Several connections between Solovay randomness and $K$, including the Chaitin type characterization of Solovay randomness, carry over to those between weak Solovay randomness and $QK$. We work towards a Levin-Schnorr type characterization of weak Solovay randomness in terms of $QK$. Schnorr randomness has a Levin-Schnorr characterization using $K_C$; a version of $K$ using a computable measure machine, $C$. We similarly define $QK_C$, a version of $QK$. Quantum Schnorr randomness is shown to have a Levin-Schnorr and a Chaitin type characterization using $QK_C$. The latter implies a Chaitin type characterization of classical Schnorr randomness using $K_C$.