Classical and quantum Merlin-Arthur automata

TL;DR

This paper introduces various classes of Merlin-Arthur automata, exploring their capabilities with different certificate lengths, and demonstrates their power in recognizing and verifying a wide range of languages, including nonstochastic, unary, and NP-complete languages.

Contribution

It defines and analyzes new MA automata models, showing how certificate length impacts their language recognition and verification power, including equivalences and efficiency results.

Findings

MA-DFAs recognize all regular languages with constant certificates.

MA-PFAs and MA-PostPFAs can recognize nonstochastic unary languages with sublinear and linear certificates.

MA-PostQFAs can verify all unary languages and some NP-complete binary languages with linear certificates.

Abstract

We introduce Merlin-Arthur (MA) automata where Merlin provides a certificate at the beginning of computation and it is scanned by Arthur before reading the input. We define Merlin-Arthur deterministic, probabilistic, and quantum finite state automata (resp., MA-DFAs, MA-PFAs, and MA-QFAs) and postselecting MA-PFAs and MA-QFAs (resp., MA-PostPFA and MA-PostQFA). We present several results using different certificate lengths. We show that MA-DFAs use constant length certificates, and they are equivalent to multi-entry DFAs. Thus, they recognize all and only regular languages, but they can be exponential and polynomial state efficient over binary and unary languages, respectively. With sublinear length certificates, MA-PFAs can recognize several nonstochastic unary languages with cutpoint 1/2. With linear length certificates, MA-PostPFAs can recognize these nonstochastic unary languages with bounded error. With arbitrarily long certificates, bounded-error MA-PostPFAs can verify every unary decidable language. With sublinear length certificates, bounded-error MA-PostQFAs can verify several nonstochastic unary languages. With linear length certificates, they can verify every unary language and some NP-complete binary languages. With exponential length certificates, they can verify every binary language.