Commuting quantum circuits: efficient classical simulations versus hardness results

  • Authors:
  • Xiaotong Ni;Maarten Van Den Nest

  • Affiliations:
  • Max-Planck-Institut für Quantenoptik, Garching, Germany;Max-Planck-Institut für Quantenoptik, Garching, Germany

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2013

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Abstract

The study of quantum circuits composed of commuting gates is particularly useful to understand the delicate boundary between quantum and classical computation. Indeed, while being a restricted class, commuting circuits exhibit genuine quantum effects such as entanglement. In this paper we show that the computational power of commuting circuits exhibits a surprisingly rich structure. First we show that every 2-local commuting circuit acting on d-level systems and followed by single-qudit measurements can be efficiently simulated classically with high accuracy. In contrast, we prove that such strong simulations are hard for 3-local circuits. Using sampling methods we further show that all commuting circuits composed of exponentiated Pauli operators eiθP can be simulated efficiently classically when followed by single-qubit measurements. Finally, we show that commuting circuits can efficiently simulate certain non-commutative processes, related in particular to constant-depth quantum circuits. This gives evidence that the power of commuting circuits goes beyond classical computation.