Simulators

Introduction to Quantum Computing

Simulators

• Unitary
  • Provides the unitary equivalent of a given circuit with only 1 shot and no measure or reset.
• QASM: Quantum Assembly Language
  • A general-purpose simulator for simulating/emulating the ideal execution of a generic quantum circuits both ideally and subject to noise modeling.  It simulates a generic circuit for a number of shots and returns measurement counts.  It is similar to an experiment. 
• StateVector
  • Simulates a quantum circuit by computing the wavefunction of the qubit’s statevector resulting from the evolution of the 0 state through that circuit as gates and instructions are applied. It supports general noise modeling and takes only 1 shot and no measure or reset.
• Pulse
• Matrix Product Operator

Quantum simulator services are advanced cloud-based classical emulators of quantum systems.  

The simulator backends have the same default number of maximum qubits and maximum number of shots.  They both accept circuits with all gates, not just the basis gates.  When simulating ideal circuits, changing the method between the exact simulation methods stabilizer should not change the simulation result (other than usual variations from sampling probabilities for measurement outcomes).

The statevector simulator returns a block of data the size of 2^N (where N is the number of qubits, and 2^N is the size of the statevector). As the number of qubits increases, the size of the statevector increases exponentially.  This should be the default choice for the simulator as it is a general-purpose solution method. 

The qasm simulator returns counts, which is a sampling of the statevector of the circuit.  It is smaller in size and won't increase in size exponentially as the number of qubits increases. 

Though they return different things, the qasm and statevector simulator are both part of the same simulator.