Introduction to OpenQASM

OpenQASM (open quantum assembly language), a machine-independent programming interface compatible with IBM® QPUs, is an imperative programming language for describing quantum circuits. OpenQASM uses the quantum circuit model to express quantum programs as ordered sequences of parameterized operations (such as gates, measurements, and resets) and real-time classical computation. In addition to quantum algorithms, OpenQASM can describe circuits intended to characterize, validate, or debug quantum processors.

As the needs of QPU development have evolved, OpenQASM's feature list has expanded in response; the latest version, OpenQASM 3, incorporates extensions including classical feed-forward flow control, gate modifiers, and pulse implementations.

OpenQASM is the choice for a variety of audiences because of its versatility. The introduction to the OpenQASM 3 paper¹ gives examples:

"Although OpenQASM is not a high-level language, many users would like to write simple quantum circuits by hand using an expressive domain-specific language. Researchers who study circuit compiling need high-level information recorded in the intermediate representations to inform the optimization and synthesis algorithms. Experimentalists prefer the convenience of writing circuits at a relatively high level but often need to manually modify timing or pulse-level gate descriptions at various points in the circuit. Hardware engineers who design the classical controllers and waveform generators prefer languages that are practical to compile given the hardware constraints and make explicit circuit structure that the controllers can take advantage of."

OpenQASM is the common interchange format among independent quantum software tools. For developers that prefer one tool for circuit construction, another for transpilation, and so forth, OpenQASM is the lingua franca that acts as a bridge among them.

The Qiskit SDK provides ways to convert between OpenQASM and the QuantumCircuit class (see OpenQASM 2 and Qiskit and OpenQASM 3 and Qiskit for instructions).

For more information, view the OpenQASM live specification.

OpenQASM code example: cat state

OPENQASM 3;
include "stdgates.inc";

const n = 3; // number of qubits
qubit[n] q; // a register 'q' of n qubits
bit[n] c; // a register 'c' of n classical bits

h q[0]; // Hadamard
for k in [0:n-1] { 
  cnot q[k], q[k+1]; // Controlled-NOT from control qubit q[k] to target qubit q[k+1] 
}

c = measure q; // measure quantum register

Next steps