OpenQASM and Assembly-Level Control in Quantum Computing

Table of Contents

1. Introduction
2. What Is OpenQASM?
3. Motivation for Assembly-Level Control
4. Versions of OpenQASM
5. OpenQASM Syntax Basics
6. Declaring Qubits and Classical Bits
7. Gate Definitions and Standard Gates
8. Custom Gates and Subroutines
9. Measurement Syntax
10. Conditional Statements and Classical Control
11. Mid-Circuit Measurement
12. Reset Operations and Initialization
13. Looping and Branching (OpenQASM 3.0)
14. Classical Expressions and Registers
15. Interfacing with Qiskit
16. Exporting Circuits as OpenQASM
17. Importing OpenQASM into Qiskit
18. Use Cases for OpenQASM
19. Limitations and Low-Level Optimization
20. Conclusion

1. Introduction

OpenQASM (Quantum Assembly Language) is an intermediate representation used to express quantum programs in a human-readable assembly-like syntax. It enables low-level access and control over quantum circuits.

2. What Is OpenQASM?

Developed by IBM, OpenQASM is the standard language for describing quantum circuits at the gate level, bridging the gap between high-level quantum programming and quantum hardware.

3. Motivation for Assembly-Level Control

• Direct control over gate execution
• Real-time measurement and feedback
• Hardware-specific optimizations
• Interoperability across platforms

4. Versions of OpenQASM

• OpenQASM 2.0: Used in IBM Quantum systems and Qiskit
• OpenQASM 3.0: Experimental, adds classical flow control and timing features

5. OpenQASM Syntax Basics

OpenQASM programs start with:

OPENQASM 2.0;
include "qelib1.inc";

6. Declaring Qubits and Classical Bits

qreg q[2];
creg c[2];

7. Gate Definitions and Standard Gates

h q[0];
cx q[0], q[1];
x q[1];

8. Custom Gates and Subroutines

gate mygate a,b {
  h a;
  cx a, b;
}
mygate q[0], q[1];

9. Measurement Syntax

measure q[0] -> c[0];

10. Conditional Statements and Classical Control

if (c==1) x q[1];

Performs a gate if a classical register equals a given value.

11. Mid-Circuit Measurement

measure q[0] -> c[0];
if (c==1) x q[1];

Supported on some devices and simulators.

12. Reset Operations and Initialization

reset q[0];

Used to reinitialize a qubit to |0⟩.

13. Looping and Branching (OpenQASM 3.0)

for i in [0:10] {
  x q[i];
}
while (c[0] == 0) {
  z q[1];
}

14. Classical Expressions and Registers

int[32] counter = 0;
bit x;
bool flag = true;

These constructs exist in OpenQASM 3.0 and enhance classical programmability.

15. Interfacing with Qiskit

Export a circuit to OpenQASM:

qasm_str = qc.qasm()

Load from OpenQASM:

from qiskit import QuantumCircuit
qc = QuantumCircuit.from_qasm_str(qasm_str)

16. Exporting Circuits as OpenQASM

with open('my_circuit.qasm', 'w') as f:
    f.write(qasm_str)

17. Importing OpenQASM into Qiskit

qc = QuantumCircuit.from_qasm_file('my_circuit.qasm')

18. Use Cases for OpenQASM

• Hardware compilation interfaces
• Program interoperability
• Custom transpilation workflows
• Debugging and verification

19. Limitations and Low-Level Optimization

• Manual gate management is error-prone
• Platform-dependent features not fully portable
• Limited abstraction compared to SDKs

20. Conclusion

OpenQASM provides precise, low-level control over quantum programs and acts as a bridge between quantum software frameworks and real quantum hardware. With Qiskit support and OpenQASM 3.0’s new features, it continues to play a vital role in quantum software development.