Measuring specific qubits with OpenQASM 3.0

The local state vector simulator and local density matrix simulator support submitting OpenQASM programs where a subset of the circuit's qubits can be measured. This is often called partial measurement. For example, in the following code you can create a two-qubit circuit and only measure the first qubit.

partial_measure_qasm = """
OPENQASM 3.0;
bit[1] b;
qubit[2] q;
h q[0];
cnot q[0], q[1];
b[0] = measure q[0];
"""

There are two qubits, q[0] and q[1] but we are only measuring qubit 0 here: b[0] = measure q[0]. Now, run the following on the local state vector simulator.

from braket.devices import LocalSimulator

local_sim = LocalSimulator()
partial_measure_local_sim_task = local_sim.run(OpenQASMProgram(source=partial_measure_qasm), shots = 10)
partial_measure_local_sim_result = partial_measure_local_sim_task.result()
print(partial_measure_local_sim_result.measurement_counts)
print("Measured qubits: ", partial_measure_local_sim_result.measured_qubits)

You can check whether a device supports partial measurement by inspecting the requiresAllQubitsMeasurement field in its action properties; if it is False, then partial measurement is supported.

AwsDevice(Devices.Rigetti.AspenM3).properties.action['braket.ir.openqasm.program'].requiresAllQubitsMeasurement

Here, requiresAllQubitsMeasurement is False, which indicates that not all qubits must be measured.