Is there a way to generate random circuits in qiskit with only certain allowed gates?

Question

It seems like I can use https://qiskit.org/documentation/stubs/qiskit.circuit.random.random_circuit.html but I would like to control the gates that are allowed.

What is an efficient way to make random circuits with a fixed set of gates?

Answer 1

I think you should report of "feature request" issue if you have a good use case for this. It should not be hard to implement and I could help with it.

If by any chance your fixed set of gates is based on the amount of qubits involved in gate, you can control that with max_operands. For example, you can exclude CCXGate and CSwapGate (3-qubit operands) with max_operand=2.

If this is not the case, the source code for random_circuit seems straightforward to modify for your need. You can remove or add the gates from *_q_ops and *_param variables on top.

def random_circuit(num_qubits, depth, max_operands=3, measure=False,
                   conditional=False, reset=False, seed=None):
    
    if max_operands < 1 or max_operands > 3:
        raise CircuitError("max_operands must be between 1 and 3")

    one_q_ops = [IGate, U1Gate, U2Gate, U3Gate, XGate, YGate, ZGate,
                 HGate, SGate, SdgGate, TGate, TdgGate, RXGate, RYGate, RZGate]
    one_param = [U1Gate, RXGate, RYGate, RZGate, RZZGate, CU1Gate, CRZGate]
    two_param = [U2Gate]
    three_param = [U3Gate, CU3Gate]
    two_q_ops = [CXGate, CYGate, CZGate, CHGate, CRZGate,
                 CU1Gate, CU3Gate, SwapGate, RZZGate]
    three_q_ops = [CCXGate, CSwapGate]

    qr = QuantumRegister(num_qubits, 'q')
    qc = QuantumCircuit(num_qubits)

    if measure or conditional:
        cr = ClassicalRegister(num_qubits, 'c')
        qc.add_register(cr)

    if reset:
        one_q_ops += [Reset]

    if seed is None:
        seed = np.random.randint(0, np.iinfo(np.int32).max)
    rng = np.random.default_rng(seed)

    # apply arbitrary random operations at every depth
    for _ in range(depth):
        # choose either 1, 2, or 3 qubits for the operation
        remaining_qubits = list(range(num_qubits))
        while remaining_qubits:
            max_possible_operands = min(len(remaining_qubits), max_operands)
            num_operands = rng.choice(range(max_possible_operands)) + 1
            rng.shuffle(remaining_qubits)
            operands = remaining_qubits[:num_operands]
            remaining_qubits = [q for q in remaining_qubits if q not in operands]
            if num_operands == 1:
                operation = rng.choice(one_q_ops)
            elif num_operands == 2:
                operation = rng.choice(two_q_ops)
            elif num_operands == 3:
                operation = rng.choice(three_q_ops)
            if operation in one_param:
                num_angles = 1
            elif operation in two_param:
                num_angles = 2
            elif operation in three_param:
                num_angles = 3
            else:
                num_angles = 0
            angles = [rng.uniform(0, 2 * np.pi) for x in range(num_angles)]
            register_operands = [qr[i] for i in operands]
            op = operation(*angles)

            # with some low probability, condition on classical bit values
            if conditional and rng.choice(range(10)) == 0:
                value = rng.integers(0, np.power(2, num_qubits))
                op.condition = (cr, value)

            qc.append(op, register_operands)

    if measure:
        qc.measure(qr, cr)

    return qc

Answer 2

You have the random_circuit function in qiskit.circuit.random to generate a random circuit of arbitrary size and form. Have a look at the documentation. This function will generate a random circuit by randomly selecting gates from the set of standard gates in qiskit.extensions.

The signature of the function with all the params is the following:

random_circuit(num_qubits, depth, max_operands=3, measure=False, conditional=False, reset=False, seed=None, *, n_qubits=None)

Following code

from qiskit.circuit.random import random_circuit

circ = random_circuit(2, 2, measure=True)
circ.draw(output='mpl')

will produce, for instance, this circuit:

Just in case you need coding your own function, the whole source code of that function is here: https://qiskit.org/documentation/stable/0.19/_modules/qiskit/circuit/random/utils.html#random_circuit