Circuits

A QuantumCircuit holds a number of qubits and an ordered list of operations (gates applied to specific qubits).

Create a circuit


from fqkit import QuantumCircuit
 
qc = QuantumCircuit(3)        # a 3-qubit circuit
print(qc.num_qubits)          # 3

Add gates


from fqkit import QuantumCircuit, Hadamard, CNOT, Qubit
 
qc = QuantumCircuit(2)
qc.add_gate(Hadamard(), [0])
qc.add_gate(CNOT(), [0, 1])

The same circuit, drawn:

Hadamard on qubit 0, then CNOT onto qubit 1.

Targets can be integer indices or Qubit objects: both work:


qc.add_gate(Hadamard(), [Qubit(0)])

Validation

add_gate checks your circuit and raises a clear ValueError if:

the number of targets doesn’t match the gate’s qubit count
a target index is negative or beyond the circuit size
a multi-qubit gate targets the same qubit twice


qc = QuantumCircuit(2)
qc.add_gate(CNOT(), [0])       # ValueError: needs 2 qubits
qc.add_gate(Hadamard(), [5])   # ValueError: index exceeds circuit size
qc.add_gate(CNOT(), [0, 0])    # ValueError: repeated target qubit

Inspect a circuit

print(qc) shows every operation:


print(qc)
# QuantumCircuit(2, [
# H on (0)
# CNOT on (0, 1)
# ])