Parameters & Binding

A Parameter is a symbolic placeholder for a number. Parameters let you build a circuit once and try many different angles: the foundation of variational algorithms like VQE and QAOA.

Define and bind


from fqkit import (
    QuantumCircuit, Hadamard, RX, Parameter,
    bind_parameters, run, measure_all
)
 
theta = Parameter("theta")
 
qc = QuantumCircuit(2)
qc.add_gate(Hadamard(), [0])
qc.add_gate(RX(theta), [1])
 
# Bind the symbol to a number
bind_parameters(qc, {"theta": 3.14159})
 
counts = measure_all(run(qc), shots=1024)
print(counts)

bind_parameters(circuit, values) walks the circuit and replaces every Parameter whose name appears in values with the given number.

Unbound parameters

If you run (or export) a circuit that still has an unbound parameter, FQkit raises a clear error instead of a confusing one:


qc = QuantumCircuit(1)
qc.add_gate(RX(Parameter("theta")), [0])
run(qc)
# ValueError: Gate RX has an unbound parameter 'theta'.
#             Call bind_parameters(circuit, {'theta': value}) ...

ℹ️

You can also pass a plain number to a rotation gate (RX(1.57)) if you don’t need a symbol.