Simulation & Measurement

Running a circuit

run(circuit) simulates the circuit and returns the final statevector: a complex NumPy array of length 2**num_qubits.

from fqkit import QuantumCircuit, Hadamard, CNOT, run
 
qc = QuantumCircuit(2)
qc.add_gate(Hadamard(), [0])
qc.add_gate(CNOT(), [0, 1])
 
state = run(qc)
print(state)   # [0.707+0j 0+0j 0+0j 0.707+0j]

The circuit always starts in |00...0> and each operation is applied in order.

How the simulator works

Each gate is applied to the statevector by tensor contraction. This is what lets multi-qubit gates act on any set of qubits, including non-adjacent or reversed ones.

Measuring

measure_all(state, shots) samples the statevector and returns a dictionary of bitstring counts. The probability of each outcome is the squared magnitude of its amplitude.

from fqkit import measure_all
 
counts = measure_all(state, shots=1024)
print(counts)   # {'00': 512, '11': 512}

Bit-order convention