Simulation Examples

Statevector Simulation

The CuStateVecStateBackend allows you to retrieve the full statevector of a quantum circuit. This example simulates a Bell State (\(|\Phi^+\rangle = \frac{|00\rangle + |11\rangle}{\sqrt{2}}\)).

import numpy as np
from pytket import Circuit
from pytket.extensions.custatevec import CuStateVecStateBackend

# 1. Define a Bell State circuit
circ = Circuit(2)
circ.H(0).CX(0, 1)

# 2. Initialize the Statevector Backend
backend = CuStateVecStateBackend()

# 3. Compile the circuit for the backend
compiled_circ = backend.get_compiled_circuit(circ)

# 4. Run and retrieve the statevector
handle = backend.process_circuit(compiled_circ)
result = backend.get_result(handle)
statevector = result.get_state()

print("Statevector:", np.round(statevector, 3))

Shot-based Sampling

The CuStateVecShotsBackend mimics a quantum computer by returning measurement counts.

from pytket import Circuit
from pytket.extensions.custatevec import CuStateVecShotsBackend

# 1. Define a circuit with measurements
circ = Circuit(2, 2)
circ.H(0).CX(0, 1)
circ.measure_all()

# 2. Initialize the Shots Backend
backend = CuStateVecShotsBackend()
compiled_circ = backend.get_compiled_circuit(circ)

# 3. Run with a specific number of shots
handle = backend.process_circuit(compiled_circ, n_shots=1000)
result = backend.get_result(handle)

# 4. Get counts
counts = result.get_counts()
print("Counts:", counts)

Calculating Expectation Values

You can calculate operator expectation values efficiently on the GPU without explicitly retrieving the full statevector.

from pytket import Circuit, Qubit
from pytket.pauli import Pauli, QubitPauliString
from pytket.utils.operators import QubitPauliOperator
from pytket.utils.expectations import get_operator_expectation_value
from pytket.extensions.custatevec import CuStateVecStateBackend

# 1. Define a circuit
circ = Circuit(2)
circ.H(0).CX(0, 1)  # Bell state

# 2. Define an operator (e.g., Z0 * Z1)
op = QubitPauliOperator({
    QubitPauliString([Qubit(0), Qubit(1)], [Pauli.Z, Pauli.Z]): 1.0
})

# 3. Calculate expectation value
backend = CuStateVecStateBackend()
expectation = get_operator_expectation_value(circ, op, backend)

print(f"Expectation value <Z0 Z1>: {expectation}")

Basis Order

By default, pytket uses ilo (Increasing Lexicographic Order). You can explicitly request dlo (Decreasing Lexicographic Order/Big Endian).

from pytket import Circuit, BasisOrder
from pytket.extensions.custatevec import CuStateVecStateBackend

# Create state |01> (qubit 0 is 0, qubit 1 is 1)
circ = Circuit(2).X(1)

backend = CuStateVecStateBackend()
compiled_circ = backend.get_compiled_circuit(circ)
result = backend.run_circuit(compiled_circ)

# pytket default (Little Endian): |01> corresponds to index 2 (binary 10 reversed)
print("ILO (Default):", result.get_state(basis=BasisOrder.ilo))

# Big Endian: |01> corresponds to index 1 (binary 01)
print("DLO:", result.get_state(basis=BasisOrder.dlo))