State Vector Filtering

Before measurement, a quantum circuit creates a state which can be described as a vector of \(2^n\) amplitudes, where \(n\) is the number of qubits. Though this state vector cannot be directly accessed on a quantum computer (as measurement destroys the state), certain quantum simulators make this information available. Classiq supports two simulators which return the full statevector, both under the Classiq provider: simulator_statevector and nvidia_simulator_statevector.

Since the data size grows exponentially, large circuits cannot be simulated. However, in certain applications such as those that use block encoding, not all of the \(2^n\) amplitudes are of interest. For example, some methods of post-processing discard certain results wholesale, and thus the amplitudes corresponding to those measured states are irrelevant.

In these instances, filtering out the amplitudes that are not of interest can greatly save memory and network footprint. Use the method ExecutionSession.set_measured_state_filter, to specify the execution output values of interest.

Example

from classiq import *
from classiq.execution import (
    ClassiqBackendPreferences,
    ClassiqNvidiaBackendNames,
    ExecutionSession,
    ExecutionPreferences,
)


@qfunc
def main(x: Output[QBit], y: Output[QNum], z: Output[QNum]) -> None:
    allocate(1, x)
    hadamard_transform(x)
    prepare_state(probabilities=[0.5, 0, 0.25, 0.25], bound=0.01, out=y)
    z |= y + 1


model = create_model(
    main,
    execution_preferences=ExecutionPreferences(
        backend_preferences=ClassiqBackendPreferences(
            backend_name=ClassiqNvidiaBackendNames.SIMULATOR_STATEVECTOR
        ),
    ),
)
quantum_program = synthesize(model)

with ExecutionSession(quantum_program) as session:
    session.set_measured_state_filter("x", lambda state: state == 1)
    session.set_measured_state_filter("y", lambda state: state == 2)
    results = session.sample()

Filtering ensures that results will contain only the amplitudes that correspond to states where x is 1 and y is 2.

Limitations

Currently, filtering is only available on the nvidia_simulator_statevector (ClassiqNvidiaBackendNames.SIMULATOR_STATEVECTOR). Filtering is only available for quantum scalars (QuantumBits and QuantumNumerics). Additionally, only a single value per variable is supported. For example, session.set_measured_state_filter("x", lambda x: x < 5) is not allowed.