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Before measurement, a quantum circuit creates a state which can be described as a vector of 2n2^n amplitudes, where nn 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 2n2^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. Use the method ExecutionSession.set_measured_state_filter, to specify the execution output values of interest.

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Example

from classiq import *


@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


quantum_program = synthesize(main)

execution_preferences = ExecutionPreferences(
    backend_preferences=ClassiqBackendPreferences(
        backend_name=ClassiqNvidiaBackendNames.SIMULATOR_STATEVECTOR
    )
)
with ExecutionSession(
    quantum_program, execution_preferences=execution_preferences
) 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.

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Amplitude Threshold

By default, state vector simulation filters out states with exactly zero amplitude from the result. You can tighten this filter by setting amplitude_threshold in ExecutionPreferences to exclude states whose amplitude magnitude is below a given threshold: 
from classiq import *


@qfunc
def main(x: Output[QNum]) -> None:
    prepare_state(probabilities=[0.5, 0, 0.25, 0.25], bound=0.01, out=x)


quantum_program = synthesize(main)

execution_preferences = ExecutionPreferences(
    backend_preferences=ClassiqBackendPreferences(
        backend_name=ClassiqSimulatorBackendNames.SIMULATOR_STATEVECTOR
    ),
    amplitude_threshold=1e-4,
)
with ExecutionSession(
    quantum_program, execution_preferences=execution_preferences
) as session:
    results = session.sample()
States with |amplitude| <= amplitude_threshold are excluded from results. This reduces the size of the result for circuits where most amplitudes are negligibly small (e.g. block-encoded circuits).
Setting include_zero_amplitude_outputs=True overrides amplitude_threshold and includes all states regardless of amplitude.
Filtering removes states from the returned state vector. As a result, the remaining amplitudes will not sum to a norm of 1. Do not rely on the filtered state vector being normalized.

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Limitations

Currently, filtering is only available on Classiq’s simulator_statevector (ClassiqSimulatorBackendNames.SIMULATOR_STATEVECTOR) and 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.