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Fix performance regression in 3 or more qubits RB #905

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Fix performance regression in 3 or more qubits RB #905

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29 changes: 23 additions & 6 deletions qiskit_experiments/library/randomized_benchmarking/rb_experiment.py
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Original file line number Diff line number Diff line change
Expand Up @@ -38,7 +38,7 @@
LOG = logging.getLogger(__name__)


SequenceElementType = Union[Clifford, Integral]
SequenceElementType = Union[Clifford, Integral, QuantumCircuit]


class StandardRB(BaseExperiment, RestlessMixin):
Expand Down Expand Up @@ -200,8 +200,7 @@ def _sequences_to_circuits(
circ.barrier(qubits)

# Compute inverse, compute only the difference from the previous shorter sequence
for elem in seq[len(prev_seq) :]:
prev_elem = self.__compose_clifford(prev_elem, elem)
prev_elem = self.__compose_clifford_seq(prev_elem, seq[len(prev_seq) :])
prev_seq = seq
inv = self.__adjoint_clifford(prev_elem)

Expand All @@ -212,13 +211,14 @@ def _sequences_to_circuits(

def __sample_sequence(self, length: int, rng: Generator) -> Sequence[SequenceElementType]:
# Sample a RB sequence with the given length.
# Return integer instead of Clifford object for 1 or 2 qubit case for speed
# Return integer instead of Clifford object for 1 or 2 qubits case for speed
if self.num_qubits == 1:
return rng.integers(24, size=length)
if self.num_qubits == 2:
return rng.integers(11520, size=length)

return [random_clifford(self.num_qubits, rng) for _ in range(length)]
# Return circuit object instead of Clifford object for 3 or more qubits case for speed
# TODO: Revisit after terra#7269, #7483, #8585
return [random_clifford(self.num_qubits, rng).to_circuit() for _ in range(length)]

def _to_instruction(self, elem: SequenceElementType) -> Instruction:
# TODO: basis transformation in 1Q (and 2Q) cases for speed
Expand All @@ -235,6 +235,21 @@ def __identity_clifford(self) -> SequenceElementType:
return 0
return Clifford(np.eye(2 * self.num_qubits))

def __compose_clifford_seq(
self, org: SequenceElementType, seq: Sequence[SequenceElementType]
) -> SequenceElementType:
if self.num_qubits <= 2:
new = org
for elem in seq:
new = self.__compose_clifford(new, elem)
return new
# 3 or more qubits: compose Clifford from circuits for speed
# TODO: Revisit after terra#7269, #7483, #8585
circ = QuantumCircuit(self.num_qubits)
for elem in seq:
circ.compose(elem, inplace=True)
return org.compose(Clifford.from_circuit(circ))

def __compose_clifford(
self, lop: SequenceElementType, rop: SequenceElementType
) -> SequenceElementType:
Expand All @@ -260,6 +275,8 @@ def __adjoint_clifford(self, op: SequenceElementType) -> SequenceElementType:
return CliffordUtils.clifford_1_qubit(op).adjoint()
if self.num_qubits == 2:
return CliffordUtils.clifford_2_qubit(op).adjoint()
if isinstance(op, QuantumCircuit):
return Clifford.from_circuit(op).adjoint()
return op.adjoint()

def _transpiled_circuits(self) -> List[QuantumCircuit]:
Expand Down