Added random single-qubit CUE and Clifford gates (#6670)

* added cue and clifford gates

* promoted rng check to the parent function

* rename variables

* minor fixes

* minor formatting fix

* added test coverage for ValueError

* more formatting change

---------

Co-authored-by: Seneca Meeks <[email protected]>

Author: gauravgyawali

cirq-core/cirq/transformers/randomized_measurements.py

@@ -13,97 +13,158 @@
 # limitations under the License.
 
 from collections.abc import Sequence
-from typing import Any, Literal
+from typing import Any
 
 import cirq
 import numpy as np
+from cirq.ops import SingleQubitCliffordGate
 from cirq.transformers import transformer_api
 
 
 @transformer_api.transformer
 class RandomizedMeasurements:
-    """A transformer that appends a moment of random rotations to map qubits to
-    random pauli bases."""
+    """A transformer that appends a moment of random rotations from a given unitary ensemble (pauli,
+    clifford, cue)"""
 
     def __init__(self, subsystem: Sequence[int] | None = None):
         """Class structure for performing and analyzing a general randomized measurement protocol.
         For more details on the randomized measurement toolbox see https://arxiv.org/abs/2203.11374
 
         Args:
             subsystem: The specific subsystem (e.g qubit index) to measure in random basis
+            rest of the qubits are measured in the computational basis
         """
         self.subsystem = subsystem
 
     def __call__(
         self,
-        circuit: 'cirq.AbstractCircuit',
+        circuit: "cirq.AbstractCircuit",
+        unitary_ensemble: str = "pauli",
         rng: np.random.Generator | None = None,
         *,
         context: transformer_api.TransformerContext | None = None,
-    ):
+    ) -> "cirq.Circuit":
         """Apply the transformer to the given circuit. Given an input circuit returns
-        a list of circuits with the pre-measurement unitaries. If no arguments are specified,
-        it will default to computing the entropy of the entire circuit.
+        a new circuit with the pre-measurement unitaries and measurements gates added.
+        to the qubits in the subsystem provided.If no subsystem is specified in the
+        construction of this class it defaults to measuring all the qubits in the
+        randomized bases.
 
         Args:
             circuit: The circuit to add randomized measurements to.
-            rng: Random number generator.
+            unitary_ensemble: Choice of unitary ensemble (pauli/clifford/cue(circular
+                unitary ensemble))
             context: Not used; to satisfy transformer API.
+            rng: Random number generator.
 
         Returns:
-            List of circuits with pre-measurement unitaries and measurements added
+            A circuit with pre-measurement unitaries and measurements added
         """
+
+        all_qubits = sorted(circuit.all_qubits())
+        if self.subsystem is None:
+            subsystem_qubits = all_qubits
+        else:
+            subsystem_qubits = [all_qubits[s] for s in self.subsystem]
         if rng is None:
             rng = np.random.default_rng()
 
-        qubits = sorted(circuit.all_qubits())
-        num_qubits = len(qubits)
-
-        pre_measurement_unitaries_list = self._generate_unitaries_list(rng, num_qubits)
-        pre_measurement_moment = self.unitaries_to_moment(pre_measurement_unitaries_list, qubits)
+        pre_measurement_moment = self.random_single_qubit_unitary_moment(
+            unitary_ensemble, subsystem_qubits, rng
+        )
 
         return cirq.Circuit.from_moments(
-            *circuit.moments, pre_measurement_moment, cirq.M(*qubits, key='m')
+            *circuit.moments, pre_measurement_moment, cirq.M(*subsystem_qubits, key="m")
         )
 
-    def _generate_unitaries_list(self, rng: np.random.Generator, num_qubits: int) -> Sequence[Any]:
-        """Generates a list of pre-measurement unitaries."""
-
-        pauli_strings = rng.choice(["X", "Y", "Z"], size=num_qubits)
-
-        if self.subsystem is not None:
-            for i in range(pauli_strings.shape[0]):
-                if i not in self.subsystem:
-                    pauli_strings[i] = np.array("Z")
-
-        return pauli_strings.tolist()
-
-    def unitaries_to_moment(
-        self, unitaries: Sequence[Literal["X", "Y", "Z"]], qubits: Sequence[Any]
-    ) -> 'cirq.Moment':
+    def random_single_qubit_unitary_moment(
+        self, unitary_ensemble: str, qubits: Sequence[Any], rng: np.random.Generator
+    ) -> "cirq.Moment":
         """Outputs the cirq moment associated with the pre-measurement rotations.
+
         Args:
-            unitaries: List of pre-measurement unitaries
+            unitary_ensemble: clifford, pauli, cue
             qubits: List of qubits
+            rng: Random number generator to be used in sampling.
 
-        Returns: The cirq moment associated with the pre-measurement rotations
+        Returns:
+            The cirq moment associated with the pre-measurement rotations
+
+        Raises:
+            ValueError: When unitary_ensemble is not one of "cue", "pauli" or "clifford"
         """
+
+        if unitary_ensemble.lower() == "pauli":
+            unitaries = [_pauli_basis_rotation(rng) for _ in range(len(qubits))]
+
+        elif unitary_ensemble.lower() == "clifford":
+            unitaries = [_single_qubit_clifford(rng) for _ in range(len(qubits))]
+
+        elif unitary_ensemble.lower() == "cue":
+            unitaries = [_single_qubit_cue(rng) for _ in range(len(qubits))]
+
+        else:
+            raise ValueError("Only pauli, clifford and cue unitaries are available")
+
         op_list: list[cirq.Operation] = []
-        for idx, pauli in enumerate(unitaries):
-            op_list.append(_pauli_basis_rotation(pauli).on(qubits[idx]))
+
+        for idx, unitary in enumerate(unitaries):
+            op_list.append(unitary.on(qubits[idx]))
 
         return cirq.Moment.from_ops(*op_list)
 
 
-def _pauli_basis_rotation(basis: Literal["X", "Y", "Z"]) -> 'cirq.Gate':
-    """Given a measurement basis returns the associated rotation.
+def _pauli_basis_rotation(rng: np.random.Generator) -> "cirq.Gate":
+    """Randomly generate a Pauli basis rotation.
+
     Args:
-        basis: Measurement basis
-    Returns: The cirq gate for associated with measurement basis
+        rng: Random number generator
+
+    Returns:
+        cirq gate
+    """
+    basis_idx = rng.choice(np.arange(3))
+
+    if basis_idx == 0:
+        gate: "cirq.Gate" = cirq.Ry(rads=-np.pi / 2)
+    elif basis_idx == 1:
+        gate = cirq.Rx(rads=np.pi / 2)
+    else:
+        gate = cirq.I
+    return gate
+
+
+def _single_qubit_clifford(rng: np.random.Generator) -> "cirq.Gate":
+    """Randomly generate a single-qubit Clifford rotation.
+
+    Args:
+        rng: Random number generator
+
+    Returns:
+        cirq gate
+    """
+
+    # there are 24 distinct single-qubit Clifford gates
+    clifford_idx = rng.choice(np.arange(24))
+
+    return SingleQubitCliffordGate.to_phased_xz_gate(
+        SingleQubitCliffordGate.all_single_qubit_cliffords[clifford_idx]
+    )
+
+
+def _single_qubit_cue(rng: np.random.Generator) -> "cirq.Gate":
+    """Randomly generate a CUE gate.
+
+    Args:
+        rng: Random number generator
+
+    Returns:
+        cirq gate
     """
-    if basis == "X":
-        return cirq.Ry(rads=-np.pi / 2)
-    elif basis == "Y":
-        return cirq.Rx(rads=np.pi / 2)
-    elif basis == "Z":
-        return cirq.I
+
+    # phasedxz parameters are distinct between -1 and +1
+    x_exponent, z_exponent, axis_phase_exponent = 1 - 2 * rng.random(size=3)
+
+    return cirq.PhasedXZGate(
+        x_exponent=x_exponent, z_exponent=z_exponent, axis_phase_exponent=axis_phase_exponent
+    )

cirq-core/cirq/transformers/randomized_measurements_test.py

@@ -14,19 +14,23 @@
 
 import cirq
 import cirq.transformers.randomized_measurements as rand_meas
+import pytest
 
 
 def test_randomized_measurements_appends_two_moments_on_returned_circuit():
     # Create a 4-qubit circuit
     q0, q1, q2, q3 = cirq.LineQubit.range(4)
-    circuit = cirq.Circuit([cirq.H(q0), cirq.CNOT(q0, q1), cirq.CNOT(q1, q2), cirq.CNOT(q2, q3)])
-    num_moments_pre = len(circuit.moments)
+    circuit_pre = cirq.Circuit(
+        [cirq.H(q0), cirq.CNOT(q0, q1), cirq.CNOT(q1, q2), cirq.CNOT(q2, q3)]
+    )
+    num_moments_pre = len(circuit_pre.moments)
 
     # Append randomized measurements to subsystem
-    circuit = rand_meas.RandomizedMeasurements()(circuit)
-
-    num_moments_post = len(circuit.moments)
-    assert num_moments_post == num_moments_pre + 2
+    unitary_ensembles = ['pauli', 'clifford', 'cue']
+    for u in unitary_ensembles:
+        circuit_post = rand_meas.RandomizedMeasurements()(circuit_pre, unitary_ensemble=u)
+        num_moments_post = len(circuit_post.moments)
+        assert num_moments_post == num_moments_pre + 2
 
 
 def test_append_randomized_measurements_leaves_qubits_not_in_specified_subsystem_unchanged():
@@ -36,10 +40,9 @@ def test_append_randomized_measurements_leaves_qubits_not_in_specified_subsystem
 
     # Append randomized measurements to subsystem
     circuit = rand_meas.RandomizedMeasurements(subsystem=(0, 1))(circuit)
-
     # assert latter subsystems were not changed.
-    assert circuit.operation_at(q2, 4) == cirq.I(q2)
-    assert circuit.operation_at(q3, 4) == cirq.I(q3)
+    assert circuit.operation_at(q2, 4) is None
+    assert circuit.operation_at(q3, 4) is None
 
 
 def test_append_randomized_measurements_leaves_qubits_not_in_noncontinuous_subsystem_unchanged():
@@ -51,5 +54,14 @@ def test_append_randomized_measurements_leaves_qubits_not_in_noncontinuous_subsy
     circuit = rand_meas.RandomizedMeasurements(subsystem=(0, 2))(circuit)
 
     # assert latter subsystems were not changed.
-    assert circuit.operation_at(q1, 4) == cirq.I(q1)
-    assert circuit.operation_at(q3, 4) == cirq.I(q3)
+    assert circuit.operation_at(q1, 4) is None
+    assert circuit.operation_at(q3, 4) is None
+
+
+def test_exception():
+    q0, q1, q2, q3 = cirq.LineQubit.range(4)
+    circuit = cirq.Circuit([cirq.H(q0), cirq.CNOT(q0, q1), cirq.CNOT(q1, q2), cirq.CNOT(q2, q3)])
+
+    # Append randomized measurements to subsystem
+    with pytest.raises(ValueError):
+        rand_meas.RandomizedMeasurements()(circuit, unitary_ensemble="coe")