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Improve __pow__ for SingleQubitCliffordGate (issue #6327) #6919

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Improve __pow__ for SingleQubitCliffordGate (issue #6327) #6919

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23 changes: 14 additions & 9 deletions cirq-core/cirq/ops/clifford_gate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -743,16 +743,21 @@ def _to_phased_xz_gate(self) -> phased_x_z_gate.PhasedXZGate:
return phased_x_z_gate.PhasedXZGate(x_exponent=x, z_exponent=z, axis_phase_exponent=a)

def __pow__(self, exponent: Union[float, int]) -> 'SingleQubitCliffordGate':
# First to check if we can get the sqrt and negative sqrt Clifford.
if self._get_sqrt_map().get(exponent, None):
pow_gate = self._get_sqrt_map()[exponent].get(self, None)
if int(exponent) == exponent:
# The single qubit Clifford gates are a group of size 24
ret_gate = super().__pow__(int(exponent) % 24)
return SingleQubitCliffordGate.from_clifford_tableau(ret_gate.clifford_tableau)
elif int(2 * exponent) == 2 * exponent:
# If exponent = k/2 for integer k, then we compute the k-th power of the square root
if exponent < 0:
sqrt_exp = -0.5
else:
sqrt_exp = 0.5
pow_gate = self._get_sqrt_map()[sqrt_exp].get(self, None)
if pow_gate:
return pow_gate
# If not, we try the Clifford Tableau based method.
ret_gate = super().__pow__(exponent)
if ret_gate is NotImplemented:
return NotImplemented
return SingleQubitCliffordGate.from_clifford_tableau(ret_gate.clifford_tableau)
return pow_gate ** (abs(2 * exponent))

return NotImplemented

def _act_on_(
self,
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2 changes: 2 additions & 0 deletions cirq-core/cirq/ops/clifford_gate_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -219,6 +219,8 @@ def test_pow():
assert cirq.SingleQubitCliffordGate.Y_nsqrt == cirq.SingleQubitCliffordGate.Y**-0.5
assert cirq.SingleQubitCliffordGate.Z_nsqrt == cirq.SingleQubitCliffordGate.Z**-0.5
assert cirq.SingleQubitCliffordGate.X_sqrt**-1 == cirq.SingleQubitCliffordGate.X_nsqrt
assert cirq.SingleQubitCliffordGate.X_sqrt**3 == cirq.SingleQubitCliffordGate.X**1.5
assert cirq.SingleQubitCliffordGate.Z**2.0 == cirq.SingleQubitCliffordGate.I
assert cirq.inverse(cirq.SingleQubitCliffordGate.X_nsqrt) == (
cirq.SingleQubitCliffordGate.X_sqrt
)
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