Add a limited FSIM gateset #2804
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- This will be equivalent to combining the sqrt iswap and syc gatesets. - Combining these two gatesets will have the following advantages: -- This can allow for parameterization of the ISWAP**0 gate -- That allows circuits to drop out iSwaps and batch efficiently -- Also allows for expansion if other FSim variants become available
cirq/google/gate_sets.py
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| MEASUREMENT_DESERIALIZER, | ||
| WAIT_GATE_DESERIALIZER, | ||
| ]) | ||
| document(SQRT_ISWAP_GATESET, |
cirq/google/common_serializers.py
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| serialized_name='theta', | ||
| serialized_type=float, | ||
| op_getter=(lambda op: cast(ops.ISwapPowGate, op.gate).exponent * | ||
| np.pi / 2)), |
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Need a minus sign here, because iswap**0.5 corresponds to fsim(-pi/4, 0).
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Good catch! I always screw up this convention.
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| @pytest.mark.parametrize(('gate', 'theta', 'phi'), [ | ||
| (cirq.ISWAP**0.5, -np.pi / 4, 0), | ||
| (cirq.ISWAP**-0.5, np.pi / 4, 0), |
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Can you explain to me how does it work, since you didn't specify serializer for ISwapPowGate with exponent equal to -0.5?
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The serializer checks for abs(exponent)==0.5 (mod 2pi)
| return True | ||
| # sqrt ISWAP and inverse sqrt ISWAP | ||
| if _near_mod_2pi(abs(theta), np.pi / 4) and _near_mod_2pi(phi, 0): | ||
| return True |
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There's a subtlety here with periodicity and absolute value; we want to accept theta = pi/4 and -pi/4, but also theta = 7pi/4 (which is equal to -pi/4 mod 2pi). The current code will not accept that. I think we should do:
if (_near_mod_2pi(theta, np.pi / 4) or _near_mod_2pi(theta, -np.pi / 4)) and _near_mod_2pi(phi, 0):
...Could also split these into separate if clauses and label them explicitly as sqrt iswap and inverse sqrt iswap.
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You are correct! I tried to be too fancy for my own good. I just made all the checks separate and explicit and added tests for the -7pi/4 and 7pi/4
cirq/google/common_serializers.py
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| op_getter=lambda e: 0) | ||
| ], | ||
| can_serialize_predicate=(lambda op: _near_mod_n( | ||
| abs(cast(ops.ISwapPowGate, op.gate).exponent), 0.5, 4 |
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here, too, need to check whether the exponent is close to 0.5 mod 4 or -0.5 mod 4; taking the absolute value misses exponent 3.5, which gives the same gate as -0.5.
-- This can allow for parameterization of the ISWAP**0 gate
-- That allows circuits to drop out iSwaps and batch efficiently
-- Also allows for expansion if other FSim variants become available