Add support for using physical-Z phase matching on 2-qubit gates #3862
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maffoo
requested review from
cduck,
vtomole,
wcourtney and
a team
as code owners
dstrain115
approved these changes
Feb 26, 2021
| @@ -437,7 +459,7 @@ def _convert_physical_z(op: ops.Operation, proto: v2.program_pb2.Operation): | |||
| op_serializer.GateOpSerializer( | |||
| gate_type=ops.FSimGate, | |||
| serialized_gate_id='fsim_pi_4', | |||
| args=[], | |||
| args=[_phase_match_arg], | |||
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This is a little confusing, since the default is to match via physical Z for sqrt iswap, correct?
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That's correct. I think it's nice to have this here for consistency with other gates. I've added a note about this in the PR description.
|
Thanks, @dstrain115. I've updated the PR description for posterity, and will include that in the commit message. |
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When run on hardware, two-qubit gates must be "phase-matched" to account for the detuning between the rest frames of the two qubits. By default, this phase matching will use "virtual" Z gates where possible, namely for gates like CZ and iSWAP that commute with single-qubit Z rotations (in the case of iSWAP, the commuted Zs swap to the other qubit). In some cases, however, we may want to use "physical" Z gates for matching, for example to avoid errors due to non-ideal gates (non-zero swap angle in CZ, or incomplete swap in iSWAP).
Here we add support for tagging 2-qubit gates with
cirq.google.PhysicalZTag()to indicate that physical Z gates should be used for matching, instead of the default matching scheme. We modify the 2-qubit gate serializers to recognize these tags and pass them through when serializing/deserializing.Note that for sqrt-iSWAP gates and others that do not commute with Z, the default behavior is already to use physical Z gates for matching. In these cases adding an explicit
PhysicalZTag()does not change the behavior, but we allow it for consistency with other 2-qubit gates.