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Add equivalence-library rules between rzz and cp #13019

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Sep 9, 2024
Merged

Add equivalence-library rules between rzz and cp #13019

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9a2add5
Add equivalence-library rules between `rzz` and `cp`
jakelishman Aug 13, 2024
bbe529b
Rewrite release note
jakelishman Aug 27, 2024
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43 changes: 43 additions & 0 deletions qiskit/circuit/library/standard_gates/equivalence_library.py
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Expand Up @@ -189,6 +189,21 @@ def _cnot_rxx_decompose(plus_ry: bool = True, plus_rxx: bool = True):
cphase_to_cu1.append(CU1Gate(theta), [0, 1])
_sel.add_equivalence(CPhaseGate(theta), cphase_to_cu1)

# CPhaseGate
#
# global phase: ϴ/4
# ┌─────────┐
# q_0: ─■──── q_0: ─■─────────┤ Rz(ϴ/2) ├
# │P(ϴ) ≡ │ZZ(-ϴ/2) ├─────────┤
# q_1: ─■──── q_1: ─■─────────┤ Rz(ϴ/2) ├
# └─────────┘
theta = Parameter("theta")
cphase_to_rzz = QuantumCircuit(2, global_phase=theta / 4)
cphase_to_rzz.rzz(-theta / 2, 0, 1)
cphase_to_rzz.rz(theta / 2, 0)
cphase_to_rzz.rz(theta / 2, 1)
_sel.add_equivalence(CPhaseGate(theta), cphase_to_rzz)

# RGate
#
# ┌────────┐ ┌───────────────────────┐
Expand Down Expand Up @@ -394,6 +409,19 @@ def _cnot_rxx_decompose(plus_ry: bool = True, plus_rxx: bool = True):
def_rzx.append(inst, qargs, cargs)
_sel.add_equivalence(RZXGate(theta), def_rzx)

# RZXGate to RZZGate
# ┌─────────┐
# q_0: ┤0 ├ q_0: ──────■───────────
# │ Rzx(ϴ) │ ≡ ┌───┐ │ZZ(ϴ) ┌───┐
# q_1: ┤1 ├ q_1: ┤ H ├─■──────┤ H ├
# └─────────┘ └───┘ └───┘
theta = Parameter("theta")
rzx_to_rzz = QuantumCircuit(2)
rzx_to_rzz.h(1)
rzx_to_rzz.rzz(theta, 0, 1)
rzx_to_rzz.h(1)
_sel.add_equivalence(RZXGate(theta), rzx_to_rzz)


# RYGate
#
Expand Down Expand Up @@ -654,6 +682,21 @@ def _cnot_rxx_decompose(plus_ry: bool = True, plus_rxx: bool = True):
rzz_to_rzx.h(1)
_sel.add_equivalence(RZZGate(theta), rzz_to_rzx)

# RZZ to CPhase
#
# global phase: ϴ/2
# ┌───────┐
# q_0: ─■───── q_0: ─■────────┤ Rz(ϴ) ├
# │ZZ(ϴ) ≡ │P(-2*ϴ) ├───────┤
# q_1: ─■───── q_1: ─■────────┤ Rz(ϴ) ├
# └───────┘
theta = Parameter("theta")
rzz_to_cphase = QuantumCircuit(2, global_phase=theta / 2)
rzz_to_cphase.cp(-theta * 2, 0, 1)
rzz_to_cphase.rz(theta, 0)
rzz_to_cphase.rz(theta, 1)
_sel.add_equivalence(RZZGate(theta), rzz_to_cphase)

# RZZ to RYY
q = QuantumRegister(2, "q")
theta = Parameter("theta")
Expand Down
17 changes: 17 additions & 0 deletions releasenotes/notes/cphase-rzz-equivalence-e8afc37b71a74366.yaml
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@@ -0,0 +1,17 @@
---
features_circuits:
- |
The standard equivalence library (:data:`.SessionEquivalenceLibrary`) now has rules that can
directly convert between Qiskit's standard-library 2q continuous Ising-type interactions (e.g.
:class:`.CPhaseGate`, :class:`.RZZGate`, :class:`.RZXGate`, and so on) using local equivalence
relations. Previously, several of these conversions would go via a 2-CX form, which resulted
in less efficient circuit generation.

.. note::

In general, the :class:`.BasisTranslator` is not guaranteed to find the "best" equivalence
relation for a given :class:`.Target`, but will always find an equivalence if one exists. We
rely on more expensive resynthesis and gate-optimization passes in the transpiler to improve
the output. These passes are currently not as effective for basis sets with a continuously
parametrized two-qubit interaction as they are for discrete super-controlled two-qubit
interactions.
1 change: 0 additions & 1 deletion test/python/circuit/test_controlled_gate.py
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Expand Up @@ -1438,7 +1438,6 @@ def test_control_zero_operand_gate(self, num_ctrl_qubits):
@data(
RXGate,
RYGate,
RZGate,
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why did you need to remove this test?

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The test is asserting that a failure occurs, and for most of the gates in the list, they shouldn't really fail - we've got enough equivalence and translation rules that it should be possible to translate the gate into one we can represent better. The add_control logic is just not super smart about gate translation.

As a side effect of this PR, the RZ path now happens to convert things to CPhaseGate rather than CRZGate (or something like that), so the control works further than it did before.

RXXGate,
RYYGate,
RZXGate,
Expand Down
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