cirq.optimizers.decompose_cphase_into_two_fsim

Decomposes CZPowGate into two FSimGates.

This function implements the decomposition described in section VII F I of https://arxiv.org/abs/1910.11333

The decomposition results in exactly two FSimGates and a few single-qubit rotations. It is feasible if and only if one of the following conditions is met:

|sin(θ)| <= |sin(δ/4)| <= |sin(φ/2)|
|sin(φ/2)| <= |sin(δ/4)| <= |sin(θ)|

where:

 θ = fsim_gate.theta,
 φ = fsim_gate.phi,
 δ = -π * cphase_gate.exponent.

Note that the gate parameterizations are non-injective. For the decomposition to be feasible it is sufficient that one of the parameter values which correspond to the provided gate satisfies the constraints. This function will find and use the appropriate value whenever it exists.

The constraints above imply that certain FSimGates are not suitable for use in this decomposition regardless of the target CZPowGate. We reject such gates based on how close |sin(θ)| is to |sin(φ/2)|, see atol argument below.

This implementation accounts for the global phase.

cphase_gate The CZPowGate to synthesize.
fsim_gate The only two qubit gate that is permitted to appear in the output.
qubits The qubits to apply the resulting operations to. If not set, defaults cirq.LineQubit.range(2).
atol Tolerance used to determine whether fsim_gate is valid. The gate is invalid if the squares of the sines of the theta angle and half the phi angle are too close.

Operations equivalent to cphase_gate and consisting solely of two copies of fsim_gate and a few single-qubit rotations.

ValueError under any of the following circumstances:

  • cphase_gate or fsim_gate is parametrized,
  • cphase_gate and fsim_gate do not satisfy the conditions above,
  • fsim_gate has invalid angles (see atol argument above),
  • incorrect number of qubits are provided.