Obtaining and Applying XX and ZZ Parity in Stim for Controlled Paulis

Question

I am attempting to perform a CNOT between two surface code qubits in Stim, based on this paper by Daniel Litinski and Felix von Oppen.

The CNOT they perform is shown in the figure below (Figure 3 from the paper).

In Stim, how do I go about obtaining the parity of the $ZZ$ and $XX$ measurements? Following this, how would one apply Pauli gates controlled by these parities in Stim?

Answer 1

Stim's supported gates documentation includes the gate MXX and the gate MZZ which are exactly the two-qubit parity measurement gates you want:

import stim

lattice_surgery_cnot = stim.Circuit("""
    RX 1
    MZZ 0 1
    MXX 1 2
    MZ 1
    CZ rec[-2] 0
    CX rec[-1] 2 rec[-3] 2
""")

# Verify it's a CNOT
assert lattice_surgery_cnot.has_all_flows([
    stim.Flow("X0 -> X0*X2"),
    stim.Flow("X2 -> X2"),
    stim.Flow("Z0 -> Z0"),
    stim.Flow("Z2 -> Z0*Z2"),
])

print(lattice_surgery_cnot.diagram())

q0: ----MZZ:rec[0]------------Z^rec[1]----------
        |
q1: -RX-MZZ--------MXX:rec[1]-M:rec[2]----------
                   |
q2: ---------------MXX--------X^rec[2]-X^rec[0]-

Stim also has MPP, which can Measure Pauli Products with arbitrarily many terms:

MPP X0*X1 Z1*Z2
MPP X2*Y3*Z5
MPP X10*X11*X12*X13*X14*X15*X16*X17
...