Too many SWAP instructions with NxQuantumProcessor

[daniel-mills-cqc]

The following code snippet:

import networkx as nx
from pyquil.quantum_processor import NxQuantumProcessor
from pyquil.api import QVMCompiler
from pyquil import Program
from pyquil.gates import CZ

coupling_map = [[0, 1], [0, 14], [1, 2], [1, 13], [2, 3], [2, 12], [3, 4], [3, 11], [4, 5], [4, 10], [5, 6], [5, 9], [6, 8], [7, 8], [8, 9], [9, 10], [10, 11], [11, 12], [12, 13], [13, 14]]

G = nx.from_edgelist(coupling_map)

quantum_processor = NxQuantumProcessor(topology=G)
compiler = QVMCompiler(quantum_processor=quantum_processor, timeout=600)

quil_circ = Program()
n_qubits = 15
for i in range(n_qubits):
    quil_circ += CZ(i%n_qubits,(i+1)%n_qubits)

compiler.quil_to_native_quil(quil_circ)

gives the error:

RPCError: Unhandled error in host program:
Too many SWAP instructions selected in a row: 1000

This is the case with the following versions:

• pyquil version 3.0.0
• quilc docker tag b9a0eb05
• qvm docker tag e4c86b28

This error does not occur if the CZs do not form a 'closed chain' . For example replacing the for loop adding the CZ gates with:

for i in range(n_qubits - 1):
    quil_circ += CZ(i%n_qubits,(i+1)%n_qubits)

does not give an error. I cannot, however, confirm that closed chains are the problem.

[karlosz]

The following Lisp code runs fine. I'm not totally sure this is logically equivalent to what the Python is doing, though.

(in-package :cl-quil)

(defun test ()
  (let ((chip
          (build-chip-from-digraph '((0 1) (0 14) (1 2) (1 13) (2 3) (2 12) (3 4) (3 11) (4 5) (4 10) (5 6) (5 9) (6 8) (7 8) (8 9) (9 10) (10 11) (11 12) (12 13) (13 14)) :architecture :cz))
        (n-qubits 15)
        (gates ()))
    (dotimes (i n-qubits)
      (push (build-gate "CZ" () (mod i n-qubits) (mod (1+ i) n-qubits))
            gates))
    (compiler-hook (make-instance 'parsed-program :executable-code (coerce (nreverse gates) 'vector))
                   chip)))

(test)

[karlosz]

Closing since issue cannot be reproduced using only quilc proper.