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Allow measurement keys with differing qubits in OpenQASM #6803

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merged 8 commits into from
Nov 22, 2024
Merged

Allow measurement keys with differing qubits in OpenQASM #6803

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0bcac30
Allow measurement keys with differing qubits in OpenQASM
dstrain115 Nov 19, 2024
8d8a20d
Merge branch 'main' into fix_classical_register_size
dstrain115 Nov 19, 2024
56ebbec
Fix merge issues.
dstrain115 Nov 19, 2024
4e2693c
Change to function and put in constructor.
dstrain115 Nov 20, 2024
a91318a
Update cirq-core/cirq/circuits/qasm_output_test.py
dstrain115 Nov 22, 2024
b75090b
Update cirq-core/cirq/circuits/qasm_output_test.py
dstrain115 Nov 22, 2024
3772025
Update cirq-core/cirq/circuits/qasm_output_test.py
dstrain115 Nov 22, 2024
5a90191
Merge branch 'main' into fix_classical_register_size
dstrain115 Nov 22, 2024
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17 changes: 10 additions & 7 deletions cirq-core/cirq/circuits/qasm_output.py
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@

"""Utility classes for representing QASM."""

from typing import Callable, Dict, Iterator, Optional, Sequence, Set, Tuple, Union, TYPE_CHECKING
from typing import Callable, Dict, Iterator, Optional, Sequence, Tuple, Union, TYPE_CHECKING

import re
import numpy as np
Expand Down Expand Up @@ -289,22 +289,25 @@ def output(text):
output(f'qubit[{len(self.qubits)}] q;\n')
# Classical registers
# Pick an id for the creg that will store each measurement
already_output_keys: Set[str] = set()
# cregs will store key -> (#qubits, comment)
cregs: Dict[str, tuple[int, str]] = {}
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If you did this in the QasmOutput constructor and stored it in QasmArgs, you could pass it into KeyCondition.qasm, and fix #5691.

The reason it's required is that KeyCondition is triggered if meas[m_key] != 0. In OpenQasm2, the only logical comparator is equality, so != 0 can only be represented if the creg is a single bit (i.e. it would convert the condition to if m_key == 1). If the creg is multiple bits, then if m_key == 1 would be incorrect, and m_key != 0 is not representable. So that's why the existing behavior is to play it safe and just fail.

Going a little bit further, in OpenQasm3, it looks like m_key != 0 is representable, so checking QasmArgs.version in KeyCondition.qasm would allow supporting multi-bit classical controls if version == '3.0'. OpenQasm3 could also potentially support serializing additional types of logic in SympyConditions.

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It might also be useful to have this be a function on Circuit or part of the measurement_keys protocol. I'd imagine "get the shapes of all the measurement keys in the circuit" would be a common need.

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Changed to put into constructor, but I am not convinced that helps, since we don't pass QasmOutput during the qasm protocol. In any case, I will deal with classical controls in a follow-up PR.

Also, Circuit has enough cruft on it that I don't think it's a good idea to add more functions onto it, so I will leave it here.

for meas in self.measurements:
key = protocols.measurement_key_name(meas)
if key in already_output_keys:
continue
already_output_keys.add(key)
meas_id = self.args.meas_key_id_map[key]

if self.meas_comments[key] is not None:
comment = f' // Measurement: {self.meas_comments[key]}'
else:
comment = ''

if meas_id not in cregs or cregs[meas_id][0] < len(meas.qubits):
cregs[meas_id] = (len(meas.qubits), comment)
for meas_id in cregs:
if self.args.version == '2.0':
output(f'creg {meas_id}[{len(meas.qubits)}];{comment}\n')
output(f'creg {meas_id}[{cregs[meas_id][0]}];{cregs[meas_id][1]}\n')
else:
output(f'bit[{len(meas.qubits)}] {meas_id};{comment}\n')
output(f'bit[{cregs[meas_id][0]}] {meas_id};{cregs[meas_id][1]}\n')

# In OpenQASM 2.0, the transformation of global phase gates is ignored.
# Therefore, no newline is created when the operations contained in
# a circuit consist only of global phase gates.
Expand Down
27 changes: 27 additions & 0 deletions cirq-core/cirq/circuits/qasm_output_test.py
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Expand Up @@ -590,3 +590,30 @@ def test_reset():
reset q[1];
""".strip()
)


def test_different_sized_registers():
qubits = cirq.LineQubit.range(2)
c = cirq.Circuit(cirq.measure(qubits[0], key='c'), cirq.measure(qubits, key='c'))
output = cirq.QasmOutput(
c.all_operations(), tuple(sorted(c.all_qubits())), header='Generated from Cirq!'
)
assert (
str(output).strip()
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== """// Generated from Cirq!

OPENQASM 2.0;
include "qelib1.inc";


// Qubits: [q(0), q(1)]
qreg q[2];
creg m_c[2];


measure q[0] -> m_c[0];

// Gate: cirq.MeasurementGate(2, cirq.MeasurementKey(name='c'), ())
measure q[0] -> m_c[0];
measure q[1] -> m_c[1];"""
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)
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