Transpiler wishlist

[stavros11]

Moved from qiboteam/qibolab#606 with some updates:

• [x] Change the single qubit native gate from U3 to GPI2 (see https://github.com/qiboteam/qibolab/issues/360#issuecomment-1604308906). Generally, decide which are really the native single-qubit gates and update NativeType. (note: The qibolab compiler already has a rule to translate GPI2 to pulses).
• [x] Provide option to the user to select which hardware qubits to use. I believe currently the transpiler is automatically using all the qubits included in the topology. If there is already way to limit to specific qubits it should be documented.
• [ ] Potentially allow a mechanism to select different single qubit natives in the transpiler, similar to what we have for two qubit natives.
• [x] Fix Sabre transpiler to work with measurements.
• [ ] When I tried the Sabre transpiler on simple circuits and the 5-qubit topology, I was getting weirdly long transpiled circuits (adding more SWAPs than I would expect). We need to check if it is working properly in these cases.
• [ ] Longer term (not trivial to implement): Currently the transpilation creates a completely new circuit. Providing a way to set variational parameters to the new circuit via the old one could be useful for variational optimization, so that we don't have to re-transpile in every optimization step.

This list is sorted in terms of priorities with lab usability in mind.

To clarify the second point (originally requested by @MatteoRobbiati ), take for example

import networkx as nx
from qibo import Circuit, gates
from qibo.transpiler.pipeline import Passes

# define transpiler on usual 5-qubit star connectivity
topology = nx.Graph()
topology.add_nodes_from([0, 1, 2, 3, 4])
topology.add_edges_from([(0, 2), (1, 2), (2, 3), (2, 4)])
transpiler = Passes(connectivity=topology)

circuit = Circuit(1)
circuit.add(gates.H(0))

transpiled_circuit, qubit_map = transpiler(circuit)

currently transpiled_circuit will be a 5-qubit circuit with a gate on qubit-0. We would like an easy way to use a different qubit. A potential way would be transpiler(circuit, qubits=[2]).

@renatomello if you have any other transpiler related issues in mind, please add them here or open new issues. We can mark them with the transpiler label to distinguish from the rest of qibo.

### Tasks
- [ ] https://github.com/qiboteam/qibo/pull/1084
- [ ] https://github.com/qiboteam/qibo/pull/1095

[Simone-Bordoni]

Currently fixing the point 4 (Fix Sabre transpiler to work with measurements) in #1084

[Simone-Bordoni]

currently fixing point 1 in #1095

[BrunoLiegiBastonLiegi]

One thing that is still missing, I believe, is some documentation for the transpiler. It would probably be good to have an example in advanced code examples detailing how to use the transpiler, and a section in api-reference illustrating its structure.

[Simone-Bordoni]

One thing that is still missing, I believe, is some documentation for the transpiler. It would probably be good to have an example in advanced code examples detailing how to use the transpiler, and a section in api-reference illustrating its structure.

There is already an example in "advanced code examples". have a look at it and tell me if it is necessary to change something. With the new PR we can also add a section in API

[BrunoLiegiBastonLiegi]

There is already an example in "advanced code examples". have a look at it and tell me if it is necessary to change something. With the new PR we can also add a section in API

Where is it exactly? I couldn't find it in the advanced examples

[MatteoRobbiati]

Where is it exactly? I couldn't find it in the advanced examples

I also was searching for it yesterday. I think there is no example for now :)

[stavros11]

Where is it exactly? I couldn't find it in the advanced examples

It's here https://github.com/qiboteam/qibo/blob/master/doc/source/code-examples/advancedexamples.rst#how-to-modify-the-transpiler. You probably cannot find it in the stable documentation online as we have not released qibo after moving the transpiler here. I guess the only way to view it for now is to compile the docs locally (or just read directly from the link above).

Also, I agree that transpiler should also be added in the API reference.

[BrunoLiegiBastonLiegi]

ah yeah that's right

[MatteoRobbiati]

It's here https://github.com/qiboteam/qibo/blob/master/doc/source/code-examples/advancedexamples.rst#how-to-modify-the-transpiler. You probably cannot find it in the stable documentation online as we have not released qibo after moving the transpiler here. I guess the only way to view it for now is to compile the docs locally (or just read directly from the link above).

Many thanks. Anyway, I think it should be better to have a notebook as a proper tutorial (like the ones we have for algorithms). As soon as I find some time to read this documentation, I can try to do it.