Circuit extraction

[matulni]

This PR introduces functionality to perform the circuit extraction algorithm presented in Ref. [1] which allows to extract a circuit without ancillas from a strongly deterministic pattern.

Context

The main result of the paper is a procedure to perform the transformation

$$U_{\mathcal{P}} \rightarrow C V(\mathbf{\theta}),$$

where $U_{\mathcal{P}}$ is the isometry implemented by the pattern $\mathcal{P}$ (a unitary when the pattern has the same number of inputs and outputs), $C$ is a Clifford map, and $V(\mathbf{\theta})$ is an ordered product of Pauli exponentials.

The structure of the extraction process reads as follows:

flowchart LR

n0["Pattern"]
n1["OpenGraph"]
n2("Focused PauliFlow")

subgraph s1["ExtractionResult"]
    s10("PauliExponentialDAG")
    s11("CliffordMap")
end

%% Junction node to merge arrows
j1(( ))

n3("Graphix circuit")

n0 --> n1
n1 --> n2
n2 --> s10
n2 --> s11

%% Merging into the junction
s10 --> j1
s11 --> j1
j1 --> n3

%% Styling the junction to be small
style j1 fill:#000,stroke-width:0px,width:10px

Loading

• The transformation Pattern -> OpenGraph -> Focused PauliFlow already exists in the current implementation of Graphix. The $O(N^3)$ Pauli-flow extraction algorithm in Ref. [2] always returns focused Pauli flows, contrary to the $O(N^5)$ version in Ref. [1] which requires an additional post-processing to focus the correction function.

• ExtractionResult is the output of the algorithm which is denoted "PdDAG" in [1]. In particular:

  • PauliExponentialDAG is a mapping between measured nodes in the open graph and Pauli exponentials, and a partial order between the measured node (the flow's partial order). Pauli exponentials are a pair of an Angle (the node's measurement angle) and a PauliString acting on the output nodes of the open graph. The corresponding Pauli string is obtained from the focused flow's correction function.

  • Clifford Map describes a linear transformation between the space of input qubits and the space of output qubits. It is encoded as a map from the Pauli-group generators ($X$ and $Z$) over the input nodes to Pauli strings over the output nodes.

    The $Z$-map is also obtained from the focused flow's correction function. The $X$-map is obtained from the focused flow's correction function of the extended open graph (see Def. C.3 in [1]).

Summary of changes

• Added the following to graphix.flow.core.PauliFlow:

  • is_focused : (method) Verifies if a Pauli flow is focused according to definition 4.3 in Ref. [1].
  • extract_circuit : (method) Returns a ExtractionResult object.
  • pauli_strings : (cached_property) Associates a Pauli string to every corrected node in the flow's correction function.

• Added new module graphix.circ_ext.extraction

  • This module contains the machinery to perfom the transformation Focused PauliFlow -> ExtractionResult.
  • We introduce the following (self-descriptive) classes:
    • ExtractionResult
    • PauliString
    • PauliExponential
    • PauliExponentialDAG
    • CliffordMap

• Added new module graphix.circ_ext.compilation

  • This module contains the machinery to perform the transformation ExtractionResult -> Graphix circuit.
  • It lays out a structure of abstract compilation passes which will allow in the future to experiment with different strategies. Currently:
    • The transformation PauliExponentialDAG -> Graphix circuit is done with the naive "star" construction (see [3]).
    • The transformation CliffordMap -> Graphix circuit relies on stim and currently only supports unitaries (i.e., patterns with the same number of inputa and outputs). To avoid introducing a dependency on stim, the StimCliffordPass is implemented in the external plugin graphix-stim-compiler. This compilation pass together with the tooling introduced in this PR allow to do a round-trip conversion circuit -> MBQC pattern -> circuit.

We note that all the transformations in the figure work with parametric objects as well.

References

[1] Simmons, 2021, arXiv:2109.05654.

[2] Mitosek and Backens, 2024, arXiv:2410.23439.

[3] https://quantumcomputing.stackexchange.com/questions/5567/circuit-construction-for-hamiltonian-simulation/11373#11373

[codecov]

Codecov Report

❌ Patch coverage is 82.69231% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.56%. Comparing base (03a89c9) to head (5a49b55).

Files with missing lines	Patch %	Lines
graphix/circ_ext/extraction.py	83.15%	16 Missing ⚠️
graphix/circ_ext/compilation.py	84.09%	14 Missing ⚠️
graphix/flow/core.py	76.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
- Coverage   88.76%   88.56%   -0.20%     
==========================================
  Files          44       46       +2     
  Lines        6308     6516     +208     
==========================================
+ Hits         5599     5771     +172     
- Misses        709      745      +36     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.