Add Open graph class

docs/source/open_graph.rst

@@ -0,0 +1,13 @@
+Open Graph
+======================
+
+:mod:`graphix.opengraph` module
++++++++++++++++++++++++++++++
+
+This module defines classes for defining MBQC patterns as Open Graphs.
+
+.. currentmodule:: graphix.opengraph
+
+.. autoclass:: OpenGraph
+
+.. autoclass:: Measurement

graphix/opengraph.py

@@ -0,0 +1,123 @@
+"""Provides a class for open graphs."""
+
+from __future__ import annotations
+
+import math
+from typing import TYPE_CHECKING, NamedTuple
+
+import networkx as nx
+
+from graphix.generator import generate_from_graph
+
+if TYPE_CHECKING:
+ from graphix.pattern import Pattern
+ from graphix.pauli import Plane
+
+
+class Measurement(NamedTuple):
+ """An MBQC measurement.
+
+ :param angle: the angle of the measurement. Should be between [0, 2)
+ :param plane: the measurement plane
+ """
+
+ angle: float
+ plane: Plane
+
+ def isclose(self, other: Measurement, rel_tol=1e-09, abs_tol=0.0) -> bool:
+ """Compares if two measurements have the same plane and their angles
+ are close.
+
+ Example
+ -------
+ >>> from graphix.opengraph import Measurement
+ >>> from graphix.pauli import Plane
+ >>> Measurement(0.0, Plane.XY).isclose(Measurement(0.0, Plane.XY))
+ True
+ >>> Measurement(0.0, Plane.XY).isclose(Measurement(0.0, Plane.YZ))
+ False
+ >>> Measurement(0.1, Plane.XY).isclose(Measurement(0.0, Plane.XY))
+ False
+ """
+ return math.isclose(self.angle, other.angle, rel_tol=rel_tol, abs_tol=abs_tol) and self.plane == other.plane
+
+
+class OpenGraph(NamedTuple):
+ """Open graph contains the graph, measurement, and input and output
+ nodes. This is the graph we wish to implement deterministically
+
+ :param inside: the underlying graph state
+ :param measurements: a dictionary whose key is the ID of a node and the
+ value is the measurement at that node
+ :param inputs: a set of IDs of the nodes that are inputs to the graph
+ :param outputs: a set of IDs of the nodes that are outputs of the graph
+
+ Example
+ -------
+ >>> import networkx as nx
+ >>> from graphix.opengraph import OpenGraph, Measurement
+ >>>
+ >>> inside_graph = nx.Graph([(0, 1), (1, 2), (2, 0)])
+ >>>
+ >>> measurements = {i: Measurement(0.5 * i, Plane.XY) for i in range(2)}
+ >>> inputs = [0]
+ >>> outputs = [2]
+ >>> og = OpenGraph(inside_graph, measurements, inputs, outputs)
+ """
+
+ inside: nx.Graph
+ measurements: dict[int, Measurement]
+ inputs: list[int]
+ outputs: list[int]
+
+ def __eq__(self, other) -> bool:
+ """Checks the two open graphs are equal
+
+ This doesn't check they are equal up to an isomorphism"""
+
+ if not nx.utils.graphs_equal(self.inside, other.inside):
+ return False
+
+ if self.inputs != other.inputs or self.outputs != other.outputs:
+ return False
+
+ if set(self.measurements.keys()) != set(other.measurements.keys()):
+ return False
+
+ return all(m.isclose(other.measurements[node]) for node, m in self.measurements.items())
+
+ @classmethod
+ def from_pattern(cls, pattern: Pattern) -> OpenGraph:
+ """Initialises an `OpenGraph` object based on the resource-state graph
+ associated with the measurement pattern."""
+ g = nx.Graph()
+ nodes, edges = pattern.get_graph()
+ g.add_nodes_from(nodes)
+ g.add_edges_from(edges)
+
+ inputs = pattern.input_nodes
+ outputs = pattern.output_nodes
+
+ meas_planes = pattern.get_meas_plane()
+ meas_angles = pattern.get_angles()
+ meas = {node: Measurement(meas_angles[node], meas_planes[node]) for node in meas_angles}
+
+ return cls(g, meas, inputs, outputs)
+
+ def to_pattern(self) -> Pattern:
+ """Converts the `OpenGraph` into a `Pattern`.
+
+ Will raise an exception if the open graph does not have flow, gflow, or
+ Pauli flow.
+ The pattern will be generated using maximally-delayed flow.
+ """
+
+ g = self.inside.copy()
+ inputs = self.inputs
+ outputs = self.outputs
+ meas = self.measurements
+
+ angles = {node: m.angle for node, m in meas.items()}
+ planes = {node: m.plane for node, m in meas.items()}
+
+ return generate_from_graph(g, angles, inputs, outputs, planes)

graphix/pyzx.py

@@ -0,0 +1,178 @@
+"""Functionality for converting between OpenGraphs and PyZX
+
+These functions are held in their own file rather than including them in the
+OpenGraph class because we want PyZX to be an optional dependency.
+"""
+
+from __future__ import annotations
+
+import warnings
+
+import networkx as nx
+import pyzx as zx
+
+from graphix.opengraph import Measurement, OpenGraph
+from graphix.pauli import Plane
+
+
+def to_pyzx_graph(og: OpenGraph) -> zx.graph.base.BaseGraph:
+ """Return a PyZX graph corresponding to the the open graph.
+
+ Example
+ -------
+ >>> import networkx as nx
+ >>> g = nx.Graph([(0, 1), (1, 2)])
+ >>> inputs = [0]
+ >>> outputs = [2]
+ >>> measurements = {0: Measurement(0, Plane.XY), 1: Measurement(1, Plane.YZ)}
+ >>> og = OpenGraph(g, measurements, inputs, outputs)
+ >>> reconstructed_pyzx_graph = og.to_pyzx_graph()
+ """
+ # check pyzx availability and version
+ try:
+ import pyzx as zx
+ except ModuleNotFoundError as e:
+ msg = "Cannot find pyzx (optional dependency)."
+ raise RuntimeError(msg) from e
+ if zx.__version__ != "0.8.0":
+ warnings.warn(
+ "`to_pyzx_graph` is guaranteed to work only with pyzx==0.8.0 due to possible breaking changes in `pyzx`.",
+ stacklevel=1,
+ )
+ g = zx.Graph()
+
+ # Add vertices into the graph and set their type
+ def add_vertices(n: int, ty: zx.VertexType) -> list[zx.VertexType]:
+ verts = g.add_vertices(n)
+ for vert in verts:
+ g.set_type(vert, ty)
+
+ return verts
+
+ # Add input boundary nodes
+ in_verts = add_vertices(len(og.inputs), zx.VertexType.BOUNDARY)
+ g.set_inputs(in_verts)
+
+ # Add nodes for internal Z spiders - not including the phase gadgets
+ body_verts = add_vertices(len(og.inside), zx.VertexType.Z)
+
+ # Add nodes for the phase gadgets. In OpenGraph we don't store the
+ # effect as a seperate node, it is instead just stored in the
+ # "measurement" attribute of the node it measures.
+ x_meas = [i for i, m in og.measurements.items() if m.plane == Plane.YZ]
+ x_meas_verts = add_vertices(len(x_meas), zx.VertexType.Z)
+
+ out_verts = add_vertices(len(og.outputs), zx.VertexType.BOUNDARY)
+ g.set_outputs(out_verts)
+
+ # Maps a node's ID in the Open Graph to it's corresponding node ID in
+ # the PyZX graph and vice versa.
+ map_to_og = dict(zip(body_verts, og.inside.nodes()))
+ map_to_pyzx = {v: i for i, v in map_to_og.items()}
+
+ # Open Graph's don't have boundary nodes, so we need to connect the
+ # input and output Z spiders to their corresponding boundary nodes in
+ # pyzx.
+ for pyzx_index, og_index in zip(in_verts, og.inputs):
+ g.add_edge((pyzx_index, map_to_pyzx[og_index]))
+ for pyzx_index, og_index in zip(out_verts, og.outputs):
+ g.add_edge((pyzx_index, map_to_pyzx[og_index]))
+
+ og_edges = og.inside.edges()
+ pyzx_edges = [(map_to_pyzx[a], map_to_pyzx[b]) for a, b in og_edges]
+ g.add_edges(pyzx_edges, zx.EdgeType.HADAMARD)
+
+ # Add the edges between the Z spiders in the graph body
+ for og_index, meas in og.measurements.items():
+ # If it's an X measured node, then we handle it in the next loop
+ if meas.plane == Plane.XY:
+ g.set_phase(map_to_pyzx[og_index], meas.angle)
+
+ # Connect the X measured vertices
+ for og_index, pyzx_index in zip(x_meas, x_meas_verts):
+ g.add_edge((map_to_pyzx[og_index], pyzx_index), zx.EdgeType.HADAMARD)
+ g.set_phase(pyzx_index, og.measurements[og_index].angle)
+
+ return g
+
+
+def from_pyzx_graph(g: zx.graph.base.BaseGraph) -> OpenGraph:
+ """Constructs an Optyx Open Graph from a PyZX graph.
+
+ This method may add additional nodes to the graph so that it adheres
+ with the definition of an OpenGraph. For instance, if the final node on
+ a qubit is measured, it will add two nodes behind it so that no output
+ nodes are measured to satisfy the requirements of an open graph.
+ .. warning::
+ works with `pyzx==0.8.0` (see `requirements-dev.txt`). Other versions may not be compatible due to breaking changes in `pyzx`
+ Example
+ -------
+ >>> import pyzx as zx
+ >>> from graphix.opengraph import OpenGraph
+ >>> circ = zx.qasm("qreg q[2]; h q[1]; cx q[0], q[1]; h q[1];")
+ >>> g = circ.to_graph()
+ >>> og = OpenGraph.from_pyzx_graph(g)
+ """
+ zx.simplify.to_graph_like(g)
+
+ measurements = {}
+ inputs = g.inputs()
+ outputs = g.outputs()
+
+ g_nx = nx.Graph(g.edges())
+
+ # We need to do this since the full reduce simplification can
+ # leave either hadamard or plain wires on the inputs and outputs
+ for inp in g.inputs():
+ nbrs = list(g.neighbors(inp))
+ et = g.edge_type((nbrs[0], inp))
+
+ if et == zx.EdgeType.SIMPLE:
+ g_nx.remove_node(inp)
+ inputs = [i if i != inp else nbrs[0] for i in inputs]
+
+ for out in g.outputs():
+ nbrs = list(g.neighbors(out))
+ et = g.edge_type((nbrs[0], out))
+
+ if et == zx.EdgeType.SIMPLE:
+ g_nx.remove_node(out)
+ outputs = [o if o != out else nbrs[0] for o in outputs]
+
+ # Turn all phase gadgets into measurements
+ # Since we did a full reduce, any node that isn't an input or output
+ # node and has only one neighbour is definitely a phase gadget.
+ nodes = list(g_nx.nodes())
+ for v in nodes:
+ if v in inputs or v in outputs:
+ continue
+
+ nbrs = list(g.neighbors(v))
+ if len(nbrs) == 1:
+ measurements[nbrs[0]] = Measurement(float(g.phase(v)), Plane.YZ)
+ g_nx.remove_node(v)
+
+ next_id = max(g_nx.nodes) + 1
+
+ # Since outputs can't be measured, we need to add an extra two nodes
+ # in to counter it
+ for out in outputs:
+ if g.phase(out) == 0:
+ continue
+
+ g_nx.add_edges_from([(out, next_id), (next_id, next_id + 1)])
+ measurements[next_id] = Measurement(0, Plane.XY)
+
+ outputs = [o if o != out else next_id + 1 for o in outputs]
+ next_id += 2
+
+ # Add the phase to all XY measured nodes
+ for v in g_nx.nodes:
+ if v in outputs or v in measurements:
+ continue
+
+ # g.phase() may be a fractions.Fraction object, but Measurement
+ # expects a float
+ measurements[v] = Measurement(float(g.phase(v)), Plane.XY)
+
+ return OpenGraph(g_nx, measurements, inputs, outputs)

requirements-dev.txt

@@ -11,3 +11,6 @@ tox
 qiskit>=1.0
 qiskit-aer
 rustworkx
+
+# Optional dependency. Pinned due to version changes often being incompatible
+pyzx==0.8.0

tests/circuits/adder_n4.qasm

@@ -0,0 +1,31 @@
+OPENQASM 2.0;
+include "qelib1.inc";
+qreg q[4];
+creg c[4];
+x q[0];
+x q[1];
+h q[3];
+cx q[2],q[3];
+t q[0];
+t q[1];
+t q[2];
+tdg q[3];
+cx q[0],q[1];
+cx q[2],q[3];
+cx q[3],q[0];
+cx q[1],q[2];
+cx q[0],q[1];
+cx q[2],q[3];
+tdg q[0];
+tdg q[1];
+tdg q[2];
+t q[3];
+cx q[0],q[1];
+cx q[2],q[3];
+s q[3];
+cx q[3],q[0];
+h q[3];
+measure q[0] -> c[0];
+measure q[1] -> c[1];
+measure q[2] -> c[2];
+measure q[3] -> c[3];