# qml.qaoa.cycle.cycle_mixer¶

cycle_mixer(graph: networkx.classes.digraph.DiGraph)pennylane.ops.qubit.hamiltonian.Hamiltonian[source]

Calculates the cycle-mixer Hamiltonian.

Following methods outlined here, the cycle-mixer Hamiltonian preserves the set of valid cycles:

$\frac{1}{4}\sum_{(i, j)\in E} \left(\sum_{k \in V, k\neq i, k\neq j, (i, k) \in E, (k, j) \in E} \left[X_{ij}X_{ik}X_{kj} +Y_{ij}Y_{ik}X_{kj} + Y_{ij}X_{ik}Y_{kj} - X_{ij}Y_{ik}Y_{kj}\right] \right)$

where $$E$$ are the edges of the directed graph. A valid cycle is defined as a subset of edges in $$E$$ such that all of the graph’s nodes $$V$$ have zero net flow (see the net_flow_constraint() function).

Example

>>> import networkx as nx
>>> g = nx.complete_graph(3).to_directed()
>>> h_m = cycle_mixer(g)
>>> print(h_m)
(-0.25) [X0 Y1 Y5]
+ (-0.25) [X1 Y0 Y3]
+ (-0.25) [X2 Y3 Y4]
+ (-0.25) [X3 Y2 Y1]
+ (-0.25) [X4 Y5 Y2]
+ (-0.25) [X5 Y4 Y0]
+ (0.25) [X0 X1 X5]
+ (0.25) [Y0 Y1 X5]
+ (0.25) [Y0 X1 Y5]
+ (0.25) [X1 X0 X3]
+ (0.25) [Y1 Y0 X3]
+ (0.25) [Y1 X0 Y3]
+ (0.25) [X2 X3 X4]
+ (0.25) [Y2 Y3 X4]
+ (0.25) [Y2 X3 Y4]
+ (0.25) [X3 X2 X1]
+ (0.25) [Y3 Y2 X1]
+ (0.25) [Y3 X2 Y1]
+ (0.25) [X4 X5 X2]
+ (0.25) [Y4 Y5 X2]
+ (0.25) [Y4 X5 Y2]
+ (0.25) [X5 X4 X0]
+ (0.25) [Y5 Y4 X0]
+ (0.25) [Y5 X4 Y0]

Parameters

graph (nx.DiGraph) – the directed graph specifying possible edges

Returns

the cycle-mixer Hamiltonian

Return type

qml.Hamiltonian