Quantum registers

In Qadence, quantum programs can be executed by specifying the layout of a register of resources as a lattice. Built-in Register types can be used or constructed for arbitrary topologies. Common register topologies are available and illustrated in the plot below.

Building and drawing registers

Built-in topologies are directly accessible in the Register methods:

from qadence import Register

reg = Register.all_to_all(n_qubits = 2)
reg_line = Register.line(n_qubits = 2)
reg_circle = Register.circle(n_qubits = 2)
reg_squre = Register.square(qubits_side = 2)
reg_rect = Register.rectangular_lattice(qubits_row = 2, qubits_col = 2)
reg_triang = Register.triangular_lattice(n_cells_row = 2, n_cells_col = 2)
reg_honey = Register.honeycomb_lattice(n_cells_row = 2, n_cells_col = 2)

Qubit coordinates are saved as node properties in the underlying NetworkX graph, but can be accessed directly with the coords property.

reg = Register.square(2)
print(reg.coords)

{0: (0.5, -0.5), 1: (0.5, 0.5), 2: (-0.5, 0.5), 3: (-0.5, -0.5)}

By default, the coords are scaled such that the minimum distance between any two qubits is 1, unless the register is created directly from specific coordinates as shown below. The spacing argument can be used to set the minimum spacing. The rescale_coords method can be used to create a new register by rescaling the coordinates of an already created register.

scaled_reg_1 = Register.square(2, spacing = 2.0)
scaled_reg_2 = reg.rescale_coords(scaling = 2.0)
print(scaled_reg_1.coords)
print(scaled_reg_2.coords)

{0: (1.0, -1.0), 1: (1.0, 1.0), 2: (-1.0, 1.0), 3: (-1.0, -1.0)}
{0: (1.0, -1.0), 1: (1.0, 1.0), 2: (-1.0, 1.0), 3: (-1.0, -1.0)}

The distance between qubits can also be directly accessed with the distances and edge_distances properties.

print(reg.distances)
print(reg.edge_distances)

Distance between all qubit pairs:
{(0, 1): 1.0, (0, 2): 1.4142135623730951, (0, 3): 1.0, (1, 2): 1.0, (1, 3): 1.4142135623730951, (2, 3): 1.0}
Distance between qubits connect by an edge in the graph
{(0, 1): 1.0, (0, 3): 1.0, (1, 2): 1.0, (2, 3): 1.0}

By calling the Register directly, either the number of nodes or a specific graph can be given as input. If passing a custom graph directly, the node positions will not be defined automatically, and should be previously saved in the "pos" node property. If not, reg.coords will return empty tuples and all distances will be 0.

import networkx as nx

# Same as Register.all_to_all(n_qubits = 2):
reg = Register(2)

# Register from a custom graph:
graph = nx.complete_graph(3)

# Set node positions, in this case a simple line:
for i, node in enumerate(graph.nodes):
    graph.nodes[node]["pos"] = (1.0 * i, 0.0)

reg = Register(graph)

print(reg.distances)

{(0, 1): 1.0, (0, 2): 2.0, (1, 2): 1.0}

Alternatively, arbitrarily shaped registers can also be constructed by providing the node coordinates. In this case, there will be no edges automatically created in the connectivity graph.

import numpy as np
from qadence import Register, PI

reg = Register.from_coordinates(
    [(x, np.sin(x)) for x in np.linspace(0, 2*PI, 10)]
)

reg.draw(show=False)

Units for qubit coordinates

In general, Qadence makes no assumption about the units for qubit coordinates and distances. However, if used in the context of a Hamiltonian coefficient, care should be taken by the user to guarantee the quantity \(H.t\) is dimensionless for exponentiation in the PyQTorch backend, where it is assumed that \(\hbar = 1\). For registers passed to the Pulser backend, coordinates are in \(\mu \textrm{m}\).

Connectivity graphs

Register topology is often assumed in digital simulations to be an all-to-all qubit connectivity. When running on real devices that enable the digital-analog computing paradigm, qubit interactions must be specified either by specifying distances between qubits, or by defining edges in the register connectivity graph.

It is possible to access the abstract graph nodes and edges to work with if needed.

from qadence import Register

reg = Register.rectangular_lattice(2,3)

reg.nodes = NodeView((0, 1, 2, 3, 4, 5))
reg.edges = EdgeView([(0, 2), (0, 1), (1, 3), (2, 4), (2, 3), (3, 5), (4, 5)])

There is also an all_node_pairs property for convencience:

print(reg.all_node_pairs)

[(0, 1), (0, 2), (0, 3), (0, 4), (0, 5), (1, 2), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), (2, 5), (3, 4), (3, 5), (4, 5)]

More details about the usage of Register types in the digital-analog paradigm can be found in the digital-analog basics section.