Broken drawing examples

[jpmoutinho]

QFT

Some weird CPHASE lines, gets worse for larger QFTs:

from qadence import *
from qadence.draw import display
display(qft(3))

HEA + QFT with tags

• Qubits not ordered
• Bottom qubit line ramps up into the HEA at the start
• Bottom qubit lines jump off a cliff at the end

block_hea = tag(hea(2, 2), "hea")
block_qft = tag(qft(5), "qft")
display(chain(block_hea, block_qft))

[jpmoutinho]

Since these seem related to the QFT, it might be useful to have a simplified standalone function that writes it:

import torch
from qadence import *
from qadence.draw import display

def qft_layer(n_qubits, layer_ix):
    qubit_range = range(layer_ix + 1, n_qubits)
    # CPHASE ladder
    cphases = []
    for j in qubit_range:
        angle = torch.pi / (2 ** (j - layer_ix))
        cphases.append(CPHASE(j, layer_ix, angle))
    # Return Hadamard followed by CPHASEs
    return chain(H(layer_ix), *cphases)

def qft_digital(n_qubits):
    return chain(qft_layer(n_qubits, i) for i in range(n_qubits))

display(qft_digital(4))

[jpmoutinho]

Here's another example following the structure of the QFT but a bit simpler:

def broken_circuit(n_qubits):
    qubit_range = range(1, n_qubits)
    ops = []
    for j in qubit_range:
        ops.append(CNOT(0, j))
    return chain(*ops)

display(broken_circuit(5))

The reordering of the qubits happens when n_qubits is odd

[jpmoutinho]

from qadence import *

def broken_circuit(n_qubits):
    qubit_range = range(1, n_qubits)
    ops = []
    for j in qubit_range:
        ops.append(CNOT(0, j))
    return chain(*ops)

display(chain(broken_circuit(4), broken_circuit(2)))

And the tilted controls happens when you chain a circuit with larger qubit support followed by one with smaller qubit support.

[jpmoutinho]

Few more example cases:

Gives error: display(chain(CNOT(0, 3), CNOT(0, 3)))
One on top of the other: display(chain(CNOT(0, 3), CNOT(1, 2)))
Qubit lines don't go to the end: display(chain(CNOT(1, 2), CNOT(0, 3)))
Control line tilted: display(chain(CNOT(0, 3), CNOT(0, 1)))
Second control qubit gets flushed to the end of the circuit: display(chain(CNOT(0, 3), CNOT(1, 3)))