How can I save the quantum circuit into disk and load from disk in PennyLane

PennyLane Help
1

Hi, I am trying to save the quantum circuit constructed by PennyLane into a file and back again. Is it supported by PennyLane now?

Looking forward to your suggestions.
Thanks.

2

Hi @Yang, it’s good to see you in the Forum!

You could use to_openqasm and from_qasm to do what you need.

import pennylane as qml

dev = qml.device("lightning.qubit",wires=1)

@qml.qnode(dev)
def circuit(x):
    qml.RX(x,wires=0)
    return qml.expval(qml.PauliZ(0))

circuit.construct([0.5], {})
# Convert to QASM (str)
t = circuit.qtape.to_openqasm()
print(t)

# Remove the measurement
t_cut = t[:-23]
print(t_cut)

This gives you a string that you can send to someone. The other person can then use the following code:

my_circuit = qml.from_qasm(t_cut) # Create a template

@qml.qnode(dev)
def new_circuit(y):
    # You can add new things to the circuit
    qml.RY(y, wires=0)
    # Add your old circuit
    my_circuit(wires=[0])
    return qml.expval(qml.PauliZ(0))

#Draw
drawer = qml.draw(new_circuit)
print(drawer(y = 0.3))

Does this work for what you need?

2 Likes
3

Yes, it is exactly what I need. Thanks very much.

4

Awesome! I’m glad this works for you.

5

Just to note, it appears that in Pennylane 0.33.1, this method changes the circuit by decomposing the constituent gates. For example, a Hadamard gate in the qasm is decomposed into a series of Ry, Rz gates in Pennylane.

After testing, I determined that this is due to the qml.from_qasm function, which reads in a Hadamard as a parametrised U gate. This is then decomposed if you extract the QASM.

I got around this by defining a custom parser which uses Python’s exec function to read in the QASM and constructs a Pennylane function from it. Obviously this is far from best practice, but it is a temporary fix.

I think this is a serious oversight which should be fixed: the Pennylane QASM parser should be able to convert all standard QASM gates into the corresponding gates in Pennylane, without defaulting to representing them as parametrised U gates.

6

Hey @babel! Welcome to the forum :slight_smile:

Can you share some code that demonstrates the behaviour you’re seeing? I tried this and it seems to behave nicely :thinking:

import pennylane as qml
​
with qml.tape.QuantumTape() as tape:
    qml.Hadamard(0)
​
qasm_repr = tape.to_openqasm()
print(qasm_repr)
​
with qml.tape.QuantumTape() as tape:
    qml.from_qasm(qasm_repr)()
​
print(tape.draw())

OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];
creg c[1];
h q[0];
measure q[0] -> c[0];

0: ──H─┤