# Custom gate with a matrix
**URL:** https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773
**Category:** PennyLane Help
**Created:** [December 4, 2023, 8:42pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773 "2023-12-04T20:42:23Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@sdas](https://discuss.pennylane.ai/u/sdas)
#### Post date: [December 4, 2023, 8:42pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/1 "2023-12-04T20:42:23Z")
HI, I am creating a custom gate using a unitary matrix A.
```auto
class transform_matrix(Operation):
num_params = 0
num_wires = 10
par_domain = None
@staticmethod
def _matrix(self):
s1 = np.matrix([[0,1],[1,0]])
ID = np.identity(2)
A_1 = np.array(np.zeros((64, 64)))
for i in range(0, 64, 1):
if i//32 == 0:
A_1[i, 2*i+1] = 1
elif i//32 == 1:
A_1[i, 64-2*(i%32)-2] = 1
A = np.kron(A_1, np.kron(np.kron(s1, s1), np.kron(s1, s1)))
return np.array(A)
```
Then I add it to the device using:
```auto
dev = qml.device('default.qubit', wires = 10)
dev.operations.add("transform_matrix")
```
When I call it as following
```auto
transform_matrix(wires=[i for i in range(10)])
```
it shows this error
```auto
pennylane.operation.MatrixUndefinedError
```
Can someone tell me where I am wrong?
---
### Author:  [@isaacdevlugt](https://discuss.pennylane.ai/u/isaacdevlugt)
#### Post date: [December 4, 2023, 9:01pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/2 "2023-12-04T21:01:00Z")
Hey @sdas!
Quick aside: I’m running the most updated version of PennyLane (v0.33) and this code won’t work because of device API changes that were made a couple releases ago. If you decide to update (which I recommend that you do!) you just need to change your device to use the old default qubit:
```auto
dev = qml.device('default.qubit.legacy', wires = 10)
```
If I change that and run on v0.33, your code works and I get
```auto
transform_matrix(wires=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
```
Having said that, I think you should create your custom operation differently 🙂. Check out this documentation page for an example: [Adding new operators — PennyLane 0.33.0 documentation](https://docs.pennylane.ai/en/stable/development/adding_operators.html?highlight=custom%20operators#creating-custom-operators).
Let me know if this helps!
---
### Author:  [@sdas](https://discuss.pennylane.ai/u/sdas)
#### Post date: [December 4, 2023, 10:56pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/3 "2023-12-04T22:56:34Z")
Thanks Issacdevlugt. If I understood correctly, I upgraded pennylane to v0.33.1 and only changed “default.qubit” to “default.qubit.legacy”. But now it shows,
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/ **init**.py”, line 337, in device
raise DeviceError(“Device does not exist. Make sure the required plugin is installed.”)
pennylane.\_device.DeviceError: Device does not exist. Make sure the required plugin is installed.
---
### Author:  [@sdas](https://discuss.pennylane.ai/u/sdas)
#### Post date: [December 4, 2023, 11:14pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/4 "2023-12-04T23:14:36Z")
Sorry there was a mistake in the upgrading process. Trying the solution now after correct upgrade.
---
### Author:  [@sdas](https://discuss.pennylane.ai/u/sdas)
#### Post date: [December 4, 2023, 11:24pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/5 "2023-12-04T23:24:54Z")
So after I update the version and change “default.qubit” to “default.qubit.legacy” and run the code, it is showing same error,
pennylane.operation.MatrixUndefinedError
In detail,
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/qnode.py”, line 1027, in **call**
res = qml.execute(
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/interfaces/execution.py”, line 616, in execute
results = inner\_execute(tapes)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/interfaces/execution.py”, line 249, in inner\_execute
return cached\_device\_execution(tapes)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/interfaces/execution.py”, line 371, in wrapper
res = list(fn(tuple(execution\_tapes.values()), \*\*kwargs))
File “/home/sd/anaconda3/lib/python3.9/contextlib.py”, line 79, in inner
return func(\*args, \*\*kwds)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/\_qubit\_device.py”, line 460, in batch\_execute
res = self.execute(circuit)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/\_qubit\_device.py”, line 279, in execute
self.apply(circuit.operations, rotations=self.\_get\_diagonalizing\_gates(circuit), \*\*kwargs)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/devices/default\_qubit\_legacy.py”, line 296, in apply
self.\_state = self.\_apply\_operation(self.\_state, operation)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/devices/default\_qubit\_legacy.py”, line 338, in \_apply\_operation
matrix = self.\_asarray(self.\_get\_unitary\_matrix(operation), dtype=self.C\_DTYPE)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/devices/default\_qubit\_legacy.py”, line 682, in \_get\_unitary\_matrix
return unitary.matrix()
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/operation.py”, line 775, in matrix
canonical\_matrix = self.compute\_matrix(\*self.parameters, \*\*self.hyperparameters)
File “/home/sd/anaconda3/lib/python3.9/site-packages/pennylane/operation.py”, line 744, in compute\_matrix
raise MatrixUndefinedError
pennylane.operation.MatrixUndefinedError
---
### Author:  [@Tom\_Bromley](https://discuss.pennylane.ai/u/Tom_Bromley)
#### Post date: [December 5, 2023, 2:39pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/6 "2023-12-05T14:39:36Z")
Hi @sdas!
The following will work with the (non-legacy) `default.qubit` and the latest release of PennyLane:
```python
import pennylane as qml
from pennylane.operation import Operation
from pennylane import numpy as np
class transform_matrix(Operation):
num_params = 0
num_wires = 10
par_domain = None
@staticmethod
def compute_matrix():
s1 = np.matrix([[0,1],[1,0]])
ID = np.identity(2)
A_1 = np.array(np.zeros((64, 64)))
for i in range(0, 64, 1):
if i//32 == 0:
A_1[i, 2*i+1] = 1
elif i//32 == 1:
A_1[i, 64-2*(i%32)-2] = 1
A = np.kron(A_1, np.kron(np.kron(s1, s1), np.kron(s1, s1)))
return np.array(A)
dev = qml.device("default.qubit")
@qml.qnode(dev)
def f():
transform_matrix(wires=range(10))
return qml.state()
f()
```
Here, the trick is to override the `compute_matrix()` static method.
---
### Author:  [@sdas](https://discuss.pennylane.ai/u/sdas)
#### Post date: [December 5, 2023, 5:25pm UTC](https://discuss.pennylane.ai/t/custom-gate-with-a-matrix/3773/7 "2023-12-05T17:25:10Z")
Hi @Tom_Bromley
It is working now, thanks!