Possibility of Qubit, Qutrit and Qudit(when qudit device added) Interaction

[ArturDomingues]

Feature details

I know that for now we don't have qudit devices, but we already have qubit and qutrit devices, but we can only use qubits interacting with qubits and qutrits interacting with qutrits. There are some physical that, for simulation purposes, may need to have a wire with a qubit and another with a qutrit and gates for their interactions

Implementation

No response

How important would you say this feature is?

2: Somewhat important. Needed this quarter.

Additional information

No response

[trbromley]

Thanks @ArturDomingues! Mixed qudit-qudit devices is a great idea and something that we have discussed about before. We're gradually increasing our $d>2$ capabilities, e.g., with an upcoming mixed-qutrit device thanks to @Gabriel-Bottrill. One option for qubit-qutrit simulation might be to use this mixed-qutrit device, encoding the qubit into one of the qutrits and interacting using a multi-qutrit QutritChannel.

However, we will not be able to get this across the line within this quarter. We definitely welcome contributions if you are looking to help out in this direction!

[Gabriel-Bottrill]

Hi, @ArturDomingues. I have also been thinking a lot about this, but a qubit/qutrit device has a lot of associated challenges. To add to @trbromley's suggestion, you could create a transform using Pennylane's transforms that converts qubit operations to qml.QutritUnitary operations. You could then apply that to any quantum function that includes both qubit and qutrit operations.