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https://github.com/qiskit-community/qiskit-qubit-reuse

A Qiskit transpiler stage plugin to enable qubit reuse via mid-circuit measurement and reset.
https://github.com/qiskit-community/qiskit-qubit-reuse

Last synced: 17 days ago
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A Qiskit transpiler stage plugin to enable qubit reuse via mid-circuit measurement and reset.

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# Qubit Reuse By Reset Plugin



This repository contains an experimental transpiler pass called `qubit_reuse` which is executed at the end of the `init` stage of transpilation. This pass is based on: Matthew DeCross et al. "Qubit-reuse compilation with mid-circuit measurement and reset" [arXiv:2210.0.08039v1](https://arxiv.org/abs/2210.08039v1)



### Background



Certain circuits can reduce the number of qubits required to produce results by resetting and re-using existent measured qubits. The order in which certain qubits are chosen is based on their **causal cones** and the order in which they are measured.



#### Causal Cones



Let's say we have qubit a x in a `DAGCircuit`. We can traverse the `DAGCircuit` from the output node of x by checking all its predecessor nodes. When checking every operation node found, if at any point x interacts with other qubits, via a multi-qubit gate, the qubits in that operation are added to a set. From that point we continue evaluating recursively all the predecessor nodes in that multi-qubit interaction and adding all qubits found into the set, until no more predecessor nodes are left. 



When the traversal ends, the set will contain **all the qubits whose interactions affect qubit x**. That is what we call the causal cone of x.



#### Order of Measurement



Qubits are re-arranged based on the length of their causal cones in ascending order, i.e. the first to be re-arranged are those with smaller causal cones. 



Before re-arranging a qubit, we need to check if there are any qubit that have been measured and is available to re-use. If so, we reset it and apply all operations onto its wire. Otherwise, a new qubit is added and the operations are passed on to that wire.



## Installation



This package is not available through pypi, but can be installed by cloning this repository:



```zsh

git clone https://github.com/qiskit-community/qiskit-qubit-reuse

```

And then installing locally:



```zsh

pip install ./qiskit-qubit-reuse

```

If you have the proper authentication keys, you can install it remotely by using:



```zsh

pip install git+https://github.com/qiskit-community/qiskit-qubit-reuse

```



## Usage



Once installed, Qiskit is able to detect the `qubit_reuse` plugin via an entry point. All that needs to be done is to specify the init method in your `transpile` call by using `init_method="qubit_reuse"`. Use the following example:



```py3

from qiskit.circuit.random import random_circuit

from qiskit import transpile

from qiskit.providers.fake_provider import FakeGuadalupeV2



qc = random_circuit(16, 4, measure=True)



transpiled_qc = transpile(qc, backend=FakeGuadalupeV2(), init_method="qubit_reuse")

```



This entry point provides the option with the least amount of qubits. If you want to specifically use the normal or dual circuit, you can specifcy that by using the `qubit_reuse_normal` or the `qubit_reuse_dual` endpoints.



**Warning: This plugin should only be used with circuits that contain measurements.**