# 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.**
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"description": "# Qubit Reuse By Reset Plugin\n\nThis 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)\n\n### Background\n\nCertain 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.\n\n#### Causal Cones\n\nLet'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. \n\nWhen 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.\n\n#### Order of Measurement\n\nQubits 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. \n\nBefore 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.\n\n## Installation\n\nThis package is not available through pypi, but can be installed by cloning this repository:\n\n```zsh\ngit clone https://github.com/qiskit-community/qiskit-qubit-reuse\n```\nAnd then installing locally:\n\n```zsh\npip install ./qiskit-qubit-reuse\n```\nIf you have the proper authentication keys, you can install it remotely by using:\n\n```zsh\npip install git+https://github.com/qiskit-community/qiskit-qubit-reuse\n```\n\n## Usage\n\nOnce 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:\n\n```py3\nfrom qiskit.circuit.random import random_circuit\nfrom qiskit import transpile\nfrom qiskit.providers.fake_provider import FakeGuadalupeV2\n\nqc = random_circuit(16, 4, measure=True)\n\ntranspiled_qc = transpile(qc, backend=FakeGuadalupeV2(), init_method=\"qubit_reuse\")\n```\n\nThis 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.\n\n**Warning: This plugin should only be used with circuits that contain measurements.**\n",
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