--- title: Path (latest version) description: API reference for qiskit_noise_learning.sequences.Path in the latest version of qiskit-noise-learning source: https://quantum.cloud.ibm.com/docs/en/api/qiskit-noise-learning/generated/sequences-path --- # qiskit\_noise\_learning.sequences.Path *class* `qiskit_noise_learning.sequences.Path(start_fragment: Iterable[T], repeatable_fragment: Iterable[T], end_fragment: Iterable[T], fragment_depth: int | None = None)` [GitHub](https://github.com/Qiskit/qiskit-noise-learning/tree/stable/0.1/qiskit_noise_learning/sequences/path.py) Bases: `BaseSequence`\[[`FidelityIndex`](/docs/api/qiskit-noise-learning/generated/sequences-fidelity-index "qiskit_noise_learning.sequences.fidelity_index.FidelityIndex")] A sequence of fidelity indices. **Parameters** - **start\_fragment** – The start of the sequence. - **repeatable\_fragment** – The repeatable middle of the sequence. - **end\_fragment** – The end of the sequence. - **fragment\_depth** – The number of repetitions of the repeatable fragment. ### \_\_init\_\_ `__init__(start_fragment: Iterable[T], repeatable_fragment: Iterable[T], end_fragment: Iterable[T], fragment_depth: int | None = None)` ## Methods | | | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------- | | [`__init__`](#qiskit_noise_learning.sequences.Path.__init__ "qiskit_noise_learning.sequences.Path.__init__")(start\_fragment, ...\[, fragment\_depth]) | | | `bind_at`(fragment\_depth) | Return a new instance with the same fragments bound to the fragment depth. | | [`extend_permutations`](#qiskit_noise_learning.sequences.Path.extend_permutations "qiskit_noise_learning.sequences.Path.extend_permutations")(instruction\_sequence) | Return an instruction sequence with extended permutations to traverse self. | | [`fragment_sign_flips`](#qiskit_noise_learning.sequences.Path.fragment_sign_flips "qiskit_noise_learning.sequences.Path.fragment_sign_flips")(instruction\_sequence) | Whether the instruction sequence fragments flip the observable sign when traversing self. | | [`is_traversed_by`](#qiskit_noise_learning.sequences.Path.is_traversed_by "qiskit_noise_learning.sequences.Path.is_traversed_by")(instruction\_sequence) | Whether or not this path is traversed by the instruction sequence. | | [`to_instruction_sequence`](#qiskit_noise_learning.sequences.Path.to_instruction_sequence "qiskit_noise_learning.sequences.Path.to_instruction_sequence")() | Return a minimally-specified instruction sequence that traverses this path. | | `unbind`() | Return a new instance with the same fragments but fragment depth set to `None`. | ## Attributes | | | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------- | | `end_fragment` | The ending fragment. | | [`end_fragment_observable_idxs`](#qiskit_noise_learning.sequences.Path.end_fragment_observable_idxs "qiskit_noise_learning.sequences.Path.end_fragment_observable_idxs") | The observable indices of the end fragment. | | `fragment_depth` | The number of repetitions of the repeatable fragment. | | `is_unbound` | Whether the sequence is unbound. | | `repeatable_fragment` | The repeatable fragment. | | [`repeatable_fragment_observable_idxs`](#qiskit_noise_learning.sequences.Path.repeatable_fragment_observable_idxs "qiskit_noise_learning.sequences.Path.repeatable_fragment_observable_idxs") | The observable indices of the repeatable fragment. | | `start_fragment` | The starting fragment. | | [`start_fragment_observable_idxs`](#qiskit_noise_learning.sequences.Path.start_fragment_observable_idxs "qiskit_noise_learning.sequences.Path.start_fragment_observable_idxs") | The observable indices of the start fragment. | ### start\_fragment\_observable\_idxs Type: [`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`int`](https://docs.python.org/3/library/functions.html#int)]] The observable indices of the start fragment. ### repeatable\_fragment\_observable\_idxs Type: [`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`int`](https://docs.python.org/3/library/functions.html#int)]] The observable indices of the repeatable fragment. ### end\_fragment\_observable\_idxs Type: [`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`list`](https://docs.python.org/3/library/stdtypes.html#list)\[[`int`](https://docs.python.org/3/library/functions.html#int)]] The observable indices of the end fragment. ### extend\_permutations `extend_permutations(instruction_sequence: InstructionSequence) → InstructionSequence | None` Return an instruction sequence with extended permutations to traverse self. Given an `instruction_sequence`, this method attempts to extend its definition by constructing a new instruction sequence with the same structure as the input, but for which the [`PartialPauliPermutation`](/docs/api/qiskit-noise-learning/generated/sequences-partial-pauli-permutation "qiskit_noise_learning.sequences.PartialPauliPermutation")s are extended to specify the minimal superset of mappings required to traverse this path. If this is not possible, the function will return `None`, indicating that the procedure failed. In terms of the specific algorithm, when encountering a [`PartialPauliPermutation`](/docs/api/qiskit-noise-learning/generated/sequences-partial-pauli-permutation "qiskit_noise_learning.sequences.PartialPauliPermutation") while jointly iterating through `self` and `instruction_sequence`, this method will attempt to extend the permutation to map the current Pauli in the path to the Pauli in the next relevant [`FidelityIndex`](/docs/api/qiskit-noise-learning/generated/sequences-fidelity-index "qiskit_noise_learning.sequences.FidelityIndex"). Note that this can lead to failures in instances when multiple permutations occur in a row, even if it is technically possible to find a super instruction sequence that traverses the path. **Parameters** **instruction\_sequence** – The base instruction sequence. **Returns** The extended instruction sequence, or `None` if an extension is not possible. **Raises** [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – If the transitions of `self` do not start and end with the identity. ### is\_traversed\_by `is_traversed_by(instruction_sequence: InstructionSequence) → bool` Whether or not this path is traversed by the instruction sequence. Whether or not a [`Path`](#qiskit_noise_learning.sequences.Path "qiskit_noise_learning.sequences.Path") is traversed by an [`InstructionSequence`](/docs/api/qiskit-noise-learning/generated/sequences-instruction-sequence "qiskit_noise_learning.sequences.InstructionSequence") is determined on a fragment-by-fragment basis. Requires the path starts and ends at the identity. **Parameters** **instruction\_sequence** – The instruction sequence. **Returns** Whether or not the path is traversed. **Raises** [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – If the path does not start and end at the identity. ### fragment\_sign\_flips `fragment_sign_flips(instruction_sequence: InstructionSequence) → tuple[bool, bool]` Whether the instruction sequence fragments flip the observable sign when traversing self. Requires the path starts and ends at the identity. This method ignores the fragment depths of the path and instruction sequence, operating only on the fragment structure. **Parameters** **instruction\_sequence** – An instruction sequence that traverses this path. **Returns** A tuple of booleans, the first indicating whether the combined action of the start and end fragments flip the sign, and the second indicating whether the repeatable fragment flips the sign. **Raises** - [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – If the path does not satisfy the assumptions. - [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – If the instruction sequence does not traverse the path. ### to\_instruction\_sequence `to_instruction_sequence() → InstructionSequence` Return a minimally-specified instruction sequence that traverses this path. The single-qubit Clifford layers between gate set elements are given as [`PartialPauliPermutation`](/docs/api/qiskit-noise-learning/generated/sequences-partial-pauli-permutation "qiskit_noise_learning.sequences.PartialPauliPermutation")s specifying only the mappings required to traverse this path. The fragment depth of the returned instruction sequence is `self.fragment_depth`. **Returns** An instruction sequence traversing this path.