Quantum State
Definition of the quantum state classes.
This package defines classes used to track quantum states in SeQUeNCe. The formalism-specific state classes are used by quantum managers, while FreeQuantumState is used by individual photons that do not use a quantum manager.
- class sequence.kernel.quantum_state.BellDiagonalState(diag_elems: list[float], keys: list[int])
Class for representing a 2-qubit EPR pair in the Bell diagonal formalism.
Has 4 diagonal elements of density matrix in Bell basis.
- state
diagonal elements of 2-qubit density matrix in Bell bases. Should be of length 4.
- Type:
np.array
- keys
list of keys (subsystems) associated with this state. Should be length 2.
- Type:
list[int]
- class sequence.kernel.quantum_state.DensityState(state: list[complex] | tuple[complex, ...] | list[list[complex]] | tuple[tuple[complex, ...], ...], keys: list[int], truncation: int = 1)
Class for representing a quantum state in the density matrix formalism.
- state
density matrix values. NxN matrix with N = d ** len(keys), where d is dimension of elementary Hilbert space. Default is d = 2 for qubits.
- Type:
np.array
- keys
list of keys (subsystems) associated with this state.
- Type:
list[int]
- truncation
maximally allowed number of excited states for elementary subsystems. Default is 1 for qubit. dim = truncation + 1
- Type:
int
- class sequence.kernel.quantum_state.FreeQuantumState
Class used by photons to track internal quantum states.
This is an alternative to tracking states in a dedicated quantum manager, which adds simulation overhead. It defines several operations, including entanglement and measurement. For memories with an internal quantum state and certain photons, such as those stored in a memory or in parallel simulation, this class should not be used. Quantum states stored in a quantum manager class should be used instead. This module uses the ket vector formalism for storing and manipulating states.
- state
list of complex coefficients in Z-basis.
- Type:
tuple[complex]
- entangled_states
list of entangled states (including self).
- Type:
list[QuantumState]
- combine_state(another_state: FreeQuantumState)
Method to tensor multiply two quantum states.
- Parameters:
another_state (QuantumState) – state to entangle current state with.
- Side Effects:
Modifies the entangled_states field for current state and another_state. Modifies the state field for current state and another_state.
- measure(basis: tuple[tuple[complex]], rng: Generator) int
Method to measure a single quantum state.
- Parameters:
basis (tuple[tuple[complex]]) – measurement basis, given as list of states (that are themselves lists of complex coefficients).
rng (Generator) – random number generator for measurement
- Returns:
0/1 measurement result, corresponding to one basis vector.
- Return type:
int
- Side Effects:
Modifies the state field for current and any entangled states.
- static measure_multiple(basis, states, rng: Generator)
Method to measure multiple qubits in a more complex basis.
May be used for bell state measurement.
- Parameters:
basis (list[list[complex]]) – list of basis vectors.
states (list[QuantumState]) – list of quantum state objects to measure.
rng (Generator) – random number generator for measurement
- Returns:
measurement result in given basis.
- Return type:
int
- Side Effects:
Will modify the state field of all entangled states.
- random_noise(rng: Generator)
Method to add random noise to a single state.
Chooses a random angle to set the quantum state to (with no phase difference).
- Side Effects:
Modifies the state field.
- set_state(state: tuple[complex])
Method to change entangled state of multiple quantum states.
- Parameters:
state (tuple[complex]) – new coefficients for state. Should be 2^n in length, where n is the length of entangled_states.
- Side Effects:
Modifies the state field for current and entangled states.
- set_state_single(state: tuple[complex])
Method to unentangle and set the state of a single quantum state object.
- Parameters:
state (tuple[complex]) – 2-element list of new complex coefficients.
- Side Effects:
Will remove current state from any entangled states (if present). Modifies the state field of current state.
- class sequence.kernel.quantum_state.KetState(amplitudes: list[complex] | tuple[complex, ...], keys: list[int], truncation: int = 1)
Class for representing a quantum state in the ket vector formalism.
- state
state vector. Should be of length d ** len(keys), where d is dimension of elementary Hilbert space. Default is 2 for qubits.
- Type:
np.array
- keys
list of keys (subsystems) associated with this state.
- Type:
list[int]
- truncation
maximally allowed number of excited states for elementary subsystems. Default is 1 for qubit. dim = truncation + 1
- Type:
int
- class sequence.kernel.quantum_state.StabilizerState(state: TableauSimulator | None, keys: list[int], seed: int | None = None)
Stim-backed stabilizer quantum state used by a quantum manager.
The state is stored internally by a
stim.TableauSimulator. Tableau accessors are provided for inspection and interoperability with Stim.- state
the internal stabilizer state simulator.
- Type:
TableauSimulator
- keys
list of keys (subsystems) associated with this state.
- Type:
list[int]
- seed
random seed used by the simulator
- Type:
int | None
- canonical_stabilizers() list[PauliString]
Return the simulator’s canonical stabilizer generators, which describes the quantum state in a normalized stabilizer-generator form.
- Distinction against forward tableau: many different Clifford circuits can produce the same stabilizer state.
Their forward tableaus may differ, but their canonical stabilizers can be the same because the final state is the same.
- Returns:
canonical stabilizer generators.
- Return type:
list[PauliString]
- copy() StabilizerState
Create a copy of this stabilizer state.
- Returns:
Copied state with copied keys and simulator.
- Return type:
- current_forward_tableau() Tableau
Return the forward tableau for user-facing state inspection, which describes the Clifford transformation
- Returns:
current forward tableau.
- Return type:
Tableau
- current_inverse_tableau() Tableau
Return current inverse tableau from simulator state.
- Returns:
current inverse tableau.
- Return type:
Tableau
- get_seed() int | None
Get the current random seed for this state.
- Returns:
Current random seed.
- Return type:
int | None
- set_seed(seed: int)
Set the random seed for this state, affecting future simulator operations.
- Parameters:
seed (int) – new random seed to set.
- classmethod zero_state(key: int, seed: int | None = None) StabilizerState
Create a single-qubit stabilizer state initialized to |0>.
- Parameters:
key (int) – Quantum-manager key for the qubit.
seed (int | None) – Seed used by stim.TableauSimulator.
- Returns:
New state bound to [key].
- Return type: