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merlin.core.process module

Quantum computation processes and factories.

class merlin.core.process.AbstractComputationProcess

Bases: ABC

Abstract base class for quantum computation processes.

abstract compute(*args, **kwargs)

Perform the computation.

Args:: *args: Positional arguments for the computation. **kwargs: Keyword arguments for the computation.`

class merlin.core.process.CircuitConverter(circuit, input_specs=None, dtype=torch.complex64, device=device(type='cpu'))

Bases: object

Convert a parameterized Perceval circuit into a differentiable PyTorch unitary matrix.

This class converts Perceval quantum circuits into PyTorch tensors that can be used in neural network training with automatic differentiation. It supports batch processing for efficient training and handles various quantum components like beam splitters, phase shifters, and unitary operations.

Supported Components:

PS (Phase Shifter)
BS (Beam Splitter)
PERM (Permutation)
Unitary (Generic unitary matrix)
Barrier (no-op, removed during compilation)

Attributes:

circuit: The Perceval circuit to convert param_mapping: Maps parameter names to tensor indices device: PyTorch device for tensor operations tensor_cdtype: Complex tensor dtype tensor_fdtype: Float tensor dtype

Example:

Basic usage with a single phase shifter:

>>> import torch
>>> import perceval as pcvl
>>> from merlin.pcvl_pytorch.locirc_to_tensor import CircuitConverter
>>>
>>> # Create a simple circuit with one phase shifter
>>> circuit = pcvl.Circuit(1) // pcvl.PS(pcvl.P("phi"))
>>>
>>> # Convert to PyTorch with gradient tracking
>>> converter = CircuitConverter(circuit, input_specs=["phi"])
>>> phi_params = torch.tensor([0.5], requires_grad=True)
>>> unitary = converter.to_tensor(phi_params)
>>> print(unitary.shape)  # torch.Size([1, 1])

Multiple parameters with grouping:

>>> # Circuit with multiple phase shifters
>>> circuit = (pcvl.Circuit(2)
...            // pcvl.PS(pcvl.P("theta1"))
...            // (1, pcvl.PS(pcvl.P("theta2"))))
>>>
>>> converter = CircuitConverter(circuit, input_specs=["theta"])
>>> theta_params = torch.tensor([0.1, 0.2], requires_grad=True)
>>> unitary = converter.to_tensor(theta_params)
>>> print(unitary.shape)  # torch.Size([2, 2])

Batch processing for training:

>>> # Batch of parameter values
>>> batch_params = torch.tensor([[0.1], [0.2], [0.3]], requires_grad=True)
>>> converter = CircuitConverter(circuit, input_specs=["phi"])
>>> batch_unitary = converter.to_tensor(batch_params)
>>> print(batch_unitary.shape)  # torch.Size([3, 1, 1])

Training integration:

>>> # Training loop with beam splitter
>>> circuit = pcvl.Circuit(2) // pcvl.BS.Rx(pcvl.P("theta"))
>>> converter = CircuitConverter(circuit, ["theta"])
>>> theta = torch.tensor([0.5], requires_grad=True)
>>> optimizer = torch.optim.Adam([theta], lr=0.01)
>>>
>>> for step in range(10):
...     optimizer.zero_grad()
...     unitary = converter.to_tensor(theta)
...     loss = some_loss_function(unitary)
...     loss.backward()
...     optimizer.step()

set_dtype(dtype)

Set the tensor data types for float and complex operations.

Args:: dtype: Target dtype (float32/complex64 or float64/complex128)
Raises:: TypeError: If dtype is not supported

to(dtype, device)

Move the converter to a specific device and dtype.

Args:: dtype: Target tensor dtype (float32/complex64 or float64/complex128) device: Target device (string or torch.device)
Returns:: Self for method chaining
Raises:: TypeError: If device type or dtype is not supported

to_tensor(*input_params, batch_size=None)

Convert the parameterized circuit to a PyTorch unitary tensor.

Return type:: Tensor

Args:

*input_params: Variable number of parameter tensors. Each tensor has shape (num_params,) or (batch_size, num_params) corresponding to input_specs order. batch_size: Explicit batch size. If None, inferred from input tensors.

Returns:

Complex unitary tensor of shape (circuit.m, circuit.m) for single samples: or (batch_size, circuit.m, circuit.m) for batched inputs.

Raises:

ValueError: If wrong number of input tensors provided. TypeError: If input_params is not a list or tuple.

class merlin.core.process.Combinadics(scheme, n, m)

Bases: object

Rank/unrank Fock states in descending lexicographic order.

Parameters:

scheme (str) – Enumeration strategy. Supported values are "fock", "unbunched", and "dual_rail".
n (int) – Number of photons. Must be non-negative.
m (int) – Number of modes. Must be at least one.

Raises:

ValueError – If an unsupported scheme is provided or the parameters violate the constraints of the selected scheme.

compute_space_size()

Return the number of admissible Fock states for this configuration.

Returns:: Cardinality of the state space.
Return type:: int

enumerate_states()

Return all admissible states in descending lexicographic order.

Returns:: State list matching iter_states().
Return type:: list[Tuple[int, …]]

fock_to_index(counts)

Map a Fock state to its index under the configured scheme.

Parameters:: counts (Iterable[int]) – Photon counts per mode.
Returns:: Rank of the Fock state in descending lexicographic order.
Return type:: int
Raises:: ValueError – If counts violates the scheme-specific constraints.

index_to_fock(index)

Map an index to its corresponding Fock state.

Parameters:: index (int) – Rank to decode.
Returns:: Photon counts per mode.
Return type:: Tuple[int, …]
Raises:: ValueError – If index is outside the valid range for the configured scheme.

iter_states()

Yield admissible states in descending lexicographic order.

Returns:: Generator over states matching the configured scheme.
Return type:: Iterator[Tuple[int, …]]

class merlin.core.process.ComputationProcess(circuit, input_state, trainable_parameters, input_parameters, n_photons=None, reservoir_mode=False, dtype=torch.float32, device=None, computation_space=None, no_bunching=None, output_map_func=None)

Bases: AbstractComputationProcess

Handles quantum circuit computation and state evolution.

compute(parameters)

Compute quantum output distribution.

Return type:: Tensor

compute_ebs_simultaneously(parameters, simultaneous_processes=1)

Evaluate a single circuit parametrisation against all superposed input states by chunking them in groups and delegating the heavy work to the TorchScript-enabled batch kernel.

The method converts the trainable parameters into a unitary matrix, normalises the input state (if it is not already normalised), filters out components with zero amplitude, and then queries the simulation graph for batches of Fock states. Each batch feeds SLOSComputeGraph.compute_batch(), producing a tensor that contains the amplitudes of all reachable output states for the selected input components. The partial results are accumulated into a preallocated tensor and finally weighted by the complex coefficients of self.input_state to produce the global output amplitudes.

Return type:: Tensor

Args:

parameters (list[torch.Tensor]): Differentiable parameters that: encode the photonic circuit. They are forwarded to self.converter to build the unitary matrix used during the simulation.
simultaneous_processes (int): Maximum number of non-zero input: components that are propagated in a single call to compute_batch. Tuning this value allows trading memory consumption for wall-clock time on GPU.

Returns:

torch.Tensor: The superposed output amplitudes with shape [batch_size, num_output_states] where batch_size corresponds to the number of independent input batches and num_output_states is the size of self.simulation_graph.mapped_keys.

Raises:

TypeError: If self.input_state is not a torch.Tensor. The simulation graph expects tensor inputs, therefore other sequence types (NumPy arrays, lists, etc.) cannot be used here.

Notes:

self.input_state is normalised in place to avoid an extra allocation.
Zero-amplitude components are skipped to minimise the number of calls to compute_batch.
The method is agnostic to the device: tensors remain on the device they already occupy, so callers should ensure parameters and self.input_state live on the same device.

compute_superposition_state(parameters, *, return_keys=False)

Return type:: Tensor | tuple[list[tuple[int, ...]], Tensor]

compute_with_keys(parameters): Compute quantum output distribution and return both keys and probabilities.

configure_computation_space(computation_space=ComputationSpace.UNBUNCHED, *, validate_input=True)

Reconfigure the logical basis according to the desired computation space.

Return type:: None

class merlin.core.process.ComputationProcessFactory

Bases: object

Factory for creating computation processes.

static create(circuit, input_state, trainable_parameters, input_parameters, reservoir_mode=False, computation_space=None, **kwargs)

Create a computation process.

Return type:: ComputationProcess

enum merlin.core.process.ComputationSpace(value)

Bases: str, Enum

Enumeration of supported computational subspaces.

Member Type:: str

Valid values are as follows:

FOCK = <ComputationSpace.FOCK: 'fock'>

UNBUNCHED = <ComputationSpace.UNBUNCHED: 'unbunched'>

DUAL_RAIL = <ComputationSpace.DUAL_RAIL: 'dual_rail'>

The Enum and its members also have the following methods:

classmethod default(*, no_bunching)

Derive the default computation space from the legacy no_bunching flag.

Return type:: ComputationSpace

classmethod coerce(value)

Normalize user-provided values (enum instances or case-insensitive strings).

Return type:: ComputationSpace

merlin.core.process.build_slos_distribution_computegraph(m, n_photons, output_map_func=None, computation_space=None, no_bunching=None, keep_keys=True, device=None, dtype=torch.float32, index_photons=None)

Construct a reusable SLOS computation graph.

Parameters:

m (int) – Number of modes in the circuit.
n_photons (int) – Total number of photons injected in the circuit.
output_map_func (callable, optional) – Mapping applied to each output Fock state, allowing post-processing.
computation_space (ComputationSpace, optional) –
keep_keys (bool, optional) – Whether to keep the list of mapped Fock states.
device (torch.device, optional) – Device on which tensors should be allocated.
dtype (torch.dtype, optional) – Real dtype controlling numerical precision.
index_photons (list[tuple[int, ...]], optional) – Bounds for each photon placement.

Returns:

Pre-built computation graph ready for repeated evaluations.

Return type:

SLOSComputeGraph

merlin.core.process.overload(func)

Decorator for overloaded functions/methods.

In a stub file, place two or more stub definitions for the same function in a row, each decorated with @overload.

For example:

@overload
def utf8(value: None) -> None: ...
@overload
def utf8(value: bytes) -> bytes: ...
@overload
def utf8(value: str) -> bytes: ...

In a non-stub file (i.e. a regular .py file), do the same but follow it with an implementation. The implementation should not be decorated with @overload:

@overload
def utf8(value: None) -> None: ...
@overload
def utf8(value: bytes) -> bytes: ...
@overload
def utf8(value: str) -> bytes: ...
def utf8(value):
    ...  # implementation goes here

The overloads for a function can be retrieved at runtime using the get_overloads() function.