quantSim/system_design_doc at main · sw478/quantSim · GitHub

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## System Design

### Architecture

* Sims contain a list of SubGates
* Each SubGate is a representation of a gate within a Sim
* Each SubGate is associated with a GateContainer and a mapping of the gate inputs to Sim indices
* A GateContainer is a dataclass containing info of a quantum gate (independent of a Sim)

### Relevant Classes

#### Sim

* Describes a quantum register with n_qbits # of qubits
* Quantum gates can be added, turned into SubGates when passed in
* to_gate(): Creates a single black box gate from the Sim
* Matrix multiplication process:
    * Does not have a single statevector throughout runtime, multiplying statevector with gates as they are added to modify this statevector
    * Instead, there is a single matrix representing all gates added previously, to enable used of to_gate()
    * Statevector and probability distribution is calculated each time they are printed
        * Currently assumes statevector is initialized with all qubits as |0>
* Options to inspect state of register:
    * print_sim(): also the repr, prints a circuit diagram of the Sim, showing gates within
        * for control gates, the control and target bits are differentiated
    * print_statevector(): prints the current statevector
    * print_prob_dist(): prints the current probability distribution based on the statevector

#### GateContainer

* Data class for containing information for a quantum gate
    * Gate matrix: Unitary matrix represented by the quantum gate
    * Gate label: String used for printouts
    * Number of control bits (num_cb): Used for printing and multi-controlled gates
    * Expected qubits: Size of gate in qubits
* Independent of any Sim it might be used in

#### SubGate

* Describes a GateContainer within a Sim (dependent on Sim)
* Data includes:
    * A gate (GateContainer)
    * i_qbits: Which qubit indices of the Sim has the gate been placed on
    * n_qbits: Number of qubits in the Sim
    * full_mat:
        * The gate matrix placed in parallel (kron) with Identity gates
        * Calculated after initialization
        * Bits are swapped into the correct order indicated by i_qbits

#### Gate

* Function definitions for predefined gates to be used
* Gates can be easily created and added here
* Common gates are hardcoded to avoid unnecessary extra computation
* Some gates have arguments (all are theta right now):
    * Phase, rotational, Ising coupling gates
* Modifiers for gates, takes in a gate as an argument, returns modified gate without change:
    * Multi-controlled (MC):
        * num_cb: number of control bits for gate
        * Control bits will be the MSB bits
        * Can also get an MC gate of an MC gate
    * Multi-targeted (MT):
        * num_tb: number of target bits
        * Accepts only single bit gates
    * Dagger (dg):
        * Returns the conjugate transpose of the gate

#### Permute

* Functional class, used for swapping bit order in gate matrices
* Permutation is performed using successive swaps on two bits
* A swap matrix is used to swap the two bits, reordering a specific set of matrix rows/cols
* These swap matrices are preprocessed using memoization
    * They are preprocessed for n_qbits up to a max number, if a permutation is needed on a larger matrix, the swap matrices need to be reprocessed