Simulation Guidance

Overview

The table below specifies details corresponding to each simulation modality.

Simulation Options

Simulator	Usage Guidance	Limitations	Scaling Behaviour
Statevector	A general-purpose and brute-force method to simulate user programs. Simulations are fast up to 20 qubits.	Qubit Constraint: \( \leq 32\) Gateset Constraints: None	Memory: \(2^{n+3}\) bytes. Runtime: \(O(n)\) in program length and \(O(2^n)\) in number of qubits.
Stabilizer	Use for Clifford-only programs, like QEC. Much cheaper than Statevector simulation.	Qubit Constraint: Very low (supports 1000s of qubits) Gateset Constraint: Clifford-only	Memory: \(O(n^2)\). Runtime: \(O(d n^3)\).
MPS (CPU)	Bond-dimension truncation (\(\chi\)). Used when statevector is unfeasible.	Qubit Constraint scales with \(\chi\).	Memory: \(n 2^{4} {\chi}^{2}\) bytes. Runtime: \(O(n \chi^3)\)
MPS (GPU)	Used when \(\chi\) > 256 and CPU simulation is slow.	Qubit Constraint scales with \(\chi\)	Same as CPU scaling
Coinflip	Used for syntax checking and cost estimation only.	—	—

Additional Considerations

1. Statevector (Stabilizer) simulation uses the opensource Quest (Stim) package, integrated with Selene as a plugin, via the extensibility API.

2. Nexus provides the following compute for simulators:

   • 1 vCPU and 4 GB Memory for statevector and MPS simulators.

   • 1 vCPU and 2 GB Memory for stabilizer and Coinflip simulators.

3. MPS (GPU) is only available with hardware-tier emulators.

4. Memory scaling is specified for complex numbers defined by 2 32-bit floating-point numbers.