Interesting article from Tennin Yan, QunaSys CEO & Co-founder. The startup is developing quantum computer algorithms and applications.
As he states, a quantum computer simulator is an essential tool for quantum algorithm research. Most researchers won’t use real quantum devices for algorithm development, in particular for the proof-of-concept initial validation because of the current quantum computers called NISQ (Noisy Intermediate-Scale Quantum Computer) are ‘noisy’.
Simulating quantum computers is exponentially difficult. If you want to simulate one more qubit, it will require x2 memory and x2 calculation time. It means that simulating 20 qubits will need about 1,000 times more memory and 1,000 times more time compared to 10 qubits. Speed is essential, especially for running a variational algorithm which is thought to be a prominent application for NISQ. Variational algorithms need many iteration processes, and calculating a gradient of parametric quantum circuits needs a lot of circuit evaluations.
Qulacs is a fast quantum computer simulator. Qunasys succeeded in simulating up to a 24-qubit quantum system for Variational Quantum Eigensolver (VQE) algorithm. On another hand, Google has developed an open-source quantum computer library, Cirq, that will be a standard interface to use Google’s quantum computing system. Cirq has a CPU based quantum computer simulator. Qunasys succeeded to run Cirq’s quantum circuits simulation at least x50 faster in all qubits and up to x500 in large qubit using GPU. (Medium)