Institute for Advanced Simulation (IAS)

Jülich Supercomputing Centre (JSC)

Ideal Quantum Computers

In an ideal quantum computer the qubits are assumed to be ideal two-state quantum systems that perform highly idealized unitary operations. However, in practice these operations are difficult to realize: disregarding decoherence, a hardware implementation of a quantum computer will perform unitary operations that are more complicated. Nevertheless, in order to get good understandings of the working principle of an ideal quantum computer, including the running of quantum algorithms, it is crucial to have a massively parallel ideal quantum computer simulator taking into account all possible states. Moreover, such a simulator cannot only be used as a reference tool but can also be used as a basis for simulating more realistic realizations of quantum computers and quantum spin systems in general. Our ideal quantum computer simulator is a gate level simulator that is:

Universal: All basic operations for the simulation of any arbitrary quantum algorithm are available.
Efficient: The algorithms used and the program code are highly optimized for low memory consumption and high computing performance. It works massively parallel on suitable hardware and shows a nearly-ideal scaling.
Portable: It runs on various kinds of hardware ranging from single PCs to high-end supercomputers with distributed and/or shared memory and is also compatible to several software environments (Linux, AIX, ...). Standard Message Passing Interface (MPI) is used for communication and optionally OpenMP can be used for parallel processing within each MPI process.

Details of the implementation and extensive performance analyses are described in the paper listed below.

Recently, together with the Computational Physics group of the University of Groningen we optimized the simulation software such that we can use JUGENE to simulate a 42-qubit ideal quantum computer, the largest ideal quantum computer ever simulated. This requires the use of 262.144 CPUs operating in parallel. As seen from the figure, the simulation software scales almost perfectly.

Shor's algorithm has been demonstrated on the 42-qubit quantum computer factorizing 15707 into 113X139, a number thousand times larger than the one factorized on the experimentally realizable quantum computers up to now.

Simulating such a large ideal quantum computer is a demanding application in that it can use a significant part of the available memory and processors and can put a heavy burden on the communication network. Therefore, this simulator also finds application as a type of benchmark to assess the computational power of high-end computer systems.