Scientists have developed algorithms for quantum computer simulations that can run efficiently on distributed systems. With classical computers, they can simulate more qubits than originally thought.
Simulations of quantum computers mostly happens on supercomputers, because of the complex calculation work. This is mainly because of the fast interconnects that connect the systems in a compute cluster of supercomputers. The algorithms of the scientists do not need such interprocess communication so they can be deployed on distributed clusters, such as those of cloud platforms.
One of the advantages that this brings is that the enormous memory quantities that are needed for quantum simulations on supercomputers are also not necessary. In 2016 researchers also from Google, predicted that the simulation of circuits of more than 48 high-depth qubits, of about 40 layers, with the supercomputers of the time was impossible. For running the algorithms on a circuit of 6 × 8 qubits, 2,252 petabytes of memory would be needed, they argued. Because of this practical limit, Google spoke of ‘quantum supremacy’: the point where classical computers are no longer sufficient and quantum computers outpace their classic counterparts.
In the current research, however, scientists ran a simulation of a grid on the Cloud Compute platform. of 7 × 8 qubits and depth of 40 layers. That only required 16TB of ram and estimated the costs at $ 35,184. Their technique is scalable to more complex circuits: they estimate that quantum circuit of 7 × 7 qubits with depth of 48 layers costs about one million dollars.
The depth concerns the number of layers in which a circuit can be divided without the gates for the qubits overlap, that is, how many logical operations can be implemented. The calculation that quantum circuits can perform is determined by grids of qubits in combination with that depth. The simulation of large numbers of qubits with limited depth on classical computer architecture has been possible for some time.
The results of the simulations, as the researchers suggest, are approximate, but that applies to real quantum circuits as well. Qubits, unlike ordinary bits, do not represent either the value 0 or a 1, but can assume a 0 and 1 at the same time. The number of different values increases exponentially with increasing the number of qubits. A quantum computer can thus perform parallel calculations at once and come to a definitive result immediately, for example by checking all outcomes at once, leaving only the right one. Scientists worldwide are experimenting with this data to develop a quantum computer that can be used for actual calculation work.
Space-time volume of a quantum circuit calculation. Source: Google