Researchers have set a new world record in simulating quantum computing power on a classical computer.
If a quantum computer were a racing car it wouldn’t so much speed past a Formula One, it would simply take a private shortcut to appear at the finishing line just after the starting gun fires. And if you walked over to look under the hood to see how it worked, the engine would promptly collapse to just one random component, like a sparkplug.
This is the weirdness of the quantum world where the normal laws of physics at the atomic level become, as Einstein put it, “spooky.”
A quantum computer exploits quantum physics to rapidly uncover the right answer to a problem by sifting through and adjusting probabilities, while a classical computer will be burning up memory and time looking at each potential answer in turn.
The research means scientists have a powerful new simulation tool to capture and understand the quantum state and develop quantum-computing software. Ultimately, it will help us understand and test the sorts of problems an eventually scaled-up quantum computer will be used for, as scientists develop quantum hardware over the next decade or so.
“The capability to simulate quantum algorithms at this level is important to learning how a quantum computer will physically operate, how the software can work, and what sort of problems it can solve,” says Lloyd Hollenberg, who leads the team and is deputy director of the Centre for Quantum Computation and Communication Technology at the University of Melbourne.
Classical computers work by programming bits, the most basic form of data. Bits are binary, being either 0 or 1 and are programmed to encode and process data. But in a quantum computer the bits, or qubits, are quantum mechanical objects like atoms.
Quantum states can also be binary and can be put in one of two possibilities, or effectively both at the same time. Quantum superposition means that two qubits can, in a sense, be all four combinations of 0 and 1 at the same time.
That unique data crunching power is further boosted by entanglement, where the state of one qubit when measured mysteriously dictates the state of another qubit.
Simulating qubits and their quantum processes, or “programs”, on a classical computer is a key step in understanding how an eventually scaled up and useful quantum computer will actually work.
The problem is that using conventional techniques to simulate an arbitrary quantum process that is significantly larger than any of the existing quantum prototypes would soon require what Hollenberg describes as “planetary scale” memory on a classical computer.
It would take a classical supercomputer more than the entire life of the universe to crack some of the security codes now in use.
To give you an idea of the huge memory capability of quantum computing, one of the largest prototypes, IBM’s new 50 qubit machine, could in principle simultaneously represent about a million billion number combinations.
To simulate a random quantum state the machine would chew up some 18 petabytes of classical computer memory, or the equivalent of more than a million 16 gigabyte RAM laptops. Researchers at IBM have so far been able to classically simulate 56 qubits in carefully chosen states.
But Hollenberg’s team have gone well beyond that and simulated the output of a 60-qubit machine for which representing the whole quantum space of numbers would have required some 18,000 petabytes, or over a billion laptops—well beyond the largest supercomputer.