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IBM Raises the Bar with a 50-Qubit Quantum Computer (technologyreview.com)
IBM said on Friday it has created a prototype 50 qubit quantum computer as it further increases the pressure on Google in the battle to commercialize quantum computing technology. The company is also making a 20-qubit system available through its cloud computing platform, it said. From a report: The announcement does not mean quantum computing is ready for common use. The system IBM has developed is still extremely finicky and challenging to use, as are those being built by others. In both the 50- and the 20-qubit systems, the quantum state is preserved for 90 microseconds -- a record for the industry, but still an extremely short period of time. Nonetheless, 50 qubits is a significant landmark in progress toward practical quantum computers. Other systems built so far have had limited capabilities and could perform only calculations that could also be done on a conventional supercomputer. A 50-qubit machine can do things that are extremely difficult to simulate without quantum technology. Whereas normal computers store information as either a 1 or a 0, quantum computers exploit two phenomena -- entanglement and superposition -- to process information differently.
We're getting closer and closer to testing quantum supremacy- the hypothesis that quantum computers can practically solve problems that classical computers cannot do https://en.wikipedia.org/wiki/Quantum_supremacy. Note that this is a practical statement; anything a quantum computer can do, a classical computer can do, but with potentially exponential slowdown. This follows from the fact that BQP https://en.wikipedia.org/wiki/BQP the set of problems that a quantum computer can do in polynomial time is within is contained in PSPACE https://en.wikipedia.org/wiki/PSPACE the set of things that a classical computer can do with polynomial space (since polynomial space calculations live in EXPTIME, the set of things requiring exponential time, the result follows).
It is very likely that before we see genuinely useful quantum computing (e.g. for factoring large numbers or simulating complicated chemical systems) we'll have an answer to the quantum supremacy question. I suspect that it is more likely that we'll have an answer in terms of boson sampling before we have an answer involving a universal quantum computer.
Essentially, boson sampling works by just looking at the distribution of bosons (well for convenience, photons) as they go through very simple optical objects. Boson sampling has two major advantages: first, we know it is actually *hard* in a technical sense for a classical computer to do unless some conjectures that pretty close to everyone believes are false. In particular, Scott Aaronson and Alex Arkipov proved that if a classical computer can do boson sampling efficiently then the polynomial hierarchy will collapse https://www.scottaaronson.com/papers/optics.pdf. For those who aren't theoretical compsci people, the polynomial hierarchy not collapsing is a statement which is only marginally stronger than P!=NP and is very widely believed. This is in contrast for example with factoring large numbers where if it turned out that classical computers could efficiently factor the only major conjecture that would turn out to be false would just be the difficulty of factoring itself. Second, boson sampling is much easier in many respects than what IBM is trying to do which requires much fancier systems, supercooled qubits, careful protection from stray particles, careful preservation of entanglement and all sorts of other stuff. Still, what they are doing is important and very necessary if we're going to actually have practical quantum computers ever.