This is largely why, even though quantum computing was first proposed by Richard Feynman in 1982 and the theory was worked out in the early 1990s, it has taken until now to make devices that can actually perform a meaningful computation. That challenge gets ever greater as the number of qubits - and hence the potential to interact with the environment - increases. Researchers seeking to build quantum computers must stave off decoherence, which they can currently do only for a fraction of a second. Interactions of a system of quantum-coherent entities with their surrounding environment create channels through which the coherence rapidly “leaks out” in a process called decoherence. To carry out a quantum computation, you need to keep all your qubits coherent. As quantum theorist Daniel Gottesman of the Perimeter Institute in Waterloo, Canada, put it, “If you have enough quantum mechanics available, in some sense, then you have speedup, and if not, you don’t.” Perhaps the safest way to describe quantum computing is to say that quantum mechanics somehow creates a “resource” for computation that is unavailable to classical devices. The equations of quantum theory certainly show that it will work: that, at least for some classes of computation such as factorization or database searches, there is tremendous speedup of the calculation.
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It’s hard to say qualitatively why quantum computing is so powerful precisely because it is hard to specify what quantum mechanics means at all. (Indeed, a strong degree of qubit entanglement isn’t essential.) There’s an element of truth in those descriptions - some of the time - but none captures the essence of quantum computing. Nor have I said that entanglement permits many calculations to be carried out in parallel. Note that I’ve not said - as it often is said - that a quantum computer has an advantage because the availability of superpositions hugely increases the number of states it can encode, relative to classical bits. This is why the difference between a 5-qubit and a 50-qubit machine is so significant. The computational resources increase in simple proportion to the number of bits for a classical device, but adding an extra qubit potentially doubles the resources of a quantum computer. This means that somehow computational operations on qubits count for more than they do for classical bits. That way, a tweak to one qubit may influence all the others. To perform a computation with many such qubits, they must all be sustained in interdependent superpositions of states - a “quantum-coherent” state, in which the qubits are said to be entangled. Quantum bits do the same, except that they may be placed in a so-called superposition of the states 1 and 0, which means that a measurement of the qubit’s state could elicit the answer 1 or 0 with some well-defined probability. Classical computers encode and manipulate information as strings of binary digits - 1 or 0. The basic story has been told many times, though not always with the nuance that quantum mechanics demands. There’s still everything to play for and no guarantee of reaching the big goal.īoth the benefits and the challenges of quantum computing are inherent in the physics that permits it. The fundamental physics of quantum computing is far from solved and can’t be readily disentangled from its implementation.Įven if we soon pass the quantum supremacy milestone, the next year or two might be the real crunch time for whether quantum computers will revolutionize computing.
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It would be tempting to conclude from all this that the basic problems are solved in principle and the path to a future of ubiquitous quantum computing is now just a matter of engineering. (When pressed for an update, a spokesperson recently said that “we hope to announce results as soon as we can, but we’re going through all the detailed work to ensure we have a solid result before we announce.”) Midway through 2017, researchers at Google announced that they hoped to have demonstrated quantum supremacy by the end of the year. Fifty qubits has long been considered the approximate number at which quantum computing becomes capable of calculations that would take an unfeasibly long time classically. But the whole point of quantum computing is that a quantum bit counts for much, much more than a classical bit. That might sound absurd when you compare the bare numbers: 50 qubits versus the billions of classical bits in your laptop. There is now talk of impending “quantum supremacy”: the moment when a quantum computer can carry out a task beyond the means of today’s best classical supercomputers.