On Friday, November 1, 2019 at 3:25:21 PM UTC-6, John Clark wrote: > > Quantum Computer expert Scott Aaronson wrote a editorial in the October > 30 2019 New York Times: > > Why Google’s Quantum Supremacy Milestone Matters > <https://www.nytimes.com/2019/10/30/opinion/google-quantum-computer-sycamore.html> > > > * Why Google’s Quantum Supremacy Milestone Matters* > * By Scott Aaronson* > > Google officially announced last week in the journal Nature that it > achieved the milestone of “quantum supremacy.” This phrase, coined by the > physicist John Preskill in 2012, refers to the first use of a quantum > computer to make a calculation much faster than we know how to do it with > even the fastest supercomputers available. The calculation doesn’t need to > be useful: much like the Wright Flyer in 1903, or Enrico Fermi’s nuclear > chain reaction in 1942, it only needs to prove a point. > > Over the last decade, together with students and colleagues, I helped > develop much of the theoretical underpinning for quantum supremacy > experiments like Google’s. I reviewed Google’s paper before it was > published. So the least I can do is to try to explain what it means. > > Until recently, every computer on the planet — from a 1960s mainframe to > your iPhone, and even inventions as superficially exotic as “neuromorphic > computers” and DNA computers — has operated on the same rules. These were > rules that Charles Babbage understood in the 1830s and that Alan Turing > codified in the 1930s. Through the course of the computer revolution, all > that has changed at the lowest level are the numbers: speed, amount of RAM > and hard disk, number of parallel processors. > > But quantum computing is different. It’s the first computing paradigm > since Turing that’s expected to change the fundamental scaling behavior of > algorithms, making certain tasks feasible that had previously been > exponentially hard. Of these, the most famous examples are simulating > quantum physics and chemistry, and breaking much of the encryption that > currently secures the internet. > > In my view, the Google demonstration was a critical milestone on the way > to this vision. At a lab in Santa Barbara, Calif., a Google team led by > John Martinis built a microchip called “Sycamore,” which uses 53 loops of > wire around which current can flow at two different energies, representing > a 0 or a 1. The chip is placed into a dilution refrigerator the size of a > closet, which cools the wires to a hundredth of a degree above absolute > zero, causing them to superconduct. For a moment — a few tens of millionths > of a second — this makes the energy levels behave as quantum bits or > “qubits,” entities that can be in so-called superpositions of the 0 and 1 > states. > > This is the part that’s famously hard to explain. Many writers fall back > on boilerplate that makes physicists howl in agony: “imagine a qubit as > just a bit that can be both 0 and 1 at the same time, exploring both > possibilities simultaneously.” If I had room for the honest version, I’d > tell you all about amplitudes, the central concept of quantum mechanics > since Werner Heisenberg, Erwin Schrödinger and others discovered it in the > 1920s. > > Here’s a short version: In everyday life, the probability of an event can > range only from 0 percent to 100 percent (there’s a reason you never hear > about a negative 30 percent chance of rain). But the building blocks of the > world, like electrons and photons, obey different, alien rules of > probability, involving numbers — the amplitudes — that can be positive, > negative, or even complex (involving the square root of -1). Furthermore, > if an event — say, a photon hitting a certain spot on a screen — could > happen one way with positive amplitude and another way with negative > amplitude, the two possibilities can cancel, so that the total amplitude is > zero and the event never happens at all. This is “quantum interference,” > and is behind everything else you’ve ever heard about the weirdness of the > quantum world. > > Now, a qubit is just a bit that has some amplitude for being 0 and some > other amplitude for being 1. If you look at the qubit, you force it to > decide, randomly, whether to “collapse” to 0 or 1. But if you don’t look, > the two amplitudes can undergo interference — producing effects that depend > on both amplitudes, and that you can’t explain by the qubit’s having been 0 > or by its having been 1. > > Crucially, if you have, say, a thousand qubits, and they can interact (to > form so-called “entangled” states), the rules of quantum mechanics are > unequivocal that you need an amplitude for every possible configuration of > all thousand bits. That’s 2 to the 1,000 amplitudes, much more than the > number of atoms in the observable universe. If you have a mere 53 qubits, > as in Google’s Sycamore chip, that’s still 2 to the 53 amplitudes, or about > 9 quadrillion. > > The goal, with Google’s quantum supremacy experiment, was to perform a > contrived calculation involving 53 qubits that computer scientists could be > as confident as possible really would take something like 9 quadrillion > steps to simulate with a conventional computer. The qubits in Sycamore are > laid out in a roughly rectangular grid, with each qubit able to interact > with its neighbors. Control signals, sent by wire from classical computers > outside the dilution refrigerator, tell each qubit how to behave, including > which of its neighbors to interact with and when. > > In other words, the device is fully programmable — that’s why it’s called > a “computer.” At the end, the qubits are all measured, yielding a random > string of 53 bits. Whatever sequence of interactions was used to produce > that string — in the case of Google’s experiment, the interactions were > simply picked at random — you can then rerun the exact same sequence again, > to sample another random 53-bit string in exactly the same way, and so on > as often as desired. > > In its Nature paper, Google estimated that its sampling calculation — the > one that takes 3 minutes and 20 seconds on Sycamore — would take 10,000 > years for 100,000 conventional computers, running the fastest algorithms > currently known. Indeed the task was so hard, Google said, that even > directly verifying the full range of the results on classical computers was > out of reach for its team. Thus, to check the quantum computer’s work in > the hardest cases, Google relied on plausible extrapolations from easier > cases. > > IBM, which has built its own 53-qubit processor, posted a rebuttal. The > company estimated that it could simulate Google’s device in a mere 2.5 > days, a millionfold improvement over Google’s 10,000 years. To do so, it > said, it would only need to commandeer the Oak Ridge Summit, the largest > supercomputer that currently exists on earth — which IBM installed last > year at Oak Ridge National Laboratory, filling an area the size of two > basketball courts. (And which Google used for some of its simulations in > verifying the Sycamore results.) Using this supercomputer’s eye-popping 250 > petabytes of hard disk space, IBM says it could explicitly write down all 9 > quadrillion of the amplitudes. Tellingly, not even IBM thinks the > simulation would be especially easy — nor, as of this writing, has IBM > actually carried it out. (The Oak Ridge supercomputer isn’t just sitting > around waiting for jobs.) > > We’re now in an era where, with heroic effort, the biggest supercomputers > on earth can still maybe, almost simulate quantum computers doing their > thing. But the very fact that the race is close today suggests that it > won’t remain close for long. If Google’s chip had used 60 qubits rather > than 53, then simulating its results with IBM’s approach would require 30 > Oak Ridge supercomputers. With 70 qubits, it would require enough > supercomputers to fill a city. And so on. > > Is there real science behind the spectacle of these two tech titans > locking antlers? Is “quantum supremacy,” divorced from practical > applications, an important milestone at all? When should we expect those > practical applications, anyway? Assuming Google has achieved quantum > supremacy, what exactly has it proved — and is it something anyone doubted > in the first place? > > Let’s start with applications. A protocol that I came up with a couple > years ago uses a sampling process, just like in Google’s quantum supremacy > experiment, to generate random bits. While by itself that’s unimpressive, > the key is that these bits can be demonstrated to be random even to a > faraway skeptic, by using the telltale biases that come from quantum > interference. Trusted random bits are needed for various cryptographic > applications, such as proof-of-stake cryptocurrencies (environmentally > friendlier alternatives to Bitcoin). Google is now working toward > demonstrating my protocol; it bought the non-exclusive intellectual > property rights last year. > > Other applications will almost certainly require more qubits, and of a > higher quality — things that Google, IBM and the other players are racing > to build. One major milestone to watch for next will be the first use of > small quantum computers to simulate the quantum physics of chemicals and > materials in a way that’s actually useful to chemists and materials > scientists. Simulating quantum mechanics — that is, overcoming the > exponential explosion of amplitudes in nature via a computer equipped with > the same power — was the original application that the physicist Richard > Feynman envisioned when he proposed the idea of a quantum computer in the > early 1980s. It’s still the most important application we know — one that > could aid in the design of everything from batteries and solar cells to > fertilizers and lifesaving drugs. > > An even bigger milestone will be the first practical demonstration of > quantum error correction — a technology that, in theory, should be able to > keep qubits alive for vastly longer amounts of time by cleverly encoding > them across many physical qubits. Quantum computing researchers think that > quantum error correction is what will ultimately let quantum computers > scale beyond a couple hundred qubits, to the million- or billion-qubit > machines that would fully realize Feynman’s dream. But this hasn’t been > demonstrated yet, and no one knows when it will be. > > In the meantime, Google’s demonstration is a crucial proof of concept. > Building a quantum computer is so hard that, ever since serious efforts > began in the mid-1990s, some distinguished scientists have argued that the > task would be impossible. Qubits, they said, will always prove too fragile > to control. If quantum mechanics seems to predict that you can harness an > exponential number of amplitudes for computation, then so much the worse > for our present understanding of quantum mechanics. > > Google’s demonstration should give these skeptics pause. To all > appearances, a 53-qubit device really was able to harness 9 quadrillion > amplitudes for computation, surpassing (albeit for a special, useless task) > all the supercomputers on earth. Quantum mechanics worked: an outcome > that’s at once expected and mind-boggling, conservative and radical. > > The computer revolution was enabled, in large part, by a single invention: > the transistor. Before transistors, we were stuck with failure-prone vacuum > tubes. Yet vacuum tubes kind of, sort of worked: they translated abstract > Boolean logic into electrical signals reliably enough to be useful. We > don’t yet have the quantum computing version of the transistor — that would > be quantum error correction. Getting there will surely require immense > engineering, and probably further insights as well. In the meantime, > though, the significance of Google’s quantum supremacy demonstration is > this: after a quarter century of effort, we are now, finally, in the early > vacuum tube era of quantum computing. > > *Scott Aaronson is David J. Bruton Centennial Professor of Computer > Science at the University of Texas at Austin, and the founding director of > UT’s Quantum Information Center. He’s the author of “Quantum Computing > Since Democritus,” and blogs about quantum computing and other topics at > Shtetl-Optimized.* > > The thing I don't get is the role of superposition. If a Qbit is neither zero or one, but a superposition of both, how can it be useful to calculate anything? AG
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