Graduate Student Solves Quantum Verification Problem

["Dave Farber" <farber () gmail com>]

Begin forwarded message:

> From: Dewayne Hendricks <dewayne () warpspeed com>
> Date: October 9, 2018 at 9:58:52 PM GMT+9
> To: Multiple recipients of Dewayne-Net <dewayne-net () warpspeed com>
> Subject: [Dewayne-Net] Graduate Student Solves Quantum Verification Problem
> Reply-To: dewayne-net () warpspeed com
>
> [Note:  This item comes from friend David Rosenthal.  DLH]
>
> Graduate Student Solves Quantum Verification Problem
> Urmila Mahadev spent eight years in graduate school solving one of the most basic questions in quantum computation: 
> How do you know whether a quantum computer has done anything quantum at all?
> By Erica Klarreich
> Oct 8 2018
> <https://www.quantamagazine.org/graduate-student-solves-quantum-verification-problem-20181008/>
>
> In the spring of 2017, Urmila Mahadev found herself in what most graduate students would consider a pretty sweet 
> position. She had just solved a major problem in quantum computation, the study of computers that derive their power 
> from the strange laws of quantum physics. Combined with her earlier papers, Mahadev’s new result, on what is called 
> blind computation, made it “clear she was a rising star,” said Scott Aaronson, a computer scientist at the University 
> of Texas, Austin.
>
> Mahadev, who was 28 at the time, was already in her seventh year of graduate school at the University of California, 
> Berkeley — long past the stage when most students become impatient to graduate. Now, finally, she had the makings of 
> a “very beautiful Ph.D. dissertation,” said Umesh Vazirani, her doctoral adviser at Berkeley.
>
> But Mahadev did not graduate that year. She didn’t even consider graduating. She wasn’t finished.
>
> For more than five years, she’d had a different research problem in her sights, one that Aaronson called “one of the 
> most basic questions you can ask in quantum computation.” Namely: If you ask a quantum computer to perform a 
> computation for you, how can you know whether it has really followed your instructions, or even done anything quantum 
> at all?
>
> This question may soon be far from academic. Before too many years have elapsed, researchers hope, quantum computers 
> may be able to offer exponential speedups on a host of problems, from modeling the behavior around a black hole to 
> simulating how a large protein folds up.
>
> But once a quantum computer can perform computations a classical computer can’t, how will we know if it has done them 
> correctly? If you distrust an ordinary computer, you can, in theory, scrutinize every step of its computations for 
> yourself. But quantum systems are fundamentally resistant to this kind of checking. For one thing, their inner 
> workings are incredibly complex: Writing down a description of the internal state of a computer with just a few 
> hundred quantum bits (or “qubits”) would require a hard drive larger than the entire visible universe.
>
> And even if you somehow had enough space to write down this description, there would be no way to get at it. The 
> inner state of a quantum computer is generally a superposition of many different non-quantum, “classical” states 
> (like Schrödinger’s cat, which is simultaneously dead and alive). But as soon as you measure a quantum state, it 
> collapses into just one of these classical states. Peer inside a 300-qubit quantum computer, and essentially all you 
> will see is 300 classical bits — zeros and ones — smiling blandly up at you.
>
> “A quantum computer is very powerful, but it’s also very secretive,” Vazirani said.
>
> Given these constraints, computer scientists have long wondered whether it is possible for a quantum computer to 
> provide any ironclad guarantee that it really has done what it claimed. “Is the interaction between the quantum and 
> the classical worlds strong enough so that a dialogue is possible?” asked Dorit Aharonov, a computer scientist at the 
> Hebrew University of Jerusalem.
>
> During her second year of graduate school, Mahadev became captivated by this problem, for reasons even she doesn’t 
> fully understand. In the years that followed, she tried one approach after another. “I’ve had a lot of moments where 
> I think I’m doing things right, and then they break, either very quickly or after a year,” she said.
>
> But she refused to give up. Mahadev displayed a level of sustained determination that Vazirani has never seen 
> matched. “Urmila is just absolutely extraordinary in this sense,” he said.
>
> [snip]
>
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