### Why Is Quantum Computing So Hard?

Blog post (and two papers) by Ross Anderson and Robert Brady. News article.

Note that I do not have the physics to evaluate these claims.

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Blog post (and two papers) by Ross Anderson and Robert Brady. News article.

Note that I do not have the physics to evaluate these claims.

It seems that quantum computers might use superconducting quantum interference devices (SQUIDs).

As usual, you can also use this squid post to talk about the security stories in the news that I haven’t covered.

Location-based encryption—a system by which only a recipient in a specific location can decrypt the message—fails because location can be spoofed. Now a group of researchers has solved the problem in a quantum cryptography setting:

The research group has recently shown that if one sends quantum bits—the quantum equivalent of a bit—instead of only classical bits, a secure protocol can be obtained such that the location of a device cannot be spoofed. This, in turn, leads to a key-exchange protocol based solely on location.

The core idea behind the protocol is the “no-cloning” principle of quantum mechanics. By making a device give the responses of random challenges to several verifiers, the protocol ensures that multiple colluding devices cannot falsely prove any location. This is because an adversarial device can either store the quantum state of the challenge or send it to a colluding adversary, but not both.

Don’t expect this in a product anytime soon. Quantum cryptography is mostly theoretical and almost entirely laboratory-only. But as research, it’s great stuff. Paper here.

Not that we need more ways to get random numbers, but the research is interesting.

This is an important development:

Shor’s algorithm was first demonstrated in a computing system based on nuclear magnetic resonance—manipulating molecules in a solution with strong magnetic fields. It was later demonstrated with quantum optical methods but with the use of bulk components like mirrors and beam splitters that take up an unwieldy area of several square meters.

Last year, the Bristol researchers showed they could miniaturize this optical setup, building a quantum photonic circuit on a silicon chip mere millimeters square. They replaced mirrors and beam splitters with waveguides that weave their way around the chip and interact to split, reflect, and transmit light through the circuit. They then injected photons into the waveguides to act as their qubits.

Now they’ve put their circuit to work: Using four photons that pass through a sequence of quantum logic gates, the optical circuit helped find the prime factors of the number 15. While the researchers showed that it was possible to solve for the factors, the chip itself didn’t just spit out 5 and 3. Instead, it came up with an answer to the “order-finding routine,” the “computationally hard” part of Shor’s algorithm that requires a quantum calculation to solve the problem in a reasonable amount of time, according to Jeremy O’Brien, a professor of physics and electrical engineering at the University of Bristol. The researchers then finished the computation using an ordinary computer to finally come up with the correct factors.

I’ve written about quantum computing (and quantum cryptography):

Several groups are working on designing and building a quantum computer, which is fundamentally different from a classical computer. If one were built—and we’re talking science fiction here—then it could factor numbers and solve discrete-logarithm problems very quickly. In other words, it could break all of our commonly used public-key algorithms. For symmetric cryptography it’s not that dire: A quantum computer would effectively halve the key length, so that a 256-bit key would be only as secure as a 128-bit key today. Pretty serious stuff, but years away from being practical.

Here’s a really good essay on the realities of quantum computing and its potential effects on public-key cryptography.

Really good blog post on the future potential of quantum computing and its effects on cryptography:

To factor a 4096-bit number, you need 72*4096^3 or 4,947,802,324,992 quantum gates. Lets just round that up to an even 5 trillion. Five trillion is a big number. We’re only now getting to the point that we can put about that many normal bits on a disk drive. The first thing this tells me is that we aren’t going to wake up one day and find out that someone’s put that many q-gates on something you can buy from Fry’s from a white-box Taiwanese special.

Singapore is setting up a $98M research center for quantum computation.

Great news, but what in the world does this quote mean?

Professor Artur Ekert, Director, Research Centre of Excellence, said: “At the moment, you can buy quantum cryptography systems, you can use it in some simple applications but somehow you have to trust companies that sell it to you or you have to test the equipment.

“The kind of quantum cryptography we develop here is probably the most sophisticated that is not available in any other countries so we have some ideas to make it so secure that you don’t even have to trust equipment that you could buy from a vendor.”

You don’t even have to turn it on:

With the right set-up, the theory suggested, the computer would sometimes get an answer out of the computer even though the program did not run. And now researchers from the University of Illinois at Urbana-Champaign have improved on the original design and built a non-running quantum computer that really works.

So now, even turning the machine off won’t necessarily prevent hackers from stealing passwords.

And as long as we’re on the topic of quantum computing, here’s a piece of quantum snake oil:

A University of Toronto professor says he can now use a photon of light to smash through the most sophisticated computer theft schemes that hackers can devise.

EDITED TO ADD (3/1): More information about the University of Illinois result is here.

Sidebar photo of Bruce Schneier by Joe MacInnis.