# Bristol boffins bring qubit computing a tiny step closer

Until now, quantum computing has suffered the same problem that vacuum tubes had in the 1950s: the hardware’s too damn big – a problem addressed by Bristol boffins who have put a reconfigurable two-qubit processor on a single chip. Just creating and manipulating a pair of qubits usually needs a tabletop’s worth of equipment, …

#### I am dead impressed.

I'm not sure how to estimate the number of orders of magnitude they have succeeded in shrinking this equipment's physical size by but it must be huge (so to speak). Just for laughs imagine Sammy marketing a 7 inch tab (Galaxy SXII. Marketing slogan: "Your quantum pocket".) based on this kind of tech in about 2030 (always assuming that Cupertino has stopped suing them by then of course -:P).

#### Vacuum Tubes

You mean prior to 1940? (a lot used in Bombs)

1939

http://www.radiomuseum.org/tubes/tube_ef39.html

1940

http://www.radiomuseum.org/tubes/tube_1t4.html

1950s (Russian) 8.5mm x 43mm

http://www.radiomuseum.org/tubes/tube_1j17b.html

(better versions in late 50s and 1960s. Family production till 1991)

1959 Usable as Triode 5.5mm x 28mm

http://www.radiomuseum.org/tubes/tube_dm160.html

Similar Russian 6.5 x 25mm

http://www.radiomuseum.org/tubes/tube_iv-15.html

1960

http://www.radiomuseum.org/tubes/tube_ck5676.html

(Family production till about 1971). Versions in Korean war

a similar size used in WWII bomb proximity fuses. One RAF raid would use up 250,000 sub-miniature tubes, a miniaturised version of 1T4 etc.

Regular mains Radio valves about 3W for the heater. By late 1950s sub-miniature military types only used 14mW for the filament heater and could use 22V HT.

Early post WWII hearing aids used valves based on the WWII sub-miniature types for proximity fuses and same size as a 1953 transistor. 0.7V filament to power two in series of 1.4V silver cell and HT a miniature 22.5V battery about same size as PP3

#### Curious..

I thought the issue was with the way it spits out data, in that it no longer becomes a processing speed and size issue, as much as a "Here are all the possible answers, now which one is the correct one?".

#### Not sure what gave you that idea

GIven that the number of incorrect answers for a given problem can be Rather Large a system that simply enumerates them would be a bit Bleedin Useless.

The problem is one of decoherency, as far as I'm aware, which has nothing to do with trying to some sort of bizarre brute force search on the results of a computation.

"Here are all the possible answers, now which one is the correct one?".

There are only 2 possible answers...

<=4

or

"A suffusion of yellow"

#### Better explanation?

So this, to me, is the kind of tech that makes quantum computing seem less like ivory tower bullshit and more like coming soon awesomeness.

But the field has moved far enough that the jargon used to simply explain what the hell this thing actually does is out of my league.

Has anyone found a nice article that goes further than reporting this, and tries to explain what exactly it can do; what all these various entangled qubit pairs and single mixed qubits are good for; and where the chip really places us w.r.t. the ability to construct a useful quantum computer that can, say, factorize a 4096-digit number?

I get the feeling this is kind of the "programmable logic cell" of quantum computing (like the 4-bit LUTs inside FPGAs). So it may be really straightforward to couple lots of these together and make something that works. But it would be awesome to get some detail on that and maybe a hint that the design rules from hereon might be simple enough for normal people (with math degrees) to grasp :)

#### Is the jargon really that bad?

Waveguide, phase shifter. Those aren't even quantum-specific concepts. Entanglement perhaps... but you can handwave that, right?

This might well end up being the building block of a more complex quantum photonic system in the future, but right now it is 'simply' something that entangles a pair of photons, performs some operation(s) on one of them, and then measures the results. The operations that are performed can be configured at experiment time.

It isn't a quantum processor. It isn't really even a component of a quantum computer. It doesn't have any 'use' outside of the domain of quantum computing research; no more than nailing a couple of bits of modified silicon together to create a nonlinear junction gives you a microprocessor, or even a logic gate.

#### Fermat theorem syndrome...

...where a question so simple has such a book-length answer: Can this thing compute extra-super-mind-boggles-fast, yes or no?

It won't matter if the chip is a bit large if it can replace 300 conventional current top-of-line processors.

#### I think size does matter

If each individual qubit-molester is a metre apart, that's a whole lot of space over which to transmit coherent quantum states, which means a whole lot of time in which coherence can be lost.

You're correct that if the damn thing works fine, size isn't much of an issue. But technical limits on our ability to handle quantum entanglement mean size is very much a barrier to getting the thing working,

#### The size doesn't really matter

(the way I see it) because an infinite number of these thingies can occupy the same snippet of the space-time continuum as the real one, whatever "real" is considered to mean in this context.

So you give the real processor, and its infinity of virtual counterparts, the question "Does A^n+B^n=C^n?." giving each one simultaneously (that's the clincher - "one at a time" is too slow) a different set of values for A, B, C and n, with n>2. If one (or more) of the processors answers YES then ipso facto Fermat's Last Theorem falls. Whether all the paper in the universe is sufficient to print out those crucial values of A, B, C and n (which every mathematician in the world would be agog to see) is another matter.

Unlike the case of Godel's machine which has two possible answers to every question: "YES" (within a computably finite time) or "Still working on it", the possible outcomes from the infinite array of Qubit processors are YES and NO, within a computably finite time. If the timeout expires with nobody saying YES then the answer is NO, and that's Fermat's Last Theorem proven.

#### But what does it meeeean?

What does all this mean to the average man in t'street? As a (very slightly) informed man on the street I understand the (very) basics of quantumy stuff, but I want to know when I can play Battlefield 29 in Super-Real-Quantum-o-vision powered by one of these things successors? This new processor might be physically big, but what can it *do*?

/sob

#### For the curious...

...yet uninitiated... there's a great little pop-science book out there called A Shortcut Through Time. Worth a read. It provides a suitable grounding so that you half-understand a third of what a quater of the article was on about.

#### Entangled photons...

Much easier to produce and control than entangled electrons, but as soon as we've worked out a way to produce entangled electrons easily then you'll have graphine chips and do the whole multi qubit shebang in a very small space, it's like the jump from transistors (electrics) to IC's (electronics) - but it's coming, and I'm excited.