German Scientists Create 5 qubit Quantum Register

CMan0 writes "In the University of Bonn, a team of scientists has built a 5 qubit register, using cesium atoms trapped by a laser-beam grid, The Register reports. They've been able to install an empty 5 bit register(i.e. all bits 0), change two of them to 1, and later read those 1s back. The next goal is to create an interaction between 2 bits. The full scientific article can be found here in PDF format."

Comments

"What's a qubit?",

[Anonymous Coward]

Noah inquired.

Re:"What's a qubit?",

[maxwell demon]

A qubit (for "quantum bit") is the basic information unit for a quantum computer (just as the classical bit is for classical computers). They are actually two-state quantum systems (just as ... Ok, I'm repeating myself :-))

The point is that quantum systems have properties which are not found in classical systems. For one, they cannot be just in the states "0" or "1" (in the usual notation for quantum states: |0> or |1>), but also in so called superpositions of those states. Such a superposition means that they are something like both states at the same time (remember Schrödinger's cat? That's exactly such a state, except that unlike atoms, cats cannot actually be brought into such a state). More importantly, such a superposition can extend over more than one qubit, in which case each single qubit doesn't have a defined quantum state at all, but only the whole set of qubit has. This is called entanglement.

Now, why is this so useful? Well, assume you create a set of, say, 8 qubits which are all "half zero, half one". And now you perform a normal calculation on them (but with quantum operations). Then you are actually performing the calculation on all 8-bit combinations, at the same time, i.e. for all numbers between 0 and 255. This remarkable effect is called quantum parallelism.

Now, of course there's a catch: You cannot read out more than one of the results (because reading out one destroys the superposition), and which one you get is essentially random. Ok, you now may think, I can effectively make the calculation just for one randomly selected number? So this is actually a disadvantage? Well, the point is that you can not just do "classical" calculations, but you can add operations which are not possible in classical computers. For example, there are several "half zero and half one" bit states, and you can do a quantum operation to convert one of them to |0> and one of them to |1>. Therefore you can extract properties of that result which depend not on just one of the results, but on several of them. And this allows you to actually reduce the numeric complexity of certain tasks. For example, you can search an unsorted database in O(sqrt(N)) time, instead of the classical O(N) time (N being the size of the database). The most famous algorithm is of course Shor's algorithm which allows factorizing large numbers in polynomial time, thus allowing to break public key encryption systems like PGP.

Now, there's not too much danger yet, since AFAIK the biggest number successfully factorized with a quantum computer is 15. But then, as long as 5 qubits are newsworthy, you cannot expect too much (imagine a message that someone managed to build a classical 5-bit computer!).

Re:"What's a qubit?",

[JohnFluxx]

Just to expand on this post, you can treat |0> and |1> as vectors. Well actually they are vectors.

So |0> is [1,0] and |1> is [0,1]

So a "superposition" is simply A*|0> + B*|1>
= [A,B]

Nothing particulary fancy or anything.

The analogy I used to explain it to my dad is this:

Imagine I have a light bulb, with a dimmer switch. I could set this to a dimmer switch to anything in between on and off. Theoritically I could store an infinite amount of information in the dimmer switch. Imagine I took a large book, converted it to hex, and turned that into one long number. Then I prepended 0. to the front.

So you get "0.1939434....". Then I set the dimmer switch to that exact value.

But, if I want to look at the light, for some reason, I can only see if it's on or off. The chance I see it as being on is the same as the dimmer switch setting. (So if it's set to 0.5, then I have a 0.5 chance of seeing it as on, and 0.5 chance of seeing it's off).
I'm stretching this analogy a bit, but you can see that despite storing anything I want, I can still only read it as on or off.

So.. how do we use this usefully? We don't really know many practical uses, but what you can do is do calculations.
Say you put two of these lights in a room. Both are set to 0.5 brightness. With the case of the lights, the total brightness is now 1. So we've gone from having probability, to something definite. You are always going to see that as being on.

The analogy doesn't quite fit, but you can see how you can use the underlying probability to do calculations and get a definite answer.

First quantum OS

[Chuns]

Should the first quantum OS be M$ or Linux? :) I like to watch people argue about OS's. Makes me smile.

Re:First quantum OS

[fstrauss]

It'll be both until you boot it.

Re:First quantum OS

[cyfer2000]

Are we still going to use mouse? Or a cat with two buttons labeled as "alive" and "dead"?

And God said....

[GillBates0]

Let there be light, and there was "1".

Re:And God said....

[metlin]

Actually, there was uncertainty.

Upon further observation, it was known to have a probability of 1 ;)

On a serious note, this is awesome. With a 5 qubit entanglement and this, we might be able to build a primitive functional Quantum Computer, for the first time.

The team is now working to create a quantum gate in which two or more qubits of the register will interact in a controlled way.

Amazing. The beginnings of a first QC. We've memory, redundancy, processing capabilities and a lot more.

Now the only problem that remains is a suitable and reliable means of error correction - which is the biggest problem thus far in QC :-(

Re:And God said....

[metlin]

So, we're what 10 - 20 years away from a QC that both gives you your answer and blue screens at the same time?

Atleast.

I would say maybe 50. It's not enough if you can get a system to do something - you need to make it reliable and scaleable.

We're still tackling the very basic problems in QC, and have a very very long way to go. Error correction is still a very big problem.

Some people, such as Alexei Kitaev, have done some pioneering work but it's still in its infancy. A long long way to go.

Older News.

[modifried]

This was covered on New Scientist and IndiaTimes a few days ago. Their articles:

-New Scientist
-IndiaTimes

Bill gates sez:

[Anonymous Coward]

"a 5 qubit register should be enough for anyone"

Cesium and Laser Beams

[blankman]

So in ten years I'll have to wear a lead apron and protective glasses when I turn on my new computer? New fashion trend for geeks that never shut their boxes off.

Re:Caesium

[johannesg]

Caesium atoms are quite cheap, especially when you need several of them. They are just looking forward to a future scenario where they might need to invest in 10 or even 15 atoms.

Not quite there!

[bWareiWare.co.uk]

Whilst I am sure this is a step forward there must still be a big step between creating a 5-qubit register and a 5-qubit entangled register. With what they have created can only do the same as a five bit digital computer, with the second you could <insert you favourite quantum hyperbole here>.

Re:Caesium

[metlin]

Several reasons - it's heavy, easier to be made neutral and easier to be trapped in a wave dipole trap (that's what they seem to be using). In a standing wave dipole trap, the first factor especially plays an important role in sustaining stability.

Plus, they've a discernible signature even in a spatially modulated environment and that helps.

Re:Quantum register vs IBM quantum "computer" ?

[qcomp]

could someone please explain in what way this is more interesting than what was achieved by IBM about 3 years ago?

NMR quantum computing as demonstrated by IBM has many drawbacks.
First, there's not a single quantum system doing the computation, but rather some 10^20 molecules in the liquid - and you need so many to generate a detectable signal.
Second, the NMR quantum register cannot be properly initialized, rather it is in a nearly random state with only a slight enhancement of "0" over "1". This is part of the reason why so many systems are needed and it prevents the currently realized systmes from displaying any entanglement.
Finally, it is not clear how to scale such a system (increase the number of nuclear spins on a molecule): the larger that number, the more difficult it is to address individual qubits.

For these reasons, liquid state NMR is not be considered to be scalable. Nevertheless, the NMR people have amazing control over the operations (logic gates) they can perform, and these ideas may (and have) fed back to other implementations. Moreover, there are attempts to overcome the mentioned difficulties (while keeping some advantages of NMR) by using nuclear spins in cold solids following Kane's proposal).

Re:Caesium

[Splab]

Strange, the words look english - but put together it makes no sence at all :)

Re:Caesium, cheap!

[Eccles]

I got 5 caesium atoms free from Ron Popeil when I ordered my Showtime Rotisserie!

It's not real caesium, though! It's qubic zirconium...

QC as a PC

[like.narly]

Unfortuantely, the way scientists see it now, we'll probably never have a desktop quantum computer (or at least for a very long time). The problem is that the interaction takes place in an extremely controlled environment. Granted, the first analog computers were large, but that's because solid state wasn't really around yet. The "parts" of most QC's are acctually on the nanometer scale.

For example, one qbit setup is to use a helium superfluid, which naturally bonds electrons to the surface. The bound electrons can then be controled with a combination of microwave radiation and an electric potential from wafer posts under the fluid. Each electron (qbit) sits on top of a post, which are spaced just a few nm apart. The system is still being developed, but the nice thing is once they get it to work, they can just build a large wafer holding millions of qbits.

However, the huge problem with the above example is that it needs to run at about 50 mK, which is very close to absolute zero and requires a dilution fridge, which is a 6 foot tall cylinder. There are similar (though more complicated) limitations to the laser trapping methods.

For a commercial unit I suppose the QC wafer, microwave source, and dilution fridge could be packaged together nicely, but it is still 6 feet tall, heavy, not well suited for a house. Even if it were possible to make one small enough, there are currently no real benefits for a home user unless they really wanted to find elements in a large array or crack PGP codes... I suppose the first computers were also only suited for a lab environment and scientists probably thought the average person would never need a computer either, so who knows what will develop in the next 50 years...

Analysis & request for help

[tbo]

Disclaimer: I'm a graduate student doing research on quantum computing in optical lattices. I'm not affiliated with the group that published this article.

This result is quite exciting, because it demonstrates the feasibility of some of the techniques necessary for an optical lattice-based quantum computer. Imagine taking their 1-D lattice and turning it into a 3-D lattice, with about 30 atoms in each direction. That's a whole lot of qubits...

So what are the next steps?
1) A new means of addressing atoms (selecting one or two atoms on which to perform operations while excluding the rest) is necessary. Their magnetic gradient technique works fine for a small 1-D lattice, but it would likely be impractical for a large 3-D lattice (Maxwell's equation div B = 0 gives one major obstacle, which would require fancy tricks to overcome).
2) One and two-qubit gates need to be demonstrated using an appropriate addressing scheme.
3) Error correction, which would likely require quantum non-demolition measurements to check to see if an atom had been lost from a lattice site. Translation: we need to be able to measure if we've lost an atom from a lattice site, without disturbing the atom's state (i.e. measuring whether it's |0> or |1>).
4) Full-blown fault-tolerant computation.

My group plans to solve (1) using an addressable optical lattice. What that means is that the lattice spacing is sufficiently large that a laser can be focused on an individual atom (in 3-D, two lasers in orthogonal directions would be used). I'm currently doing simulations of one-qubit gates in this scheme.

That brings me to my question for slashdot: Some of the simulations I'll be doing (specifically, studying decoherence in the one and two qubit gates) will be very computationally intensive. They're also embarrassingly parallel, as they're essentially quantum Monte Carlo simulations. Would people be interested in a BOINC-based distributed computing project (a la SETI@home) to help develop quantum computers? If so, what kinds of things would help you get involved? Would you be interested in helping develop the software (it's C++)?

I probably won't be at that stage for another six months to a year, but it would be helpful to me to start planning now. I have just (last night) completed the core simulation engine, and would need to add code for decoherence, as well as adapt it to BOINC. The code will be GPL'd, of course.