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# Quantum Computing with Ions [Re-Post]

Researchers are taking the first steps toward building ultrapowerful computers that use individual atoms to perform calculations

To make a conditional logic gate between two ion qubits, we require a coupling between them—in other words, we need them to talk to each other. Because both qubits are positively charged, their motion is strongly coupled electrically through a phenomenon known as mutual coulomb repulsion. In 1995 Juan Ignacio Cirac and Peter Zoller, both then at the University of Innsbruck in Austria, proposed a way to use this coulomb interaction to couple indirectly the internal states of the two ion qubits and realize a CNOT gate. A brief explanation of a variant on their gate goes as follows.

First, think about two marbles in a bowl. Assume that the marbles are charged and repel each other. Both marbles want to settle at the bottom of the bowl, but the coulomb repulsion causes them to come to rest on opposite sides, each a bit up the slope. In this state, the marbles would tend to move in tandem: they could, for instance, oscillate back and forth in the bowl along their direction of alignment while preserving the separation distance between them. A pair of qubits in an ion trap would also experience this common motion, jiggling back and forth like two pendulum weights connected by a spring. Researchers can excite the common motion by applying photon pressure from a laser beam modulated at the natural oscillation frequency of the trap.

More important, the laser beam can be made to affect the ion only if its magnetic orientation is up, which here corresponds to a qubit value of 1. What is more, these microscopic bar magnets rotate their orientation while they are oscillating in space, and the amount of rotation depends on whether one or both of the ions are in the 1 state. The net result is that if we apply a specific laser force to the ions for a carefully adjusted duration, we can create a CNOT gate. When the qubits are initialized in superposition states, the action of this gate entangles the ions, making it a fundamental operation for the construction of an arbitrary quantum computation among many ions.

Researchers at several laboratories—including groups at the University of Innsbruck, the University of Michigan at Ann Arbor, the National Institute of Standards and Technology (NIST) and the University of Oxford—have demonstrated working CNOT gates. Of course, none of the gates works perfectly, because they are limited by such things as laser-intensity fluctuations and noisy ambient electric fields, which compromise the integrity of the ions’ laser-excited motions. Currently researchers can make a two-qubit gate that operates with a “fidelity” of slightly above 99 percent, meaning that the probability of the gate operating in error is less than 1 percent. But a useful quantum computer may need to achieve a fidelity of about 99.99 percent for error-correction techniques to work properly. One of the main tasks of all trapped-ion research groups is to reduce the background noise enough to reach these goals, and although this effort will be daunting, nothing fundamental stands in the way of its achievement.

Ion Highways
But can researchers really make a full-fledged quantum computer out of trapped ions? Unfortunately, it appears that longer strings of ions—those containing more than about 20 qubits—would be nearly impossible to control because their many collective modes of common motion would interfere with one another. So scientists have begun to explore the idea of dividing the quantum hardware into manageable chunks, performing calculations with short chains of ions that could be shuttled from place to place on the quantum computer chip. Electric forces can move the ion strings without disturbing their internal states, hence preserving the data they carry. And researchers could entangle one string with another to transfer data and perform processing tasks that require the action of many logic gates. The resulting architecture would somewhat resemble the familiar charge-coupled device (CCD) used in digital cameras; just as a CCD can move electric charge across an array of capacitors, a quantum chip could propel strings of individual ions through a grid of linear traps.

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1. 1. zetalimit 07:31 AM 8/11/08

I^mz .... I = infinite ^ angle of mz = largest and smalles measurement programmed ga programming

2. 2. Quasimodo 09:27 AM 8/11/08

Is the editor on vacation?
"...technological advances have dramatically boosted the..." Is that the beginning of the sentence? If so, why isn't the 't' capitalized??
Are we just throwing out the basic grammar rule book now? Shame on Sci Am. Don't publish if you aren't doing to do it right.

3. 3. SciMike in reply to Quasimodo 11:34 AM 8/11/08

Quasimodo-Were you picked on a lot as a kid? The article is concise and informative. Get over yourself and take the benefit from this article that it most definitely offers. Besides I do not know what article you read but the capitalization and punctuation is fine. The sentence starts:

"Over the past several decades technological advances have dramatically boosted the speed and reliability of computers."

So you have made an even bigger fool of yourself than I first assumed.

4. 4. Fabrice LOTY 03:53 PM 8/11/08

**The field of quantum information science promises to radically change the rules of computing.** As scientists still work on small size quantum gates, they cannot afford arguing about new computing rules at present. Designing the new rules simply belongs to another realm of understanding. Similarly, what evolutionism could prove so far is so inexistent that its early allegations about origin of life are boldly presumptuous. So far, origin of life is not a scientific debate.

5. 5. Quasimodo in reply to SciMike 05:21 PM 8/11/08

SciMike,

Professionals write professionally, got it? Too bad if you don't. You answered my question with a question. That's pretty unprofessional.
Either answer my question, or hush up. My question had Nothing to do with the actual facts put forth in the body of the article. Grow up and smell the decade.

6. 6. Quantum dot 06:40 AM 8/12/08

I have a question, if anyone here can help me out on this. Considering qubits that are based on the magnetic orientation of a trapped ion, as I understand it, in order to make a particle (say a proton) "flip" in a magnetic field from one orientation to another, we need to apply an amount of energy (by using a wave of particular frequency) that is exactly equivalent to the energy gap between the two spin states (orientations). So, how exactly is it possible to make the particle be in the superposition state? Do we apply half the energy or twice as much (which doesn't make sense), or what exactly do we do? I guess this is the whole point of quantum mechanics...but I dont really get it.

7. 7. Quantum dot 06:47 AM 8/12/08

Oh yeah, and something else regarding logic gates....maybe I misunderstood, but how is it possible to form classic logic gates like AND and OR gates using qubits? I mean, these gates are based on determinable deductive logic, but in quantum mechanics, there is always uncertainty (heisenberg's principle) and logic is based on probability (am I right?), so how is that possible?

8. 8. supraquanta 06:58 AM 8/12/08

It is of certain intrigue how the evolutionary procedure of this technological branch would reach the ironic imminence of its possible outcomes, which will set forward a whole new perspective on the computational processes within modern electronics. But considering the constant repression the elite has over the introduction of ideas that can potentially compromise their agenda, this technology would still be far from reaching consumer applications.

9. 9. kevino819 in reply to Quantum dot 11:43 AM 8/12/08

Oh yeah, and something else regarding logic gates....maybe I misunderstood, but how is it possible to form classic logic gates like AND and OR gates using qubits? I mean, these gates are based on determinable deductive logic, but in quantum mechanics, there is always uncertainty (heisenberg's principle) and logic is based on probability (am I right?), so how is that possible?

Quantum dot: Are you trying to understand how the gates work or are you just asking how classical gates can be used at all? If you are asking how a classical logic gate works in this situation I will explain that. The logic behind the gates (as I see it) works on a basic principle of quantum mechanics: when you measure something, it changes. Uncertainty doesn't apply in this instance. I see this as a similarity to the double slit experiment where a physical medium is used to measure a quantum state, which forces it to choose and orientation.

10. 10. Fabrice LOTY 05:36 PM 8/12/08

ID-based COMPLEX GATES : ID global approach holds the overall answer (God), to be verified in subsequent details. Conversely, evolutionism starts from details (fossils), with no serious intention of reaching a significant answer(Void). Similarly, quantum gates computing rules are determined from software design, while Hardware approach deals with details void of meaning. For instance, the elementary quantum gate described here may get two meanings: CNOT (already described) and XOR. Let us call D1, D2 respectively the first and second data of the gate and A the XOR result. If D1 is 1 and D2 is 1, D2 holds the result as 0: A=D2=0**D1=1, D2=0: A=D2=1** D1=0, D2=1: A=D2=1 **D1=0, D2=0: A=D2=0 ** It is therefore software level interpretation (as XOR or CNOT) that gives the gate its actual meaning, hardware approach simply describing the gate maravellous potential.

11. 11. jscix1 in reply to Quasimodo 01:04 PM 8/13/08

Quasimodo , I find it hilarious that your incorrect rant about an error in their article, is followed with this statement by you:

"Shame on Sci Am. Don't publish if you aren't doing to do it right. "

Hilarious! Shame on you, don't comment if you aren't going to do it correctly!

12. 12. Quantum dot in reply to kevino819 06:16 PM 8/13/08

Kevino819, thank you for your response. You mentioned the basic quantum mechanical principle of: when we measure something, it changes, but what I was trying to say is I just don't understand how its possible to run an algorithm or a program, or to program the logic gates themselves, when the outcome is unknown and based on the measurement, there is no expected result, do you see what I'm trying to say? I mean, its like trying to use logic gates for something where there is no logic.

13. 13. wbrigg 10:19 PM 8/13/08

With the AND and OR gates, you'd do it kinda like it's done like it's done in a normal computer. AND: measure two qubits. if they're both in the 1 state, then your AND thing gets a 1 as a return. for the OR gate you'd do the same thing but return a 1 if either qubit was a 1.

Oh, and i think that the qubits can use any of their properties as the data carrying ones, it doesn't have to be their magnetic alignment, it can be anything.

Something that would be very cool about this technology (if i'm assuming correctly) is that you could entangle particals and seperate them between a pair of machines, creating a wireless connection between the two of infinite range. It would work in a tunnel, underground, on the moon! Or, you could go a stage further and link all the processors of all Quantum computers in the world so that they share their computing power amongst all the users!

14. 14. hggreen 09:58 AM 9/2/08

I will be the first one implements commercial Qcomputer

15. 15. hggreen 09:59 AM 9/2/08

I will be the first one implements commercial Qcomputer.

16. 16. alfredeva 01:15 PM 12/21/10

17. 17. jtdwyer 02:44 PM 10/9/12

What's most interesting is how conditional programs might be able to use probabilistic results of conditional tests - will this be accomplished through probabilistic branch functions? 'If taxable income >72% Then 48% Perform tax calculation...'

18. 18. frankblank in reply to Quasimodo 06:01 PM 10/9/12

Quasi, SciMike was right. You were and are wrong. And, oddly, you persisted in being wrong.

19. 19. quantumxdt 10:06 AM 10/15/12

I have been puzzled of late. With the Higgs Boson "found" does that mean we have M-theory to ponder. I mean the standard model is vindicated and there is a bridge to quantum effects within the standard model. If this is correct the M-theory is valid; please correct me if I have strayed. What I want to know is what does an electron, for instance, look like in any of the other 7 dimensions and can that be computed using this method. Can we develop sensors and emitters to augment our simple four dimensions? Does that form the basis of what dark matter is? This has had me in a transfixed state since the Higgs was defined. If anyone can elucidate more please feel free!

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