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@Neolander, thanks for the explanation, although if I could request of you a more specific explanation of quantum computing, that would be great.

From my understanding regular computing would have to be based on the same laws of mechanics and properties of our world as everything else, its not as if we can step outside of it, simply because we are ignorant of it.

So, there must be something special about quantum computing that separates it from merely a theoretical description of how things work - to a practical engineering difference.

I'm not going to pretend that I didn't already go to wikipedia and try to make some sense of entanglement and superposition. The problem is - as much as they might try to be simple and easy to understand, they aren't quite using the right words for me.

Regular computers use transistors and attempt to represent information in binary, 1' and 0's. My understand is quantum computers attempt to use quantum properties to represent and manipulate data.

Ok, but then that's not enough to get it for me, and frankly I was joking earlier, but since you all are trying to help - what's the missing piece here?

It's not enough to just state the states can be 1, 0, or inbetween, what are the states, how do you manipulate them?

I can program in assembler just a little bit - so at least in my head, I get how regular computers work - shouldn't I be able to understand quantum computers. Lets say I want to program a quantum computer - what are my steps to do so?

From my understanding regular computing would have to be based on the same laws of mechanics and properties of our world as everything else, its not as if we can step outside of it, simply because we are ignorant of it.

So, there must be something special about quantum computing that separates it from merely a theoretical description of how things work - to a practical engineering difference.

As I mentioned above, it takes rather small systems for quantum phenomena to become significant. More precisely, what you actually need is something that has as little physical interactions as possible with its surroundings.

That is because when quantum systems are coupled to a macroscopic environment, it causes many quantum effects to vanish over time, in a process known as decoherence. As an example, if you put a system in a "half-zero, half-one" state, it will decay either to the zero or one state. The stronger the coupling, the faster the decay. One way to describe it from the point of view of physics is as information or energy being diffused into the environment and never coming back.

Thus, people who try to build quantum computers have the difficult task of building quantum chips whose state may easily be read on demand, but which are still too weakly linked to the outside world for the aforementioned decoherence effects to become problematic.

Regular computers use transistors and attempt to represent information in binary, 1' and 0's. My understand is quantum computers attempt to use quantum properties to represent and manipulate data.

Ok, but then that's not enough to get it for me, and frankly I was joking earlier, but since you all are trying to help - what's the missing piece here?

It's not enough to just state the states can be 1, 0, or inbetween, what are the states, how do you manipulate them?

A state, in a quantum mechanical sense, is a set of physical parameters that fully describe a system. As an example, electrons in solids can be described by their momentum, energy, and magnetic "spin", while a photon of light can be described by its momentum and polarization.

Often, these parameters are only allowed to take a discrete range of value, which as you can probably guess is particularly useful for information processing purpose : just make somehow sure that only two parameter values are accessible, and you got yourself your "zero" and "one" physical states. And since these are quantum states which we are talking about, nothing prevents a system from being in both states at the same time, with a certain probability of finding it in each during a measurement.

Effectively, such two-level systems have been built in a wide range of physical systems, ranging from atoms in optically reflective cavities to bunch of ions trapped using electrostatic fields, with superconducting micro-circuits somewhere inbetween. The two states in question are often chosen of different energy, so that the system cannot trivially switch between both without exchanging energy with its environment.

But at this point, I have to point out that such a quantum memory or "qubit" is useless by itself. Sure, your qubit may be simultaneously in its "zero" and "one" states, but since you need to perform lots of measurements to know for sure, nothing has really been gained. Besides, as mentioned above, quantum information storage tends to be short-lived due to decoherence effects, with data remaining reliably stored for at most a few miliseconds in the best experiments if memory serves me well.

So the next difficulty in building a quantum computer is to make quantum memories interact with each other, so as to perform computations. And just like with classical computing, the simplest thing which one can think of building this way is logic gates.

Where things become interesting here is that a quantum logic gate is able to act on all possible states of its input at the same time, and return a statistical superposition of all the results at the same time. So if we take, as an example, an unsorted database search algorithm of the form "examine every database entry, return a pointer to the entry that possesses a given key and NULL otherwise", then the only nonzero result is the pointer to our database entry, and we may naively imagine a quantum circuit which finds the right database entry in constant time.

Things sadly aren't quite as simple in practice because all logic gates which destroy information (such as, say, AND and OR) are forbidden in the quantum world. One thus has to deal with other logic gates like the CNOT and Hadamard gates, which arguably reorder information more than they do computation, and thus make programmers' lives more difficult. Due to this, an actual quantum database search algorithm, as derived by Grover et al., ends up with an algorithmic complexity of O(sqrt(N)), which is still a quadratic speedup over classical ones.

Other kinds of search-oriented tasks can benefit from a quantum device's ability to act on multiple inputs at the same time. Perhaps the most well-known one is Shor's algorithm, which factorizes numbers in O((log N)^3) time and is the worst nightmare of everyone who ever implemented RSA-based cryptographic systems. That being said, not all of cryptography is based on the difficulty of factorizing prime numbers, and so far some algorithms have proven to be pretty robust against the minds of quantum computing theoreticians.

Did that answer some of your questions ?

At this point, I don't think anyone has actually built a truly programmable quantum computer, so it's hard to answer this question. For now, most works I've heard of are based on special-purpose circuitry which aims at implementing a specific algorithm, so programming a quantum computer would be kind of like "writing" code using a bunch of logic gates wired as you wish and synced against a common clock signal.

I think that implementing a programmable quantum ALU is a dream of many quantum computing experimentalists, but is still out of reach for now. Theoreticians tend to be ahead in this domain, so I'm sure multiple designs have already been published in scientific literature.

Lots of "I think" in this last part, I know, but I haven't followed this realm closely enough to be perfectly sure about the state of current research. My current field of research (superconductivity) sure involves QM, but it's not quantum computing itself

*Edited 2013-03-24 21:49 UTC*

Member since:

2010-03-08

Let me try to help you a bit understand what QM is and how it works.

One of the core tasks of physics is to describe what makes things move and how, whether the things in questions are tiny electrons or huge nebulas. Newton's laws of motion are a simple description of this which works pretty well at our scale, whereas Einstein's special and general theories of relativity are more accurate at high speed and large scales, and quantum mechanics (or QM) is the best at small scales.

QM and relativity can both be seen as extensions of Newton's laws to extreme scales, since if we try to apply them to "regular" objects, we'll get similar results as with Newton's laws. However, merging them into one single unified theory has proved to be extremely difficult for theoretical physicists. At this point, finding a satisfactory quantum description of gravitation, as described by the theory of general relativity, remains an open problem.

Now, how does the quantum description of the world differs from the one offered by classical mechanics? In a nutshell, it allows for some physical situations which are perfectly impossible in a Newtonian world, and simultaneously forbids some things which Newton's laws are perfectly fine with. Here are a few examples:

-Quantum objects' properties are not assumed to be perfectly known. What QM describes instead is the probabilities of finding something in a given state: at a given position, with a given speed...

-All these possible states of a system are not just statistical odds. Unless a measurement procedure leads the system to collapse in a single state, multiple "possible" states of a system exist simultaneously, and may thus interact with each other.

-QM's laws of motion are based on this latter effect, describing the probabilities of finding a system in a given state as changing in space and time in a manner similar to that a sound or like wave.

-All properties of a system are not independent, and it's impossible to simultaneously know all of them at once. This stem from the mathematics used to describe quantum states, which happen to be matching some real-world behaviors.

The reason why we're dealing with a theory that violates common sense in such a brutal way is that we have failed to find a saner description of the world which matches experiment so far. As an example, if the microscopic world followed Newton's laws of motion or their relativistic equivalents, electrons would crash into atom nuclei, many magnetic materials would have totally different properties, and we would still have to argue upon whether light is a wave or a stream of particles instead of having a maths that unify both descriptions.

Edited 2013-03-22 21:07 UTC