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Quantum Logic Gate Created Using Excitons
Roland Piquepaille writes "In this article, PhysicsWeb reports that researchers in the U.S. "have taken another important step towards making a quantum computer. [They] have created a logic gate using two electron-hole pairs -- also known as "excitons" -- in a quantum dot." According to Wikipedia, "an exciton is a combination of an electron and a hole in a semiconductor or insulator in an excited state These physicists from the University of Michigan and other labs made a quantum dot by using a thin gallium arsenide layer stuck between two aluminium gallium arsenide barriers. And electrons trapped in the middle layer were excited by light to create a quantum logical gate with four states. The group says this could be useful "in other approaches to quantum computing based on the optical control of electron-spin qubits in quantum dots.." This summary contains more details."
an exciton is a combination of an electron and a hole in a semiconductor or insulator in an excited state
I love it when physicists talk dirty to me.
Please be advised that by posting this with my quantum computer, I am able to make all possible lewd comments about the above sentence simultaneously.
In Soviet Rush, today's Tom Sawyer gets high on you.
Physicists in the US have taken another important step towards making a quantum computer. Duncan Steel of the University of Michigan and co-workers have created a logic gate using two electron-hole pairs - also known as "excitons" - in a quantum dot (X Li et al. 2003 Science 301 809).
Classical computers deal with binary logic and the bits being processed must be either "0" or "1". Quantum computers, on the other hand, exploit the ability of quantum particles to be in two or more states at the same time. A quantum bit or "qubit" can therefore be "0" or "1" or any combination of the two. This means that a quantum computer could, in principle, outperform a classical computer for certain tasks. However, all the quantum computers demonstrated so far have only contained a handful of qubits.
Although qubits have been made with trapped photons, atoms and ions, it is generally thought that it should be easier to build working devices with solid-state systems. Several teams have made significant progress with the superconducting approach to solid-state quantum computing. Now Steel and co-workers at Michigan, Michigan State, the Naval Research Laboratory and the University of California at San Diego have demonstrated the first all-optical quantum gate in a semiconductor quantum dot.
Exciton transitions
Steel and co-workers grew a thin gallium arsenide layer 4.2 nm thick between two 25 nm aluminium gallium arsenide barriers to make a quantum dot. Electrons are trapped in the dot because the gallium arsenide layer has a smaller energy band-gap than the surrounding material. When excited by light, electrons from the valence band in the dot move to higher energy levels. The excited electron and the 'hole' it leaves behind combine to form an exciton. The system has four states: a ground state containing two unexcited electrons; two states containing one exciton; and a state containing two excitons (see figure). The two single-exciton states can be distinguished from each other because the excitons have different polarizations.
The researchers showed that they can drive Rabi oscillations between the ground state and the one-exciton states, and also between the one-exciton states and the biexciton state, with lasers. In particular they showed that the quantum-dot system behaves like a controlled-NOT gate in which the value of one qubit is reversed (the NOT operation) if - and only if - the value of the other qubit is 1.
Although it will not be possible to scale up the system, the group says that many of the ideas and techniques they have developed could be useful in other approaches to quantum computing based on the optical control of electron-spin qubits in quantum dots.
Many of todays (hoge comparetively) processes suffer from metal migration and huge static power dissapation. If the molecule sized transistors are going to take off they have to solve there problems first or these products will have a lifetime of a few hours.
Mouse powered Chips, Open source Processors and Lego
those guys who managed to factor "14" into 7 and 2 with Shors algorithm on an actual quantum computer implementation?
Heard anything more from them? I googled, but couldn't find anything.
How small a thought it takes to fill a whole life
So they have a quantum gate. But is it just a transistor replacement, which you still would use to build traditional computers, chomping through processor instructions, processing binary (or base 4 or whatever) numbers? Or is for quantum computers working on a completely different paradigm?
It's nicely done, but not the breakthrough that means quantum computers for all. It is, after all, only a NOT gate. I barely consider NOT as a logic function...more like half a function.
...
Was I the only person who read the line
The group says this could be useful "in other approaches to quantum computing based on the optical control of electron-spin qubits in quantum dots..
and thought "I'm sure I've seen that on a Powerpoint presentation somewhere". This is clearly uber-smart stuff by uber-smart people, but they are beginning to sound like clueless PHBs dressing things up in techno-babble.
Maybe this is the fundamental essence of quantum computers, something maybe smart or idiotic depending on the reader, the actual quality of work is only resolved when viewed by multiple individuals.
I hearby copyright the phrase Quantum-Powerpoint, and the resolution process of determining presentation value which I shall call "De-spinning Qubits"
An Eye for an Eye will make the whole world blind - Gandhi
Does it run Linux?
I know nothing about quantum computing except that it looks really hard, and I'm betting a lot of other people are in a similar situation. When traditional silicon (or other semiconductor) components have gotten as small and fast as feasibly possible, will quantum computers be anywhere near as usable as the desktops of today are? Meaning, will I be able to use a fast computer without a PhD?
-phish
Excitons - the particle formed by the collision of Porntons and GNUtrons.
If they can harness the amazing power of the Gravitron, then I and Joe Sixpack will stand up and take notice.
This space for rent.
...were excited by light to create a quantum logical gate with four states.
Quantum computer? Great, it's in all four states at once. Gotta love a computer that gives you infinitely different results depending on what universe you're in.
(First application: generating airfares.)
Excitons to be Bohring.
ok, i'll kick my own ass for that one.
Call me when they make NAND gates with a way to couple them together, then I'll get excited.
Since a Quantum computer can process millions of operations *simultaneously*, how much do I need to pay a million-CPU licence from SCO if I want to use Linux ?
Ok, so if we pair electrons and holes, we get excitons. But what do we get when we pair protons and neutrons? Do we get hardons?
the joke in the parent post is funny.
I'm going to go sit in a corner and weep, now.
*honk*
This is my sig. It's prescription, I swear. I need it for reading things... on the other side of things
After all, they say they're using "two electron-hole pairs", so doesn't that translate into 2 two-bit "registers" instead of a single four-bit one? End result is the same, mind you, but the method is different.
Have EVDO, will travel.
Wow, I never knew quantum computers would help keep flooded networks afloat!
See, qubit~=cubit.
See, like Noah's ark.
Oh, never mind.
Rrom what I understand it, a qbit is in all states simultaniously, even if it is only a single bit (0 and 1 instead of 0 or 1). Tou can emulate it using normal bits, but the way I've understood it's still "opposite", and so it's a completely different paradigm:
... (about 2^43 lines skipped)
Classic:
Is 0 the answer? FALSE
Is 1 the answer? TRUE
Quantum:
Qbit x = TestFor(answer) (test all states)
Read x = 1
Classic:
Is 0000000000000000000000000000000000000000000 the answer? FALSE
Is 1111111111111111111111111111111111111111111 the answer? FALSE
Quantum:
Qbit xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx = TestFor(answer) (test all states)
Read xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx = 1010010100010101010010010101001001000101011
However, noone has been able to get a large number of quantum bits operating. And for few qbits, you'd do faster by simply doing a classic search. A computer using low-qbit "transistors" wouldn't be operating like a base 4 classic computer, but it wouldn't be this wonderful supercomputer either. A cluster of qbit transistors would as I understand simply scale linearly. Two 10qbit transistors would have twice the power of one 10qbit transistor. While on the other hand one 20 qbit transistor would have the power of 2^10 10qbit transistors.
Kjella
Live today, because you never know what tomorrow brings
Just to educate the masses further, a quantum CNOT(controlled NOT)/XOR gate is not equivalent to a classical XOR gate. A CNOT gate includes a control bit, and retain the information of this bit at the end of the gate.
What is fun in Quantum Computing is that you do not need a lot of basic gates(AND, OR, XOR, NOT, etc.), you only need a small number of basic gates to make up the Universal gate.
Furthermore, ALL the elementary gates in QC are reversible!! Unlike classical gates, like XOR, the quantum CNOT, for example, is fully reversible
no no no no no no no no no no no... no. Any symmetric cipher is perfectly safe. You can factor and solve discrete log problems, but not much else. Just in case you are still confused about the answer to your question, the answer is... NO!
That's great. I suppose I'll hold off on getting that 3.0 GHz machine I was looking at, then. How big is the hard drive gonna be?
"We shall party like the Greeks of old! You know the ones I mean." - HedonismBot
Um... correct me if I'm wrong... but this would imply that it is impossible to construct an AND gate, right? Because it is impossible to extend a conventional AND gate (or OR, or NOR, or NAND) so that it is reversible. That is, you can't make a reversible gate that takes n inputs to n outputs, such that one of the outputs is the AND of all the inputs.
All this means for quantum computing, is that in order to emulate an irreversible gate like AND, you have to also keep around enough extra information in the output so that you can still reverse the computation. (A, B, 0) --> (A, B, A^B), for example, could be a valid quantum gate. This restriction only applies for as long as you want to maintain a superposition of values in A and B. There are tricks to try to keep this explosion of storage needs under control, but it will be a significant problem with large algorithms.