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."

Spare the Poor Server and read this

[Suhas]

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.

Re:PHB does physics...

[CausticWindow]

Just wait and see till you graduate.

The real world is all about fronting, not about keeping it real.

Re:In the future!

[MindStalker]

Thats why people make Operating Systems so you don't actually have to use the computer. Anyways the first quantum computers will probably be co-processors as the normal logic based programs wouldn't run on them.

Re:What about..

[Anonymous Coward]

Google [google.com]

Re:In the future!

[DrWho520]

How much about a standard computer must you know to run anything today? If you are coding in assembly, admittedly you must have an understanding, but do you need to know the base logic of the computer? Do you really have to understand how a transistor or a JK-flip-flop works to write C, C++ or Java? You certainly do not to build a machine. Its just a bunch of black boxes.

The true power of quantum computing is the idea of a mixed state, the shades of gray if you will, that will be possible with quantum elements. While logic gates take strictly binary inputs (bits), quantum gates will take superpositions of the 1 and 0 states (qubits). Ask a simple question, is it cloudy outside? A bit either says yes or know depending upon a threshold of some sort. Who sets the threshold, does everyone agree on the threshold, and how accurately is the threshold mesured? A qubit can give you a mixture of yes and know, relaxing the systems. Its very similar to fuzzy sets, as elements are not strictly in or out of a set.

There will be a learning curve. Unfortunately, until there are a large number of gates of a specific type, a deffinitive logic process (fuzzy logic, if you will) cannot be decided upon. (Maybe there will be serveral types, and Intel works with one type of qubit logic and AMD works with a different.) But the logic system is what you will need to understand, that is what people understand now. Is it really a simple process to break down everything into yes or know? You don't need a PhD. for that. I think the fuzziness of a quantum system is much closer to reality than that of binary.

I picked up my PhD. (Posthole Digger) at the hardware store.

Re:Is this for quantum or electronic computers?

[xyvimur]

Check the section called: `The Potential and Power of Quantum Computing' from the page: About quatum computing [caltech.edu]

It's still a qbit...

[Kjella]

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:

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
... (about 2^43 lines skipped)
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

Re:XOR

[Karhgath]

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

Re:Quantum?

[YouMakeMeSoANGRY]

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!

Re:XOR

[zCyl]

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.