Paradoxical features of quantum mechanics asset for future quantum computing?Heidelberg 23 jun 2001 Last key note speaker at the SC 2001 Conference was Dr. Peter Zoller from the University of Innsbruck in Austria, who introduced himself as the exotic bird among the audience of computer experts and engineers since he is a physicist. His talk addressed Information Technology for the 21st century, in the knowledge that the technologies of the future are the basic research of today. In physics, quantum mechanics is a big revolution. The innovative concept may be used for a new paradigm in HPC also, which can be described as quantum computing to solve complex problems. Dr. Zoller however stressed that the fate of visionaries is that their predictions usually are either outperformed or not achieved.
Dr. Rolf Landauer has defined information as physical and computing today indeed is physical, according to the speaker. Objects are stored as physical objects on conceptually classical computers. The present computers are processing information following the laws of classical physics. By 2017, the storage of a single bit will reach the size of a single atom. At a fundamental level, nature obeys the laws of quantum theory. In computing, this will lead to the birth of new algorithms for new complexity classes.
In order to demonstrate the potential of quantum computing, Dr. Zoller decided to give the audience a crash course in quantum mechanics. There are some strange features in this concept. To build a statistical theory, the state of the system is measured. The theory only talks of the probabilities. For instance, to measure the interference of light particles, you have to solve the wave equation but what is happening between the preparation of the light particles and their measurement? Each attempt to detect the path destroys the interference. In the same way, a microscope influences the system and alters it.
In any case, the strange features of quantum mechanics such as interference, superposition, and entanglement also hold in store much promise for future applications in computing, as Dr. Zoller pointed out. The underlying hardware
in quantum computing has to obey to quantum mechanics and quantum dynamics are influenced by quantum measurement. A quantum bit, referred to as qubit can be
1, 0, or a superposition of zero and 1. This superposition amounts to strange combinations.
A qubit can be visualised as a vector on a sphere. There exists an infinite number of quantum states. The quantum information is included in the qubit but you are not allowed to look at it because the state alters with the measurement
The quantum register stores all the numbers in the digital notation but as stated, it is impossible to look at it without changing them. This can only be solved by applying the measurement afterwards.
However, the potential of quantum memory through parallellism is enormous. There are 300 elementary particles, which involves a space of 2 to factor 300 atoms in the universe. You can use this memory space in the form of algorithms
for quantum parallel processing but you cannot apply it in the naive form
because the read out process alters everything, Dr. Zoller explained. Therefore, a division has to be made into quantum logical gates. In fact,
quantum computing is a sort of reversible computation from the very beginning.
The gates can be decomposed in two types. More complicated are the two-qubit quantum gates, since swapping of the states is possible. The computational paths can interfere, which can amplify the states or wipe them out. This depends on the phases. Dr. Zoller described the concept of decoherence as the bad guy. The noise from the environment randomises the quantum phases. If we lose the interference. this is the death of quantum computing so we should build systems which are isolated from the environment.
The applications of quantum computing are factoring, database search, simulation, distributed quantum computation, etc. Factorising numbers is a difficult computational task, which takes a long time. Peter Shor developed an algorithm for factorising, so that it takes only a few minutes in quantum computing. And this is merely the top of the iceberg, as Dr. Zoller stated.
The building of a quantum computer is like a man-on-the-moon project and requires a serious investment. At the time, Dr. Schroedinger found it ridiculous to experiment with single particles but now, the idea of bits written on 1 atom is no longer ridiculous, as Dr. Feinman once thought.
A quantum computer needs quantum memory, gates, and isolation from the environment. The conditions are to scale things up and also increase the speed. Experiments have been performed at the University of Innsbruck to take atoms and store the ions by forming ion traps. Through individual manipulation with laser pulses, you can cause interaction via collective phonon nodes. Scalability is possible using ions as qubits in microtraps.
Dr. Zoller also noted that Nuclear Magnetic Resonance (NMR) experiments are very attractive because it is something like computing in a cup of coffee and constitutes a well established technique for analysis. The qubits are nuclear spins of molecules, e.g. coffein, and they are addressing microwaves with good decoherence times. In the read-out, you get a bulk signal but there is only deviation from the thermal state.
In the solid state, a free design is possible and this might be the future. The problem is thermal noise from the rest of the system. Quantum dots may be a candidate in solid state NMR, according to the Kane proposal. Is quantum mechanics able to control decoherence and can we perform error correction? Dr. Zoller believes the answer to these questions is positive so there is no
fundamental obstacle for quantum computing. Nonetheless, there will be no more than few tens of qubits in the coming years so big scale quantum computing is far in the future.
In the meanwhile, we can identify a few qubit applications, such as long distance secure quantum communications over the Internet, for example. How can this be realised at best? Dr. Zoller concluded that there is no obvious best candidate but there shoud be possible a merging of ideas and different fields.
Leslie Versweyveld |
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