What are qubits?

Abstreiter: Traditional bits are based upon two states, classified as 0 and 1. Viewed physically in the computer, the classifications exist as a charged or uncharged capacitor. However, cubits are based upon the quantum-mechanical superposition of two states. Unlike traditional bits, this mixed state can be varied continuously. For example, if vector arrows represent the pure states 0 and 1, where one points up and one points down, then all other directions are superpositions of both pure conditions (quantum states).

Basic research is striving for a qubit-based computer, or quantum computer. What’s the difference between today’s computers and a quantum computer, and how is a quantum computer superior?

Abstreiter: In quantum computers, information is processed based upon quantum-mechanical principles. The principles contain new designs without traditional parallels, such as the superposition of states. In the last ten years, it has become clear that this technique opens fascinating and new perspectives for information processing and communications. For example, the exploitation of massive parallelism can handle computationally intense tasks much more quickly than traditional computers. The examples often mentioned include the factoring of large numbers: separating them into prime numbers with the Shor algorithm and searching in large databases with the Glover algorithm.

Here I’d like to dampen the enthusiasm for application just a little, since right now there are very few examples of applications where quantum computing would really be advantageous. Traditional computers are not yet ready for retirement – not by a long shot. And right now it’s completely unclear if a quantum computer that could be put to practical use can even be built. But questions of quantum information processing have led to a global synergy between theoretical and experimental physicists, electrical engineers, computer scientists, and mathematicians. Quantum information processing is an extremely fertile source of inspiration for several extremely interesting scientific questions – both theoretical and applied.

How can quantum conditions be used to carry information?

Abstreiter: The smallest unit for quantum information is a qubit. As noted, a qubit is the quantum-mechanical superposition of two states. To use a qubit, we need systems that have two quantum-mechanical conditions. Nature offers several options, only a few of which I’ll mention: individual atoms or ions in traps or in electronic or magnetic resonance, Cooper pairs or flux quanta in super conductivity, electron-hole pairs, or excitons in semiconductor quantum dots. Which system is best suited to quantum information processing is still a completely open question. What’s important, however, is the property of being able to manipulate and abridge the quantum-mechanical level of freedom in a controlled manner and at an exact time. Here, the long decoherence times of the quantum systems play an important role. Simply put: the system must be able to remember the starting condition for a long enough period. Only then can a series of coherent quantum manipulations be performed. And you must be able to couple several qubits as well.

What’s the current status of quantum computers and what challenges have to be overcome?

Abstreiter: Quantum information theory, which is now trying to develop quantum algorithms, such as the Shor or Grover algorithm, is far ahead of any technological realization. So far only simple operations with a few qubits have been demonstrated in the lab. For example, the number 15 was separated into its prime numbers: 3 and 5. That is an initial proof that the principle works. Many of the systems used, such as atomic traps or nuclear magnetic resonance, are difficult to expand into larger systems of qubits or even into integrated circuits. In most cases, the scalability required for practical applications is not present. That’s why many groups around the world are working on solid-state systems.

What are the current approaches to research in semiconductor physics, and what are their goals?

Abstreiter: Our projects based upon semiconductors are concentrated on electrons, excitions, photons, and characteristics of electronic spin in semiconductor quantum dots. In the past year, we have been able to prove that it’s possible to read out the coherent manipulation of an exciton in a quantum dot electrically. But this realization of an optically controlled qubit with electric contacts is only the very first step and cannot solve many problems for semiconductor-controlled quantum information processing. Another approach uses nuclear or electronic spin as qubits. There are a lot of ideas for this. In principle, they’re even compatible with traditional microelectronics based upon silicon. Another approach, one that’s currently much closer to application, is the realization of a single photon source, which would be required for tap-proof transmission of information. In the area of this quantum cryptography, I expect to see significant advances and the first serviceable components in the coming years.

How can this research influence IT?

Abstreiter: With all the questions posed above, right now it’s difficult to foresee the influence on IT. The development of microelectronics will advance over the next 15 to 20 years with the same speed it now enjoys. The size of transistors will then be smaller than 10 billionths of a meter. At that stage, I can imagine combining microelectronics with concepts of quantum information processing. The precondition for it, however, is that by then we can control quantum-mechanical states exactly. Then, continuous development would be imaginable: wherever quantum-mechanical principles would offer advantages, they would be implemented.

Applications in tap-proof information processing can be expected much earlier. As soon as the appropriate single photon sources and detectors are available, they will be applied to optical transmission. Personally, I don’t believe that quantum computers will replace traditional computers in the foreseeable future. But quantum information processing is nonetheless a fascinating area that’s sure to produce several surprises.

What’s your personal motto?

Abstreiter: Do the possible, try the impossible!