The second key principle of quantum computers is superposition: a special characteristic of qubits that allows them to assume two or more states simultaneously for a certain period (coherence time). The quantum mechanical coherence time, which can be a fraction of a second up to more than an hour, is immediately followed by the loss of superposition (and entanglement) – also called decoherence. With a single qubit this may not be impressive; it gets exciting with the addition of more qubits: With two qubits, four states are simultaneously possible, with ten already more than 1000, and with 20 even more than a million. With each qubit, the amount of information that can be stored in the system doubles. With just a few dozen qubits, quantum computers exceed the storage capacity of the most efficient classical computers. In combination with entanglement, which makes it possible to compute on all these states at the same time, it becomes quite an efficient system.
This also solves the two questions of the introductory text, namely what superpositions have to do with the operation mode and why a quantum computer would have read through this article already: due to superposition, a quantum computer could in theory have analyzed every paragraph of this article at the same time and just as quickly drawn its conclusions from it. Another example of superposition is the participation of a quantum computer in a lottery draw. Whereas we would have to fill out a huge number of lottery tickets to have a good chance of winning the main prize (statistically around 1 in 140 million), a quantum computer leaves nothing to chance and always has six correct numbers and the super number with a single ticket. Almost magically, the right crosses always appear when you pull the lottery ticket out of the envelope. A very talented lottery player, isn't it? The superposition principle thus holds enormous potential in many areas and provides an initial explanation for the worldwide race to developing the most efficient quantum computers.
- 1935: The German physicist Albert Einstein, US physicist Boris Podolsky and US-Israeli physicist Nathan Rosen cast doubt on the quantum mechanical conception of the world in a scientific paper (Can quantum-mechanical description of physical reality be considered complete?). An assumption that was to be refuted in the following decades.
- 1959: Richard Feynman, US physicist and Nobel Prize winner, gives a lecture at the University at California Institute of Technology on how technology can be brought to a microscopic level.
- 1980: Paul Benioff, also a US physicist and one of the pioneers of quantum information theory, proposes the first theoretical basis of a quantum computer.
- 1985: At the University of Oxford, David Deutsch, an Israeli-British physicist in the field of quantum information theory, describes how the model of a conventional computer can be simulated with quantum theory. This is the birth of descriptions of a universally usable quantum computer.
- 1994: Peter Shor, an American mathematician and computer scientist, invents Shor's algorithm, which can be used as a decomposition of numbers and is a groundbreaking development. With his algorithm, quantum computers can now decode theoretically complicated encryptions efficiently.
- 1995: Peter Zoller, an Austrian theoretical physicist, and Ignacio Cirac, a Spanish physicist, propose the implementation of a quantum computer with cold ions. A first computing operation based on this principle is demonstrated in the same year by Christopher Monroe and a group of American scientists at NIST (National Institute of Standards and Technology).
- 1998: Jonathan A. Jones, British physicist, and Michele Mosca, Canadian mathematician, present the first quantum computer with 2 qubits at Oxford University. All good things come in twos, so that Richard Feynman's first theoretical idea from 1959 can now be put into practice.
- 1999: Two becomes three – one year later, IBM presents the first quantum computer with 3 qubits at the Almaden Research Center in California.
- 2000: A first functional 5-qubit computer is demonstrated at the Technical University of Munich.
- 2005: After 5 years, the next level is reached – the first quantum computer with 8 qubits is presented at the University of Innsbruck by a working group led by the Austrian physicist Reiner Blatt.
- 2006: Even faster is the following development of a quantum computer with 12 qubits, which researchers from the Institute for Quantum Computing, MIT and the Perimeter Institute for Theoretical Physics
- 2010: A group of physicists led by Dr. Arno Rauschenbeutel at the Johannes Gutenberg University in Mainz realized a fiber-optic-based quantum interface between light and atoms that is suitable for the transmission of quantum information.
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