Posted on Apr 5, 2011

Physicists at the University of Innsbruck in Austria have achieved controlled entanglement of 14 quantum bits (qubits), realizing the largest quantum register ever produced, almost doubling the record for the number of entangled quantum bits created experimentally (a property of the quantum mechanical state of a system containing two or more objects, where the objects that make up the system are linked in such a way that the quantum state of any of them cannot be adequately described without full mention of the others). They confined 14 calcium atoms in an ion trap (similar to a quantum computer), and then manipulated them with laser light. The internal states of each atom formed single qubits and a quantum register of 14 qubits.

They discovered that the decay rate of the atoms is not linear, but is proportional to the square of the number of the qubits. When several particles are entangled, the sensitivity of the system increases significantly in a process known as superdecoherence, which has rarely been observed in quantum processing.

In the classical model of a computer, the fundamental building block, the bit, can only exist in one of two distinct states, a 0 or a 1.

In a quantum computer, not only can a 'quantum bit', referred to as a 'qubit', exist in the classical 0 and 1 states, it can also be in a coherent superposition of both. When a qubit is in this state it can be thought of as existing in two universes, as a 0 in one universe and as a 1 in the other. An operation on such a qubit effectively acts on both values at the same time.

The significant point being that by performing the single operation on the qubit, quantum computing performs the operation on two different values. Likewise, a two-qubit system would perform the operation on 4 values, and a three-qubit system on eight. Increasing the number of qubits therefore exponentially increases the 'quantum parallelism' obtained with the system. With the correct type of algorithm it is possible to use this parallelism to solve certain problems in a fraction of the time taken by a classical computer.

The very thing that makes quantum computing so powerful, its reliance on the bizarre subatomic phenomena governed by the rules of quantum mechanics, also makes it very fragile and difficult to control. For example, consider a qubit that is in the coherent state. As soon as it measurably interacts with the environment it will decohere and fall into one of the two classical states. This is the problem of decoherence and is a stumbling block for quantum computers as the potential power of quantum computers depends on the quantum parallelism brought about by the coherent state.

This problem is compounded by the fact that even looking at a qubit can cause it to decohere, making the process of obtaining a solution from a quantum computer just as difficult as performing the calculation itself.

Quantum computers, conceived in 1982 by the Nobel prize-winning physicist Richard Feynman, have the potential to solve problems that would take a classical computer longer than the age of the universe.

Oxford Professor David Deutsch, quantum-computing pioneer, wrote in his controversial masterpiece, *Fabric of Reality*: "quantum computers can efficiently render every physically possible quantum environment, even when vast numbers of universes are interacting. Quantum computers can also efficiently solve certain mathematical problems, such as factorization, which are classically intractable, and can implement types of cryptography which are classically impossible. Quantum computation is a qualitatively new way of harnessing nature."

Casey Kazan via http://www.doc.ic.ac.uk/~nd/surprise_97/journal/vol4/spb3/

Ref.: Rainer Blatt et al., 14-Qubit Entanglement: Creation and Coherence, Physical Review Letters, March 31, 2011

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