Unfortunately there are limits to this apparent life line for the development of quantum computing. However Leo Kouwenhoven has said that “the whole show stops at 15 qubits”. This is because the magnetic signal that the NMR apparatus receives generates the answer to the calculation that the qubits have been programmed to perform. Due to decoherence the signal fades rapidly demanding more molecules to be present in the NMR fluid. Due to the huge number of molecules scientists are unable to handle, at present more than 10-12 qubits at a time.
It has been shown that a quantum computer is destined to only perform simple sums, like factorising 15, until the number of qubits, reaches near 100. – The image above represents how an oscillating field affects the orientation of the spin. A 180-degree pulse, shown left will cause the spinning nucleus to entirely flip over and a 90-degree pulse, shown right will cause the spinning nucleus to turn upon it side. In this example the spinning nucleus is represented by a spinning top. Quantum entanglement Another element, which is helping scientists and quantum computer developers, is the phenomena of quantum entanglement.
This is another strange quality of already baffling quantum behaviour. When an external force is applied to two atoms, they are said to become entangled. This causes the two atoms to resemble the little bar magnets so utilized by NMR. One atom will pick one spin or one value, while the other picks the exact opposite value. Without nuclear entanglement a single atom will spin in all directions constantly changing its rotation. The metaphor of two dice being thrown, whose total value will always be, for example, eight is used to describe entanglement.
The value of each separate dice isn’t determined before the first throw of one of the dice, if it for example lands on two the value for the other dice is then known to be six in this case. This allows scientists to determine the value or spin of a qubit without affecting it. This again comes back to the idea of an excess of molecules being used in order to allow scientist to determine the state of some atoms to determine the state of their “Quantum twin”. Dr. Jeff Kimble of the Caltech Corporation said after the first successful teleportation experiment, “entanglement means if you tickle one the other laughs”.
Chloroform (CHCl3) is an example of a two qubit computer. The carbon in chloroform has one extra neutron than regular carbon twelve causing a spin over the atom. If the carbon atom starts with a value of one, that is with a known spin, lets say pointing up parallel to the magnetic field and the hydrogen atom starts in the same state. A pulse is then applied causing the carbon atom to rotate 90-degrees causing it to process around the vertical. The speed at which the carbon atom spins depends upon the orientation of the hydrogen atom, in this case it spins fast, if the hydrogen atom was oppositely orientated the rotation would be slower.
If the rotation was fast then when another pulse is emitted causing the carbon atom to flip by 90-degrees, so that its orientation has changed by a total of 180-degrees, if the rotation was slow then the carbon atom flips back to its original position by 90-degrees. This demonstrates another quality of quantum computers NOT gates, which can operate on a combination of seemingly incompatible information. Classical computers require two input gates as well as a simpler NOT gate to perform similar operations on classically compatible inputs.
Alternatives to NMR Ions seem to be the answer to NMR’s problems. There energy states can be determined using light and they can be controlled, by laser super cooling to micro Kelvin temperatures and positioned in ultra-high vacuum by electromagnetic forces that carefully control their 2 dimensional positions but allow 3 dimensional movement. Limits to Quantum Computing It is unlikely that quantum computers will ever be completely commercially viable and may only ever be used by big corporations using the factorisation algorithm.
This is because the quantum computer is not suited to word processing, design programmes, or any form of internet and email jobs. However it is suited to large scale cryptography and constructing and efficiently searching large scale computers. At present it will be difficult to build a quantum computer of any size as the more qubits interacting the harder it is to control and stop the qubits dissipating useful information to the surrounding environment. There are also technical issues of working on a sub-atomic and single-photon scale.