What a Quantum computer can do for you
In order to continue the advance in computing a new type of technology needed to be exploited. According to quantum physics a subatomic particle can’t be said to exist, there are only probabilities of it’s existence and position until its definite state and position is discovered, then its probabilities collapse. Quantum physics breaks down the classically binary nature of a bit, with the invention of a quantum bit or qubit that can exist in coherent superposition, i.e. as a 0, 1 or simultaneously as a 1 and 0, with a numerical coefficient representing the probability of each state. The qubit is represented by the nuclear spins of each individual atom, for example the change in energy state. When you perform a calculation using an electron existing in both states you are performing two calculations, when another superposed qubit is added four calculations can be performed at once and so on. This exponential increase means that the time taken to carry out calculations rapidly decreases. The time to carry out calculations also decreases as atoms change energy states far quicker than even the fastest computer processors. With only a few hundred qubits it is possible to represent more numbers than there are atoms in our universe.
It also increases storage capacity exponentially, as N qubits can store 2 numbers at once. Imagine the qubits are atoms whose different electronic states can be controlled by a tuned laser; this will change their state allowing in only one computational step a calculation to be performed on 2 different input numbers encoded in coherent superpositions of N qubits.
The actual space a quantum computer will take up will be significantly smaller than present day desk tops, allowing the further development of sophisticated, efficient palm held computers. This is because given the right calculation each qubit can take the place of an entire processor, meaning that 100 barium ions could take the place of 100 computer processors.
Aside from computers quantum technology has developed rapidly in the last ten years. In June 2002 a team of Australian scientist were able to teleport a laser beam, causing it to disappear and be regenerated 3 ft away, the results are yet to be confirmed but if they are sound this development could in a matter of years be able to teleport actual objects significant distances.
Nuclear Magnetic Resonance
After Shor’s discovery quantum computing construction began in earnest, however due to the phenomena of decoherence no atom or photon, being the qubit, can be of an undetermined state after being detected, the probabilities collapse and its state becomes definite. This makes further calculations impossible as the exponential element of the qubit has been lost, causing it to behave as a regular analogous bit. In order to keep the coherence of the qubit the inner workings of a quantum computer must be separate from the outside environment to stop any interactions that may determine the state of the qubit from occurring, but also be accessible so that calculations can be carried out and results obtained.
A solution was Nuclear Magnetic Resonance (NMR) a technique developed in the 1940s, which is widely used in medical imagery and chemistry. Experiments were carried out, during the mid 1990s and it was found using a classical fluid made of many (1023) selected molecules allowed each qubit to be represented by many molecules allowing interactions to take place between some of the molecules but still maintaining the coherence of the qubit.
NMR treats the “spins” of qubits as tiny bar magnets that will line up when a magnetic field is applied, allowing manipulation of the qubits. Two alternative alignments are generated one parallel to the external field and one anti-parallel to the field, corresponding to two different quantum states. NMR procedures also use an oscillating electromagnetic field, which is specially selected according to the properties of the molecules used. This causes certain spins in the liquid to be rotated, causing them to perform the different calculations required.
Example
Hydrogen nuclei placed in a magnetic field of 10 tesla, change orientation at a frequency of 400 megahertz (radio frequency) Due to decoherence the pulse is only turned on for a few millionths of a second but can cause the spins to rotate by 180 degrees, a pulse half that length causes a 90 degree spin. This causes the spins to be of state 1 or 0 with equal probability. This causes the spin to rotate about the magnetic field, as shown in the image on the right. This rotation emits a weak radio signal, which is picked up by the NMR apparatus.
Limits to NMR
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
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.
