A quantum computer promises to deliver unparalleled performance, but trying to construct one has proven to be far from easy. The aim of this article is to report on the progress being made towards building a practical quantum computer.
The notion of the quantum computer has existed since 1982 when Richard Feynman outlined how such a device might operate. Since then research groups around the world have been racing to pioneer a practical system. If realised, a quantum computer promises to outperform the classical computer (at some tasks) as thoroughly as the classical computer has outperformed the abacus. It would be able to do this by using the phenomenon of quantum parallelism, a mechanism whereby quantum coexistence enables multiple calculations to be performed simultaneously.
This strange effect is brought about by building the computer from special bits known as qubits. These qubits can exist in the classical '0' or '1' states and in a 'coherent' state where they are both '0' and '1' at the same time. If every qubit in a quantum computer is in the coherent state then the computer can be thought of being in every possible state that those qubits can represent. With the right type of algorithm this property can be utilised to perform calculations on all possible values in parallel.
The main stumbling block that has been holding researchers back is the problem of decoherence. The coherent state of a qubit is perhaps the most fragile thing imaginable. Almost anything, even a stray electron, can cause a coherent qubit to collapse into one of the two classical states.
The power of quantum parallelism only exists whilst the qubits of the quantum computer are in the coherent state. As soon as they collapse to a definite state the computer is fixed to a single existence and the parallelism is lost. This means that the number of useful calculations that can be performed on a quantum computer is directly related to the time that the qubits can remain coherent.
The problem of decoherence is compounded by the fact that even measuring a qubit can cause it to collapse. This poses researchers the problem of how they might extract the answer from a calculation without destroying it in the process.
An example of an implementation of the qubit is the 'quantum dot' which is basically a single electron trapped inside a cage of atoms. When the dot is exposed to a pulse of laser light of precisely the right wavelength and duration, the electron is raised to an excited state; a second burst of laser light causes the electron to fall back to its ground state. The ground and excited states of the electron can be thought of as the '0' and '1' states of the qubit and the application of the laser light can be regarded as a controlled NOT function as it knocks the qubit from '0' to '1' or from '1' to '0'.
If the pulse of laser light is only half the duration of that required for the NOT function, the electron is placed in a superposition of both ground and excited states simultaneously, this being the equivalent of the coherent state of the qubit. More complex logic functions can be modeled using quantum dots arranged in pairs. It would therefore seem that quantum dots are a suitable candidate for building a quantum computer. Unfortunately there are a number of practical problems that are preventing this from happening:
Quantum dots are not the only implementation of qubits that have been experimented with. Other techniques have attempted to use individual atoms or the polarisation of laser light as the information medium. The common problem with these techniques is decoherence. Attempts at shielding the experiments from their surroundings, by for instance cooling them to within a thousandth of a degree of absolute zero, have proven to have had limited success at reducing the effects of this problem.
The latest development in quantum computing takes a radical new approach. It drops the assumption that the quantum medium has to be tiny and isolated from its surroundings and instead uses a sea of molecules to store the information. When held in a magnetic field, each nucleus within a molecule spins in a certain direction which can be used to describe its state; spinning upwards can signify a '1' and spinning down ,a '0'. Nuclear Magnetic Resonance (NMR) techniques can be used to detect these spin states and bursts of specific radio waves can flip the nuclei from spinning up ('1') to spinning down ('0') and vice-versa.
The quantum computer in this technique is the molecule itself and its qubits are the nuclei within the molecule. This technique does not however use a single molecule to perform the computations; it instead uses a whole 'mug' of liquid molecules. The advantage of this is that even though the molecules of the liquid bump into one another, the spin states of the nuclei within each molecule remain unchanged. Decoherence is still a problem, but the time before the decoherence sets in is much longer than in any other technique so far. Researchers believe a few thousand primitive logic operations should be possible within the decoherence time.
Dr. Gershenfield from the Massachusetts Institute of Technology, is one of the pioneers of the NMR technique. His research team has already been able to add one and one together, a simple task which is way beyond any of the other techniques being investigated. The key to being able to perform more complex tasks is to have more qubits but this requires more complex molecules with a greater number of nuclei, the caffeine molecule being a possible candidate. Whatever the molecule, the advancement to 10 qubit systems is apparently straightforward. Such a system, Dr. Gershenfield hopes, will be possible by the end of this year, and should be capable of factoring the number 15.
Advancing beyond a 10 qubit system may prove to be more difficult. In a given sample of 'computing liquid' there will be a roughly even number of up and down spin states but a small excess of spin in one direction will exist. It is the signal from this small amount of extra spin, behaving as if it were a single molecule, that can be detected and manipulated to perform calculations while the rest of the spins will effectively cancel each other out. This signal is extremely weak and grows weaker by a factor of roughly 2 for every qubit that is added. This imposes a limit on the number of qubits a system may have as the readable output will be harder to detect.
The recent work on the NMR technique pioneered by Dr Gershenfield and Dr Chuang (Los Alomos National Laboratory, New Mexico), has given quantum computing a promising future. In fact, Dr Gershenfield believes that a quantum co-processor could be a reality within 10 years if the current pace of advancement continues. Other techniques, such as quantum dots, may also yield similar results as our technology advances. The optimist will point out that the problems being experienced by researchers appear to be technical rather than fundamental.
On the other side of the argument, is the topic of decoherence. This problem has not been resolved and many people, including Rolf Landauer of IBM's Thomas Watson Research Center, believe that the quantum computer is unlikely to progress beyond the 10 qubit system described above, as they are too fragile to be practical.
The answer to the question posed by the title of this article is yes, a quantum computer can be built as illustrated by the work of Dr Gershenfield and others; perhaps a more pertinent question would be 'Will we ever be able to build a practical quantum computer?'. The answer to this question is typically quantum: 'Well... maybe!'.