Unlike conventional computers which by their nature perform one task at a time, quantum computers strong ace will be that they will be able to perform an unimaginable large amount of computational tasks in parallel at each time cycle of their operation; this technological knack is to be made possible by quantum mechanics strange laws which govern each and every element of our subatomic world.
So how do we build them? Unlike a normal Turing machine that uses bits, that is, electrical circuits that can be put into two different electrical states, 0 and 1, albeit only one at a time, quantum computers will have to be build using Qubits. The difference between a Qubit and a bit is that the former one can store the value 0, or 1, or any combination of the two values at the same time, whilst a bit can only memorize a single value, 1 or 0, at the same time. Let me clarify this! Qubits can have 76% the value 1 and 24% the value 0, or 43.23% the value 0 and 56.77% the value 1, all at the same time. And you can dream of any other combination. This translates to : in 76% of the parallel universes belonging to the Multiverse the Qubit which Ive exemplified earlier will poses the value 1 and in the rest of 24% it will poses the value 0. In the other example the Qubit will poses the value 0 in 43.23% of the parallel universes and in the other 56.77% it will poses the value 1. Thats a lot of parallel computing considering the total number of parallel universes calculated to be possibly perceivable by an observer lies around the figure of 10^10^16 universes
Whats all this clap-trap about parallel universes and the multiverse? Quantum mechanics, the field of physics that deals with sub-atomic particles has solved and clarified some of our most mysterious experimental data that even Einstein couldnt get the grips with. The interactions between all subatomic particles can well be explained and predicted with an accuracy worthy of envy by using the well worked mathematical model of Quantum Mechanics. But, although we can predict and calculate the subatomic world with very delicate precision, paradoxically, we do not understand why subatomic particles behave the way they do. As renowned quantum physicist Richard Feynman said : If you think you understand quantum mechanics, you dont understand quantum mechanics. And why is that? Its because atoms, electrons, protons, and all the rest behave in strange ways; they can be in multiple places at the same time; they sometimes behave like particles, sometimes like waves; they can travel back in time and even be connected one with another in ways that defy classical physics logic. Many explanations exist on the scientific highway as to why the quantum world behaves the way it does, but the most widely known, the Many Worlds Interpretation (MWI, Parallel Universes , Multiverse) first introduced by physicist Hugh Everett is converting with an ever increasing rate physicists from all kinds of fields. If you read Russell Standishs The Theory of Nothing you can find that in a survey done by David Raub on quantum mechanics experts and cosmologists 58% of them believed that the MWI is true.
So, in order to build a quantum computer we must have bits that can find themselves in multiple universes at the same time and these are called, as weve stated earlier, Qubits. The fact that they must be made to work together in order to show their prowess, just like normal bits do, is a real impediment because quantum systems are easily vulnerable to the outside world; their small size and the risk of interference with other particles gives a real challenge to the experimentalists in this field.
Practically the challenge is to build a set of isolated-from-the-outside-world qubits, which are all interconnected with one another through what we call Entanglement. Unlike bit computing systems, which are relatively easy to put together and to integrate with one another because they are very large so they are immune to interference from other particles therefore they can be easily manipulated through classical physics methods, qubit quantum systems are very hard to make practical. Qubit systems must be read, written, and protected from alien particles all at the same time without disturbing the already established quantum entangled system. Entanglement, or spooky action at a distance is one of the strangest phenomenon that governs the quantum world. For example, when two particles are entangled, modifying a certain quantum property belonging to one of the two particles will also instantaneously modify the same property belonging to the other particle no mater the distance between the two; to translate this into quantum computing terms, measuring a certain property of a certain particle will instantaneously give you information about the property of the other entangled particle. Entangling a sufficient number of qubits (ions, electrons, photons, superconducting devices solid-state qubits), and keeping them entangled and isolated from all the outside world an adequate amount of time is the main challenge that lies ahead in building the quantum computer.
What is the progress so far? Well, in 2010 we saw the first three solid-state qubit system. A team lead by Robert Schoelkopf, Professor of Physics Applied Physics at Yale, and a team of physicists from UC Santa Barbara separately demonstrated the first entanglement of three solid state qubits. Solid state qubits, unlike qubits made from electrically charged atoms (ions), photons, or electrons, are build using superconducting quantum circuits (SQC). These are computing chips manufactured using the same technology as the current computer chip industry is using. These chips, made from hundreds of millions of atoms, can be made to behave as a single atom by cooling them to just a few fractions of a degree above absolute zero in order to behave quantum mechanically; these so called artificial atoms promise to be the most easy to implement qubits because the manufacturing infrastructure is already present. But, because of their relatively big size, compared to qubits made from atoms, photons and electrons, their quantum state is easily disturbed by interference with the outside world so further theoretical and experimental research into the quantum behavior of superconducting devices needs to occur in order to fully exploit all the possibilities that could be offered by this kind of qubit system.
Things look good for smaller scale qubits also. Molecules could provide all the advantages of easy entanglement with each other because theyre not so vulnerable to disruption as the smaller atomic qubits are, and because they are smaller than solid-state qubits they are not so vulnerable to all the added interference from the surroundings as larger qubits would be. A team from Yale University managed to cool a molecule of strontium monofluoride around the temperature of absolute zero using beams of photons released by a laser; the streams of light coming from multiple directions reduced the random velocities of the molecule and thus proved how we could more easily control molecules in order to use them as qubits.
Qubits, as Ive stated in the prologue of the article, need to be read, written, and preserved for long amounts of time in order to be implementable in a working quantum computer. Major steps have been made concerning this issue also. A team of physicists from UC Santa Barbara used a crystal of diamond, which had a defect the size of an atom, in order to trap electrons that were further hit by photons. The interaction between the electrons and the photons created a mixture of light and matter that had some lucrative qualities. Incredibly, by measuring properties of the light contained in that mixture they were able to measure the quantum state of the electrons (i.e. read) without destroying their configuration . Furthermore, by using light measurement to discretely analyze the electrons state they managed to modify their properties (i.e. write) without destroying their configuration.
In the September 2010 Issue of the Nature Journal, physicists from Universities in Australia and Finland presented a paper in which they managed to create a quantum circuit capable of reading the spin of an electron from a phosphorus atom contained in a block of silicon. This single electron reader is capable of reading with high-fidelity the spin of an electron and it is the first stand alone device with such abilities. Further research needs to be done in order to build a circuit that would also be capable of writing (i.e modifying) electron spins at the same time.
Even if quantum computing hardware easily stretches the limits of human perfection, several constraints that were once thought to render the task of building a quantum computer impossible have now started to sound like easy-cake. Ulrik Andersen successfully demonstrated in a 2010 issue of the Nature Photonics journal how a possible quantum error correcting code might work in a quantum computer that uses qubits made from photons. In another research paper, physicists led by Dr Barrett from from University of Queensland in Brisbane successfully demonstrated, theoretically, how 25% of the qubits from a specially arranged lattice of qubits could be lost without significant loss of information; the remaining 75% qubits could be used to extract the lost ones.
Even with all this positive information added up, how and when we will manage to build the quantum computer are not easy questions, but it will sure as hell depend on whether sufficient money are pumped into the scientific engine. With news about fund cutbacks on science all around the world i dont believe we will see it built anytime soon, but i sure as hell hope to see it in my lifetime.