Quantum Computers: The Next Genre of Computers

Human race has progressed in various frontiers; specially science and technology at an incredible pace. Invention of computers proved to be a milestone in this ever-expanding realm of technology. The computer technology has itself evolved from different physical realizations from gears to relays to valves to transistors to integrated circuits. Today's technology has enabled us to create micro wafer chips (state of the art IBM chips 0.25 micron wide containing nearly 200 million transistors).

Shrinking to nano scale
Computer power has grown at a phenomenal rate. The key to this growth lies in continuous miniaturization of the computer's most elementary component the silicon magic transistor. The size is shrinking and the increase in power is exponential. This miniaturization has reached saturation point. A quantum threshold beyond which the transistor shows unexpected behaviour. The chips are currently being fabricated on the nano scale. Further reduction in size would disrupt the operation due to the quantum phenomena of electrons, tunneling effect. The most logical solution is to design components, harnessing the power of quantum mechanics. Hence, ”computers devised with components only functioning on quantum effects but algorithms still following the classical approach may be called- 'nanoscale-quantum computers'. Soon it was realized that even if the algorithms were made to function using the same quantum effects; the power they would yield would be far more powerful than any classical scheme. Such a computer would be a quantum computer in its true nature.

Fundamentals
As we all know, Bit is the fundamental unit of information. In a classical system it may exist in any one of the two logical states-on or off, true or false, or simply 0 or 1. This is physically realized by an uncharged or charged capacitor or a transistor in an on or off state.
In a quantum computer, fundamental unit of information is qubit. Qubit is strictly governed by the laws of quantum physics, in comparison to the classical physics. A Qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words,” a qubit can exist as 0,1,or simultaneously as both 0 and 1 with a numerical coefficient representing the probability for each state”. Qubit is a purely quantum phenomenon and has no equivalent of this superposition in classical physics.

Correlation And Entanglement
Two atoms are said to be correlated: if the first has decayed the second will also have decayed, and if the first atom has not decayed, neither has the second. Such a correlation is a 100% correlation. Similarly, “qubits are correlated so that if the first is in the superposition state, the second will also be in the same state”. Great number of correlations may exist between the atoms than expected classically. This kind of quantum super-correlation is called “entanglement”. “A change in the state of one of the entangled atoms would also cause a change in the corresponding entangled atoms”.

Quantum Communication System
It can be explained with the help of an example. Consider person A, with two photons in an entangled state. Now, 'A' stores one of these and sends the other one to person 'B'. As B decides to transmit some information back to A, he performs an operation on his stored photon. Since the photons are in an entangled state, the change is reflected amongst the quantum states of both the particles. Thus A also observes a change on his stored particle. (This could be practiced from either end and it effectively doubles the peak capacity of an information channel.) Operations performed should be unitary (from one end at a time) so as to maintain stable quantum superposition state of the entangled system.

Parallel Computational Power Unleashed
Consider a register in a classical system consisting of three bits. At any instant, it may hold any one of the eight (23) possible combinations i.e. 000,001,010…On the other hand; imagine a register of three qubits. It may then have the capability of storing all the eight possible numbers in a quantum superposition at any given instant of time. Remarkably all the eight numbers are physically present in the register (because of the property of qubit which enables it to be zero and one at the same time). And a register prepared in superposition of different numbers allows operations to be performed on all of the numbers. We may conclude: ”A Quantum computer in any single computational step may perform mathematical operations on 2L different members; organized in coherent superpositions of L Qubits”. In comparison a classical computer would require 2L different processors working in parallel, or repeat the same computation 2L times. Thus quantum computers offer an enormous gain in terms of vital resources such as time and memory.

Obstacles
Quantum mechanics tells us that directly measuring the state of a qubit invariably destroys the superposition of states in which it exists, forcing it to become either zero or one.
Decoherence or the tendency of a quantum state to decay into an incoherent state as it interacts with the variables of the external system. These interactions are difficult to avoid thereby leading to erroneous results. Thus to harness the vast potential of quantum computing, steps are to be taken to overcome decoherence initially.
Researchers at MIT managed to spread a qubit across three nuclear spins in each molecule of alanine (or trichloroethylene) utilizing NMR techniques. Spreading of information in such a manner made it resistant to corruption. The technique of spreading and the property of entanglement allowed researchers to study interactions between states indirectly (thereby avoiding direct measurement of states). Thus a higher level of coherence could also be maintained in the quantum state.

Uses
Dense coding, secured cryptography and applications to communication complexity (entanglement property) are some of the many practical applications where such a technology finds its use….

(Information has been researched and obtained from various sources)