What is the difference between classical and quantum computer?
Classical computers which we use in our day to day life uses bits that is 0’s and 1’s also known as binary language to process data. Every character, number, special characters, are defined by ASCII code which is followed by classical computers. The processor which is main component in a computer is made up of billions of transistors which turn on and off in sync to transmit and process data. Billions of transistors on such a small chip! Imagine how small each transistor is in a processor.
Moores law states that the number of transistors on a chip doubles every 18 months. So now we are actually approaching the limit a transistor can be shrunk. A transistor is made up of junctions and as the transistors are becoming smaller the width of junction is approaching the atomic scale, currently the latest processors have the junction width of few tens of atoms. If we shrink the transistor more quantum effects like quantum tunneling comes into play. So there will come a point when reducing the size of transistors is not possible.
Quantum computers use quantum mechanical phenomenon like superposition and entanglement. The major difference between classical and quantum computers is that they use qubits over bits. Bits are 0’s and 1’s that is on state or off state. These both states are well defined. A single qubit can represent a one, a zero, or any quantum superposition of those two qubit states; a pair of qubits can be in any quantum superposition of 4 states, and three qubits in any superposition of 8 states. In general, a quantum computer with n qubits can be in an arbitrary superposition of up to 2^{n} different states simultaneously (this compares to a normal computer that can only be in one of these 2^{n} states at any one time). A quantum computer operates on its qubits using quantum gates and measurement (which also alters the observed state). That is we cannot observe the quantum process as observing it would collapse the process and it would be just like any other classical computer.
Physical representation of a qubit is any system or particle having 2 quantum states. For example an electron can be a qubit with spin up as one state and spin down as other state, superposition of these two states is also possible in a qubit as mentioned before. Other physical implementations of qubits are : Photons, Nucleus, Optical lattice, Josephson Junction, Quantum dot etc. So if we cannot observe the quantum process how do we manipulate qubits to get the desired output? For this we use the phenomenon of quantum entanglement, in simple words pairs or system of particles are in the same quantum state but away from each other.Measurements of physical properties such as position, momentum, spin, and polarisation, performed on entangled particles are found to be appropriately correlated.
One more important thing that may cause a disturbance in quantum computing is Quantum Decoherence. In quantum mechanics particles such as electrons are described by wave functions. If a quantum system is not perfectly isolated but is in contact with the surrounding then the quantum behaviour is lost. Decoherence can be viewed as loss of information to the surrounding. The superimposed wave functions acquire phases from their immediate surrounding.
This is a very active and wide field of research and lot of development is needed to make quantum computing practical. Some benefits of quantum computing are that more number of operations can be done at the same time in parallel way, large-scale quantum computers would theoretically be able to solve certain problems much more quickly than any classical computers that use even the best currently known algorithms, they can be used for research in genetics as it requires rigorous analysis, etc. The quantum realms are deep and unexplored and wonderful things happen at quantum level.