In this digital age, as we all know the information is digitally handled as bits, that is zeros and ones. Each of this information chunks are called "bit". For last five decades, we have been playing with this so called bit and today all of our digital life, even real life is connected to these two numbers, 0 and,1 called bits. Each of this bit can be intimated to the states of an electric switch. The on state of the switch can be considered as 1 and off state is 0. Analogs to this classical approach, we can materialize the representation of information in quantum computing using units called quantum bits or abbreviated as qubits. The quantum version of classical bits. Qubit is a two-state or two-level quantum mechanical representation of information. The fundamental difference between a qubit and classical bit is that, classical bit can only be in either one of its states or level whereas qubit can be in coherent superposition of both states/levels. The term qubit was originally introduced by Benjamin Schumacher, an American theoretical physicist working in the area of quantum information theory, in his 1995 paper, Quantum Coding published in Physical Review A. The paper deals with the compression of quantum information created by a qubit, and it is called Schumacher compression.
The classical information processed as bits using a CPU is implemented by one of the two levels of a low DC voltage. This binary logic is represented by two voltage states in different logic families, for example, the voltage level for TTL family for logical 0 is
and for logical 1 is
. Similarly, for CMOS family of logic, 0 is 0.5 V and 1 is 2 Vcc. For all these logic families, information is represented by either 0 or 1 or a combination of both. A qubit or quantum bit is used in quantum computing and it is different from normal bits. Consider representing data using an atom. The ground state of electron in the atom can be taken as state
and the excited state of the atom to another energy level can be taken as
. In this format, a qubit can simultaneously represent the states
and
. Using this property, if we use 4 qubits to represent information, the qubit can represent all the data simultaneously and it is 16 times faster than a normal computer. Table 1—the table represents quantum equivalent of classical computers. A quantum computer with 300 qubit can replace all the supercomputers on the planet
Quantum Bits | Equivalent Classical Bit |
1 | 1 |
10 | 1024 |
50 | 1.125899907E17 |
300 | 2.037035976E90 |
There are various proposals that have been made to physically implement a qubit. Many researches are going on to find a good method to realize qubit. A quantum mechanical system with two-levels can be used as a qubit. We can also use multilevel systems provided if they have two levels that can be decouple from other levels. Researchers are successfully implemented systems that can be approximated two-levels with various degrees of accuracy. The various technologies and techniques to produce a qubit is will serve various purposes in a practical quantum machine. The following list will be an incomplete set of technologies practically implemented by researchers:
Physics | Terminology | Information |
|
|
Coherent state of light | Squeezed Light | Quadrature | Early | Late |
Photon | Number of Photons | Fock State | Vacuum | Single Photon State |
Electron | Spin/Number | Spin/Charge | Up/No Charge | Down/Charge |
Josephson Junction | Superconducting Charge/Flux/Phase Qubit | Charge/Current/Energy | Uncharged Island/Clockwise Current/Ground State | Charged Island/Counter Clockwise Current/First Excited State |
