Due to technical obstacles, till date, a quantum computer has not yet been realized. But the concepts and ideas of quantum computing has been demonstrated using various methods. Here, we discuss four of the most important technologies that are used to demonstrate quantum computing. They are

1. Nuclear Magnetic Resonance
2. Ion Trap
3. Quantum Dot
4. Optical Methods

While reading the following "top four technologies", two things should be kept in mind. The first is that the list will change over time. Some of the approaches valuable for exploring quantum computing in the laboratory are fundamentally un-scalable, and so will drop out of contention over the next few years. The second thing to keep in mind is that although there are a bewildering number of proposed methods for demonstrating quantum computing (a careful search will yield many more options that what is listed here); all of them are variations on three central themes:

(a) manipulating the spin of a nucleus or subatomic particle
(b) manipulating electrical charge
(c) manipulating the polarization of a photon.

In variation "a" a qubit is derived from superposition of up and down spins. In variation "b" a qubit is derived from superposition of two or more discrete locations of the charge. In the last variation a qubit is derived from superposition of polarization angles. Of the three, the manipulation of spin is generally viewed as the most promising for practical large-scale application. Let�s now see each of these techniques in detail.

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