A classical computer has a memory made up of bits, where each bit is represented by either a one or a zero. A quantum computer maintains a sequence of qubits. A single qubit can represent a one, a zero, or any quantum superposition of those two qubit states;[11]:13–16 a pair of qubits can be in any quantum superposition of 4 states,[11]:16 and three qubits in any superposition of 8 states. In general, a quantum computer with n {\displaystyle n} 📷 qubits can be in an arbitrary superposition of up to 2 n {\displaystyle 2^{n}} 📷 different states simultaneously[11]:17 (this compares to a normal computer that can only be in one of these 2 n {\displaystyle 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). An algorithm is composed of a fixed sequence of quantum logic gates and a problem is encoded by setting the initial values of the qubits, similar to how a classical computer works. The calculation usually ends with a measurement, collapsing the system of qubits into one of the 2 n {\displaystyle 2^{n}} 📷 pure states, where each qubit is zero or one, decomposing into a classical state. The outcome can therefore be at most n {\displaystyle n} 📷 classical bits of information (or, if the algorithm did not end with a measurement, the result is an unobserved quantum state). Quantum algorithms are often probabilistic, in that they provide the correct solution only with a certain known probability.[12] Note that the term non-deterministic computing must not be used in that case to mean probabilistic (computing), because the term non-deterministic has a different meaning in computer science.
An example of an implementation of qubits of a quantum computer could start with the use of particles with two spin states: "down" and "up" (typically written | ↓ ⟩ {\displaystyle |{\downarrow }\rangle } 📷 and | ↑ ⟩ {\displaystyle |{\uparrow }\rangle } 📷, or | 0 ⟩ {\displaystyle |0{\rangle }} 📷 and | 1 ⟩ {\displaystyle |1{\rangle }} 📷). This is true because any such system can be mapped onto an effective spin-1/2 system.
I believe both have its own set of limitations and possibilities, quantum computer will be the better choice for those particular type of computational problems where the principle of linear superposition and quantum entanglement actually provides a better solution than its classical counterpart, either in terms of computational accuracy or substantial reduction in computational time, i certainly do not believe that quantum computers will be panacea for computational problems, for that to happen the postulates of quantum mechanics need to be understood more deeply and probed more deeply from philosophical, mathematical and experimental points of view.
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