## Qubit

Quantum Computing has entered my radar. I am a little ashamed that I ignored this subject for so long.

We are entering a new era. In the forthcoming blogposts I will try to inform you about the recent technological breakthroughs in quantum computing. But, in this introductory blogpost, I would like to start with the concepts.

A classical computer uses bits (implementations of “1 or 0”, “yes or no”, “up or down”, “on or off” in tiny electrical circuits in semiconductor substrates) to implement algorithms. In the future, a quantum computer will use qubits instead of bits to accomplish the same.

A qubit is not to be confused with an ancient unit of length called cubit. A qubit is a quantum-mechanical system where 2 states are in quantum superposition. Such a system can represent both a zero and a one simultaneously.

There are many candidates for qubit implementations. In most of them a qubit stays in quantum superposition for a fraction of a second. Thermal noise disturbs the qubit and collapses (decoheres) the qubit wavefunction to either 1 or 0.

Interaction and measurement is the same thing. We cannot measure a superposition. When a measurement/interaction takes place quantum superposition is broken. When we measure a qubit we get either 1 or 0 not both. Similarly, when there is thermal noise the qubit interacts with the environment and collapses (decoheres) to either 1 or 0.

One of the most promising candidates for qubit implementation is a special type of superconducting circuit involving Josephson junctions. These superconducting circuits have to be cooled to 20 milli-Kelvins to minimize the chances of decoherence.

One word that is not mentioned – but implied – in the explanations of quantum superposition is “parallelism.” In the electrical-circuit representation of the classical bit the electrical circuit has to switch from 1 to 0 or from 0 to 1. Switching is a classical physics process and therefore takes time. In contrast, for a qubit in quantum superposition there is no switching. Quantum superposition, in a way, accelerates time in the form of parallelism. Problem is  that quantum superposition is very short-lived. It is as if the Nature is saying: “I will allow you to accelerate time but for a very short time.”

A single qubit is meaningless! In terms of computing, we cannot achieve anything meaningful with a single qubit. Nature provides another puzzle/opportunity for us. The puzzle/opportunity in this case is quantum entanglement.  Quantum-entangled objects are correlated (not coupled) in a way that have no classical analog. If we  can correlate many qubits using quantum entanglement and let the wavefunction of the qubit ensemble evolve then we can achieve more time acceleration.

The key word here is the “evolution” of the qubit ensemble wavefunction. I will write more about this in the future.

Teams of physicists and engineers at IBM, Google, Microsoft, Amazon and D-Wave are making progress towards achieving stable qubits as well as progress towards assembling entangled qubits. I will report about these developments in future blogposts.