Quantum noise model analysed, its capacity to transfer message without noise quantified
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New Delhi: Researchers have analysed an existing model studying noise in quantum systems and have devised a method to quantify its quantum capacity, which is the number of qubits of information a quantum system is capable of safely transmitting per use of fibre.

The researchers from University of Amsterdam, The Netherlands, and Cornell University, US, have published their method in the journal Nature Photonics.

The existing model, called the bosonic dephasing channel, studies noise in quantum systems and how the noise affects the transmission of quantum information.

The model represents the dephasing acting on a single node of light at definite wavelength and polarisation.

Dephasing in quantum systems, which is the altering or tampering with the phase of a quantum system, thereby producing ‘environmental noise’, can be detrimental to the success of quantum computing.

In addition to quantifying the capacity, the new study also identifies the amount of redundancy needed to be introduced in a quantum message in order to protect it from the dephasing noise. This, the researchers said, is significant.

Quantum computing uses the principles of quantum mechanics to perform calculations. Unlike classical computers, which use bits that can be either 0 or 1, quantum computers use quantum bits, or qubits, which can be in a superposition of 0 and 1 simultaneously, allowing a quantum computer to factor very large numbers in a fraction of the time it would take a classical computer.

The researchers said that while a quantum computer is advantaged to perform numerous parallel calculation, the reality is much more complicated. The quantum wave function of the quantum computer, which represents its physical state, possesses several branches, each with its own phase.

A phase can be thought of as the position of the hand of a clock, which can point in any direction on the clockface.

At the end of its computation, the quantum computer recombines the results of all computations it simultaneously carried out on different branches of the wave function into a single answer.

“The phases associated to the different branches play a key role in determining the outcome of this recombination process, not unlike how the timing of a ballerina’s steps play a key role in determining the success of a ballet performance,” explained Ludovico Lami, University of Amsterdam.

In everyday devices such as optical fibres, which transfer information in the form of light, the researchers said that the light rays travelling through a fibre could take different paths, each associated with a specific phase.

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The lack of knowledge about the path taken amounts to an effective dephasing noise, they said.

To overcome the effects of noise, one can incorporate redundancy in the message to ensure that the quantum information can still be retrieved at the receiving end. This is similar to saying “Alpha, Beta, Charlie” instead of “A, B, C” when speaking on the phone. Although the transmitted message is longer, the redundancy ensures that it is understood correctly.

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