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Formula predicts effects of noise on quantum information

Researchers from Cornell Engineering and the University of Amsterdam have derived a formula that predicts the effects of environmental noise on quantum information – an advancement crucial for designing and building quantum computers capable of working in an imperfect world.

Environmental noise is a significant obstacle to advancing quantum computing because it alters the phase of different branches of a wave function in an unpredictable way. This process of tampering with the phase of a quantum system is called dephasing, and can be detrimental to the success of a quantum computation.

Light can travel through an optical fiber via different paths, as depicted by the blue and red lines. The impossibility of knowing the exact path a light ray has taken leads to an effective dephasing noise.

Dephasing can occur in everyday devices such as optical fibers, which are used to transfer information in the form of light. Light rays traveling through an optical fiber can take different paths; since each path is associated to a specific phase, not knowing the path taken amounts to an effective dephasing noise.

In their new publication in Nature Photonics, Ludovico Lami, assistant professor at the University of Amsterdam and at the research center QuSoft, and Mark M. Wilde, associate professor of electrical and computer engineering at Cornell, analyze a model called the bosonic dephasing channel to study how noise affects the transmission of quantum information. It represents the dephasing acting on a single mode of light at definite wavelength and polarization.

The number quantifying the effect of the noise on quantum information is the quantum capacity, which is the number of qubits that can be safely transmitted per use of a fiber. The new publication provides a full analytical solution to the problem of calculating the quantum capacity of the bosonic dephasing channel for all possible forms of dephasing noise.

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.

The new study quantifies exactly how much redundancy needs to be added to a quantum message to protect it from dephasing noise. This is significant because it enables scientists to quantify the effects of noise on quantum computing and develop methods to overcome these effects.

This article was adapted from an original version with permission from the University of Amsterdam.

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