Shiroman Prakash completed his M.Sc. in Physics from Indian Institute of Technology, Kanpur, and his Ph.D. in Physics from Tata Institute of Fundamental Research, in theoretical physics.

His research interests include string theory, quantum field theory and quantum information/computation.

“Implications and Applications of Quantum Contextuality”

Abstract

In this talk we will discuss implications that research in quantum computing – particularly the subject of fault-tolerant quantum computing – has for scientific and philosophical questions associated with consciousness.

Quantum mechanics is the most successful scientific theory ever proposed, remaining unfalsified after over a century of experimental tests of ever-increasing rigor. Quantum mechanics is also mysterious and counter-intuitive. Some aspects of quantum mechanics, such as the uncertainty principle and the measurement problem, are self-evident from the initial applications of the theory to atomic physics. However, in the decades since its discovery, researchers in quantum foundations have learned that quantum mechanics is fundamentally different from classical physics in ways that aren’t immediately obvious.

John Preskill describes quantum computing as “putting weirdness to work.” Computers that make full use of quantum physics are known as quantum computers – and are, in theory, able to solve certain specific computational tasks much faster than classical computers.

Fault-tolerant quantum computing concerns the practical question of how to build a large- scale quantum computer that can preserve and process information stored in delicate quantum superpositions in despite ever-present environmental noise. To make sure the task is no more challenging than absolutely-necessary, we must understand exactly what kind of quantum “weirdness’’ we need (and how much of it we require) to achieve a quantum speedup. Over 50 years ago, Kochen, Specker and Bell identified contextuality as a fundamental difference between classical and quantum mechanics. Unlike the uncertainty principle and the measurement problem, contextuality remained obscure, barely mentioned in any quantum mechanics textbook and unknown to most physicists. However, in recent years, quantum computing researchers have identified and began to quantify contextuality as the essential resource for quantum computation.

What are the implications for the interpretation of quantum mechanics? While some of the weird features of quantum mechanics – observer-dependence and the uncertainty principle

– could conceivably be eliminated by modifying and reformulating the theory, contextuality is a mathematical property of the probabilities that quantum mechanics predicts. Any observer-independent reformulation or deterministic modification of quantum mechanics that agrees with experiments must reproduce probabilities that exhibit at least some form of apparent contextuality – and one can argue that this would make such an observer- independent reformulation practically unintelligible. Contextuality is thus the essential reason why quantum mechanics is best understood as a first-person theory, reminding us that we cannot forever ignore our own existence as conscious observers participating with the universe.

“Do biological systems, such as brains, show quantum behavior?” is another difficult question to answer. Brains are hot, humid and noisy, so, at first glance, the answer seems to be no. But brains are also extremely fine-tuned over billions of years of evolution, capable of doing remarkable things, and widely accepted to be the most complex systems known to us. Moreover, our understanding of quantum information is still evolving, and only a few decades ago, many physicists thought quantum-error correction was impossible. So, for many scientists, the question remains open. But how can we experimentally demonstrate quantum brain biology? One hope is that a deeper understanding of contextuality can lead to unambiguous signatures of quantum behavior, though much work remains to be done.