11 - Introduction to Quantum Information and Quantum Computing

Summary

When Einstein died, his greatest rival, Bohr, found for him words of moving admiration. When a few years later Bohr in turn died, someone took a photograph of the blackboard in his study. There's a drawing on it. A drawing of the ‘light-filled box’ in Einstein's thought experiment. To the very last, the desire to challenge oneself and understand more. And to the very last: doubt.

—Carlo Rovelli

In this chapter we shall study how quantum entanglement is tested in a laboratory experiment, and how it empowers us to boost computing powers to unprecedented levels. We develop our notion of reality from our day-to-day experiences. Quantum theory is largely counterintuitive because it conflicts with our naïve and untutored perceptions of position and momentum. It accounts for physical events in the universe with enduring cogency; but it demands reconciliation with the principle of uncertainty and a consequent statistical description of nature. Quantum theory has impacted science, technology, and also human lifestyle, notwithstanding the fact that relations such as Eq. 1.105 (Chapter 1) and Eq. 3.3 (Chapter 3) characterize quantum theory as essentially probabilistic. Einstein's famous quote “God does not play dice” grossly undervalues his unease, and also his insight, in quantum physics. Bohr's proverbial response “it is not your job to tell God what to do,” on the other hand, underscores not just his confidence in quantum theory but also his extraordinary insight in an exhaustive discernment of the laws of nature. The previous ten chapters are inspired by the triumph of quantum mechanics. In the present chapter, we revisit a few elements of the Bohr–Einstein deliberations and also the works of John Bell three decades later, which provided a methodology, based on which experiments could be performed to obtain clarity on the probability conundrum in quantum theory. In the meantime, intellectual churning over half a century since the Bohr–Einstein debates led to a deeper understanding of the principle of superposition and entanglement. The Bohr–Einstein debates immortalized the fifth Solvay conference held in 1927 [1], but continued through subsequent years, with two major publications in 1935 – one by Einstein, Podolsky, and Rosen [2], and the other by Bohr [3] – providing major landmarks. In-depth analysis of the works triggered by Einstein and Bohr, and later by John Bell [4], has now emerged as a robust cornerstone of quantum information science and quantum computing, heralding the second quantum revolution.