Last year, a smart friend told me to learn about quantum computing, because it’s going to be the next big thing. My physics education ended in high school, so I found the subject intimidating. Still, I decided to give it a go and bought an introductory book on the subject: Schrödinger’s Killer App: Race to Build the World’s First Quantum Computer by Jonathan Dowling1. Here are my thoughts on the first chapter (with further chapters to come if I feel like it).
I’ll start in the style of a book review, so you can decide whether to read it yourself.
This chapter is a brief introduction to quantum theory and its history. The author is determined to keep the tone lighthearted with frequent jokes and asides, especially about the whimsical world of academia. Overall, I’d say he keeps the material accessible, and many of his anecdotes are interesting, but often they last so long it distracts from the lesson. At one point he spends more than a page describing a debate at a particular conference, not because the individuals or arguments were valuable to the chapter, but just to illustrate that sometimes scientists disagree.
Even this early, I sense limits on describing quantum mechanics for a mass audience. At times the author admits an idea is too difficult to explain and simply bypasses it. Worse, he occasionally fails to specify whether something is unknown or merely complicated. I hope in future chapters he makes clear which questions are mysteries even among experts, which have possible answers that are still debated, and which are considered solved but too complex to explain to laymen.
Here, I’ll summarize the content that seemed most pertinent to quantum computing. Key concepts are bolded.
One of the landmark papers in quantum theory is “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete”, published by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935. It is known as the EPR Paper, based on its authors’ surnames. Einstein hated quantum theory (although he helped to invent it), and this was his last attempt to discredit it.
The EPR paper proposes that there are three conditions at the heart of classical physics: reality, locality and certainty. These are illustrated with a thought experiment about two watches in different cities.
- Reality means each watch always keeps a particular time. The user can consult their watch to find the local time.
- Locality means one watch cannot affect the other. If one watch breaks, the other remains intact.
- Certainty means the watches can keep time with arbitrary accuracy. In other words, winding a watch in a consistent fashion (keeping all conditions the same like winding tension, temperature, air resistance, etc.) should always cause the same mechanical outcome. Observing a watch won’t change its timekeeping.
Quantum theory subverts these conditions. The book explains the subversion with different thought experiments involving a pair of “quantum” watches containing a pair of quantum-entangled ions. Each watch’s hour hand follows the north pole of its ion. Due to the nature of entanglement, the hands of the watches must always face opposite directions (when one watch reads 3 o’clock, the other will read 9 o’clock) and this direction is random until observed (it could point at any hour). These experiments reflect new quantum conditions: unreality, nonlocality, and uncertainty.
- Unreality means the ion (and thus the hour hand) is pointing in all directions until observed. Observing the watch causes the ion to jump (or “collapse”) into a particular alignment.
- Nonlocality means the watches are not independent. Reading one watch at 3 o’clock instantaneously causes the other to swing to 9 o’clock, even if the second watch hasn’t yet been observed.
- Uncertainty means the observations will show completely random directions no matter how carefully the input conditions are repeated.
These conditions have been proven experimentally! Entanglement has been shown between ions separated by a maximum of a few inches (as of the book’s 2013 publication), while photons have been entangled along fiber optic cables at 100 kilometers.
Einstein despised this “spooky action at a distance”. He was certain quantum theory was an incomplete description of reality and suggested some mundane physical principle was causing all of the strange experimental results. Once this Hidden Variable Theory was found, Einstein was sure that physics would be real, local, certain, and finally complete.
Hidden variable theory was disproved in 1964 by John Bell using statistics. By running many tests with an atom smasher, he found more spooky results than any proposed hidden variable could explain, while quantum theory predicted the results very well. This work was called Bell’s Theorem.
These are all the notes that didn’t seem critical to learning quantum computing but were still interesting.
- In one aside about the limits of the scientific method, the author shares a theory for an inverted Earth (see pages 31-32). According to this theory, Earth as we know it is inside-out: a hollow sphere inside an infinite block of dirt. We live on Earth’s inner surface. Inside this hollow sphere is the entirety of outer space. All Newtonian laws are present but inverted; no physical test can disprove the theory. This premise took some time for me to grasp, but in the end I found it the most thought-provoking idea in the whole chapter. Wikipedia mentions the theory2, but the book does a much better job at explaining it.
- There are still competing quantum theories, but Bell ensured they must all be unreal, nonlocal, and uncertain (ie, equally bizarre). These alternatives include Copenhagen quantum theory, Bohm quantum theory, and Many-Worlds theory. The author styles himself a referee or cataloger of these debating schools of thought. He even wrote an article on them, “Interpreting the Interpretations”3.
- There is also a faction known as operationalists who adopt no theory and strictly focus on math and experiments. The author disapproves, calling their attitude “a barren state of mind”.
- Einstein’s famous line: “God does not play dice with the universe” is often misunderstood. It refers to his belief that the universe was deterministic and measurable.
- Erwin Schrödinger first conceived of quantum entanglement using the German word Verschränkung. He personally translated this as “entanglement”. However, the author speculates that Schrödinger may have mistranslated. Verschränkung is closer to “interconnected” or “entwined”, like clasped hands. It suggests an organized condition, not something messy.
- Schrödinger’s cat paradox has several proposed solutions. The most popular is that the experiment requires a perfectly sealed environment (even protected from cosmic rays), and bigger objects are exponentially more difficult to isolate. Perhaps you could shield quantum behavior in single ions long enough for human tools to measure, but any cat-sized quantum activity will end almost instantly as something in the environment “observes” the interior.
- An astronomer in the mid-1800s named Le Verrier noticed the path of Mercury’s orbit violated then-mainstream Newtonian physics. Over fifty years later, Einstein’s theory of general relativity explained Mercury’s orbit by showing gravity is limited by the speed of light. The fun part is that Le Verrier had his own theory: that a hidden planet was bending Mercury’s orbit. He named this planet Vulcan, and an image of a solar system including planet Vulcan was placed on his statue at the Paris Observatory, only to be erased once Vulcan’s existence was conclusively disproved. Later this prediction would inspire the name and homeworld of the vulcans in Star Trek.
- Max Planck discovered Planck’s constant, h, in 1900, setting the groundwork for quantum theory.
- Paul Dirac formalized quantum mechanics in 1930, winning the Nobel Prize in 1933. The author heard him give a speech in 1980. Dirac was 78 and talked for an hour with no slides or notes.
- Radio signals travel farther at night when the Earth’s upper atmosphere, the ionosphere, has less interruption from radiation from the Sun.
- Nyquist noise is a type of electronic noise that plagues the accuracy of trapped-ion atomic clocks.
1 Dowling, J. P. (2013). Schrödinger’s Killer App: Race to Build the World’s First Quantum Computer. CRC Press.