Schrödinger's Web

Race to Build the Quantum Internet

Jonathan P. Dowling

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000080230
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Optics & Light

As the race to build the world's first quantum computer is coming to an end, the race to build the quantum internet has just started. This book leverages the author's unique insights into both the Chinese and American quantum programs. It begins with the physics and history of the quantum internet and ends with the latest results in quantum computing and quantum networks.

• The Chinese quantum Sputnik moment.
• The U.S. National Quantum Initiative.
• What's up with Quantum Computing Supremacy?
• The Race to Build the Quantum Internet.
• Where will Quantum Technology be Tomorrow?

Written by a renowned quantum physicist, this book is for everyone who is interested in the rapidly advancing field of Quantum Technology — The Second Quantum Revolution. The 2016 launch of the Chinese quantum satellite Mozi was a quantum Sputnik moment. The United States went from thinking it was ten years ahead of the Chinese to the realization that it was ten years behind them. This quantum gap led to the U.S. National Quantum Initiative, launched in 2018. Since then, the race to build the quantum internet has taken off at breakneck speed.

Chapter 1

Many Hands Make Light Work

¹

In the beginning, God created the heaven and the earth. And the earth was without form and void, and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. And God said, “Let there be light,” and there was light. And God saw the light, and it was good, and God divided the light from the darkness.²

1 Sistine Chapel ceiling, “The Separation of Light and Darkness,” by Michelangelo di Lodovico Buonarroti Simoni. Work is in the public domain.

2 Genesis 1.5, King James version of the English Bible. Work is in the public domain.

The Darkened Room

The current internet (and the future quantum internet) runs on light. What is light? This is a question that humans have been asking themselves for thousands of years, with the first written record in western culture coming from The Book of Genesis. A precursor to the modern camera (the pinhole camera) may have been discovered accidentally as far back as 30,000 years ago. The pinhole camera is a darkened box with a pinhole poked in it. I made one of these out of a cylindrical, cardboard, Quaker Oats container as an undergrad in a beginning physics lab. I placed a circular piece of undeveloped photographic film at one end of the tube (in a dark room), taped a piece of aluminum foil over the other end, and then poked a hole in the tinfoil with a pin. I took the contraption up to the roof of the physics building, pointed the pinhole at the University of Texas tower, opened the cardboard shutter, counted to ten, shut it up and went back down, and developed the film. There appeared a ghostly image of that infamous owl-faced tower.³

There are theories that ancient cave drawings were inspired by such images, shining through small cracks in the cave wall and into the cave itself, where the image would appear (upside down) on the opposite wall. All the inhabitants had to do is to trace the image with coal or other pigments. Once they developed artistic skills of their own, the theory goes, they had no further need for the pinholes.

Such pinhole cameras, making images of the sun, have been described in ancient Chinese writings as far back as 1046 BCE. Pinhole cameras are one of the safest ways to view a solar eclipse or a sunspot – as the image of the eclipse is projected onto a sheet of paper and not on the back of your eyeball where it could fry your retina. (Never look directly into the sun.⁴)

The first details and accurate written descriptions of the pinhole camera and the science behind the imaging process go back to the writings of the Chinese Philosopher, Mozi, from around 500 BCE. He understood that the pinhole acted like a lens, and his drawings of the camera are similar to what you would find in a modern book of optics. The name Mozi became synonymous with science and the study of light in Chinese culture. As a foreshadowing of where I am going with this, there is now (as I type) a Chinese quantum optical communications satellite named Mozi. It is currently orbiting the earth, and its handlers just announced today (Friday 16 June 2017), the results of experiments transmitting quantum entangled states of light from the satellite to the ground. The spacecraft uses a transmitter that produces a rainbow of different-colored photons – what I call “Schrödinger’s Rainbow” (Figure 1.1).

3 The real story of the owl in the University of Texas clock tower, https://alcalde.texasexes.org/2012/01/legendary-ut/.

4 “Yes, Donald Trump really did look into the sky during the solar eclipse,” by Chris Cillizza, CNN (2017), www.cnn.com/2017/08/21/politics/trump-solar-eclipse/index.html.

Figure 1.1 Schrödinger’s Rainbow – separating the light from the darkness. Photo of quantum entangled photons produced in my former group’s laboratory at the NASA Jet Propulsion Laboratory, circa 2003.⁵

An older name for these pinhole cameras, used for popular entertainment in Europe hundreds of years ago (some can still be found today⁶), was a camera obscura, which is Latin for a darkened room. These devices were human-sized darkened rooms with a hole punched in the side. You would locate actors in daylight outside the chamber, and the gap would project ghostly images of them inside onto the wall (upside down) for the entertainment of the audience in the room. Mozi understood that the image became inverted as a result of ray tracing the light beams (Figure 1.2). In the 1800s, when we invented photographic film, we shrunk the camera obscurae (darkened rooms) down to the size of a shoebox. The images became permanently written onto photographic film, which, when developed, gave you a permanent shot of the object or person (just like with my oatmeal box).

5 Photo of entangled photons generated by spontaneous parametric down-conversion, taken by Dmitry Strekalov, circa 2003, in our lab at the NASA Jet Propulsion Laboratory. The photo was taken by a government contractor in the course of his work duties and not copyrightable under U.S. law. For details on how this image was created, see https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion.

6 You can visit one of the few remaining large camera obscurae in Bristol, UK. See https://en.wikipedia.org/wiki/Clifton_Observatory.

Figure 1.2 Schematic of an 18th-century camera obscura. The dotted lines trace three beams of light, showing that the soldier’s hat (A) is mapped to the bottom of the camera, his shoes (B) to the top, and his belt (D) to the center (C), demonstrating that the image becomes inverted. The darkened room is to the right (where the audience sat), and the pinhole is located at C.⁸

The shoebox devices were still called camera obscurae, but eventually, they became merely known as cameras (because nobody could pronounce obscurae). Hence our English word “camera” simply means “room”. One problem with the pinhole camera is that the pinhole lets very little light through, and so the image takes a long time to record on the photographic film. Small children, having their portraits taken, would have to sit completely still for minutes. Photographers developed elaborate clamps to attach to the sides of their heads under their hair to keep them from moving. Hence this camera was not ideal for use on toddlers in their terrible twos (and the clamps possibly amounted to child abuse). Camera enthusiasts started making the pinholes bigger to let in more light, to shorten the exposure time, but the image was out of focus. So, they added glass lenses to compensate and to make the image arrive at the viewfinder and the film right-side up. In the 1990s and 2000s, we finally replaced the film with charge-coupled devices, and the modern digital camera was born.⁷ Interestingly, on most cellphone cameras, they still use pinholes and not lenses because the digital detecting arrays are very sensitive to even feeble rays of light, and there is not much room for lenses in your ultra-slim iPhone.

If we look at how we trace the rays of light in Figure 1.2, we see how the beams cross over each other at the pinhole, and so it would appear that light moves in perfectly straight lines – like machine-gun bullets. It would be very natural to assume that is what light consists of – particles akin to swiftly moving bullets. That view agrees with this diagram and even agrees if you start adding lenses and mirrors (at least up to a point).⁹ From such observations, a popular point of view evolved that light was composed of fleet little bullets of energy that moved in very straight lines. This bullet idea was called the particle theory of light, and a famous English scientist, apple-fell-on-his-head Isaac Newton, was its’s champion.¹⁰

8 Figure from https://commons.wikimedia.org/wiki/File:001_a01_camera_obscura_abrazolas.webp. Author unknown. This work is in the public domain in its country of origin and other countries and areas where the copyright term is the author’s life plus 70 years or less. This work is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before 1 January 1923.

7 Charge-coupled device https://en.wikipedia.org/wiki/Charge-coupled_device.

9 For more information on the ray tracing of light, see https://en.wikipedia.org/wiki/Ray_tracing_(physics).

10 You can find a biography of Newton here https://en.wikipedia.org/wiki/Isaac_Newton.

Newton’s Bullets

The challenge facing Newton was to get all his optics experiments, carried out in the late 1600s, to agree with his particle theory. Newton had developed an entire approach to mechanics where objects such as apples and the earth could be treated as point particles, so why not light? To explain how light moves through optical devices with lenses such as microscopes, telescopes, and eye...