Paul is President of Quantum Computing at ColdQuanta, where he leads the team building the world’s most useful quantum computer.
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So much of modern life relies on GPS that it has been estimated that an outage would cost the U.S. economy over $1 billion per day. Such an outage isn’t too far-fetched. We have seen recent examples of GPS being blocked in conflict zones, spoofed for nefarious purposes and disrupted by solar storms. At its heart, GPS is a timing system. To understand how timing relates to navigation, and how quantum technology will enable us to overcome the limitations of GPS, we need to go back in history a few hundred years.
Dava Sobel’s book Longitude recounts the tale of John Harrison’s mechanical clock that revolutionized maritime navigation and global trade. In the same way that Harrison’s clock accelerated economic growth through the 18th and 19th centuries, optical clocks are poised to radically transform industries as diverse as transportation, financial services, communication and energy.
Fundamentally, building an accurate clock entails finding something that ticks fast with a very high degree of regularity and stability. Harrison’s H4 clock, completed in 1759, was a mechanical device that ticked five times per second and kept time to within one second over a period of a month. Today we are on the cusp of commercial optical clocks that tick over one hundred trillion times per second and are accurate to within one second over the entire age of the universe. This astounding improvement in timing precision is arguably the most substantial technological leap in human history.
Time orders the events of our daily lives. What time is my next meeting? How many minutes do I have to catch my flight? What time does the game start? We measure events’ times in hours, minutes and seconds. In the case of an NBA buzzer-beater, perhaps we’ll get down to tenths of a second. However, many technologies that we rely on utilize vastly more precise time measurements.
For example, we take GPS for granted. Just pull up an app on your phone and you know your location anywhere on earth to within a few meters. GPS is essentially a timing system. Each GPS satellite holds four atomic clocks that transmit an integrated time signal. GPS receivers (like the ones on our phones) use the small differences in time signals from multiple satellites—in combination with their local time reference—to triangulate your position.
The international definition of the second is based on the frequency of the ground-state quantum transition of the Cesium-133 atom, which is over 9 billion “ticks” per second. The first Cesium clock was built in 1955, and this frequency has remained the global timing standard ever since. Some quick math illustrates how this tick rate determines the positional accuracy of GPS. The GPS radio signal travels at the speed of light, which travels approximately 30 cm. in a billionth of a second. So in the time of a single Cesium “tick,” light travels approximately 3 cm. Although the practical reality is more nuanced, this “back of envelope” gives a sense of the maximum positional accuracy of GPS.
Highly accurate, deployable quantum clocks would enable a reliable and stable source of time, independent of GPS, allowing precise navigation even when GPS is unavailable (e.g., in space, under the ocean or in the mountains), denied or otherwise contested. To achieve this goal, we need a timing source that ticks faster than Cesium. Enter optical clocks, which utilize Strontium or Rubidium atoms, with frequencies that “tick” over 100 trillion times per second, more than 10,000 times faster than Cesium. These atoms can be cooled close to absolute zero in a lattice grid of laser light, reducing noise and improving timing stability.
Realizing the importance of accurate timing to the future of its maritime leadership, the UK Longitude Act of 1714 offered prizes worth over $2 million today to catalyze the development of accurate and stable marine clocks. Today, governments are similarly funding the development of deployable optical clocks that can be used to address real-world applications.
The uses for these clocks go far beyond navigation. This year the world will create, store and access over 100 Zettabytes of data (a Zettabyte is 1,000 trillion GB), much of which is stored in large, globally distributed databases. Every database transaction must be time-stamped to ensure that data doesn’t get out of sync. For example, if two copies of a financial transaction are stored in separate databases on different continents, with vast numbers of daily transactions, without accurate time-stamping, it would be impossible to reconcile a source of truth. Modern data centers house atomic clocks to provide a local timing source, integrated with GPS timing and with each other to ensure accurate time synchronization.
Database time-stamping resolution is largely a function of clock frequency—the faster the clock, the more transactions can occur in a given time interval, enabling higher volume applications and more efficient data center scaling as the world’s data needs accelerate.
Einstein’s General Theory of Relativity explains how a gravitational field slows time. Optical lattice clocks have been used at the University of Wisconsin–Madison and at the University of Colorado Boulder to measure this gravitational time dilation on sub-centimeter scales. The ability to accurately measure minute changes in gravity will transform fields such as mineral exploration, earthquake prediction and national security.
Over 300 years have passed since the Longitude Act catalyzed clock development and revolutionized the global economy. We stand on the cusp of commercial optical clocks with accuracies that would have been literally unimaginable to our forebears. These clocks will open up new domains of time with tremendously exciting implications for new discoveries, applications and economic growth. Tick-tock—let’s get started!
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