An atomic clock’s projected error allowance is about one second per 100 million years – a reminder of just how rudimentary our beloved mechanical watches actually are. To put it in perspective, one of the most accurate modern quartz wristwatches, the Citizen Caliber 0100, is a feat of engineering that allows for an error of roughly one second per year. That means that over 100 million years, there’d be an expected error of just over three years.

That’s, uh, a pretty big gap. And thanks to a recent breakthrough at MIT, the atomic clock is now even more accurate.

Physicists in the university’s Research Laboratory of Electronics have developed an atomic clock that takes advantage of the way atoms behave when they’ve been quantumly entangled.

A typical atomic clock works by measuring the vibrations at the atomic level of cesium-133 atoms, which oscillate with absolute consistency, 9,192,631,770 times a second. Traditionally, lasers measure a cloud of these randomly oscillating, cooled-down cesium-133 atoms. MIT’s new clock, however, uses ytterbium atoms that have been entangled, meaning the atoms behave in a uniform fashion, with the atoms oscillating in sync rather than randomly. Ytterbium’s oscillation rate is 100,000 units faster per second than cesium-133, and these oscillations can be tracked more precisely, allowing the team to measure even smaller intervals of time. In other words, they’ve achieved increased accuracy.

Quantum entanglement creates a situation where the ytterbium atoms in this atomic clock defy the laws of classical physics. The team says this new method of atomic timekeeping is not only more accurate, but it’s also a step toward learning more about complex ideas like dark matter and the behavior of gravitational waves.

No one can explain the science better than the folks who developed it. So we asked four scientists from the project to break it all down. The answers below come from MIT’s Research Laboratory of Electronics, Lester Wolfe Professor of Physics Vladan Vuletić and Postdoctoral Researchers Edwin Pedrozo-Peñafiel and Simone Colombo, along with Chi Shu, a Ph.D. candidate from the MIT-Harvard Center for Ultracold Atoms (CUA).

**HODINKEE: **In order to understand how your new clock works, I think it’s important to understand how a standard atomic clock works first.

**MIT: **To keep time, people need something that is very regular to measure against. For many centuries, people used the motion of the earth around the sun. The year and day became the standard at first, then the hour and the minute. But the truth is that it isn’t exactly regular. The planets are pulling on the earth a little bit and creating slight variations.

So in the 1960s, it was established that one could use the oscillation of atoms to measure time instead. And it proved to be more stable than the motion of the earth around the sun or any quartz oscillator, and for that matter, *any* mechanical device used to measure time.

In some sense, the oscillation of atoms can be considered “mechanical,” but on a very, very small scale. The reason atoms are so good as oscillators is because we have learned how to keep atoms in space, away from everything else. So take individual atoms, keep them in space, and now we can even isolate and trap them. In the 1960s, the second was established as a certain number of oscillations – around 9 billion – of a cesium atom. And that standard has been kept for almost 60 years.

Recently, there’s been a kind of revolution where people no longer use these oscillations. Now we can measure microwave frequencies and observe about 10 billion oscillations per second. Even further, we can now observe atoms in a state where the oscillations are a hundred thousand times faster. So we are now talking about a hundred trillion oscillations per second. We’ve learned how to use lasers to count these oscillations, and it’s redefined what we know by using these so-called “optical frequencies” which allow us to measure with much more precision.

Atomic clocks now are, by far, the most accurate instruments that mankind has ever made. Modern atomic clocks are so good that if you ran them for the age of the universe, which we count since the big bang, they would be off by only about 10 seconds. They are far more accurate than any other instrument that mankind has ever made.

**HODINKEE: **So we’ve come this far in terms of the sort of accuracy we can achieve. Now, how are you advancing atomic timekeeping even further?

**MIT: **There’s something very specific about the behavior of atoms as quantum mechanical oscillators. If we compare the atom to a pendulum, it would only be observable at one of the two further points on the swing; we are not able to observe the entire motion of the “swinging” pendulum when it comes to examining the positions of the oscillations in these atoms. We can only decide in a binary fashion if it’s at its furthest point one way or another. When you measure it, sometimes it’s at one “turning point” or the other turning point.

It’s a lot like tossing a coin: You get heads or tails each time. When you toss a coin, each toss is independent, it results in a heads or tails. When you toss a hundred coins, you’d expect a 50/50 distribution between heads and tails if the coin is unbiased, right? But in reality, we know that it’s something like 49/51 or 52/48. There’s some sort of randomness in how the results are averaged. The more you average, the closer you would get to 50/50 probability, however. But “entanglement” is like magical coins. Imagine a hundred coins that somehow know about each other so that each individual coin is random, but they will always decide to average to exactly 50 percent heads and 50 percent tails.

**HODINKEE: **For those of us who aren’t familiar with quantum mechanics, can you explain what “entanglement” means?

**MIT: **It has to do with quantum correlations. Einstein very much disliked it; it’s this so-called “spooky action at a distance.” So imagine that I have a box with a red and blue ball in it. I shake the box and I cut it into two parts. One contains the red ball, and one contains the blue ball.

Now imagine that I give you one box and I keep the other box. You don’t know what is in your box. It could be a blue or red ball, but what you definitely know is that whatever is in your box, I have the opposite color, right? So if you have a blue ball then I have the red ball. But it’s not actually about the outcome, but rather the correlations between two things. It’s about how particles behave relative to one another.

In quantum mechanics, these correlations can be much stronger than in classical physics.

**HODINKEE:** So it sounds like, at the most basic level, quantum mechanics looks at the relationship between two things whereas classical physics isn’t as concerned with it. And you’re using entanglement to take advantage of these mysterious relationships and reduce randomness in the system, therefore allowing a more accurate reading of oscillations, and in turn more precise measurements of time.

**MIT:** Yes. That’s a very nice description.

**HODINKEE: **You hinted that your research could answer age-old questions and reveal more about dark matter and gravitational waves. What sort of questions might it answer? Is this one step closer to finding an answer to the question, “What is the meaning of life?”

**MIT: **Well, in a physicist’s mind, the big questions have more to do with how the universe came to be and what has occurred over this timespan. How does time actually flow? We’re not saying that when we conclude our research we’ll be able to answer this, but what we will know is that we’ll be one step closer.

There are many questions in fundamental science that we don’t know the answers to simply because we don’t have devices that could measure them. This new atomic clock *can *measure things that we haven’t been able to before. It can measure time far more precisely. We already accept some constants as true, but now we can measure if they *actually are* constant or not.

Take Einstein’s theory of relativity for example. We’re already seeing examples of it being true in GPS devices. If you have one clock here on earth, and you compared it with the clock that the GPS sees without taking into account some relativity, you’d have some problems. There are about 46 microseconds difference between them, and that sounds small, but that translates to variation in precision. Let’s say the GPS is off by two meters one day because of those 46 microseconds – not long after, it will be off by ten meters, and after that, it will be completely useless.

The actual flow of time is changing, and by measuring that, we can get closer to answering the bigger questions. Gravity has an effect on time, and there are even larger oscillations happening in spacetime. Tiny changes and shifts happen in time, and we don’t necessarily have an answer as to why.

**HODINKEE: **So to sum it up, there are practical applications for more accurate atomic timekeeping, of course, but there are larger, sometimes philosophical questions we have yet to answer – and your breakthrough is helping us get there.

**MIT:** Exactly. And I think it’s important to mention that we like to think of time as absolute, but it really depends on gravity. In our everyday life, sure, it is seemingly absolute, but the effects of relativity still do exist. Even the gravitational field of your body can change the time, and even though it’s incredibly small, it’s present. Now we’re closer to being able to measure that more accurately, and there will be a very practical benefit to that. This idea sort of challenges what a clock constitutes.