Quantum and Dance: It Takes 2 to Entangle

Credit: B. Hayes/NIST

One lecture in an undergraduate quantum physics class changed my life forever.

It was 2001, and the professor introduced the idea that physicists could use quantum particles to build a new kind of superpowerful computer. I was so captivated by the idea that quantum physics could lead to fundamentally new approaches to technology that I spent the rest of the semester devouring every resource I could find on the topic. I became so engrossed I nearly failed the class! However, that fascination has grown into a lifelong passion for exploring the quantum world.        

The rules that govern the smallest particles in nature are known as quantum mechanics, and quantum mechanics allows us a glimpse into how the tiniest things in our universe behave. 

I’ve always been curious about how the world works, so it isn't just the deep philosophical issues intertwined with quantum physics that interest me. I’m also fascinated by how emerging technologies based on quantum principles are set to have a potentially revolutionary impact on our society.

A few years after that undergraduate class, I moved to Toronto to pursue a Ph.D. in experimental quantum physics. One night, I took a break from working in the lab and went out with a friend to a jazz club. There was a couple dancing together on a postage-stamp-sized dance floor. The kinetic way they danced and the joy they exuded deeply moved me. Afterward, I peppered them with questions. The dance, they said, was the original swing dance called Lindy Hop. I stayed up all night watching Lindy Hop videos and trying to find dance classes I could take. (This was before YouTube, so you really had to search to find these things!)

The Poetry of the Quantum World

A goal of physics is to try to understand the rules that govern how the world works. Take gravity, for example. Sir Isaac Newton discovered that any two objects that have mass will be attracted to one another. This is why objects fall. Objects are attracted to the Earth, which has a great deal of mass. This simple principle that objects with mass are attracted to one another explains not just why objects fall, but also why Earth orbits the Sun and how distant galaxies are formed.

In contrast, when we study the atomic and subatomic worlds, we work with very tiny particles. One type of small particle of light is called a photon.

In this world, we find that the rules governing it are vastly different from those of our everyday experiences. Imagine throwing a ball at a wall, and instead of bouncing back, it occasionally appears on the other side. This phenomenon, called quantum tunneling, occurs because particles like photons or electrons can behave like waves, allowing them to pass through barriers that would normally stop them.

Quantum processes are also inherently unpredictable. Take, for example, the behavior of an atom when it encounters a barrier. There's no way to know in advance whether it will bounce back or tunnel through, as the outcome is governed by chance.

While the quantum world may seem bizarre, especially in popular books and movies, I view quantum mechanics as a poem the universe has written about its most fundamental workings. The language of this poem is mathematics. One definition of a poem is that it cannot be fully translated, and quantum mechanics is no exception. Translating the math and experimental observations of the quantum world into everyday experiences is challenging, which is why many quantum phenomena appear strange.

The Dance of Photons

Lindy Hop is a partner dance, so it is fitting that my research involves choreographing a different type of dance using photons. We use a sophisticated optical system to generate pairs of photons. These photons have properties that are intertwined with one another, and we say that they are entangled.

This entanglement is a much stronger connection than anything we experience in our everyday lives — stronger even than the connection any two dance partners share on the floor. Even if the photons are taken to opposite ends of the galaxy, measuring one photon will have an instantaneous effect on the state of the other. This is what Albert Einstein referred to as “spooky actions at a distance.”

One important concept to understand with entanglement is that we cannot use it to communicate faster than the speed of light. That’s because when one entangled photon is having an effect on the other, that effect is unpredictable. So we can’t use it to communicate faster than the speed of light because of that randomness element. 

Entanglement is one of the key ideas for understanding why the rules governing the quantum world are different. It is also one of the core principles that powers many emerging technologies based on quantum mechanics. While it is challenging to explain in lay terms, I've worked with both magicians and dancers in the past to try to convey the essential ideas and concepts of quantum mechanics and entanglement. While dance and magic may not be as precise as mathematics, they can powerfully convey a certain feeling of how complex concepts work.

Entangling Our Way to New Tech

As a scientist at NIST, I am especially excited about how quantum technologies can allow us to make better and more precise measurements. In particular, I'm interested in building technologies that can connect and entangle remote quantum systems. These advancements could revolutionize disaster response with ultrasensitive seismic sensors for earthquakes or create communication systems immune to cyberattacks. This can pave the way for a more secure and connected world.            

Building a quantum network that allows devices and systems at remote locations to communicate is challenging. Entangled photons are incredibly fragile, easy to lose and very faint. Typical optical networks, such as the ones used to send internet traffic long distances, use bright lasers to transmit data. These light signals can overwhelm the single photons we wish to send. It is like taking a swimming pool full of sand and trying to keep track of a single grain.

Right now, because entangled photons are so fragile, we can’t use them at any large scale. A large part of my research focuses on building high-quality sources of entangled photons that will enable long-distance quantum communication. This work has the potential to power new kinds of secure communications methods.

While quantum networks are still in their infancy, we have already demonstrated how they can do things that conventional (non-quantum) networks cannot. We recently used entanglement to build the best random number generator that nature allows.

True randomness plays a critical role in many of our secure communications systems. A public source of randomness can also be used in tasks that require fairness, like jury duty selection or assigning students to schools. Our team, in partnership with the University of Colorado Boulder, recently launched a randomness service that is based on our entangled photons. This service is designed to readily integrate into a wide variety of applications and is compatible with the concept of the next version of the web, known as Web3.

Building these quantum systems is hard, but NIST is the ideal place to carry out this work. I'm surrounded by many of the world's experts in a wide range of scientific areas. Often, whenever I encounter a problem, there is an expert just down the hall who can answer my question. This greatly accelerates the pace of research breakthroughs and is why NIST has been the epicenter of so many quantum-related innovations.

I'm proud of the work my colleagues and I are doing to help prepare for a quantum future. As we push the boundaries of what’s possible in quantum mechanics, I’m reminded that every great journey begins with a single step — or in my case, a dance.