He Seeks Mystery Magnetic Fields With His Quantum Compass

By Charlie Wood May 17, 2024

Collected at: https://www.quantamagazine.org/he-seeks-mystery-magnetic-fields-with-his-quantum-compass-20240517/

Atomic physicists “are jacks of all trades,” according to Alex Sushkov. “You have to have the idea, design the experiment, build the experiment, run the experiment, fix everything, take data, analyze data, write up the paper. You do everything,” and that “suits my personality.”

In his lab at Boston University, the Russian-born Australian is supercharging a 50-year-old tool for new purposes. Quantum particles have a property called spin, which points in a particular direction. Sushkov’s experiments use the spins of atoms as miniature compass needles that can sense other quantum particles through their magnetic influence. Researchers have long leveraged this phenomenon, called magnetic resonance, to spy inside bodies, identify chemicals and hunt for oil reserves. But Sushkov has been pushing magnetic resonance to its limits.

He has developed one of the most targeted magnetic resonance experiments to date, one that uses a defect in a diamond to feel the tickle of a magnetic field from an individual atom or molecule. In another experiment, Sushkov aims to achieve unprecedented sensitivity. He’s orchestrating troops of lead atoms in hopes that they’ll respond to the elusive magnetism of a hypothetical particle called the axion.

First proposed in the 1970s, axions are conceived as ripples in a field that would extend throughout the universe. The presence of such a field would make the strong nuclear force, which holds protons and neutrons together, symmetric in a way that’s observed in experiments. On top of that, axions are also perhaps the leading candidate for the identity of the invisible “dark matter” that appears to mold galaxies and galaxy clusters.

Experiments searching for axions are under development or already underway in roughly 30 labs worldwide. If Sushkov and his team succeed, their magnetic resonance experiment will be one of the most comprehensive of any axion searches.

Quanta recently visited Sushkov in Boston to discuss quantum compasses, doing experiments with diamonds, and the hunt for axions. The interview has been condensed and edited for clarity.

Introduction

One of your quantum compasses sits inside a diamond. Why a diamond? 

It’s not an accident. There are several special things about diamonds that make them great for quantum physics. Humans love diamonds because diamonds are transparent and sparkly, which is related to the fact that it’s hard to knock an electron loose from its host atom. Diamonds are also very hard, which means at room temperature there are very few vibrations in the atomic lattice.

These properties let us create an isolated qubit — a simple quantum system with a few different states — inside the diamond. You don’t need tunable lasers. You don’t need to create a vacuum. Everything works at room temperature. We make our qubit by bombarding the diamond with nitrogen ions. This punches out a bunch of gaps in the carbon lattice. Occasionally, a nitrogen ion will stick to a gap, becoming a “defect” in the lattice, with two extra electrons. This whole thing forms a qubit with different spin states, and it’s easy to do basic operations like flipping or measuring the spin.

What makes that qubit useful?

We can detect the magnetic field of a single atom or molecule. We create the defects in the lattice about 5 nanometers below the diamond’s surface, and then we sense the magnetic fields of things on the surface. In recent experiments, we’ve been able to detect single protein molecules, as well as interactions between particles floating around on the surface of the diamond.

Doing research with diamonds doesn’t sound cheap.

[Sushkov retrieves a plastic cylinder from a cabinet and shakes a diamond flake a few millimeters wide onto his desk.] This one probably cost a few thousand dollars, so don’t sneeze. They do tend to fly off. I’ve heard stories of them dropping into a bolt-hole in an optical table and it taking a week to try to extract them. But in the grand scheme of academic research, which includes graduate student salaries and a half-million-dollar dilution refrigerator, our diamond budget is negligible.

Introduction

Your other experimental platform will look for axions. How?

The original point of axions was to solve a mystery involving the strong force. To solve that problem, axions need to interact with protons and neutrons. And one way that axions would interact with protons and neutrons is basically to tip the spins of those particles. This tipping is the defining interaction of axions, and we can search for it with magnetic resonance.

How does magnetic resonance work?

You start with a particle that has spin. A spinning particle is basically like a compass needle; it likes to point “north” along any magnetic field in the background. But there’s one big difference between a compass needle and a quantum spin. If you tip a compass needle away from north, it swings straight back to north. Whereas if you tip a quantum spin, it will kind of spiral back toward north — it wants to “precess” about the magnetic field.

There are various games you can play with magnetic resonance, but the key idea is this: First, you tip the needle with a magnetic field, specifically one that “resonates” with the needle. Then, after the tip, you watch the needle precess as it returns to north. It might do this faster or slower than you’d expect, which tells you that there are more magnetic fields in the vicinity, created by other nearby atoms with spin. And you can tell what is making them.

For example, in magnetic resonance imaging, the compass needles are protons in your blood. The MRI machine lines the needles up, tips them, and lets them spiral back. When there are oxygen molecules around, their magnetic fields make these protons spiral up faster, so the MRI machine can map out the oxygen in your body. In the case of axions, the hope is that we can get atoms to resonate with any axion fields that might be out there.

I recently had an MRI due to a foot injury. Why didn’t my MRI find any axions?

Even if the MRI magnet was calibrated to get your protons precessing at the right frequency to resonate with axions, the machine wouldn’t be nearly sensitive enough to detect the tiny tipping that might result.

One little fact I like is the following. Suppose I had a single spin. It’s sitting there, and I have everything set up just right so the axions are interacting with this spin, and their field is tipping the compass needle away from north. It would take 1 million years to tip the spin all the way south. That’s how weak this interaction would be.

That’s a long time to spend in an MRI machine. Or doing an experiment.

We don’t want to wait for a million years. In the lifetime of the experiment, the tipping angle will be very small, perhaps 0.00000000001 degrees.

Introduction

How could you possibly measure such a small change?

Rather than focusing on a single spin, we focus on a lot of spins, like a coin-sized chunk of a lead-based compound with tons of atoms. If axions tip a lot of the spins, even if only by a tiny amount, we’ll be able to sense it. Then the question is: What is the fundamental limit of sensitivity with which you can measure all those spins?

The rules of quantum mechanics dictate that we can’t do it perfectly. Whenever we measure a bunch of spins, regardless of whether they’re actually tipped, we’re almost always going to pick up what looks like a little tipping just due to quantum randomness. That “quantum noise” is our floor. It imposes a limit on the sensitivity you can get, even with a perfectly sensitive detector.

But if we can eliminate every other source of noise, an axion-induced tipping should be detectable. So our ultimate goal is to build an experiment that’s limited by this quantum noise.

Last year you published a paper with an “O.P. Sushkov.” Any relation?

That’s my dad. He’s a theoretical condensed matter physicist, and he helped us analyze a new material that might be more advantageous than our lead-based compound for axion hunting. We crunched the numbers for this alternate material, and it looks promising.

What was collaborating with him like?

Sadly, he’s in Australia and I’m here, so most of it was over Zoom. But he came to visit last summer. And during those three weeks we wrote a paper and built a deck for my house. I guess that’s how we bond. We write physics papers, and we build decks.

Introduction

Axion hunting is becoming a bit of an industry these days. How does your experiment relate to other attempts to pin down these dark matter candidates?

I always think in terms of complementarity rather than competition. If axions exist, the span of masses they might have — and therefore frequencies we need to search — is huge. There’s plenty of room for everybody.

All experiments work by scanning the range of possible axion frequencies and looking for an interaction, a bit like you might scan an AM radio and look for a radio station. A lot of the other experiments scan by physically making their devices bigger and bigger. What makes our experiment unique is that we search for the radio station by tuning the magnetic field, which basically amounts to turning a dial.

Between you and the other dozen or so labs looking for axions, how much of the mass range will you be able to cover in the foreseeable future?

Eventually, perhaps over several decades, I think we collectively stand a good chance of covering most of the possible mass range for the axion.

 What do you think the odds are you’ll find axions?

That’s like me asking you what the odds are of you winning the lottery. Except that you know someone will win the lottery. In dark matter, we don’t even know if it’s axions or something else. The odds may be zero, or they may be high.

 What motivates you to pursue something so uncertain? 

It’s what motivates anyone to explore the unknown. It’s curiosity.

When I was in grad school at Berkeley, my grand professor (that’s my professor’s professor) and I were having a similar discussion at some point. And he said, “Alex, life is too short to do experiments whose outcome you already know.”