Fermilab on the trail for a new building block of matter and quantum computing power

The electromagnetic ring used to measure the muon rotation frequency finds a new home at Fermilab. (Reidar Hahn/FERMILAB)

By Sarah Anderson and Yuliya Klochan
Medill Reports

Researchers transported a gigantic electromagnetic ring from Brookhaven National Laboratory on Long Island to Fermilab near Chicago eight years ago in the search for a new building block of matter.  While it wasn’t the secret spaceship bystanders thought it was, it did allow scientists to explore fundamental questions about our universe.

The ring was needed to confirm an experimental result that had intrigued particle physicists for 20 years.  The subject of the experiment was the muon, one of the 17 fundamental particles of nature.  The muon has the same negative charge as an electron, but the mass of about 200 electrons.  Muons behave like tiny spinning tops that generate their own magnetic field.

In 2001, scientists at Brookhaven National Laboratory measured the frequency at which muons rotated in an external magnetic field.  This rotation frequency is used to calculate a g factor—a scaling constant that relates the magnetic strength and rotational momentum of the muon.  The g factor is important because it can indicate the presence of other particles that block the muons’ interaction with the applied magnetic force.

The researchers observed that the experimental rotation frequency produced a g factor greater than the value predicted by the standard theoretical model of physics.  The Standard Model accounts for all the known fundamental particles and forces of nature, so the Brookhaven result hinted at the existence of undiscovered particles or forces.

“If these two numbers don’t agree with each other, it’s the space in the middle where the new physics can lie,” said Chris Polly, a senior scientist for the muon experiment at Fermilab.

The results of the Fermilab and Brookhaven muon experiments do not match the Standard Model prediction, hinting at the existence of an undiscovered particle or force. (Ryan Postel/FERMILAB)

Fermilab combined its muon-generating particle accelerator with Brookhaven’s electromagnetic ring to repeat Brookhaven’s initial experiment on a much larger scale.  They again observed that the measured rotation frequency did not align with the theoretical g factor, suggesting that the Standard Model may need to be overhauled.   There is only a 1 in 40,000 probability that the results differed by chance, providing further evidence of new physical forces or particles in the universe.

“Maybe there’s monsters lurking out there that we haven’t even imagined yet,” Polly said.

As experimental physicists at Fermilab work to replicate this result, theoretical physicists across the world are using simulations to scrutinize their theoretical models.  And they need powerful computers to do so.

Although it’s not yet ready to be used for the muon experiment, researchers at Fermilab are also working to develop technology for quantum computers that can solve such complex problems exponentially faster than standard computers.

Think of it this way. If someone gave you a list of locations and told you they had stashed a pile of cash at one of them, you would have no choice but to search one location, and then the next, and so on until you found it.  Standard computers are subject to this same limitation.  Just as you can only be in one place at a time, the system can only occupy one of two defined states (represented by the ones and zeroes you see in computer hacking movies) at a given moment.

Fermilab’s Quantum Lab features an environmental apparatus for testing superconducting qubits. (Reidar Hahn/FERMILAB)

But what if you could search many locations at the same time?  That’s essentially what a quantum computer does.  Its system can occupy multiple superimposed quantum states simultaneously, allowing the computer to consider many possible solutions to a problem at once.

“It actually is extraordinarily valuable in terms of being able to traverse through the entire computation space much more rapidly than a traditional computer,” said Akshay Murthy, a postdoctoral research associate at Fermilab.

Murthy and his colleagues are researching computer technology called superconducting qubits (quantum bits) that use electromagnetic radiation to access the higher-energy quantum states.  Specifically, they are working to prolong the qubits’ coherence time—the amount of time that the system can live in the quantum space and perform calculations.  Right now, we’re getting poofed out of the “everywhere at once” mode before we can find the cash.  In fact, the coherence times of qubits need to be 1,000 to 1 million times longer before they can be used for quantum computing.

To extend coherence times, the team is examining the qubits under a powerful microscope and analyzing the chemical composition of their surfaces to look for any defects that might cause occupation of the quantum states to come crashing down prematurely.  They are also exploring modifications that could be made to the external environment, such as shielding the qubit in a freezing cold chamber to prevent temperature fluctuations that might destabilize the system.

“This technology is truly transformational if we’re able to deliver on its promises,” Murthy said.

Sarah Anderson is a health, environment and science reporter at Medill and a Ph.D. chemist.  Follow her on Twitter @seanderson63.
Yuliya Klochan is a health, environment and science reporter at Medill.