Let’s use qubits to look for dark matter.

Examining Akash Dixit’s qubit-coupled cavity in the Schuster Laboratory (http://schusterlab.uchicago.edu/). Photo credit: Reidar Hahn (http://vms.fnal.gov/asset/detail?recid=1955845)

We’re starting a new research program and I’m excited to tell you about it!

Let’s start with the big picture, and then we’ll use subsequent posts to zoom in on the details. The big picture goes like this:

  1. 95% of the mass-energy in the universe is a huge mystery. Everything we see around us — pizza, dinosaur bones, galactic clusters — is made of atoms. But as best we can tell, atoms account for only 5% of the mass-energy in the observable universe. The rest is a big mystery that we call dark matter and dark energy. It would be fair to say that physicists are very interested in sorting out the remaining 95%. Aren’t you? If your everyday senses only reveal 5% of the known universe, what are you missing out on? Imagine a cosmic auditor who can tally up all the mass/energy (E=mc^2) in the universe.
  2. Dark matter is very probably made of particles, and we think a great candidate is a particle called the axion. The axion is cool because it solves other problems in physics in addition to dark matter. (We’ll talk about it later, but you can skip ahead and look up the “strong CP problem” if you want.) You may not have heard of axions yet, but the scrabble game on my telephone sure has.
  3. Axions are hard to detect. They’re a bit like neutrinos, in that they’re (probably) all around us but they hardly ever interact with normal matter. You detect them by converting them into photons and then looking for the photons. (Physicists are pretty good at counting photons; we’ve had a century of practice at it.) To further complicate matters, our theorists have narrowed down the axion mass to a window that spans three orders of magnitude. We have to design an axion search that is sensitive to potential masses (or equivalently, photon energies) between meV and ╬╝eV.
  4. So: a metaphor. Imagine you’re driving down a desert highway in an old car and you want to listen to the radio. Since you’re way out in the middle of nowhere, the radio is mostly static. How do you find a station? You’d probably tune the radio dial a little bit, listen for a while to see if you could pick out any signal in the noise, tune the dial, listen, tune, and so on. Eventually, if you started to pick out some faint music in the static, you’d know you were getting close. Same for us! In fact, our colleagues have an experiment called the Dark Matter Radio. The key point here is distinguishing signal from noise.
  5. Reduce, reduce, reduce the noise that competes with the signal. If you give me $100M for a 30-tesla magnet, I can give you a real strong, healthy axion signal. If you don’t have that kind of magnet money laying around, I’ll have to figure out some way to keep noise out of my experiment. One way to do this is to use quantum bits. I’ll get into this in a later post, but the technology we borrow from quantum information science gives us the ability to suppress experimental noise by four orders of magnitude. Not bad!

So there you go: the broad strokes for finding dark matter using quantum bits. Slowly but surely, we’ll take all those individual menu items above and expand on them in future posts. Exciting times!

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