Control of Cold Molecular Ions

On Tuesday, Dzmitry Matsukevich gave a talk on “Control and Manipulation of Cold Molecular Ions”. He just arrived here at the CQT, coming from Christopher Monroe’s Trapped Ion Quantum Information Group at the University of Maryland.

Cold molecules can be used to study:

• Quantum information
• Precision measurements
• Quantum chemistry
• Strongly interacting degenerate gases

But most work uses neutral molecules; work on cold molecular ions is a bit new. The advantage of working with ions is that since they’re electrically charged, they can be trapped in radio-frequency Paul traps. This allows them to be isolated from the environment for days or weeks, and methods developed for ion trap quantum computations can be applied to them. On the downside, it’s hard to find spectroscopic data on most molecular ions.

Molecules and molecular ions can wiggle in various ways, with different characteristic frequencies:

• Vibrational modes: about 30 terahertz

• Rotational modes: about 10-100 gigahertz

• Hyperfine modes: about 1 gigahertz

Room temperature corresponds to a frequency of about 6.25 gigahertz, so lots of modes are excited at this temperature. To make molecular ions easier to understand and manipulate, we’d prefer them to jump around between just a few modes: those of least energy. For this, we need to cool them down.

How can we do this? Use sympathetic cooling: our molecules can lose energy by interacting with trapped atomic ions that we keep cool using a laser!

(It might not be obvious that you can use a laser to cool something, but you can. The most popular method is called Doppler cooling. It’s basically a trick to make moving atoms more likely to emit photons than atoms that are standing still.)

Once our molecular ions are cold, how can we get them into specific desired states? Use a mode locked pulsed laser to drive stimulated Raman transitions.

Huh? As far as I can tell, this means “blast our molecular ion with an extremely brief pulse of light: it can then absorb a photon and emit a photon of a different energy, while itself jumping to a state of higher or lower energy.”

Here “extremely brief” can mean anywhere from picoseconds (10^-12 seconds) to femtoseconds (10^-15 seconds).

Once we’ve got our molecular ion in a specific state, it’ll get entangled with neighboring atomic ions thanks to their collective motion. This lets us try to implement quantum logic operations. There’s a large available Hilbert space: many qubits can be stored in a single molecule.

This paper shows how to use stimulated Raman transitions to create entangled atomic qubits:

• D. Hayes, D. N. Matsukevich, P. Maunz, D. Hucul, Q. Quraishi, S. Olmschenk, W. Campbell, J. Mizrahi, C. Senko, and C. Monroe, Entanglement of atomic qubits using an optical frequency comb, Phys. Rev. Lett. 104 (2010) 140501.

Precision control of molecular ions also lets us do precision measurements! Hyperfine modes depend on the mass of the electron and the fine structure constant. Vibrational and rotational modes depend on the mass of the proton. This allows accurate measurement of the ratio of the proton and electron mass.

(People looking at quasars in different parts of the sky see different drifts in the fine structure constant. One observation in the Northern hemisphere sees the fine structure constant changing, while one in the Southern hemisphere sees that it’s not. It’ll probably turn out nothing real is happening — at least that’s my conservative opinion — but it’s worth studying.)

What molecular ions are good to use?

• ionized silicon oxide, SiO⁺: convenient transition wavelengths, no hyperfine structure, and… umm… almost diagonal Frank-Condon factors.

• ionized molecular chlorine, Cl_2⁺: the fine structure splitting is close to the vibrational splitting, so this is good for precision measurements of the variation of the fine structure constant.

Matsukevich’s goals in the next 3 years:

• build an ion trap apparatus for simultaneously trapping Yb⁺ atomic ions and SiO⁺ molecular ions: the ytterbium lets us do sympathetic cooling.

• develop methods to load them, cool them, and do cool things with them!