Quantum control of molecular ions

Seven beryllium ions and
two molecular ions (BeH+).

Our group develops quantum control techniques for molecules. Diatomic molecules are particularly interesting systems with potential applications in quantum memories, tests of molecular quantum theory, fundamental symmetry tests, and searches for time-variation of fundamental constants.

Lowest electronic potentials of the O2+ ion.

Control of molecular quantum states is complicated by the typical molecule's lack of suitable means for cooling or for preparing or detecting its quantum states. Our approach is to leverage advances in quantum information processing using trapped atomic ions. Here, we co-trap a diatomic molecular ion with an atomic ion. The atomic ion is chosen such that we can use it for all dissipative tasks, including translational cooling, initial-state creation, and internal-state detection. The interaction between the two charged particles acts as an information bus, where the presence or absence of motion provides information about the state of the molecular ion.

As an example of the sort of measurement enabled by these techniques, we are investigating the use of nonpolar diatomic molecular ions in searches for time-variation of the proton-to-electron mass ratio. A variation of this parameter would indicate new physics and is predicted by some theories of quantum gravity such as string theory. Molecules are excellent candidates for such a search because their vibrational motion is directly sensitive to the mass-ratio variation. Nonpolar molecules (for example homonuclear diatomic molecules) should be less sensitive to many systematic effects. Our current molecule of interest is O2+, and we have published a paper in Physical Review A describing its features in detail (pdf) (PRA link).

The lab

Our lab at Amherst College.

Our experiment is built around a radiofrequency ion trap. The trap is housed in an ultrahigh vacuum chamber, which has access for lasers, imaging, and electrical connections. We load our atomic ions (beryllium) by electron-impact ionization of neutral beryllium, which come from a hot beryllium wire. We have also loaded molecular ions such as O2+ by electron-impact ionization of neutral molecules admitted to the chamber through a precision leak valve. We have formed molecules like BeH+ by chemically reacting the trapped Be+ ions with a background gas. We are working to photoionize O2+ into a known ro-vibrational state with a pulsed dye laser.

Quantum control of the Be+ ions uses a laser at 313 nm. We made this laser by generating the third-harmonic of an amplified 939 nm diode laser. Our implementation is described in an article in Optics Express (pdf) (OE link). We use acousto-optic modulators for fine control of the laser frequency and of the duration of laser pulses. Quantum control of the molecular ions will be from a variety of sources such as a commercial femtosecond laser, quantum cascade lasers, and radiofrequency sources.

Student involvement

The Qiao ion trap

Undergraduate students are important participants in this research. Shenglan Qiao '13 designed the trap electrodes and mounting structure and worked with the College's machinist to fabricate them on our CNC mill. Cheyenne Teng '14E built a system that removes long-term frequency drift of our diode laser by referencing it to a stabilized helium–neon laser. Phyo Aung Kyaw '14 worked on the vacuum chamber and built a radiofrequency resonator that provides high voltage to our trap electrodes.

Dust in the paperclip trap
(illuminated by a green laser)

Celia Ou '13 worked on nonlinear optics for the beryllium laser system. Jiajun Shi '15E worked on control for the acousto-optic modulators. Ned Kleiner '16 and David Lane '17 took some of our first data with Be+ ions, aided in molecular-theory calculations, and participated in some of our first experiments with molecular ions. You can read about their projects in the theses section of our publications page. Many other students have helped with smaller projects such as building photodetectors, setting up a data-logger for ambient lab conditions, and building a radiofrequency signal-conditioning box for a Pound-Drever-Hall lock. Students have even built a demonstration "paperclip trap" that uses 6 kV at 60 Hz to trap dust particles with the same technique we use for atomic and molecular ions.

Current poster

Here is a link to our current poster.

Ryan Carollo and Alex Frenett at DAMOP 2018 in Ft. Lauderdale, FL.

Prior work

National Institute of Standards and Technology

As a postdoc with David Wineland in the Ion Storage Group at NIST, I contributed to combining for the first time all the fundamental elements required for scalable quantum computing using qubits stored in internal states of trapped atomic ions. In addition, we demonstrated the first programmable two-qubit quantum processor. At NIST, we used 9Be+ to store and manipulate quantum information. We co-trapped 24Mg+ ions, which we laser-cooled to sympathetically cool the 9Be+ ions without disturbing the qubit state. This dual-species approach is a key tool informing our work at Amherst. The other techniques used, such as ground-state-cooling of the ions' motion and laser-based quantum gates, also heavily influence our work.

Harvard University

As a graduate student with Gerald Gabrielse at Harvard, I contributed to two new measurements of the electron magnetic moment (the "g-value"). These measurements probe the electron's interaction with the fluctuating vacuum. With a quantum electrodynamics calculation, they provide the most accurate determination of the fine structure constant. Given an independent determination of the fine structure constant, such measurements set limits on electron substructure and other extensions to the Standard Model of particle physics. Our second measurement, which formed the basis of my Ph.D. thesis, measured the moment at a relative uncertainty of 0.28 parts-per-trillion. It yielded a value of the fine structure constant with a relative accuracy of 0.37 parts-per-billion, over an order of magnitude smaller uncertainty than the next-best methods at the time. Our techniques included trapping a single electron with static electromagnetic fields, cooling it to the quantum ground-state of its fastest motion, and resolving single quantum jumps of this motion and single spin-flips.