A 96-ion barium quantum computer
In Chris Monroe and Crystal Noel’s group at Duke, we’re building a trapped-ion quantum computer that holds 96 barium-137 ions in a single chain. My part is designing the trap internals and building the machine itself.
Trap internals & building the machine
A trapped-ion quantum computer is mostly an ultra-high-vacuum chamber full of carefully engineered hardware, with an optics table’s worth of lasers pointed into it. Everything inside the vacuum has to be clean, non-magnetic, and electrically well-behaved, because the ions sit only a few hundred microns from the electrodes and feel every stray field.
I design the parts that live inside that chamber: the blade-trap electrode assembly, the structure that holds and aligns it, and the wiring that carries RF and DC voltages from the outside world to the trap. I also work on assembling and bringing up the full system, from vacuum to lasers to the first trapped ions.
How a trapped-ion quantum computer works
- Trap the ions. You can’t hold a charge in place with static fields alone (Earnshaw’s theorem), so a Paul trap uses a fast oscillating RF field to make an effective confining potential in two directions, and DC voltages to confine along the third. Ions that are laser-cooled to near rest repel each other and settle into a straight line, like beads on a string.
- Store a qubit in each ion. Each ion’s qubit is two internal energy levels. In 137Ba+ we use two hyperfine “clock” states of the ground level whose energy difference is first-order insensitive to magnetic-field noise, so they keep their phase for a long time.
- Entangle through shared motion. The ions are coupled by their mutual Coulomb repulsion, so the chain vibrates in collective normal modes. A laser can push on an ion differently depending on its qubit state. Choreograph that push so the chain’s motion traces a closed loop and returns to where it started, and the qubits come out entangled. This is the Mølmer–Sørensen gate. The explorer at the bottom of this page shows those modes.
- Read it out. Shine 493 nm light on the chain. Ions in one qubit state scatter millions of photons a second and glow. Ions in the other stay dark. A camera or photon counter sees which is which.
Why barium-137?
Most of barium’s useful transitions are in the visible. That means mature lasers, low-loss fibers, and cameras with good quantum efficiency, which matters a lot when you are trying to collect single photons.
The 5D5/2 state lives for roughly 30 seconds. A narrow 1762 nm laser can move (“shelve”) one qubit state there before readout, which makes the bright-versus-dark measurement very hard to confuse.
The odd isotope 137Ba has nuclear spin 3/2, which gives the ground state the hyperfine structure needed for a clock-state qubit. It’s only about 11% of natural barium, so loading has to be isotope-selective.
| Wavelength | Transition | What it does |
|---|---|---|
| 493 nm | 6S1/2 ↔ 6P1/2 | Doppler cooling and fluorescence detection |
| 650 nm | 5D3/2 ↔ 6P1/2 | Repump: the P1/2 state sometimes decays to D3/2, and this pulls it back into the cooling cycle |
| 1762 nm | 6S1/2 ↔ 5D5/2 | Narrow quadrupole transition used to shelve population for high-fidelity readout |
| 614 nm | 5D5/2 ↔ 6P3/2 | Deshelving: returns the ion from D5/2 to the ground state |
Normal-mode explorer
The gates talk to each other through these modes, so knowing their frequencies and shapes is half the battle in designing a gate. This solves the real equilibrium positions and mode spectrum for N ions in a harmonic trap. Frequencies are in units of the axial center-of-mass frequency ωz.