Laser Control of Qubits
Trapped ions, neutral atoms, and NV centers are controlled with laser light, using resonant, Raman, or Rydberg transitions to drive gates.
Light as the control field
Atomic qubits are manipulated with laser light rather than microwaves. Depending on the encoding, a laser can directly drive an optical transition, drive a two-photon Raman transition between ground-state sublevels, or excite an atom to a Rydberg state for entangling gates. The laser's frequency, intensity, phase, and duration set the operation.
Optical and Raman transitions
Optical qubits use a narrow-linewidth laser resonant with a ground-to-metastable transition; the laser's own frequency stability directly limits gate quality, so it is locked to an ultra-stable reference cavity. Hyperfine qubits are driven by two laser beams whose frequency difference matches the qubit splitting, a Raman transition that inherits the excellent stability of a microwave-frequency difference while using optical beams that can be focused onto a single atom.
Cooling and initialization
- Doppler and resolved-sideband cooling bring atoms near their motional ground state
- Optical pumping initializes the qubit to a known internal state
- Fluorescence detection reads the qubit by state-dependent scattering of light
Two-qubit gates
For ions, lasers couple internal states to shared motion to run Molmer-Sorensen gates. For neutral atoms, lasers drive the Rydberg excitation that creates the blockade interaction. In both cases beam pointing, intensity noise, and phase stability set the fidelity ceiling.
Engineering demands
Laser control requires stable, well-characterized optical systems: frequency locks, low-noise amplitude control, precise beam delivery to individual atoms, and integration of many beams as qubit counts grow. Photonic integration of optics onto trap chips is an active effort to make laser control scale the way microwave control has for superconducting devices.