Trapped Ion
Qubits encoded in individual atoms suspended by electromagnetic fields, offering high fidelity and long coherence times.
Trapped ion quantum computers use individual atoms confined by electromagnetic fields as qubits. The electronic states of these ions encode quantum information.
How It Works
Trapping
Ions are held in place by oscillating electric fields (Paul trap or linear trap):
- Radio-frequency fields create an effective potential well
- Ions levitate in vacuum, isolated from the environment
- Can trap chains of tens of ions
Qubit Encoding
Two approaches:
| Type | States | Typical Splitting |
|---|---|---|
| Hyperfine | Nuclear spin states | ~GHz |
| Optical | Ground + metastable excited | ~THz |
Example: Ytterbium-171 (Yb+) hyperfine qubit uses two nuclear spin states.
Control
Single-Qubit Gates
- Apply focused laser beams to individual ions
- Stimulated Raman transitions between qubit states
- Can also use microwave pulses
Two-Qubit Gates
- Ions interact via shared motional modes
- Laser pulses create spin-dependent forces
- Mølmer-Sørensen gate: ~99.9% fidelity demonstrated
Readout
- Shine resonant laser
- Ion in bright state: fluoresces (measure 1)
- Ion in dark state: no fluorescence (measure 0)
Advantages
| Advantage | Details |
|---|---|
| Identical qubits | All atoms of same species are identical |
| Long coherence | T2 up to seconds or minutes |
| High fidelity | Best gate fidelities achieved |
| All-to-all connectivity | Any ion can interact with any other |
| Optical interface | Natural for quantum networks |
Specifications
| Parameter | Typical Value |
|---|---|
| T1 time | Seconds to minutes |
| T2 time | Seconds (with dynamical decoupling) |
| 1-qubit gate fidelity | >99.99% |
| 2-qubit gate fidelity | >99.9% |
| Gate time (1Q) | ~1-10 μs |
| Gate time (2Q) | ~100-500 μs |
| Readout fidelity | >99.9% |
Challenges
| Challenge | Issue |
|---|---|
| Speed | Gates are slower than superconducting |
| Scaling | Adding ions to chain becomes harder |
| Optics complexity | Many laser beams needed |
| Vacuum requirements | Ultra-high vacuum needed |
Scaling Approaches
Quantum CCD
Move ions between trapping zones using electrode voltages. Demonstrated by Quantinuum.
Photonic Interconnects
Connect separate ion traps via optical fiber using photon-mediated entanglement.
2D Arrays
Trap ions in 2D grids rather than linear chains.
Major Players
- Quantinuum (Honeywell + Cambridge Quantum): QCCD architecture
- IonQ: Linear chains, photonic scaling
- Alpine Quantum Technologies: European player
- University labs: NIST, Duke, Oxford, Innsbruck
See also: Qubit, T1 Time, T2 Time, Quantum Gate