Self-Assessment Quiz: Trapped Ion Qubits

Twenty questions on ion traps, laser-driven gates, all-to-all connectivity, and the platform with the highest fidelities. Aim for 16+.

Question 1

Trapped-ion qubits are encoded in:

A) Internal electronic states of individual atomic ions B) Superconducting circuits C) Photon polarization D) Nuclear spins in silicon

Question 2

A major advantage over superconducting qubits is:

A) Qubits are identical by nature — no fabrication variation B) Faster gates C) Higher temperature operation D) Simpler control electronics

Question 3

Ions are confined by:

A) Oscillating electric fields in a Paul trap B) Magnetic bottles only C) Physical walls D) Optical fibres

Question 4

Two-qubit gates couple ions via:

A) Shared motional modes of the ion chain B) Direct electrostatic contact C) Microwave resonators D) Photon exchange only

Question 5

The canonical two-qubit gate is the:

A) Mølmer–Sørensen gate B) CNOT by direct coupling C) CZ via flux D) Toffoli

Question 6

Because all ions share motional modes, connectivity is:

A) All-to-all within a chain B) Nearest-neighbour only C) One-dimensional with degree 2 D) Random

Question 7

That connectivity advantage means:

A) Far fewer SWAP gates, so lower effective circuit depth B) Faster gates C) More qubits D) Lower error rates automatically

Question 8

Typical trapped-ion two-qubit gate times are:

A) ~100 ns B) ~100 μs C) ~1 ms D) ~1 s

Question 9

Compared to superconducting gates, that is:

A) Roughly 100–1,000× slower B) Faster C) Equivalent D) 10× faster

Question 10

Trapped-ion coherence times are typically:

A) Seconds to minutes B) Microseconds C) Nanoseconds D) Hours

Question 11

State preparation and measurement use:

A) State-dependent fluorescence — the ion scatters photons only in one state B) Dispersive readout C) Charge sensing D) Tunnelling

Question 12

SPAM error in leading trapped-ion systems is around:

A) $10^{-1}$ B) $10^{-2}$ C) $10^{-3}$ to $10^{-4}$ D) $10^{-6}$

Question 13

The main scaling obstacle for a single long chain is:

A) Motional mode crowding and slower gates as the chain lengthens B) Fabrication C) Cooling power D) Photon loss

Question 14

The QCCD architecture addresses this by:

A) Physically shuttling ions between separate trap zones B) Adding more lasers C) Cooling further D) Using microwaves only

Question 15

Quantinuum's H-series machines are notable for:

A) Highest quantum volume and gate fidelities, with modest qubit counts B) Largest qubit counts C) Room-temperature operation D) Lowest cost

Question 16

True or false: Trapped ions require dilution refrigerators.

Question 17

True or false: All-to-all connectivity means trapped ions always outperform superconducting devices.

Question 18

True or false: Slow gates are a fundamental limitation with no path to improvement.

Question 19

Short answer. Explain the central trade between trapped ions and superconducting qubits.

Question 20

Short answer. Why does all-to-all connectivity matter more than a raw gate-fidelity comparison suggests?

Answer Key

Q Ans Note
1 A Hyperfine or optical transitions in ions such as $^{171}$Yb$^+$ or $^{40}$Ca$^+$.
2 A Every $^{171}$Yb$^+$ ion is exactly like every other — no frequency spread, no per-qubit fabrication calibration.
3 A RF Paul traps; Earnshaw's theorem forbids purely static 3D confinement.
4 A Laser pulses couple internal states to the shared vibrational modes, mediating entanglement.
5 A The MS gate, robust to motional-state temperature.
6 A Any pair in the chain can be entangled directly — the platform's signature advantage.
7 A No routing overhead; a circuit needing all-to-all runs as written.
8 B Tens to hundreds of microseconds.
9 A The platform's signature disadvantage.
10 A Hyperfine qubits can have coherence times of seconds or longer — orders of magnitude beyond superconducting.
11 A Cycling transitions give near-perfect state discrimination by collecting scattered photons.
12 C Among the best of any platform, and far better than superconducting readout.
13 A More ions means more motional modes, closer together, requiring slower gates to avoid exciting the wrong one.
14 A Quantum Charge-Coupled Device: small chains in zones, ions shuttled between them.
15 A Consistently highest QV, with tens rather than hundreds of qubits — the fidelity-over-count strategy.
16 False They need ultra-high vacuum and laser systems; many operate at room temperature or ~4 K for cryopumping, not millikelvin.
17 False Slow gates mean fewer operations per unit time, and shuttling adds overhead. Which platform wins depends on whether the workload is depth-limited, connectivity-limited, or throughput-limited (Ch. 24's architecture case study).
18 False Gate speed is limited by motional frequencies and laser power; faster gates have been demonstrated using stronger drives and shaped pulses, at the cost of higher error. It is a real trade, not a hard wall.
19 — Superconducting: fast gates (~100× faster), lithographic scalability, but short coherence, limited connectivity, and fabrication variation requiring per-qubit calibration. Trapped ions: near-perfect qubit uniformity, coherence orders of magnitude longer, all-to-all connectivity, and the best gate and SPAM fidelities — but gates 100–1,000× slower and scaling requiring shuttling or photonic interconnects rather than lithography. Neither dominates; they trade speed and manufacturability against fidelity and connectivity.
20 — Because a fidelity comparison per gate ignores how many gates a circuit needs. On limited-connectivity hardware, a circuit requiring non-local interactions pays 3 CNOTs per SWAP plus routing depth — often doubling or tripling the two-qubit gate count (Ch. 7). A device with 10× worse per-gate fidelity but no routing overhead can deliver a better end-to-end result on connectivity-hungry circuits. Compare total circuit fidelity on the transpiled circuit, not per-gate error rates.