Self-Assessment Quiz: Photonic, Neutral Atom, and Other Approaches

Twenty questions on the platforms beyond superconducting and trapped ions, and why no approach has yet won. Aim for 16+.

Question 1

Neutral-atom qubits are held by:

A) Optical tweezers — focused laser beams B) RF electric fields C) Superconducting loops D) Physical cavities

Question 2

Entangling gates between neutral atoms use:

A) Rydberg blockade B) Shared motional modes C) Josephson coupling D) Photon exchange only

Question 3

The Rydberg blockade works because:

A) An excited atom shifts its neighbours' energy levels, preventing simultaneous excitation B) Atoms collide C) Photons are absorbed D) Magnetic fields align

Question 4

The distinctive advantage of neutral atoms is:

A) Reconfigurable geometry — atoms can be physically rearranged B) Fastest gates C) Room-temperature operation D) No lasers needed

Question 5

That reconfigurability makes them attractive for:

A) qLDPC codes requiring non-local stabilizers B) Surface codes only C) Classical computing D) Cryptography

Question 6

A significant neutral-atom challenge is:

A) Atom loss during operation, requiring reloading B) Fabrication variation C) Wiring heat load D) Charge noise

Question 7

Photonic qubits encode information in:

A) Photon properties such as polarization, path, or time-bin B) Atomic states C) Superconducting phase D) Electron spin

Question 8

A major photonic advantage is:

A) Room-temperature operation and natural networking B) Deterministic two-qubit gates C) Long-lived memory D) High gate rates

Question 9

The central photonic difficulty is:

A) Two-qubit gates are probabilistic without strong nonlinearity B) Photons decohere quickly C) Photons cannot be measured D) Photons are too slow

Question 10

Measurement-based / one-way quantum computing uses:

A) A large entangled cluster state consumed by adaptive measurements B) Gates only C) No entanglement D) Classical bits only

Question 11

Photonic approaches often rely on:

A) Cluster states plus fusion gates, with heavy multiplexing to overcome probabilistic success B) Deterministic CNOTs C) Ion shuttling D) Flux tuning

Question 12

Silicon spin qubits are attractive because:

A) They may leverage existing semiconductor manufacturing B) They are the fastest C) They need no cooling D) They have all-to-all connectivity

Question 13

Topological qubits aim to:

A) Encode information non-locally so local noise cannot corrupt it B) Run at room temperature C) Eliminate measurement D) Avoid entanglement

Question 14

The status of topological quantum computing is:

A) Still unproven — definitive demonstration of the required quasiparticles remains contested B) Commercially deployed C) Fully demonstrated D) Mathematically impossible

Question 15

Quantum annealing (e.g. D-Wave) is:

A) A different computational model without gate-model universality or proven speedup B) Equivalent to gate-model computing C) Faster than all classical methods D) The basis of Shor's algorithm

Question 16

True or false: One hardware platform has clearly won.

Question 17

True or false: Photonic quantum computers need no cryogenics at all.

Question 18

True or false: Neutral-atom systems have demonstrated the largest numbers of entangled logical qubits.

Question 19

Short answer. Explain why probabilistic gates are a fundamental obstacle for photonics and how the field addresses it.

Question 20

Short answer. Why has no platform won, and what would winning look like?

Answer Key

Q Ans Note
1 A Arrays of optical tweezers, each holding one atom.
2 A Excitation to a high-lying Rydberg state produces a strong, distance-dependent interaction.
3 A The blockade shifts the doubly-excited state out of resonance, producing a conditional phase.
4 A Atoms are moved between operations, giving effectively programmable connectivity.
5 A Non-local stabilizers become implementable, which is why qLDPC and neutral atoms are frequently discussed together (Ch. 23–24).
6 A Atoms are lost from traps during operation; continuous reloading and loss-tolerant protocols are required.
7 A Several encodings are used; dual-rail path encoding is common.
8 A No cryogenics for the photons themselves, and photons are the natural carrier for networking.
9 A Photons barely interact, so linear-optical two-qubit gates succeed only probabilistically.
10 A Cluster-state computing: entangle first, then compute by measuring adaptively.
11 A Fusion-based architectures with multiplexing to convert probabilistic success into near-deterministic operation.
12 A The manufacturing argument — CMOS-compatible fabrication at scale.
13 A Braiding non-Abelian anyons would give inherent protection at the hardware level.
14 A Claims have been made and contested; the field treats it as unproven.
15 A Annealing solves optimization heuristically; it is not gate-model universal and has no proven speedup.
16 False Superconducting and trapped ions lead on different metrics, neutral atoms lead on logical-qubit demonstrations, and photonics and silicon have long-term structural advantages. It remains genuinely open.
17 False The photons need none, but high-efficiency single-photon detectors typically require ~1–4 K. Photonics reduces cryogenic demands substantially rather than eliminating them.
18 True QuEra's 2026 result — 96 logical qubits from 448 physical atoms using high-rate codes — is the largest verified logical-qubit count reported.
19 — Photons do not interact appreciably, so with linear optics alone a two-qubit gate can only be implemented probabilistically — the KLM scheme's CNOT succeeds with probability well below 1, heralded by ancilla detection. Cascading such gates would make success probability vanish exponentially. The field's answer is multiplexing: run many probabilistic attempts in parallel and route successful outcomes forward with fast switches, converting a low per-attempt probability into near-deterministic operation at the cost of enormous component counts and stringent loss requirements. Fusion-based architectures organize this into a scalable scheme.
20 — Because the platforms have complementary rather than comparable strengths: superconducting is fast and lithographic but wiring-limited; ions have the best fidelity but slow gates; neutral atoms have reconfigurable connectivity but suffer atom loss; photonics networks naturally but has probabilistic gates; silicon may manufacture at scale but lags in fidelity. Winning would mean demonstrating a clear path to thousands of error-corrected logical qubits — sustained below-threshold operation, logical two-qubit gates, real-time decoding, and an architecture whose scaling constraint is understood and surmountable. No platform has shown all of it.