Can a laser fix mistuned qubits on a quantum chip?
Briefly heating individual qubit junctions with a laser sets their frequencies precisely enough to avoid clashes between neighbours, without harming how long the qubits stay coherent.
Source
High-performance superconducting quantum processors via laser annealing of transmon qubits
Study at a glance
- Design
- Other — Laboratory process study: Josephson junctions on multiqubit chips were laser-annealed to target resistances, then chips were cooled and qubit frequencies, coherence and gate errors measured; Monte Carlo and gate simulations supplement the data.
- N
- No single N: 390 qubits were tuned in the precision experiment, 241 qubits from seven Falcon and two Hummingbird processors were used for frequency-assignment analysis, and 221 qubits on four partially tuned chips were used for the coherence comparison.
- Population
- Fixed-frequency superconducting transmon qubits on IBM 27-qubit Falcon and 65-qubit Hummingbird processors
- Outcome
- Frequency-equivalent tuning precision, tuning success rate, qubit coherence times (T1, T2), and two-qubit gate fidelity
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What they did
The team used a focused green laser to anneal the tiny tunnel junctions of fixed-frequency transmon qubits, nudging each junction's resistance (and so its frequency) down toward a planned target. They tuned 390 qubits to test how precise the process is, then cooled whole processors to measure actual qubit frequencies, coherence times, and two-qubit gate errors. They compared tuned and untuned qubits on the same chips and used simulations to estimate yield and gate error before and after tuning.
What they found
About 89.5% of qubits reached their target, and the tuning itself was precise to about 4.7 MHz in frequency terms. Once chips were cleaned, bonded and cooled, the practical frequency precision was about 18.5 MHz, dominated by imperfect prediction of cold frequency from room-temperature resistance rather than by the laser step. Tuned and untuned qubits had essentially the same coherence (aggregate T1 about 79 microseconds), and a tuned 65-qubit processor had all 72 two-qubit gates working with a median fidelity of 98.7%.
The limits
What it doesn't show
The work comes from one company's fabrication line and processor designs, so it does not show how well the method transfers to other qubit architectures such as tunable-frequency qubits. The yield gains and the predicted drop in gate error rely on Monte Carlo and gate-error models rather than on a direct before-and-after measurement of the same chip. The method can only lower frequencies (resistance only increases, up to about 14%), and it does not address errors from next-nearest-neighbour or spectator qubits, which the authors flag as future work.
Key terms
- Transmon qubit
- A superconducting circuit built around a Josephson junction whose two lowest energy levels act as a quantum bit.
- Josephson junction
- A thin insulating barrier between two superconductors; its resistance at room temperature predicts the qubit's frequency when cold.
- Frequency collision
- When neighbouring qubits have frequencies too close together (or in other bad relationships), causing unwanted interactions and gate errors.
- Laser annealing (LASIQ)
- Heating a junction with a laser to raise its resistance in small steps, lowering the qubit frequency to a chosen value.
- Coherence time (T1, T2)
- How long a qubit keeps its energy (T1) or its phase information (T2) before noise destroys it.
- Cross-resonance gate
- A two-qubit entangling gate for fixed-frequency transmons driven by microwaves, whose quality depends on the frequency spacing between the two qubits.
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Quiz yourself
What physical quantity does the laser directly change?
Common questions
Why not just make the junctions accurately in the first place?
Fabrication spreads junction resistance by a few percent, which scatters qubit frequencies by around 100 MHz; that is too large to avoid collisions on big chips, so a post-fabrication trim is needed.
Does heating the qubits with a laser damage them?
The authors compared tuned and untuned qubits on the same chips and found no statistically significant difference in T1 or T2 coherence times.
What limits the final frequency precision?
Mostly the imperfect mapping from room-temperature resistance to cold qubit frequency, not the laser tuning, which on its own is precise to about 5 MHz.
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