Millisecond lifetimes and coherence times in 2D transmon qubits
NatureTL;DR
Imagine a qubit is like a tiny, spinning top. Its spin holds special quantum information. The problem is that this top is incredibly wobbly and easily disturbed by the 'table' it's sitting on. The slightest vibration or imperfection in the table can make it fall over and lose its information. This is called 'decoherence'. Scientists have been searching for the perfect material for this table. This research discovered that using a super-pure silicon wafer as the table, instead of the more common sapphire, makes the top spin for a much, much longer time. A longer spin time means we can perform more calculations before the qubit forgets what it's doing, which is essential for a working quantum computer.
Materials improvement is a powerful approach to reducing loss and decoherence in superconducting qubits, because such improvements can be readily translated to large-scale processors. Recent work improved transmon coherence by using tantalum as a base layer and sapphire as a substrate1. The losses in these devices are dominated by two-level systems with comparable contributions from both the surface and bulk dielectrics2, indicating that both must be tackled to achieve substantial improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon markedly decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Qavg) of 9.7 × 106 across 45 qubits. For our best qubit, we achieve a Qavg of 1.5 × 107, reaching a maximum Q of 2.5 × 107, corresponding to a lifetime (T1) up to 1.68 ms. This low loss also allows us to observe decoherence effects related to the Josephson junction, and we use an improved, low-contamination junction deposition to achieve Hahn echo coherence times (T2E) exceeding T1. We achieve these materials improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single-qubit gates with 99.994% fidelity. The tantalum-on-silicon platform comprises a simple material stack that can potentially be fabricated at the wafer scale and therefore can be readily translated to large-scale quantum processors.
- 1Replacement of sapphire with high-resistivity silicon in qubits markedly decreased bulk substrate loss.
- 2Achieved time-averaged quality factor (Qavg) of 9.7 × 10^6 across 45 qubits.
- 3Maximum qubit lifetime (T1) reached up to 1.68 ms with high fidelity of 99.994%.
- 4Retention of qubit architecture while incorporating standard quantum control gates.
- 5Potential for fabrication at wafer scale enabling large-scale application.
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