What Is A Transmon Qubit? Principle, Benefits And Drawbacks
What is Transmon Qubit?
To overcome charge noise sensitivity, the transmon qubit is essential to superconducting quantum computer development. A major step towards real quantum computing systems.
“Transmission line shunted plasma oscillation qubit” refers to the superconducting charge qubit transmon. In 2007, Yale and University de Sherbrooke scientists Jens Koch, Terri M. Yu, Jay Gambetta, and Robert J. Schoelkopf created it. The transmon is a macroscopic artificial quantum system between 300 and 500 micrometres, unlike other qubit approaches that use natural two-level quantum systems.
Multilevel superconducting qubits use Josephson junctions to connect superconducting islands. Transmon design is preferred for readout, coherence, and coupling.
Basic Design and Principle
A transmon qubit uses a large capacitor to switch a single or adjustable Josephson junction.
This notion is similar to the “Cooper-pair box,” an earlier charge qubit. The key difference is the Josephson energy (EJ) to charging energy (EC) ratio. Transmons have a high ratio, indicating that EJ/EC is more than 1 due to a large shunting capacitor.
The key benefit of this high EJ/EC ratio is better charge noise immunity. Early charge qubits may experience energy shifts and decoherence due to offset charge changes across the Josephson junction. Raising the shunting capacitance extends the qubit's dephasing period, making the transmon's energy levels roughly independent of this offset charge. The transmon's insensitivity makes it a better quantum processor qubit.
Pros and Cons
The transmon design's charge noise-insensitive coherence times are its main benefit. This challenge persists in superconducting quantum computing since coherence time has improved by more than five orders of magnitude since qubits were invented.
One drawback of this design is reduced anharmonicity. Anharmonicity is the qubit's energy gap between levels. Despite being a multilayer device, the transmon is often handled as a qubit by focussing on the lowest two energy levels.
Less anharmonicity makes it harder to selectively excite only the first excited state without filling higher energy levels since the first and second excited states have a smaller energy difference. This challenge is solved by advanced microwave pulse design that uses destructive interference to terminate stimulation at higher energy levels.
High-fidelity qubit state control requires charge noise suppression to increase coherence time, which usually outweighs the reduction in anharmonicity.
Fabrication and Materials
Transmon qubits made with cutting-edge semiconductor technologies are accelerating superconducting quantum computing. Processes often deposit superconducting layers on sapphire substrates.
Niobium (Nb) and aluminium (Al) have been used as basis superconductors due to their stable characteristics and well-established production methods.
Recent investigations using tantalum (Ta) sheets as superconductors have made significant progress. Tantalum increases transmon qubit coherence time beyond 0.3 milliseconds, especially in the BCC alpha-phase. One study found that tantalum transmons had a best T1 lifetime of 503 microseconds, outperforming niobium and aluminium qubits made using the same design and manufacturing methods. Tantalum transmons perform better due to its surface oxide, Ta2O5, which appears to be the single component.
Dry etching is needed to make qubits. Even though early tantalum research showed better results with wet etching, dry etching offers many benefits for scalable quantum circuits, including high anisotropy, automation, decreased material usage, and enhanced industrial hygiene. Research has improved dry etching Ta film methods to generate clean, smooth edges for circuit manufacture.
Josephson junctions—Al-AlOx-Al trilayer tunnel junctions—are often made by double-angle evaporation. Coplanar capacitors are utilised to make transmon-shutted capacitors; bigger pad surfaces reduce surface losses and electric field density. Some compact devices use micromachined vacuum-gap capacitors to conserve space and maintain vacuum participation.
Improvement of Coherence Time
Qubit performance depends on coherence time, which measures how long a qubit can stay quantum. T1 coherence times for planar on-chip transmon qubits have typically been 30–40 microseconds. Recently, T1 has exceeded 0.3 milliseconds after intensive effort to increase these durations.
Quasiparticles and TLS errors at material interfaces generate transmon decoherence. Interfaces include metal-substrate (MS), metal-metal (MM), and metal-air (MA). Qubit coupling with surface oxide flaws on aluminium or niobium at ambient temperatures can cause decoherence. Handling, chemical cleaning, and annealing substrates reduces MS and MM interface losses. Before junction manufacture, metal surfaces are cleaned.
Surface oxides are crucial at the metal-air interface, a serious issue. Ta transmons have lengthy coherence durations due to tantalum oxide (Ta2O5) properties. By eliminating surface oxide layers and using vacuum packaging to prevent recontamination, additional progress is envisaged.
Designing three-dimensional superconducting cavities instead of transmission line cavities improves T1 times beyond material selection and manufacture. Modern transmon designs often reduce ambient noise by lowering electrodes and increasing capacitor pad size to lower surface loss and electric field density.
Ability to operate, control, and scale
Circuit quantum electrodynamics (cQED) is used to measure and manipulate transmons with microwave resonators. Equipped with powerful electromagnetic fields, these resonators are capacitively coupled to qubits. This allows exact qubit manipulation and reading.
Realistic quantum computers require large-scale qubit integration. Individual transmons can attain long coherence times, but control complexity and external noise make scaling to multi-qubit computers problematic. However, employing dry etching and optimised material platforms like tantalum sheets, medium- to large-scale superconducting quantum circuits with longer lifetimes could be made for usable quantum computers.
Compact vacuum-gap transmon qubits, measuring 36 × 39 μm2, enable scaling up quantum processors by reducing parasitic cross-coupling and footprint, which can limit performance due to radiation losses. Due to large vacuum participation ratios, these designs can accurately detect dielectric loss tangents and probe superconductor surface losses.
Transmons as Qudits
Transmons can be studied as qudits, which are quantum systems with more than two energy levels, as well as qubits. Since transmons have more energy levels than the lowest two, researchers have considered using the lowest three to form a “qutrit” (a 3-dimensional qudit). With theoretical and simulation work expanding to qudits of any size, transmon-based single-qutrit quantum gates and two-qutrit entangling gates have been implemented. This adaptability boosts the transmon's quantum computing capability.
