Material
Aluminum, the junction metal
Yes, aluminum is a superconductor when cooled below its superconducting transition, which Chinese Science Bulletin gives as about 1.2 kelvin. [1] Aluminum films and their oxide barriers are widely used to make Josephson junctions for qubits.
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Why ordinary aluminum is not superconducting
An aluminum object at room temperature is a normal electrical conductor. Cooling can bring it into a superconducting state, provided magnetic field and current remain within the material's limits. The transition temperature is therefore a condition for superconductivity, rather than a property that makes every aluminum wire resistance-free in ordinary use. Film processing and impurities can change the transition. [1][2]
An operating qubit needs more than a superconducting metal. It must also suppress unwanted thermal excitation and energy loss in the circuit. Qubit refrigeration therefore works well below the metal's transition. The dilution refrigerator, shielding, filtering, and microwave connections are part of the device, even though aluminum itself can become superconducting without that entire apparatus. [3]
The oxide that makes a junction
A Josephson tunnel junction places a thin insulating barrier between superconducting electrodes. Aluminum is useful because controlled exposure to oxygen forms an aluminum oxide barrier. The junction supports a supercurrent through quantum tunneling and supplies a nonlinear circuit element. That nonlinearity allows a qubit circuit to distinguish its quantum energy levels. [1]
Chinese Science Bulletin describes junction fabrication by angled aluminum evaporation through a suspended mask, with oxidation between deposition steps. The study adjusts oxygen exposure to control electrical properties. [1] A native oxide is not automatically a suitable junction barrier: the fabrication process must control electrode overlap, barrier quality, contamination, and electrical continuity. Reproducibility belongs to the process as well as the material.
Where aluminum loses energy
Zero DC resistance does not eliminate microwave loss. Defects in amorphous oxides and at interfaces can interact with the circuit's electric field. Unwanted quasiparticles and magnetic flux can also introduce dissipation. Review of Scientific Instruments describes superconducting resonators as tools for measuring these losses under conditions relevant to quantum circuits. [3]
The same oxide that provides a junction barrier can introduce unwanted loss when it appears on surfaces carrying microwave fields. Device geometry determines how strongly those fields sample each interface. Cleaning, deposition, oxidation, and packaging therefore influence performance. Choosing aluminum over another superconductor does not remove the need to measure the assembled device at its operating power and temperature. [1][3]
A circuit metal, not a high-field magnet wire
Aluminum's usefulness in quantum circuits comes from its superconductivity and compatible fabrication methods. Those advantages differ from the current and field capacity needed by a large magnet. Niobium-based wires and cuprate conductors serve that role with different operating limits and composite structures. A low-loss junction electrode is judged by different requirements from a mechanically loaded winding. [1][3]
Aluminum alloys can also be engineered for sensors. A NIST-hosted Applied Physics Letters study demonstrates manganese-doped aluminum as a transition-edge sensor material, with the transition deliberately suppressed. [2] This is a different use from an aluminum qubit electrode. It illustrates why composition and fabrication must accompany the material name when interpreting superconducting measurements or selecting a device film.
Sources
- Character and fabrication of Al/Al2O3/Al tunnel junctions for qubit applicationChinese Science Bulletin, 2012
- Superconducting transition edge sensor using dilute AlMn alloysApplied Physics Letters, 2004
- Materials loss measurements using superconducting microwave resonatorsReview of Scientific Instruments, 2020
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