Quantum computing is based on three main properties of quantum mechanics: superposition and entanglement. The basic unit of information in a quantum computer is a quantum bit or qubit, which can be in two possible states. Many different technologies are currently being explored for the basic physical implementation of qubits. While qubits with single ions and atoms in cavities and traps can be controlled with exquisite precision, it would be difficult to scale up these techniques to thousands of atoms or ions. On the other hand, for solid-state qubits, the microfabrication tools used in the semiconductor industry could provide a route to mass fabrication. Among the solid-state qubits, the superconducting approach is currently the most advanced.
We plan to follow the superconducting route for the implementation of qubits. Superconducting qubits are implemented from resonant modes in non-linear electronic microcircuits tailored from superconducting inductors, capacitors, and Josephson tunnel junctions. In standard practice, a Josephson junction consisting of a thin (a few nanometers) layer of aluminum oxide (insulator), sandwiched between two layers of superconducting aluminum (below 1.2 K), is used. Besides this standard approach, we will also explore various oxides and superconducting materials for the fabrication of Josephson junctions. Superconducting circuits will be manufactured using a multi-step fabrication process involving lithographic patterning, metal deposition, etching, and controlled oxidation of thin two-dimensional films of a superconductor such as aluminum or niobium. Circuits will be fabricated on silicon or sapphire substrates, which are compatible with silicon CMOS manufacturing. Defects at various film interfaces and within the oxide layer can produce parasitic two-level systems, which limit device coherence. Materials research in this area will also serve to investigate decoherence sources in disordered superconductors.
Research will be carried out exploring other possibilities for qubits utilizing superconductivity, such as, Majorana bound states (MBSs) in solid-state systems of atomic chains on the surface of superconducting materials (for example, Fe chains on Pb surface). Such investigations will be carried out with controlled deposition of atoms on a clean surface of a superconducting material under ultrahigh vacuum condition and in-situ scanning tunneling microscopy and spectroscopy. The wavefunction localization of MBSs is crucial for their future implementation as qubits. This experimental facility will also be used for the investigation of atomic-scale physics of topological phases of matter and electronic properties of fabricated topological superconductor devices.