Quantum computing is based on three main properties of quantum mechanics: coherence, 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 began 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 nanometres) layer of aluminium oxide (insulator), sandwiched between two layers of superconducting aluminium (Tc = 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 films of a superconductor such as aluminium 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. Our ongoing materials research in this area will also serve to investigate decoherence sources and explore newer materials for superior qubits.
Building a superconducting quantum computer requires a complex interplay of hardware and software, including specialized components, extreme cooling systems, and robust control electronics. The superconducting quantum processor, containing superconducting qubits, must operate at temperatures near absolute zero and be shielded from environmental noise. We have built a basic setup for a superconducting quantum computer with a dilution refrigerator (Bluefors), which provides 10 milliKelvin temperature, cryogenic electronic components (Low Noise Factory) and control electronics (Quantum Machines). We have been working with a single-qubit processor, which is a 3D cavity plus a superconducting transmon qubit system. Various characterizations of this system, such as cavity spectroscopy, qubit spectroscopy, Rabi oscillations, Ramsey interferometry, Hahn echo measurement, implementation of various one-qubit gates etc. have been carried out. Beyond this, we have also carried out experiments on noise spectroscopy (Fourier transform noise spectroscopy) on this qubit. Currently we are set to work with a five-qubit superconducting quantum processor.