Quantum Computing Technologies

Pol Forn-Díaz

The QCT group develops superconducting circuits for applications in quantum computation. By employing Josephson-junction technology, the group engineers superconducting qubits for the implementetion of quantum algorithms. Part of the research is focused on improving the qubit quality by mitigation of environmental noise sources, while at the same time the circuits are used to investigate fundamental physics of quantum optics phenomena.

Demonstration of superconductivity in nitridized aluminum thin films.

By injecting nitrogen during the deposition process of aluminum, we have achieved a new superconducting material consisting of nitridized aluminum, or NitrAl, with critical temperature higher than bare aluminum, up to 3.5K. The material has very good prospects of becoming very useful in applications for superconducting qubits in quantum computing. This work is a collaboration with IMB-CNM-CSIC who fabricated the samples.

Figure 1: Critical temperature of NitrAl samples as function of the nitrogen concentration during film deposition. For reference, a dashed line shows the bare aluminum reference. Beyond 13.3%, the samples do not show superconductivity and become insulating.

Coherence times observed in the first superconducting flux qubits at IFAE.

Within the AVaQus FET-Open project led by IFAE, the first coherent dynamics have been observed in a superconducting flux qubit device by the IFAE team. The sample was fabricated by the Royal Holloway University London fabrication team. This result is an important milestone in the goal of the IFAE QCT group of performing analog quantum computation with superconducting flux qubits.

Figure 2: Flux qubit circuit, showing the qubit in the center of the image coupled to the readout resonator (right) and flux control line (bottom).

Figure 3: Zoom in on qubit. The qubit loop can be observed as the diamond-shape coupled to its shunting capacitor (to the left).

Figure 4: Circuit wiring to control qubit inside the coldest stage of the dilution refrigerator where temperatures near 0.02 K are attained. Sample coil is mounted on the qubit sample box to generate magnetic fields. Coaxial cables guide microwave signals to control and readout the qubit state.

Figure 5: Coherent control of the qubit state. Shown is a measurement of Ramsey oscillations, where a qubit superposition is prepared and its evolution over time is shown as function of the wait time. The coherence is of order 100ns at this flux bias point, where flux noise strongly damps the oscillations, as expected. This metric is being improved in later devices with better tuned device parameters.