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.

2025 Activities

1. Expansion of the QCT Laboratory with a Second Dilution Refrigerator

Installation of second dilution refrigerator in the QCT lab in October 2025, funded by the Next Generation funds from the Plan Complementario “Comunicaciones Cuánticas” coordinated by ICFO. The new fridge is a large-size unit, with a lowest-temperature stage of 50cm diameter, reaching temperatures of 11 millikelvin. The system completed the QCT laboratory, 6.5 years after its inception. The new refrigerator will permit multiple experiments cooled down simultaneously, and multi-qubit experiments that require a significant amount of cabling for controlling the device.

2. Ultrastrong Coupling Demonstrated in a Superconducting Circuit Fabricated at IFAE

Demonstration of ultrastrong coupling with the first 3-junction flux qubit device fabricated fully at IFAE. This work was part of Alba Torras’ PhD thesis, and connects to the already completed project Quantera SiUCs, as the main goal of IFAE as coordinator of that consortium. The experiment demonstrated ultrastrong coupling between the qubit and a resonator using a shared superinductance made from granular aluminum. The device showed a coupling strength 13% of the resonator frequency, placing it in the perturbative ultrastrong coupling regime. The work was published in Applied Physics Letters, and is going to be extended in an improved device produced by Alba shortly before the end of her PhD thesis defense.

3. Cosmic Radiation Detection with NTD Bolometers in Dilution Refrigerators

Collaboration with group from IJCLab Orsay led by Prof. Andrea with the installation of an NTD bolometer in the IFAE dilution refrigerator and demonstration of detection of cosmic radiation events. The bolometer will be installed on a qubit device in 2026, as the final experiment of the project ICRQ (Interaction of Cosmic Radiation with Qubits) funded by the Ministry of Science and Innovation.

4. Launch of the Euro-QHPC-Integration Project

-Kick off of the Euro-QHPC-Integration project, where IFAE is a partner through the Spain branch coordinated by the Barcelona Supercomputing Center. The project will develop applications and functionality for the 6 quantum computers installed in European HPC centers, including BSC. IFAE will provide expertise on analog quantum computing devices for the new BSC analog quantum computer provided by Qilimanjaro.

5. Completion of the NESQQ Project and Development of NitrAl Superconducting Technology

Closing of the project NESQQ: “Nanofabrication Enhanced Superconducting Quantum Processors”, within the Plan Complementario “Comunicaciones Cuánticas” coordinated by ICFO. Within the project the QCT group demonstrated the new superconductor nitridized aluminum (NitrAl, in short) with which it is currently working to demonstrate its potential to produce quantum circuits such as qubits, resonators and amplifiers.

Ultrastrong Coupling in a Superconducting Circuit: First Fully In-House QCT Publication at IFAE

First paper of QCT group entirely made at IFAE with only IFAE people (no collaboration): Superinductor-based ultrastrong coupling in a superconducting circuit, Alba Torras-Coloma, Luca Cozzolino, Ariadna Gómez-del-Pulgar-Martínez, Elia Bertoldo, P. Forn-Díaz Appl. Phys. Lett. 127, 214002 (2025).

The paper describes the design and fabrication of a superconducting qubit circuit in which a 3-junction flux qubit is galvanically coupled to an LC resonator. The coupling is provided with a shared granular aluminum wire, which is a novelty in the field. The granular aluminum contributes its high kinetic inductance leading to a coupling above 500 MHz, leading to a ratio coupling/resonator frequency of 13%, indicating that the system is in the perturbative ultrastrong coupling (above 10%). This work paves the way for future devices in the ultrastrong coupling regime with coherent qubits, which is an important missing goal in the field.

Figure 1: Images of the device used in this work, along with circuit schematics.

Characterization of Cosmic Radiation for Superconducting Qubit Experiments

Publication of the work on characterization of Cosmic Radiation for superconducting qubit experiments. The work was led by Dr. Elia Bertoldo and was published in New Journal of Physics. In this project, cosmic radiation was characterized at several sites using a portable detector constructed by Manel Martínez. Measurements were conducted at IFAE, at the deepest Barcelona subway station, at the Collserola tunnels and the sub-basement of the ALBA synchrotron. The angular dependence of muon flux was characterized. The work is a collaboration with the University of Zaragoza, with Dr. Maria Martínez and Víctor Pérez, who performed GEANT4 simulations, and with the group of Prof. Andrea Giuliani from Orsay, on the characterization of cosmic radiation detected by NTD bolometers. The conclusion of the work is the presence of energy deposition in the sub-MeV from cosmic muons for devices oriented looking to the zenith, and multi-MeV events for devices oriented looking to the horizon, placing an important constraint on the best orientation of devices for limiting cosmic radiation impacts.

Figure 2: Image of the portable cosmic muon counter made by Manel Martínez, including its own power supply, memory, and display.

Figure 3: Data taken by the muon counter at different depths. TMB: El Coll-la Teixonera subway stop. Vallvidrera: Collserola road tunnel. ALBA: sub-basement of the ALBA synchrotron.

Figure 4: Data collected at Orsay in the laboratory of Prof. Giuliani using the calibration of NTD detectors oriented vertically and horizontally, where muon impacts were recorded.