Periodic Reporting for period 2 - FERMIcQED (Manipulating single fermions with light in cQED architectures)
Reporting period: 2023-03-01 to 2024-08-31
The field of circuit quantum electrodynamics (cQED) has revolutionized our understanding of quantum physics and holds great promise for quantum information technologies. It is based on both tunnel Josephson junctions, which are used as two-level systems called superconducting qubits, and microwave photons trapped inside superconducting resonators. These two strongly interacting quantum objects provide an elementary and versatile platform that allows one to test the basic rules of quantum physics. Superconducting qubits being intrinsically bosonic by nature, traditional cQED experiments somehow consist in measuring “the interplay between light and light”. A promising direction involves leveraging mature cQED architectures and techniques to probe novel quantum systems and phenomena in well-controlled environments. Along this line, the overarching aim of project FERMIcQED is to interface low-dimensional quantum conductors with microwave light at the level of the single fermion and photon. The idea consists in isolating an individual fermionic degree of freedom within a hybrid Josephson junction – a quantum dot connected to two superconductors. Due to the superconducting proximity effect, entangled electron-hole states – called the Andreev bound states (ABS) – form in the quantum dot and depend on the superconducting phase difference. By enclosing the hybrid Josephson junction inside a superconducting photonic cavity, one can couple these fermionic states to microwave light and probe their quantum properties in a well-controlled environment. The first objectives of project FERMIcQED are to (i) implement hybrid Josephson junctions based on carbon nanotubes, interface them with superconducting photonic cavity, perform photonic spectroscopy of the ABS and investigate their spin degree of freedom. The following goals are to (ii) demonstrate quantum control of the ABS and (iii) investigate the topological properties that can appear in double quantum dot geometries. In parallel (iv), the project aims at implementing a carbon nanotube-based bosonic qubit that is gate-tunable and explore the interplay between bosonic and fermionic degrees of freedom.
During the first half of project FERMIcQED, the team has executed a two-pronged strategy, which consists in developing and characterizing hybrid Josephson junctions of high-quality on the one hand, and building cQED architectures that are compatible with low-dimensional conductors on the other. A novel nanofabrication technique was first developed, which allows for the production of hybrid Josephson junction devices based on encapsulated carbon nanotubes. Multiple devices were fabricated and measured at cryogenic temperatures. Quantum transport experiments demonstrated gate-dependent supercurrents, as large as 8 nA, and spectroscopic signatures of the Andreev bound states. These measurements led to the discovery of new physics, with the observation of quantum phase transitions in a four-fold degenerate superconducting quantum dot. In parallel, cQED architectures were developed and characterized at microwave frequencies. A quantum control experiment was first performed using a tunnel junction-based transmon qubit that displayed large coherence times, thus demonstrating that the experimental setup is ready for more exotic qubits. Carbon nanotube-based junctions were then integrated in these architectures, in order to detect the Andreev bound states with photons. To do so, we developed a novel two-tone spectroscopy technique that is based on the AC Josephson effect and that can operate at millimeter-wave frequencies. Finally, a carbon nanotube-based bosonic qubit could be implemented. Radio-frequency measurements were performed and demonstrated a gate-tunable qubit spectrum, consistent with Andreev physics. This work culminated with the quantum control of this newly developed qubit, with modest though promising coherent times of ~ 40 ns. In parallel, theoretical works were performed in order to investigate the hybridization of the Andreev bound sates in double quantum dot geometries and the resulting non-reciprocal Josephson effect.
Several progresses have been made and are already beyond the state of the art. The nanofabrication technique of encapsulated carbon nanotubes is novel and allows for the implementation of ultra-clean carbon nanotube-based Josephson junctions. This pristineness translates into a single-electron electrostatic control, over ranges of more than 100 periods. The quality of the devices is also demonstrated by the four-fold degeneracy associated with spin and orbit degrees of freedom that could be observed in the gate-dependence of the supercurrent. Another significant progress is the two-tone Josephson spectroscopy technique, that allows one to operate at millimeter-wave frequencies in a cQED architecture, and that is compatible with Andreev physics. Finally, the implementation and quantum control of a carbon nanotube-based transmon, exhibiting significant coherence times, is also well beyond the state of the art. The next steps in the project include improving the coherence times of this carbon nanotube-based bosonic qubit by careful engineering of the microwave architecture, as well as performing the photonic spectroscopy of the Andreev bound states. Until the end of the project, it is expected to successfully interface the Andreev bound states to microwave photons. The coherent manipulation of such a superconducting fermionic qubit is within experimental reach. The double quantum dot geometry, which allows one to play with the Andreev bound states parity and lower-down the fermionic qubit frequency will also be addressed.