PhD - Leuven | About a week ago
Quantum dot-based spin qubits have typically a qubit energy splitting of the order of 100 MHz and a coherence time that can be up to a few ms . Transmon qubits, on the other hand, have a qubit splitting of the order of a few GHz and a coherence time of the order of a few μs . A combination of both systems could thus be an outstanding solution to the major problems that quantum computation is facing at the moment. Hence the large amount of research devoted in coupling both systems. However, the unmatched energy splitting between spin qubits and superconducting qubits (or resonators) makes the coupling between them too slow . Many proposals exist to use charge or hybrid quantum dot-based qubits instead of spin qubits, but these are highly sensitive to electric noise.
A solution to this could thus be to couple a transmon qubit to a highly tunable singlet-only spin qubit . A highly tunable singlet-only spin qubit has two features that can be very beneficial in a transmon-dot coupling: (1) the qubit splitting can be easily modulated from nearly zero to several tens of μeV, thus allowing for a fast capacitive coupling with resonators and transmon qubits; and (2) the singlet-only nature of the qubit—that is, both qubit states are singlet states and therefore not coupled to each other by any intrinsic or external magnetic noise—provides a decoherence-free subspace, where the spin system is expected to have a long coherence time.
Quantum dot-based spin qubits are simple systems that can be successfully modeled with a Hubbard Hamiltonian , whereas the capacitive coupling between the quantum dots and the transmon qubits is well described by the Jaynes-Cummings or the Tavis-Cummings Hamiltonian .
In this PhD an extensive investigation of the aforementioned coupling scheme will be performed by modeling and simulation. The outcome will result in several proposals for designing a transmon-spin qubit system being more insensitive to decoherence.
 M. Russ and G. Burkard, J. Phys.: Condens. Matter 29, 393001 (2017).
 P. Krantz, M. Kjaergaard, F. Yan, et al. Appl. Phys. Rev. 6, 021318 (2019).
 V. Srinivasa, J. M. Taylor, and C. Tahan, Phys. Rev. B 94, 205421 (2016). A. Sala, J. H. Qvist, and J. Danon, Phys. Rev. Research 2, 012062(R) (2020)
Required background: Physics, Engineering physics, Electrical Engineering
Type of work: 80% modeling 20% literature
Supervisor: Christian Maes
Co-supervisor: Bart Soree
Daily advisor: Bart Soree
The reference code for this position is 2021-058. Mention this reference code on your application form.