Readout and energy dispersion of a germanium hole quantum dot hybrid qubit

Quantum dot hybrid qubits can be viewed as a hybrid of a spin qubit and a charge qubit, with increased protection against charge noise in the spin-like region and shorter manipulation times in the charge-like region [1, 2]. The quantum dot hybrid qubit has previously been realized with electrons, which in silicon have an unpredictable valley splitting that has hindered further development of the qubit. Here, we measure the dispersion of a quantum dot hybrid qubit formed with holes in germanium, which have the advantage of no valley splitting. In the charge-like regime, we use photon-assisted tunneling to measure the energy dispersion of the qubit. We extend this dispersion into the spin-like regime by applying microwave driving of the qubit and using Elzerman readout for measurement. In addition to the qubit energy dispersion, we are able to extract the tunnel couplings relevant for the hybrid qubit. As expected for holes in germanium, we find significant variation in the lowest energy anticrossing as a function of gate voltages, corresponding to tunnel couplings in the range of 20 μeV to 50 μeV.

[1] Shi, Z. et al. Phys. Rev. Lett. 108, 140503 (2012).

[2] Kim, D. et al. Nature 511, 70–74 (2014).