Fully tunable longitudinal spin-photon interactions in Si and Ge quantum dots

Abstract

Spin qubits in silicon and germanium quantum dots are promising platforms for quantum computing, but entangling spin qubits over micrometer distances remains a critical challenge. Current prototypical architectures maximize transversal interactions between qubits and microwave resonators, where the spin state is flipped by nearly resonant photons. However, these interactions cause back-action on the qubit, that yield unavoidable residual qubit-qubit couplings and significantly affect the gate fidelity. Strikingly, residual couplings vanish when spin-photon interactions are longitudinal and photons couple to the phase of the qubit. We show that large longitudinal interactions emerge naturally in state-of-the-art hole spin qubits. These interactions are fully tunable and can be parametrically modulated by external oscillating electric fields. We propose realistic protocols to measure these interactions and to implement fast and high-fidelity two-qubit entangling gates. These protocols work also at high temperatures, paving the way towards the implementation of large-scale quantum processors.

Hole spin qubits in thin curved quantum wells

“…One viable approach is to couple a long quantum dot to a high-impedance resonator by shaking the dot along the smooth confinement direction. In analogy to Ge/Si core/shell nanowires [18,21], this approach enables a strong spin-photon interaction in a single quantum dot, and it is especially appealing in our system when the magnetic field is applied along the zdirection, where the qubit is insensitive to charge noise, see Sec. IV B.…”

“…Moreover, the strong SOI in long hole quantum dots is predicted to enable a strong transversal [18] and longitudinal [21] coupling to high-impedance microwave resonators [22][23][24][25][26][27], where the strength of the interaction exceeds the coherence time of the qubits and the photons. Reaching the strong coupling regime in hole quantum dots will enable long-range connectivity of distant qubits [28] as well as quantum error correcting architectures [29], and will be a big step towards scaling up spin-based quantum computers.…”

“…Reaching the strong coupling regime in hole quantum dots will enable long-range connectivity of distant qubits [28] as well as quantum error correcting architectures [29], and will be a big step towards scaling up spin-based quantum computers. In contrast to state-of-the-art experiments, where multiple quantum * stefano.bosco@unibas.ch dots encode a single qubit [30][31][32][33][34][35], hole spin qubits only need a single quantum dot [18,21,36,37], significantly diminishing the complexity of the architecture. However, to enhance the spin-photon interactions, the zero-point field of the photon needs to be aligned to the long direction of the dot [18,21], and thus the plunger electrode capacitively coupling the dot and the resonator has to be misaligned from center of the dot, reducing the geometric lever arm to this gate and limiting the maximal spin-resonator coupling strength.…”

Anomalous zero-field splitting for hole spin qubits in Si and Ge quantum dots

“…The compatibility of localized spins in semiconducting quantum dots (QDs) [1] with the welldeveloped CMOS technology is pushing these architectures to the front of the race towards the implementation of scalable quantum computers [2][3][4][5][6]. Spin qubits based on hole states in silicon (Si) and germanium (Ge), in particular, are gaining increasing attention in the community [5,6] because of their large spin-orbit interaction (SOI) [7][8][9][10], enabling fast and power-efficient all-electric gates [11][12][13] and strong transversal and longitudinal coupling to microwave resonators [14][15][16][17][18]. Also, significant steps forward in material engineering [19,20] as well as fast spin read-out and qubit initialization protocols [21][22][23][24] facilitated the implementation of high-fidelity twoqubit gates [25,26] and of a four-qubit quantum processor with controllable qubit-qubit couplings [27].…”