Entangling gates for electron spins in semiconductor quantum dots are generally based on exchange, a shortranged interaction that requires wavefunction overlap. Coherent spin-photon coupling raises the prospect of using photons as long-distance interconnects for spin qubits. Realizing a key milestone for spin-based quantum information processing, we demonstrate microwave-mediated spin-spin interactions between two electrons that are physically separated by more than 4 mm. Coherent spin-photon coupling is demonstrated for each individual spin using microwave transmission spectroscopy. An enhanced vacuum Rabi splitting is observed when both spins are tuned into resonance with the cavity, indicative of a coherent spin-spin interaction. Our results demonstrate that microwave-frequency photons can be used as a resource to generate long-range two-qubit gates between spatially separated spins.

We demonstrate a low loss, chip-level frequency multiplexing scheme for readout of scaled-up spin qubit devices. By integrating separate bias tees and resonator circuits on-chip for each readout channel, we realise dispersive gate-sensing in combination with charge detection based on two rf quantum point contacts (rf-QPCs). We apply this approach to perform multiplexed readout of a double quantum dot in the few-electron regime, and further demonstrate operation of a 10-channel multiplexing device. Limitations for scaling spin qubit readout to large numbers of multiplexed channels is discussed.Scaling-up quantum systems to the extent needed for fault-tolerant operation introduces new challenges not apparent in the operation of single or few-qubit devices. Spin qubits based on gate-defined quantum dots [1] are, in principle, scalable, firstly because of their small (sub-micron) footprint, and secondly, since spins are largely immune to electrical disturbance, they exhibit low crosstalk when densely integrated [2]. At the fewqubit level, readout of spin-states is via quantum point contact (QPC) or single electron transistor (SET) charge sensors, proximal to each quantum dot [3][4][5][6][7][8]. These readout sensors pose a significant challenge to scale-up however, in that they require separate surface gates and large contact leads, crowding the device and tightly constraining the on-chip architecture.The recently developed technique of dispersive gatesensing (DGS) overcomes this scaling limitation by making use of the gates, already in place to define the quantum dots, as additional charge sensors [9]. The gates act as readout detectors by sensing small changes in the quantum capacitance associated with the tunnelling of single electrons. In turn, shifts in capacitance are measured by the response of a radio-frequency (rf) LC resonator that includes the gate. In principle, all of the quantum dot gates used for electron confinement can also be used as dispersive sensors, simultaneously collecting more of the readout signal that is spread over the total device capacitance and thus increasing the signal to noise ratio. Enabling all-gate readout, as well as multichannel rf-QPC or rf-SET charge sensing, requires the development of multiplexing schemes that scale to large numbers of readout sensors and qubits.Here we report an on-chip approach to frequency multiplexing for the simultaneous readout of scaled-up spin qubit devices. We demonstrate 3-channel readout of a few-electron double quantum dot, combining two rfQPCs and a dispersive gate-sensor as well as the operation of a 10-channel planar multiplexing (MUX) circuit. Similar approaches to frequency multiplexing have been demonstrated for distributed resonators in the context of kinetic inductance detectors [10], superconducting qubits [11,12] and rf-SETs [13][14][15]. The present work advances previous demonstrations by lithographically integrating the feed-lines, bias tees, and resonators, which are fabricated on a sapphire chip using low-loss superconducting niob...

The compound semiconductor gallium-arsenide (GaAs) provides an ultra-clean platform for storing and manipulating quantum information, encoded in the charge or spin states of electrons confined in nanostructures. The absence of inversion symmetry in the zinc-blende crystal structure of GaAs however, results in a strong piezoelectric interaction between lattice acoustic phonons and qubit states with an electric dipole, a potential source of decoherence during charge-sensitive operations. Here we report phonon generation in a GaAs double quantum dot, configured as a single-or two-electron charge qubit, and driven by the application of microwaves via surface gates. In a process that is a microwave analogue of the Raman effect, phonon emission produces population inversion of the two-level system and leads to rapid decoherence of the qubit when the microwave energy exceeds the level splitting. Comparing data with a theoretical model suggests that phonon emission is a sensitive function of the device geometry.

We propose and analyze a novel "flopping-mode" mechanism for electric dipole spin resonance based on the delocalization of a single electron across a double quantum dot confinement potential. Delocalization of the charge maximizes the electronic dipole moment compared to the conventional single dot spin resonance configuration. We present a theoretical investigation of the floppingmode spin qubit properties through the crossover from the double to the single dot configuration by calculating effective spin Rabi frequencies and single-qubit gate fidelities. The flopping-mode regime optimizes the artificial spin-orbit effect generated by an external micromagnet and draws on the existence of an externally controllable sweet spot, where the coupling of the qubit to charge noise is highly suppressed. We further analyze the sweet spot behavior in the presence of a longitudinal magnetic field gradient, which gives rise to a second order sweet spot with reduced sensitivity to charge fluctuations. arXiv:1904.13117v2 [cond-mat.mes-hall]