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]

Traditional approaches to controlling single spins in quantum dots require the generation of large electromagnetic fields to drive many Rabi oscillations within the spin coherence time. We demonstrate "flopping-mode" electric dipole spin resonance, where an electron is electrically driven in a Si/SiGe double quantum dot in the presence of a large magnetic field gradient. At zero detuning, charge delocalization across the double quantum dot enhances coupling to the drive field and enables low power electric dipole spin resonance. Through dispersive measurements of the single electron spin state, we demonstrate a nearly three order of magnitude improvement in driving efficiency using flopping-mode resonance, which should facilitate low power spin control in quantum dot arrays.Recent advances in silicon spin qubits have bolstered their standing as a platform for scalable quantum information processing. As single-qubit gate fidelities exceed 99.9% [1], two-qubit gate fidelities improve [2][3][4][5][6], and the field accelerates towards large multi-qubit arrays [7,8], developing the tools necessary for efficient and scalable spin control is critical [9]. While it is possible to implement single electron spin resonance in quantum dots (QDs) using ac magnetic fields [10], the requisite high drive powers and associated heat loads are technically challenging and place limitations on attainable Rabi frequencies [11]. As spin systems are scaled beyond a few qubits, methods of spin control which minimize dissipation and reduce qubit crosstalk will be important for low temperature quantum information processing [12].Electric dipole spin resonance (EDSR) is an alternative to conventional electron spin resonance. In EDSR, static gradient magnetic fields and oscillating electric fields are used to drive spin rotations [13]. The origin of the effective magnetic field gradient varies across implementations: intrinsic spin-orbit coupling [14][15][16], hyperfine coupling [17], and g-factor modulation [18] have been used to couple electric fields to spin states. The inhomogeneous magnetic field generated by a micromagnet [19,20] has been used to create a synthetic spin-orbit field for EDSR, enabling high fidelity control [1]. Conveniently, this magnetic field gradient gives rise to a spatially varying Zeeman splitting, enabling spins in neighboring QDs to be selectively addressed [11,19,[21][22][23][24][25].In this Letter, we demonstrate a novel mechanism for driving low-power, coherent spin rotations, which we call "flopping-mode EDSR". In conventional EDSR, the electric drive field couples to a charge trapped in a single quantum dot, leading to a relatively small electronic displacement [16]. We instead drive single spin rotations in a DQD close to zero detuning, = 0, where the electric field can force the electron to flop back and forth between the left and right dots, thereby sampling a larger variation in transverse magnetic field. We call this configuration the "flopping-mode".Neglecting spin, the Hamiltonian describing a singl...

We demonstrate a scalable device architecture that facilitates indirect exchange between singlettriplet spin qubits, mediated by an intermediate quantum state. The device comprises five quantum dots, which can be independently loaded and unloaded via tunnelling to adjacent reservoirs, avoiding charge latch-up common in linear dot arrays. In a step towards realizing two-qubit entanglement based on indirect exchange, the architecture permits precise control over tunnel rates between the singlet-triplet qubits and the intermediate state. We show that by separating qubits by ∼ 1 µm, the residual capacitive coupling between them is reduced to ∼ 7 µeV.

Coherent charge-photon and spin-photon coupling has recently been achieved in silicon double quantum dots (DQDs). Here, we demonstrate a versatile split-gate cavity-coupler that allows more than one DQD to be coupled to the same microwave cavity. Measurements of the cavity transmission as a function of level detuning yield a charge cavity coupling rate of gc/2π= 58 MHz, a charge decoherence rate of γc/2π= 36 MHz, and a cavity decay rate of κ/2π= 1.2 MHz. The charge cavity coupling rate is in good agreement with device simulations. Our coupling technique can be extended to enable simultaneous coupling of multiple DQDs to the same cavity mode, opening the door to long-range coupling of semiconductor qubits using microwave frequency photons.

We report the use of dispersive gate sensing (DGS) as a means of probing the charge environment of heterostructure-based qubit devices. The DGS technique, which detects small shifts in the quantum capacitance associated with single-electron tunnel events, is shown to be sensitive to pockets of charge in the potential-landscape likely under, and surrounding, the surface gates that define qubits and their readout sensors. Configuring a quantum point contact (QPC) as a localised emitter, we show how these charge pockets are activated by the relaxation of electrons tunnelling through a barrier. The presence of charge pockets creates uncontrolled offsets in gate-bias and their thermal activation by on-chip tunnel currents suggests further sources of charge-noise that lead to decoherence in semiconductor qubits.