Frequency multiplexing for readout of spin qubits

Abstract

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...

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