Single-Shot Spin Readout in Semiconductors Near the Shot-Noise Sensitivity Limit

Abstract

Fault-tolerant quantum computation requires qubit measurements to be both high fidelity and fast to ensure that idling qubits do not generate more errors during the measurement of ancilla qubits than can be corrected. Towards this goal, we demonstrate single-shot readout of semiconductor spin qubits with 97% fidelity in1.5μs. In particular, we show that we can engineer donor-based single-electron transistors (SETs) in silicon with atomic precision to measure single spins much faster than the spin decoherence times in isotopically purified silicon (270μs). By designing the SET to have a large capacitive coupling between the SET and target charge, we can optimally operate in the “strong-response” regime to ensure maximal signal contrast. We demonstrate single-charge detection with a signal-to-noise ratio (SNR) of 12.7 at 10 MHz bandwidth, corresponding to a SET charge sensitivity (integration time forSNR=2) of 2.5 ns. We present a theory of the shot-noise sensitivity limit for the strong-response regime which predicts that the present sensitivity is about one order of magnitude above the shot-noise limit. By reducing cold amplification noise to reach the shot-noise limit, it should be theoretically possible to achieve high-fidelity, single-shot readout of an electron spin in silicon with a total readout time of approximately 36 ns.

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Plain Language Summary

To create a practical quantum computer, it is essential to correct the errors that naturally occur during each step of the computation. To achieve this, researchers require the ability to read out the state of the quantum bits (qubits) accurately and quickly. The speed at which the qubit state is read out must be much faster than the rate at which information on the qubits is lost. By engineering highly sensitive detectors at the nanoscale, we have reached a new “strong-response” regime, allowing us to read out the qubit state in microseconds compared to milliseconds while retaining high accuracy, a 2-orders-of-magnitude improvement over previous results.

The small size of our detector allows us to switch from “fully on” to “fully off” states which, coupled with our ability to position the qubit and detector with atomic-level precision, allows for strong interactions and a high signal-to-noise ratio. In contrast to previous detectors that focused on detecting small shifts in the signal, this new strong-response regime distinguishes the discrete states of our single-electron qubits. This allows us to read out a single electron spin on phosphorus-atom qubits in silicon in just1.5μs. Theoretical analysis shows that we are within one order of magnitude of the shot-noise limit, where, with straightforward technical improvements, we anticipate further improvements in readout times down to about 400 ns.

These results open the door to realistic error correction in silicon-based quantum computation.