State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot

Daniel R Ward,Max G Lagally,Ryan H Foote,Dohun Kim,Mark A Eriksson,Donald E Savage,Mark Friesen,Susan N Coppersmith

doi:10.1038/npjqi.2016.32

Abstract

AbstractUniversal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. We further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau–Zener–Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.

Highlights

We focus here on conditional coherent operations of a charge qubit, the measurement strategy and strong inter-qubit coupling deduced from the present study can be directly applied to singlet–triplet[25] or hybrid quantum dot qubits,[18,20] where strong capacitive coupling will have an essential role in the realisation of fast two-qubit gates
The dots are formed under the gates D1 through D4, approximately under the dashed line shown in Figure 1a, and for the experiments we report here, it is useful to describe the quadruple quantum dot as a pair of double quantum dots
The right double dot (RDD), formed under the gates D3 and D4, forms a charge qubit that will be manipulated coherently based on the charge state of the left

Summary

INTRODUCTION

Since being proposed theoretically,[1,2] much experimental and theoretical progress has been made towards the development of a scalable quantum-computing architecture using electrically gated semiconductor quantum dot-based spin qubits.[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] Twoqubit gates are essential, and capacitive coupling has been used in. We present measurements of a quadruple quantum dot formed in an undoped Si/SiGe heterostructure and demonstrate fast and charge-state-conditional coherent manipulation of two strongly coupled double quantum dots. We show that the strong capacitive coupling (418 GHz) between two sets of double quantum dots enables charge-state-conditional coherent Landau–Zener–Stückelberg (LZS) interference with a conditional π phase flip time of approximately 80 ps, demonstrating progress towards realizing high-fidelity two-qubit control. We focus here on conditional coherent operations of a charge qubit, the measurement strategy and strong inter-qubit coupling deduced from the present study can be directly applied to singlet–triplet[25] or hybrid quantum dot qubits,[18,20] where strong capacitive coupling will have an essential role in the realisation of fast two-qubit gates

RESULTS

DISCUSSION

MATERIALS AND METHODS