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Quantum control and process tomography of a semiconductor quantum dot hybrid qubit


The similarities between gated quantum dots and the transistors in modern microelectronics1,2—in fabrication methods, physical structure and voltage scales for manipulation—have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring19. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets11, dynamic pumping of nuclear spins12 or the addition of a third quantum dot17. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit’s charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantum dot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).

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Figure 1: Si/SiGe hybrid qubit device, energy levels and measurement of quantum oscillations.
Figure 2: State tomography and universal gate control of the hybrid qubit.
Figure 3: Quantum process tomography of the hybrid qubit.


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This work was supported in part by ARO (W911NF-12-0607), the NSF (PHY-1104660) and by the Laboratory Directed Research and Development programme at Sandia National Laboratories. Sandia National Laboratories is a multi-programme laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Development and maintenance of the growth facilities used for fabricating samples is supported by the US Department of Energy (DE-FG02-03ER46028). This research used US National Science Foundation-supported shared facilities at the University of Wisconsin-Madison. D.K. acknowledges conversations with X. Wu and K. Rudinger.

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Authors and Affiliations



M.A.E. and S.N.C. had the idea for the experiment. D.K. developed pulse sequences for qubit operation and tomography, performed electrical measurements and numerical simulations with the aid of Z.S., and analysed the data with M.A.E. and S.N.C. C.B.S. fabricated the quantum dot device. J.R.P. and D.R.W. developed hardware and software for the measurements. T.S.K., J.K.G. and M.F. helped with the theoretical analysis. D.E.S. and M.G.L. prepared the Si/SiGe heterostructure. D.K., S.N.C. and M.A.E. wrote the manuscript with the contributions of all authors.

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Correspondence to Mark A. Eriksson.

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The authors declare no competing financial interests.

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Kim, D., Shi, Z., Simmons, C. et al. Quantum control and process tomography of a semiconductor quantum dot hybrid qubit. Nature 511, 70–74 (2014).

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