Abstract 18 November 2015

Quantum control of a high-speed semiconductor quantum dot hybrid qubit

Mark A. Eriksson, Department of Physics, University of Wisconsin-Madison

One of the remarkable features of qubits formed by spins in the solid state is the enormous range of time-scales over which coherent manipulation is possible. If one considers gate-controlled manipulation of nuclear spins at one extreme [1], and exchange-driven multi-electron qubits at the other extreme [2,3], coherent control of semiconductor qubits with over 9 orders of magnitude variation in manipulation time has already been demonstrated.

In this talk, I present recent work on the high-speed end of this spectrum using quantum dots in Si/SiGe heterostructures [4]. I will discuss a three-electron, double quantum dot-based qubit and demonstrate its control by means of abrupt (~100 ps) changes in the detuning energy between the quantum dots. While particularly simple to implement, using only quasi-dc changes in detuning for qubit manipulation is not optimal: it exposes the qubit to regimes in which it is sensitive to electric-field noise, and the qubit dephases relatively rapidly. For this reason, I discuss recent experiments that make use of microwave pulses, enabling the operation of this qubit entirely at detunings for which the qubit is well-protected from charge noise, yielding much improved coherence [5].

Because this qubit has internal degrees of freedom – two tunnel couplings that describe the two lowest-energy anticrossings in the double dot energy spectrum – the coherence times of this qubit, both TRabiand T2*, can be increased by appropriate tuning of the device gate voltages. This capability implies that hardware design can have a direct impact on the coherence properties of quantum dot qubits, offering the potential to enhance qubit speed while simultaneously maintaining or even increasing qubit coherence, something that is usually very challenging to do.

[1] J. T.Muhonen, et al., Nat. Nanotechn. 9, 986 (2014).

[2] D. P. Divincenzo, et al., Nature 408, 339 (2000).

[3] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012).

[4] D. Kim, et al., Nature 511, 70 (2014).

[5] D. Kim, et al., npj Quant. Inf. 1, 15004 (2015).



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