Coherent singlet-triplet oscillations in a silicon-based double quantum dot

Abstract

Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors.

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

“…We can extract the electron g-factor using the relation: , based on numerical simulation of the stray magnetic field from the micromagnet at the estimated dot location (Supplementary Section S2). Surprisingly, when measuring the EDSR peak at a sufficiently low power to avoid power broadening, we resolve two lines, separated by T in III-V dots 6,5,7,8 , and several times longer than the * 2 T measured before in Si/SiGe dots 21,23 . This dephasing timescale can be attributed to the random nuclear field from the 5% 29 Si atoms in the substrate with standard deviation μT 9.6 = B σ , consistent with theory 24 .…”

“…2a. We can extract the electron g-factor using the relation: T in III-V dots 6,5,7,8 , and several times longer than the * 2 T measured before in Si/SiGe dots 21,23 . This dephasing timescale can be attributed to the random nuclear field from the 5% 29 Si atoms in the substrate with standard deviation μT 9.6 = B σ , consistent with theory 24 .…”

“…Most progress was made in well-controlled III-V quantum dots, where spin manipulation with two 6,11 , three 12 and four 13 dots has been realized, but gate fidelities and spin coherence times are limited by the unavoidable interaction with the fluctuating nuclear spins in the host substrate 5,6 . While the randomness of the nuclear spin bath could be mitigated to some extent by feedback techniques 14 , eliminating the nuclear spins by using group IV host materials offers the potential for extremely long electron spin coherence times that exceed one second in P impurities in bulk 28 Si 15,16 .Much effort has been made to develop stable spin qubits in quantum dots defined in carbon nanotubes 17,18 , Ge/Si core/shell nanowires 19 , Si MOSFETs 20,21 and Si/SiGe 2D electron gases 16,22,23 . However, coherent control in these group IV quantum dots is so far limited to a Si/SiGe singlet-triplet qubit with only single-axis control 23 and a carbon nanotube single-electron spin qubit, with a Hahn echo decay time of 65 ns 17 .…”

“…1a). Compared to conventional, doped heterostructures, this technology strongly improves charge stability 23 . First, accumulation gates ( mV 150 a + V ) are used to induce a twodimensional electron gas (2DEG) in a 12 nm wide Si quantum well 37 nm below the surface.…”

“…However, coherent control in these group IV quantum dots is so far limited to a Si/SiGe singlet-triplet qubit with only single-axis control 23 and a carbon nanotube single-electron spin qubit, with a Hahn echo decay time of 65 ns 17 .…”