Tunable Spin Loading andT1of a Silicon Spin Qubit Measured by Single-Shot Readout

Abstract

The remarkable properties of silicon have made it the central material for the fabrication of current microelectronic devices. Silicon's fundamental properties also make it an attractive option for the development of devices for spintronics [1] and quantum information processing [2][3][4][5]. The ability to manipulate and measure spins of single electrons is crucial for these applications. Here we report the manipulation and measurement of a single spin in a quantum dot fabricated in a silicon/silicon-germanium heterostructure. We demonstrate that the rate of loading of electrons into the device can be tuned over an order of magnitude using a gate voltage, that the spin state of the loaded electron depends systematically on the loading voltage level, and that this tunability arises because electron spins can be loaded through excited orbital states of the quantum dot. The longitudinal spin relaxation time T 1 is measured using single-shot pulsed techniques [6] and found to be ∼ 3 seconds at a field of 1.85 Tesla. The demonstration of single spin measurement as well as a long spin relaxation time and tunability of the loading are all favorable properties for spintronics and quantum information processing applications.Silicon is a material in which spin qubits are expected to have long coherence times, thanks to the predominance of a spin-zero nuclear isotope and relatively weak spin-orbit coupling. However, silicon quantum dots have yet to demonstrate the reproducibility and controllability achieved in gallium arsenide devices [7][8][9][10]. Here, we demonstrate the control and manipulation of spin states of single electrons in a silicon/silicon-germanium (Si/SiGe) quantum dot and report the first single-shot measurements of the longitudinal spin relaxation time T 1 in such devices. We also show that the presence of a relatively low-lying spin-split orbital excited state in the dot can be exploited to increase the speed and tunability of the loading of spins into the dot. Our results demonstrate that Si/SiGe quantum dots can be fabricated that are sufficiently tunable to enable single-electron manipulation and measurement, and that long spin relaxation times are consistent with the orbital and/or valley excitation energies in these systems.The measurements we report were performed on a gate-defined quantum dot with the gate configuration shown in Fig. 1a, tuned to be in the single-dot regime. The dot is measured at low temperature and in a parallel magnetic field. As shown in Fig. 1b, an electron can be loaded into one of four energy eigenstates; we denote the states, in order of increasing energy, as | ↓ g , | ↑ g , | ↓ e , and | ↑ e , where the first index refers to spin (↓ having lower energy than ↑ ) and the second to the ground (g) and excited (e) orbital levels. We obtain an experimental map of these states by measuring the differential current dI QPC /dV L through a charge sensing quantum point contact while applying square voltage pulses to gate L. The grayscale plots of dI QPC /dV L in Fig. 1c,d arXiv:10...

Theory of spin relaxation in two-electron laterally coupled Si/SiGe quantum dots

“…34,35,50,53,54,[56][57][58][59][60][61][62][63][64][65][66][67] Our work completes these findings by a global, quantitative understanding of two-electron lateral silicon double quantum dots. We investigate the spin-orbit and hyperfine-induced relaxation rate as a function of interdot coupling, detuning, and the magnitude and orientation of the external magnetic field for zero and finite temperatures, and for natural and isotopically purified silicon.…”

“…If this is the case, the multivalley system can be reduced to an effective single-valley qubit, a potentially nuclear-spin-free analog to the well-known GaAs counterpart. 50,51 In fact, many recent experiments performed on Si/SiGe quantum dots have no evidence of valley degeneracy, [33][34][35][52][53][54] indicating that the splitting is large enough to justify a single-valley treatment. On the other hand, a recent proposal of valley-defined qubits uses the valley degree of freedom as a tool for gate operations.…”

Effects of interface disorder on valley splitting in SiGe/Si/SiGe quantum wells

“…[6][7][8][9][10][11] Such designs rely on exact control of qubit states by electronic gates, hence a non-degenerate ground state is preferred. 3,12 However, Si has six equivalent conduction minima, which brings additional complexity to qubit gate operations.…”

Unconventional Transport in the “Hole” Regime of a Si Double Quantum Dot

“…In GaAs devices, spin-flip times range from ∼200 µs for a two-electron dot [16], to ∼0.85 ms (at 8 T [17]) and > 1 s (at 1 T and 120 mK [18]) for single electron dots. Recent experiments report spin-flip times in single electron dots in Si [19][20][21][22] ranging from 40 ms (at 2 T [19]) to 6 s (at 1 T [20]), at low temperatures. The tunnel coupling for the same Si double dot studied here was found to be 10 ns (25 ns) in the elastic (inelastic) tunneling regime [13].…”