Transport through an impurity tunnel coupled to a Si/SiGe quantum dot

Abstract

Achieving controllable coupling of dopants in silicon is crucial for operating donor-based qubit devices, but it is difficult because of the small size of donor-bound electron wavefunctions. Here we report the characterization of a quantum dot coupled to a localized electronic state, and we present evidence of controllable coupling between the quantum dot and the localized state. A set of measurements of transport through this device enable the determination of the most likely location of the localized state, consistent with an electronically active impurity in the quantum well near the edge of the quantum dot. The experiments we report are consistent with a gate-voltage controllable tunnel coupling, which is an important building block for hybrid donor and gate-defined quantum dot devices.Donors in silicon are a natural choice for qubits 1 , because electron spins bound to donors have very long coherence times 2-4 . Phosphorus donors also have nuclear spins with extremely long coherence times [5][6][7] . Although donor-based quantum devices can be fabricated with near-atomically precise placement of donors 8,9 , even when well-placed, donors are very small, making it difficult to control and change the tunnel couplings between them with gate voltages. In contrast, tunnel couplings are easily tunable in gate-defined quantum dots, and high-quality quantum dots hosting at least four different types of spin qubits have been demonstrated semiconductor materials [10][11][12][13][14][15][16][17][18][19][20][21][22] . An important feature of gatedefined quantum dots is that the electrons they contain can be displaced laterally simply by changing the voltages of the gates on the surface 23 . Because of the ease of spatial control of the wavefunction in quantum dots, coupling quantum dots to localized states in semiconductors, if possible, would enable a path of a wide range of hybrid donor/quantum dot technologies.In this letter, we report the observation of a controllable tunnel coupling between a localized electronic state and a gate-defined quantum dot formed in a Si/SiGe heterostructure. We present measurements of transport through the device, demonstrating controllable tunnel coupling between the quantum dot and the localized state. A set of stability diagram measurements enable a determination of the relative magnitude of the capacitance between the surface gates and both the quantum dot and the localized state. We report the expected electron density profiles in the quantum dot and the neighboring reservoirs. Combining the experimental results with 3D capacitive modeling based on the electron density profiles, we determine the most likely location of the * rhfoote@wisc.edu † maeriksson@wisc.edu localized state in the device. The results reported here demonstrate that it is possible to control the tunnel rate between localized states and quantum dots, even though there is a dramatic difference in the characteristic length scale between these two electronic systems. A gate-defined quantum dot was fabricated in a Si...

Transport spectroscopy of a spin-orbit-coupled spin to a quantum dot

“…These peaks are sensitive to the spinorbit interaction which can be tuned using electrostatic gate electrodes 4 . Similar studies have performed transport measurements through an impurity state tunnel-coupled to a quantum dot in a silicon-germanium heterostructure 6 . Motivated by these studies we considered a quantum dot coupled to a single spin via SOI and examined the current flowing through the dot.…”

“…Similar magneto-conductance measurements through a double dot formed in the channel of a silicon transistor device have demonstrated the appearance of extra peaks in the leakage current as a result of an unwanted spin interacting with the double dot in the presence of spin orbit coupling 5 . An impurity tunnel-coupled to a single quantum dot in a silicon-germanium heterostructure has also been reported 6 , and this situation may also be relevant to other silicon devices which make use of dopants to confine spins 7,8 . In these dot systems the spin is unintentionally coupled to the dot, but fullerene and in general molecular spins intentionally coupled to dot systems can be fabricated in the laboratory and have shown exceptionally long coherence times 9 .…”

Electrical Control and Quantum Chaos with a High-Spin Nucleus in Silicon

“…8.2). This can be achieved either in MOSFET structures [164] or in Si/SiGe devices, where the ability to electrically detect a single dopant atom coupled to a quantum dot has also been recently demonstrated [165]. All the above options will deliver a fixed value of strain, set by the thermal expansion of the metallic electrodes and/or the built-in strain in the substrate.…”

Theoretical formulations on thermodynamics of quantum impurity systems

“…For quantum impurity systems, such as quantum dots, one could continuously adjust λ the system-bath coupling strength. [55][56][57][58][59][60] One can also measure the impurity spectral densities, 61,62 resulting in the local Green's function, G SS (ω; λ). Another ingredient g(ω) in Eq.…”

“…Again, the key issues would be the tunability with respect to the system-bath coupling strength. [55][56][57][58][59][60]…”