[Paper Review] Microwave-frequency scanning gate microscopy of a Si/SiGe double quantum dot

Artem O. Denisov, Seong W. Oh+7 more|arXiv (Cornell University)|Mar 11, 2022

Quantum and electron transport phenomena55 references16 citations

TL;DR

This study demonstrates microwave-frequency scanning gate microscopy on a Si/SiGe double quantum dot by coupling a metallic AFM tip to a lithographically defined device with an integrated charge sensor. The technique enables local dc and microwave control of electron occupancy and resolves excited states via photon-assisted tunneling, revealing a 64 µeV energy splitting consistent with valley splitting in silicon heterostructures—enabling spatially resolved spectroscopy of quantum states critical for spin qubit applications.

ABSTRACT

Conventional quantum transport methods can provide quantitative information on spin, orbital, and valley states in quantum dots, but often lack spatial resolution. Scanning tunneling microscopy, on the other hand, provides exquisite spatial resolution of the local electronic density of states, but often at the expense of speed. Working to combine the spatial resolution and energy sensitivity of scanning probe microscopy with the speed of microwave measurements, we couple a metallic probe tip to a Si/SiGe double quantum dot that is integrated with a local charge detector. We first demonstrate that a dc-biased tip can be used to change the charge occupancy of the double dot. We then apply microwave excitation through the scanning tip to drive photon-assisted tunneling transitions in the double dot. We infer the double dot energy level diagram from the frequency and detuning dependence of the photon-assisted tunneling resonance condition. These measurements allow us to resolve $\sim$65 $\mu$eV excited states, an energy scale consistent with typical valley splittings in Si/SiGe. Future extensions of this approach may allow spatial mapping of the valley splitting in Si devices, which is of fundamental importance for spin-based quantum processors.

Motivation & Objective

• To develop a scanning probe technique that combines high spatial resolution with microwave-frequency sensitivity for studying quantum devices.
• To address the lack of spatially resolved measurement techniques for valley splitting in Si/SiGe heterostructures, a key challenge for scalable spin qubits.
• To demonstrate local control and spectroscopy of a double quantum dot using a biased and microwave-excited AFM tip.
• To enable quantitative mapping of excited states and energy level structure in silicon-based quantum dots.

Proposed method

• A metallic AFM tip is coupled to a Si/SiGe double quantum dot (DQD) via a bias-T, allowing simultaneous dc bias and microwave excitation.
• The DQD is integrated with a quantum dot charge sensor to detect electron occupancy changes with high sensitivity.
• Microwave excitation through the tip drives photon-assisted tunneling (PAT) transitions, enabling spectroscopy of excited states.
• Charge stability diagrams are extracted from current measurements (ID and IS) to map electron occupancy as a function of gate voltages.
• A capacitance matrix is extracted from PAT data to quantify tip coupling to the DQD, with the form Q = C·V.
• A three-level Hamiltonian model is used to fit PAT data, including ground and excited states in the left dot and the right dot ground state.

Experimental results

Research questions

• RQ1 Can a scanning gate tip be used to locally tune electron occupancy in a Si/SiGe double quantum dot with high spatial resolution and microwave-frequency control?
• RQ2 What is the energy scale of excited states in the DQD, and can they be resolved using microwave-driven photon-assisted tunneling?
• RQ3 Is the observed energy splitting consistent with valley splitting in silicon, and can it be spatially mapped?
• RQ4 How does the tip's coupling to the DQD affect the energy level structure, and can this be quantitatively modeled?
• RQ5 Can this technique enable spatially resolved spectroscopy of valley splitting in Si/SiGe heterostructures?

Key findings

• The microwave-frequency scanning gate microscopy successfully resolves excited states in the double quantum dot with an energy resolution of ∼65 µeV.
• A 64 µeV energy splitting between the ground and excited states in the left dot is extracted from PAT data, consistent with typical valley splittings in Si/SiGe heterostructures.
• The technique enables local manipulation of electron occupancy via the dc-biased tip, functioning as a movable plunger gate.
• The capacitance matrix was experimentally extracted, showing a tip-to-dot capacitance of ∼0.023 aF, confirming weak but measurable coupling.
• Photon-assisted tunneling resonances appear as distinct stripes in the charge stability diagram when microwave excitation is applied, confirming coherent microwave control.
• The three-level model fit yields an interdot tunnel coupling of 16 µeV and a transition energy of 64 µeV, matching literature values for valley splittings.