Semiconductor spin qubits: device physics and control
OverviewCopyright: T. Botzem
Our goal is to demonstrate scalable device- and material-platforms for spin qubits where the DiVincenzo criteria are robustly fulfilled. Our activities explore different material platforms, such as GaAs and SiGe, and are based on a closed feedback loop between advanced simulations, device operation, and optimization.
Key Research Areas
- Spin qubits in GaAs (Principal investigator: Hendrik Bluhm)
- Spin qubits in SiGe (Principal investigator: Lars Schreiber)
BackgroundCopyright: P. Cerfontaine
Realizing a quantum processor requires replacing classical bits with quantum bits (qubits). A natural choice for implementing a qubit is the electron spin, which is intrinsically a two-level system. In our group, we are studying spin qubits based on gate-defined quantum dots in GaAs (o, more precisely, GaAs/(Al,Ga)As) and in Si/SiGe. In both cases, the quantum dots are realized starting from a two-dimensional electron gas created via band-structure engineering in the semiconductor heterostructures and using metallic gates on the surface of the heterostructure to locally confine the electrons. This type of qubits have the great advantages of allowing complete electrical control of the state of the qubit, which can be accurately manipulated using electrical signals applied to the gates, and of having good prospects for scalability.
GaAs is a mature material system in which all the basic requirements for qubit operations have been demonstrated. In our group, we concentrate on two-electron-spin qubits, where only the two m=0 states of the two spins are used to encode a qubit. The exchange interaction between the two spins enables control of the qubit using electric pulses. Highlights of earlier activities include record-breaking coherence times  and the demonstration of entanglement of two such qubits . Further optimization of one- and two-qubit operations and the scaling to a larger number of qubits are among the main goals of our group [3,4].
An important aspect of these qubits is the interaction of electrons with the nuclear spins of the host lattice. Every electron is coupled to a few million nuclear spins, which create a fluctuating effective magnetic field. These fluctuations are a major source of dephasing but, they can also be used as a resource for qubit control . The combined system exhibits rich quantum dynamics  and can serve as a model system for fundamental studies of decoherence and open quantum systems.
Nowadays spin qubits in GaAs are mostly seen as a rich playground to test fundamental concepts and architecture strategies for quantum computation, and to develop specific applications as spin-to-photon coupling where the direct band-gap might be beneficial. However, a lot of attention has now turned to materials that are not inherently plagued by dephasing due to nuclear spins.Copyright: L. Schreiber
The most promising material in this respect is silicon [7,8]. Indeed, silicon contains less than 5% of isotopes carrying a nuclear spin in its natural composition and can be made nuclear-spin free by isotopic purification. Furthermore, in silicon the spin-orbit interaction is low, piezo-electric phonon coupling is absent and device manufacturing can build on the know-how developed for conventional integrated circuits – which represents a major asset in terms of scalability.
In our group, we investigate mostly spin qubits in strained-engineered Si/SiGe heterostructures [9,10,11]. In this approach, electrons are vertically confined by strain in a thin layer of isotopically purified 28Si epitaxially grown between two layers of SiGe. Lateral confinement is provided by planar gate structures on the substrate surface. This approach promises low disorder, good reproducibility and good tunability of the devices.
A list of relevant group publications is given below (see External Links).
-  Dephasing time of GaAs electron-spin qubits coupled to a nuclear bath exceeding 0.2 ms, Bluhm et al., Nature Physics (2010).
-  Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits, Shulman et al., Science (2012).
-  High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits in GaAs, P. Cerfontaine et al., Phys. Rev. Lett. (2014).
-  A High-Fidelity Gateset for Exchange-Coupled Singlet-Triplet Qubits, P. Cerfontaine et al., arXiv:1901.00851, (2019)
-  Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarizations, Foletti et al., Nature Physics (2009).
-  Quadrupolar and anisotropy effects on dephasing in two-electron spin qubits in GaAs, T. Botzem et al., Nature Communications (2016).
-  Quantum computation: Silicon comes back, Schreiber and Bluhm, Nature Nanotechnology (2014).
-  Toward a silicon-based quantum computer, Schreiber and Bluhm, Science (2018).
-  Single-Shot Measurement of Triplet-Singlet Relaxation in a Si/SiGe Double Quantum Dot, Prance et al., Phys. Rev. Lett. (2012).
-  Excitation of a Si/SiGe quantum dot using an on-chip microwave antenna, Kawakami et al., Appl. Phys. Lett. (2013).
-  Simulation of micro-magnet stray-field dynamics for spin qubit manipulation, Neumann and Schreiber, J. Appl. Phys. (2015).