Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some very useful properties which can be utilized for spin qubit-based quantum computing. Here […]

*
Papers *

## Nature Communications: Semiconductor-inspired design principles for superconducting quantum computing

March 16th, 2016 | by admin | published in News, Papers, Quantum Computing

## Paper: Spin-orbit coupling and operation of multi-valley spin qubits

November 5th, 2015 | by admin | published in News, Papers, Quantum Computing

Spin qubits composed of either one or three electrons are realized in a quantum dot formed at a Si/SiO_2-interface in isotopically enriched silicon. Using pulsed electron spin resonance, we perform coherent control of both types of qubits, addressing them via an electric field dependent g-factor. We perform randomized benchmarking and find that both qubits can […]

## Nature Communications: Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor

July 2nd, 2014 | by admin | published in News, Papers

Superconducting circuits are exceptionally flexible, enabling many different devices from sensors to quantum computers. Separately, epitaxial semiconductor devices such as spin qubits in silicon offer more limited device variation but extraordinary quantum properties for a solid-state system. It might be possible to merge the two approaches, making single-crystal superconducting devices out of a semiconductor by […]

## Phys. Rev. B: On-chip cavity quantum phonodynamics with an acceptor qubit in silicon

August 30th, 2013 | by admin | published in All, Nanotechnology, News, Papers, Phonitons, Quantum Computing

We describe a chip-based, solid-state analog of cavity-QED utilizing acoustic phonons instead of photons. We show how long-lived and tunable acceptor impurity states in silicon nanomechanical cavities can play the role of a matter nonlinearity for coherent phonons just as, e.g., the Josephson qubit plays in circuit QED. Both strong coupling (number of Rabi oscillations ≲100) and strong dispersive coupling (0.1–2 MHz) regimes can be reached in cavities in the 1–20-GHz range, enabling the control of single phonons, phonon-phonon interactions, dispersive phonon readout of the acceptor qubit, and compatibility with other optomechanical components such as phonon-photon translators. We predict explicit experimental signatures of the acceptor-cavity system.

## Introducing the phoniton: A sound-based analogue of cavity-QED, a tool for controlling sound at the quantum level

November 28th, 2011 | by admin | published in All, Blog, Featured, Highlights, Nanotechnology, Papers, Phonitons, Research

## Quantum phase transitions in photonic cavities with two-level systems

May 22nd, 2008 | by admin | published in All, Papers, Research, Solid Light

Phys. Rev. A, 77, 5, 053819 (2008)

Systems of coupled photonic cavities have been predicted to exhibit quantum phase transitions by analogy with the Hubbard model. To this end, we have studied topologies of few (up to six) photonic cavities each containing a single two-level system. Quantum phase space diagrams are produced for these systems, and compared to mean-field results. We also consider finite effective temperature, and compare this to the notion of disorder. We find the extent of the Mott lobes shrink analogously to the conventional Bose-Hubbard model.

## Quantum Fluctuations, Temperature and Detuning Effects in Solid-Light Systems

May 20th, 2008 | by admin | published in All, Papers

Phys. Rev. Lett. 100, 216401 (2008)

The superfluid to Mott insulator transition in cavity polariton arrays is analyzed using the variational cluster approach, taking into account quantum fluctuations exactly on finite length scales. Phase diagrams in one and two dimensions exhibit important non-mean-field features. Single-particle excitation spectra in the Mott phase are dominated by particle and hole bands separated by a Mott gap. In contrast to Bose-Hubbard models, detuning allows for changing the nature of the bosonic particles from quasi-localized excitons to polaritons to weakly interacting photons. The Mott state with density one exists up to temperatures $T/g\gtrsim0.03$, implying experimentally accessible temperatures for realistic cavity couplings $g$.

## Controllable valley splitting in silicon quantum devices

July 13th, 2007 | by admin | published in All, Papers

Nature Physics 3, 41 (2007)

Silicon has many attractive properties for quantum computing, and the quantum dot architecture is appealing because of its controllability and scalability. However, the multiple valleys in the silicon conduction band are potentially a serious source of decoherence for spin-based quantum dot qubits. Only when these valleys are split by a large energy does one obtain well-defined and long-lived spin states appropriate for quantum computing. Here we show that the small valley splittings observed in previous experiments on Si/SiGe heterostructures result from atomic steps at the quantum well interface. Lateral confinement in a quantum point contact limits the electron wavefunctions to several steps, and enhances the valley splitting substantially, up to 1.5 meV. The combination of electronic and magnetic confinement produces a valley splitting larger than the spin splitting, which is controllable over a wide range. These results improve the outlook for realizing spin qubits with long coherence times in silicon-based devices.

## Valley Splitting Theory of SiGe/Si/SiGe Quantum Wells

November 17th, 2006 | by admin | published in All, Papers

Phys. Rev. B 75, 115318 (2007)

We present an effective mass theory for SiGe/Si/SiGe quantum wells, with an emphasis on calculating the valley splitting. The theory introduces a valley coupling parameter, $v_v$, which encapsulates the physics of the quantum well interface. The new effective mass parameter is computed by means of a tight binding theory. The resulting formalism provides rather simple analytical results for several geometries of interest, including a finite square well, a quantum well in an electric field, and a modulation doped two-dimensional electron gas. Of particular importance is the problem of a quantum well in a magnetic field, grown on a miscut substrate. The latter may pose a numerical challenge for atomistic techniques like tight-binding, because of its two-dimensional nature. In the effective mass theory, however, the results are straightforward and analytical. We compare our effective mass results with those of the tight binding theory, obtaining excellent agreement.