Christoph Klöffel


Department of Physics
University of Basel
Klingelbergstrasse 82
CH-4056 Basel, Switzerland

email:view address

tel: +41 (0)61 207 3756 (office)
fax:+41 (0)61 267 1349

Short CV

Since July 2014  Postdoctoral associate in the group of Prof. Dr. Daniel Loss at the University of Basel
2010 - 2014PhD student in the Condensed Matter Theory group at the University of Basel, under the supervision of Prof. Dr. Daniel Loss
2009 - 2010 Research in the Nano-Photonics group of Prof. Dr. Richard J. Warburton (University of Basel and Heriot-Watt)
2008 - 2009 Master of Physics at Heriot-Watt University in Edinburgh
2005 - 2008 Undergraduate studies at the University of Wuerzburg

Further information can be found on my website


Show all abstracts.

1.  Heavy Hole States in Germanium Hut Wires
H. Watzinger, C. Kloeffel, L. Vukusic, M. D. Rossell, V. Sessi, J. Kukucka, R. Kirchschlager, E. Lausecker, A. Truhlar, M. Glaser, A. Rastelli, A. Fuhrer, D. Loss, and G. Katsaros.
Nano Lett. 16, 6879 (2016); arXiv:1607.02977.

Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so-far unexplored type of nanostructure. Low-temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function of heavy-hole character. A light-hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors compared with those for pure heavy holes. Given this tiny light-hole contribution, the spin lifetimes are expected to be very long, even in isotopically nonpurified samples.

2.  Long-Range Interaction between Charge and Spin Qubits in Quantum Dots
Marcel Serina, Luka Trifunovic, Christoph Kloeffel, and Daniel Loss.

We analyze and give estimates for the long-distance coupling via floating metallic gates between different types of spin qubits in quantum dots made of different commonly used materials. In particular, we consider the hybrid, the singlet-triplet, and the spin-1/2 qubits, and the pairwise coupling between each type of these qubits with another hybrid qubit in GaAs, InAs, Si, and Si_{0.9}Ge_{0.1}. We show that hybrid qubits can be capacitively coupled strongly enough to implement two-qubit gates, as long as the dimensions of the dots and their distance from the metallic gates are small enough. Thus, hybrid qubits are good candidates for scalable implementations of quantum computing in semiconducting nanostructures.

3.  Phonon-Assisted Relaxation and Decoherence of Singlet-Triplet Qubits in Si/SiGe Quantum Dots
Viktoriia Kornich, Christoph Kloeffel, and Daniel Loss.

We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and the decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may suppress the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.

4.  Acoustic Phonons and Strain in Core/Shell Nanowires
Christoph Kloeffel, Mircea Trif, and Daniel Loss.
Phys. Rev. B 90, 115419 (2014); arXiv:1405.4834.

We study theoretically the low-energy phonons and the static strain in cylindrical core/shell nanowires (NWs). Assuming pseudomorphic growth, isotropic media, and a force-free wire surface, we derive algebraic expressions for the dispersion relations, the displacement fields, and the stress and strain components from linear elasticity theory. Our results apply to NWs with arbitrary radii and arbitrary elastic constants for both core and shell. The expressions for the static strain are consistent with experiments, simulations, and previous analytical investigations; those for phonons are consistent with known results for homogeneous NWs. Among other things, we show that the dispersion relations of the torsional, longitudinal, and flexural modes change differently with the relative shell thickness, and we identify new terms in the corresponding strain tensors that are absent for uncapped NWs. We illustrate our results via the example of Ge/Si core/shell NWs and demonstrate that shell-induced strain has large effects on the hole spectrum of these systems.

5.  Phonon-Mediated Decay of Singlet-Triplet Qubits in Double Quantum Dots
Viktoriia Kornich, Christoph Kloeffel, and Daniel Loss.
Phys. Rev. B 89, 085410 (2014); arXiv:1311.2197.

We study theoretically the phonon-induced relaxation (T_1) and decoherence times (T_2) of singlet-triplet qubits in lateral GaAs double quantum dots (DQDs). When the DQD is biased, Pauli exclusion enables strong dephasing via two-phonon processes. This mechanism requires neither hyperfine nor spin-orbit interaction and yields T_2 << T_1, in contrast to previous calculations of phonon-limited lifetimes. When the DQD is unbiased, we find T_2 \simeq 2 T_1 and much longer lifetimes than in the biased DQD. For typical setups, the decoherence and relaxation rates due to one-phonon processes are proportional to the temperature T, whereas the rates due to two-phonon processes reveal a transition from T^2 to higher powers as T is decreased. Remarkably, both T_1 and T_2 exhibit a maximum when the external magnetic field is applied along a certain axis within the plane of the two-dimensional electron gas. We compare our results with recent experiments and analyze the dependence of T_1 and T_2 on system properties such as the detuning, the spin-orbit parameters, the hyperfine coupling, and the orientation of the DQD and the applied magnetic field with respect to the main crystallographic axes.

6.  Circuit QED with Hole-Spin Qubits in Ge/Si Nanowire Quantum Dots
Christoph Kloeffel, Mircea Trif, Peter Stano, and Daniel Loss.
Phys. Rev. B 88, 241405(R) (2013); arXiv:1306.3596.

We propose a setup for universal and electrically controlled quantum information processing with hole spins in Ge/Si core/shell nanowire quantum dots (NW QDs). Single-qubit gates can be driven through electric-dipole-induced spin resonance, with spin-flip times shorter than 100 ps. Long-distance qubit-qubit coupling can be mediated by the cavity electric field of a superconducting transmission line resonator, where we show that operation times below 20 ns seem feasible for the entangling square-root-of-iSWAP gate. The absence of Dresselhaus spin-orbit interaction (SOI) and the presence of an unusually strong Rashba-type SOI enable precise control over the transverse qubit coupling via an externally applied, perpendicular electric field. The latter serves as an on-off switch for quantum gates and also provides control over the g factor, so single- and two-qubit gates can be operated independently. Remarkably, we find that idle qubits are insensitive to charge noise and phonons, and we discuss strategies for enhancing noise-limited gate fidelities.

7.  Tunable g factor and phonon-mediated hole spin relaxation in Ge/Si nanowire quantum dots
Franziska Maier, Christoph Kloeffel, and Daniel Loss.
Phys. Rev. B 87, 161305(R) (2013); arXiv:1302.5027.

We theoretically consider g factor and spin lifetimes of holes in a longitudinal Ge/Si core/shell nanowire quantum dot that is exposed to external magnetic and electric fields. For the ground states, we find a large anisotropy of the g factor which is highly tunable by applying electric fields. This tunability depends strongly on the direction of the electric field with respect to the magnetic field. We calculate the single-phonon hole spin relaxation times T1 for zero and small electric fields and propose an optimal setup in which very large T1 of the order of tens of milliseconds can be reached. Increasing the relative shell thickness or the longitudinal confinement length prolongs T1 further. In the absence of electric fields, the dephasing vanishes and the decoherence time T2 is determined by T2 = 2 T1.

8.  Prospects for Spin-Based Quantum Computing in Quantum Dots
Christoph Kloeffel and Daniel Loss.
Annu. Rev. Condens. Matter Phys. 4, 51 (2013); arXiv:1204.5917.

Experimental and theoretical progress toward quantum computation with spins in quantum dots (QDs) is reviewed, with particular focus on QDs formed in GaAs heterostructures, on nanowire-based QDs, and on self-assembled QDs. We report on a remarkable evolution of the field where decoherence, one of the main challenges for realizing quantum computers, no longer seems to be the stumbling block it had originally been considered. General concepts, relevant quantities, and basic requirements for spin-based quantum computing are explained; opportunities and challenges of spin-orbit interaction and nuclear spins are reviewed. We discuss recent achievements, present current theoretical proposals, and make several suggestions for further experiments.

9.  Strong Spin-Orbit Interaction and Helical Hole States in Ge/Si Nanowires
Christoph Kloeffel, Mircea Trif, and Daniel Loss.
Phys. Rev. B 84, 195314 (2011); arXiv:1107.4870.

We study theoretically the low-energy hole states of Ge/Si core/shell nanowires. The low-energy valence band is quasidegenerate, formed by two doublets of different orbital angular momenta, and can be controlled via the relative shell thickness and via external fields. We find that direct (dipolar) coupling to a moderate electric field leads to an unusually large spin-orbit interaction of Rashba type on the order of meV which gives rise to pronounced helical states enabling electrical spin control. The system allows for quantum dots and spin qubits with energy levels that can vary from nearly zero to several meV, depending on the relative shell thickness.

10.  Controlling the Interaction of Electron and Nuclear Spins in a Tunnel-Coupled Quantum Dot
C. Kloeffel, P. A. Dalgarno, B. Urbaszek, B. D. Gerardot, D. Brunner, P. M. Petroff, D. Loss, and R. J. Warburton.
Phys. Rev. Lett. 106, 046802 (2011); arXiv:1010.3330.

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability. For a sigma(+) pump, the emission switches from sigma(+) to sigma(-) at a particular detuning of the laser. Simultaneously, the nuclear spin polarization switches from positive to negative. Away from the bistability, the nuclear spin polarization can be changed continuously from negative to positive, allowing precise control via the laser wavelength.