Peter Stano
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Research Interests
 Semiconductor spin qubits: spinorbit effects in lateral quantum dots
 Mesoscopic spintronics: nonlinear efects in transport and spin current measurement
 Quantum decoherence: electron/hole spin in nuclear spin bath
Short CV
2012now  SCIEX fellow with Prof. Dr. Daniel Loss 
20092011  postdoc with Prof. Philippe Jacquod, University of Arizona, USA 
20082009  postdoc with Prof. RNDr. Vladimir Buzek DrSc., Slovak Academy of Sciences, Bratislava 
2007:PhD  in condensend matter theory with Prof. Dr. Jaroslav Fabian, Universty of Regensburg, Germany 
2003:Master  in theoretical and matematical physics, Comenius University, Bratislava, Slovakia 
Publications
Show all abstracts.1.  Topological Superconductivity and Majorana Fermions in RKKY Systems 
Jelena Klinovaja, Peter Stano, Ali Yazdani, and Daniel Loss. arXiv:1307.1442
We consider quasi onedimensional RKKY systems in proximity to an swave
superconductor. We show that a $2k_F$peak in the spin susceptibility of the
superconductor in the onedimensional limit supports helical order of localized
magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The
magnetic helix is equivalent to a uniform magnetic field and very strong
spinorbit interaction (SOI) with an effective SOI length $1/2k_F$. We find the
conditions to establish such a magnetic state in atomic chains and
semiconducting nanowires with magnetic atoms or nuclear spins. Generically,
these systems are in a topological phase with Majorana fermions. The inherent
selftuning of the helix to $2k_F$ eliminates the need to tune the chemical
potential.
 
2.  Circuit QED with HoleSpin Qubits in Ge/Si Nanowire Quantum Dots 
Christoph Kloeffel, Mircea Trif, Peter Stano, and Daniel Loss. 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). Singlequbit gates can be driven through electricdipoleinduced
spin resonance, with spinflip times shorter than 100 ps. Longdistance
qubitqubit 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 squarerootofiSWAP gate. The
absence of Dresselhaus spinorbit interaction (SOI) and the presence of an
unusually strong Rashbatype SOI enable precise control over the transverse
qubit coupling via an externally applied, perpendicular electric field. The
latter serves as an onoff switch for quantum gates and also provides control
over the g factor, so that single and twoqubit gates can be operated
independently. Remarkably, we find that idle states are insensitive to charge
noise and phonons, and we discuss strategies for enhancing noiselimited gate
fidelities.
 
3.  Local Spin Susceptibilities of LowDimensional Electron Systems 
Peter Stano, Jelena Klinovaja, Amir Yacoby, and Daniel Loss. arXiv:1303.1151
We investigate, assess, and suggest possibilities for a measurement of the
local spin susceptibility of a conducting lowdimensional electron system. The
basic setup of the experiment we envisage is a sourceprobe one. Locally
induced spin density (e.g. by a magnetized atomic force microscope tip) extends
in the medium according to its spin susceptibility. The induced magnetization
can be detected as a dipolar magnetic field, for instance, by an
ultrasensitive nitrogenvacancy center based detector, from which the spatial
structure of the spin susceptibility can be deduced. We find that
onedimensional systems, such as semiconducting nanowires or carbon nanotubes,
are expected to yield a measurable signal. The signal in a twodimensional
electron gas is weaker, though materials with high enough $g$factor (such as
InGaAs) seem promising for successful measurements.
 
4.  Suppression of Interactions in Multimode Random Lasers in the Anderson Localized Regime 
Peter Stano and Philippe Jacquod. Nature Photonics 7, 66 (2013); arXiv:1210.6462.
Understanding random lasing is a formidable theoretical challenge. Unlike
conventional lasers, random lasers have no resonator to trap light, they are
highly multimode with potentially strong modal interactions and they are based
on disordered gain media, where photons undergo random multiple scattering.
Interference effects notoriously modify the propagation of waves in such random
media, but their fate in the presence of nonlinearity and interactions is
poorly understood. Here, we present a semiclassical theory for multimode random
lasing in the strongly scattering regime. We show that Anderson localization, a
waveinterference effect, is not affected by the presence of nonlinearities. To
the contrary, its presence suppresses interactions between simultaneously
lasing modes. Using a recently constructed theory for complex multimode lasers,
we show analytically how Anderson localization justifies a noninteracting,
singlepole approximation. Consequently, lasing modes in a strongly scattering
random laser are given by longlived, Anderson localized modes of the passive
cavity, whose frequency and wave profile does not vary with pumping, even in
the multimode regime when mode overlap spatially.
 
5.  Spin ordering in magnetic quantum dots: From corehalo to Wigner molecules 
Rafal Oszwaldowski, Peter Stano, Andre G. Petukhov, and Igor Zutic. Phys. Rev. B 86, 201408(R) (2012); arXiv:1210.6422.
The interplay of confinement and Coulomb interactions in quantum dots can
lead to strongly correlated phases differing qualitatively from the Fermi
liquid behavior. We explore how the presence of magnetic impurities in quantum
dots can provide additional opportunities to study correlation effects and the
resulting ordering in carrier and impurity spin. By employing exact
diagonalization we reveal that seemingly simple twocarrier quantum dots lead
to a rich phase diagram. We propose experiments to verify our predictions, in
particular we discuss interband optical transitions as a function of
temperature and magnetic field.
 
6.  Spinorbit coupled particle in a spin bath 
Peter Stano, Jaroslav Fabian, and Igor Zutic. PRB 87, 165303 (2013); arXiv:1208.5606.
We consider a spinorbit coupled particle confined in a quantum dot in a bath
of impurity spins. We investigate the consequences of spinorbit coupling on
the interactions that the particle mediates in the spin bath. We show that in
the presence of spinorbit coupling, the impurityimpurity interactions are no
longer spinconserving. We quantify the degree of this symmetry breaking and
show how it relates to the spinorbit coupling strength. We identify several
ways how the impurity ensemble can in this way relax its spin by coupling to
phonons. A typical resulting relaxation rate for a selfassembled Mndoped ZnTe
quantum dot populated by a hole is 1 $\mu$s. We also show that decoherence
arising from nuclear spins in lateral quantum dots is still removable by a spin
echo protocol, even if the confined electron is spinorbit coupled.
 
7.  Transition from fractional to Majorana fermions in Rashba nanowires 
Jelena Klinovaja, Peter Stano, and Daniel Loss. Phys. Rev. Lett. 109, 236801 (2012); arXiv:1207.7322.
We study hybrid superconductingsemiconducting nanowires in the presence of
Rashba spinorbit interaction as well as helical magnetic fields. We show that
the interplay between them leads to a competition of phases with two
topological gaps closing and reopening, resulting in unexpected reentrance
behavior. Besides the topological phase with localized Majorana fermions (MFs)
we find new phases characterized by fractionally charged fermion (FF) bound
states of JackiwRebbi type. The system can be fully gapped by the magnetic
fields alone, giving rise to FFs that transmute into MFs upon turning on
superconductivity. We find explicit analytical solutions for MF and FF bound
states and determine the phase diagram numerically by determining the
corresponding Wronskian null space. We show by renormalization group arguments
that electronelectron interactions enhance the Zeeman gaps opened by the
fields.
 
8.  Theory of Spin Relaxation in TwoElectron Lateral Coupled Si/SiGe Quantum Dots 
Martin Raith, Peter Stano, and Jaroslav Fabian. Phys. Rev. B 86, 205321 (2012); arXiv:1206.6906.
Highly accurate numerical results of phononinduced twoelectron spin
relaxation in silicon double quantum dots are presented. The relaxation,
enabled by spinorbit coupling and the nuclei of 29Si (natural or purified
abundance), are investigated for all relevant parameter regimes, the interdot
coupling, the magnetic field magnitude and orientation, and the detuning. We
calculate all relaxation rates for zero and finite temperatures (100 mK),
concluding that all findings for zero temperature qualitatively remain valid
also for 100 mK. We confirm the same anisotropic switch of the axis of
prolonged spin lifetime with varying detuning as recently predicted in GaAs.
However, there is a striking difference compared to the GaAs counterpart. In
silicon, the hyperfineinduced relaxation rate is negligible in all cases we
studiedeven for natural silicon. The spinorbit coupling, although weak, is
the dominant contribution, yielding anisotropic relaxation rates of at least
two order of magnitude lower than in GaAs.
 
9.  Theory of Spin Relaxation in TwoElectron Lateral Coupled Quantum Dots 
Martin Raith, Peter Stano, Fabio Baruffa, and Jaroslav Fabian. Phys. Rev. Lett. 108, 246602 (2012); arXiv:1111.6724.
A global quantitative picture of the phononinduced twoelectron spin
relaxation in GaAs double quantum dots is presented using highly accurate
numerical calculations. Wide regimes of interdot coupling, magnetic field
magnitude and orientation, and detuning are explored in the presence of a
nuclear bath. Most important, the unusually strong magnetic anisotropy of the
singlettriplet relaxation can be controlled by detuning switching the
principal anisotropy axes: a protected state becomes unprotected upon detuning,
and vice versa. It is also established that nuclear spins can dominate spin
relaxation for unpolarized triplets even at high magnetic fields, contrary to
common belief. These findings are central to designing quantum dots geometries
for spinbased quantum information processing with minimal environmental
impact.
 
10.  Nonlinear spin to charge conversion in mesoscopic structures 
Peter Stano, Jaroslav Fabian, and Philippe Jacquod. Phys. Rev. B 85, 241301(R) (2012); arXiv:1201.0249.
Motivated by recent experiments [VeraMarun et al., arXiv:1109.5969], we
formulate a nonlinear theory of spin transport in quantum coherent conductors.
We show how a mesoscopic constriction with energydependent transmission can
convert a spin current injected by a spin accumulation into an electric signal,
relying neither on magnetic nor exchange fields. When the transmission through
the constriction is spinindependent, the spincharge coupling is nonlinear,
with an electric signal that is quadratic in the accumulation. We estimate that
gated mesoscopic constrictions have a sensitivity that allows to detect
accumulations much smaller than a percent of the Fermi energy.
 
11.  Measuring Spin Accumulations with Current Noise 
Jonathan Meair, Peter Stano, and Philippe Jacquod. Phys. Rev. B 84, 073302 (2011); arXiv:1104.2353.
We investigate the timedependent fluctuations of the electric current
injected from a reservoir with a nonequilibrium spin accumulation into a
mesoscopic conductor. We show how the current noise power directly reflects the
magnitude of the spin accumulation in two easily noticeable ways. First, as the
temperature is lowered, the smallbias noise saturates at a value determined by
the spin accumulation. Second, in the presence of spinorbit interactions in
the conductor, the current noise exhibits a sampledependent mesoscopic
asymmetry under reversal of the electric current direction. These features
provide for a purely electric protocol for measuring spin accumulations.
 
12.  SpintoCharge Conversion of Mesoscopic Spin Currents 
Peter Stano and Philippe Jacquod. Phys. Rev. Lett. 106, 206602 (2011); arXiv:1012.1831.
Recent theoretical investigations have shown that spin currents can be
generated by passing electric currents through spinorbit coupled mesoscopic
systems. Measuring these spin currents has however not been achieved to date.
We show how mesoscopic spin currents in lateral heterostructures can be
measured with a singlechannel voltage probe. In the presence of a spin
current, the charge current $I_{\rm qpc}$ through the quantum point contact
connecting the probe is odd in an externally applied Zeeman field $B$, while it
is even in the absence of spin current. Furthermore, the zero field derivative
$\partial_B I_{\rm qpc}$ is proportional to the magnitude of the spin current,
with a proportionality coefficient that can be determined in an independent
measurement. We confirm these findings numerically.
 
13.  Theory of Single Electron Spin Relaxation in Si/SiGe Lateral Coupled Quantum Dots 
Martin Raith, Peter Stano, and Jaroslav Fabian. Phys. Rev. B 83, 195318 (2011); arXiv:1101.3858.
We investigate the spin relaxation induced by acoustic phonons in the
presence of spinorbit interactions in single electron Si/SiGe lateral coupled
quantum dots. The relaxation rates are computed numerically in single and
double quantum dots, in inplane and perpendicular magnetic fields. The
deformation potential of acoustic phonons is taken into account for both
transverse and longitudinal polarizations and their contributions to the total
relaxation rate are discussed with respect to the dilatation and shear
potential constants. We find that in single dots the spin relaxation rate
scales approximately with the seventh power of the magnetic field, in line with
a recent experiment. In double dots the relaxation rate is much more sensitive
to the dot spectrum structure, as it is often dominated by a spin hot spot. The
anisotropy of the spinorbit interactions gives rise to easy passages, special
directions of the magnetic field for which the relaxation is strongly
suppressed. Quantitatively, the spin relaxation rates in Si are typically 2
orders of magnitude smaller than in GaAs due to the absence of the
piezoelectric phonon potential and generally weaker spinorbit interactions.
 
14.  Spinorbit coupling and anisotropic exchange in twoelectron double quantum dots 
Fabio Baruffa, Peter Stano, and Jaroslav Fabian. Phys. Rev. B 82, 045311 (2010); arXiv:1004.2610.
The influence of the spinorbit interactions on the energy spectrum of
twoelectron laterally coupled quantum dots is investigated. The effective
Hamiltonian for a spin qubit pair proposed in F. Baruffa et al., Phys. Rev.
Lett. 104, 126401 (2010) is confronted with exact numerical results in single
and double quantum dots in zero and finite magnetic field. The anisotropic
exchange Hamiltonian is found quantitatively reliable in double dots in
general. There are two findings of particular practical importance: i) The
model stays valid even for maximal possible interdot coupling (a single dot),
due to the absence of a coupling to the nearest excited level, a fact following
from the dot symmetry. ii) In a weak coupling regime, the HeitlerLondon
approximation gives quantitatively correct anisotropic exchange parameters even
in a finite magnetic field, although this method is known to fail for the
isotropic exchange. The small discrepancy between the analytical model (which
employes the linear Dresselhaus and BychkovRashba spinorbit terms) and the
numerical data for GaAs quantum dots is found to be mostly due to the cubic
Dresselhaus term.
 
15.  Spindependent tunneling into an empty lateral quantum dot 
Peter Stano and Philippe Jacquod. Phys. Rev. B 82, 125309 (2010); arXiv:1005.0024.
Motivated by the recent experiments of Amasha {\it et al.} [Phys. Rev. B {\bf
78}, 041306(R) (2008)], we investigate single electron tunneling into an empty
quantum dot in presence of a magnetic field. We numerically calculate the
tunneling rate from a laterally confined, fewchannel external lead into the
lowest orbital state of a spinorbit coupled quantum dot. We find two
mechanisms leading to a spindependent tunneling rate. The first originates
from different electronic $g$factors in the lead and in the dot, and favors
the tunneling into the spin ground (excited) state when the $g$factor
magnitude is larger (smaller) in the lead. The second is triggered by
spinorbit interactions via the opening of offdiagonal spintunneling
channels. It systematically favors the spin excited state. For physical
parameters corresponding to lateral GaAs/AlGaAs heterostructures and the
experimentally reported tunneling rates, both mechanisms lead to a discrepancy
of $\sim$10% in the spin up vs spin down tunneling rates. We conjecture that
the significantly larger discrepancy observed experimentally originates from
the enhancement of the $g$factor in laterally confined lead.
 
16.  Theory of anisotropic exchange in laterally coupled quantum dots 
Fabio Baruffa, Peter Stano, and Jaroslav Fabian. Phys. Rev. Lett. 104, 126401 (2010); arXiv:0908.2961.
The effects of spinorbit coupling on the twoelectron spectra in lateral
coupled quantum dots are investigated analytically and numerically. It is
demonstrated that in the absence of magnetic field the exchange interaction is
practically unaffected by spinorbit coupling, for any interdot coupling,
boosting prospects for spinbased quantum computing. The anisotropic exchange
appears at finite magnetic fields. A numerically accurate effective spin
Hamiltonian for modeling spinorbitinduced twoelectron spin dynamics in the
presence of magnetic field is proposed.
 
17.  Coexistence of quantum operations 
Teiko Heinosaari, Daniel Reitzner, Peter Stano, and Mario Ziman. J. Phys. A 42, 365302 (2009); arXiv:0905.4953.
Quantum operations are used to describe the observed probability
distributions and conditional states of the measured system. In this paper, we
address the problem of their joint measurability (coexistence). We derive two
equivalent coexistence criteria. The two most common classes of operations 
Luders operations and conditional state preparators  are analyzed. It is shown
that Luders operations are coexistent only under very restrictive conditions,
when the associated effects are either proportional to each other, or disjoint.
 
18.  Notes on Joint Measurability of Quantum Observables 
Teiko Heinosaari, Daniel Reitzner, and Peter Stano. Foundations of Physics 38, 11331147 (2008); arXiv:0811.0783.
For sharp quantum observables the following facts hold: (i) if we have a
collection of sharp observables and each pair of them is jointly measurable,
then they are jointly measurable all together; (ii) if two sharp observables
are jointly measurable, then their joint observable is unique and it gives the
greatest lower bound for the effects corresponding to the observables; (iii) if
we have two sharp observables and their every possible two outcome
partitionings are jointly measurable, then the observables themselves are
jointly measurable. We show that, in general, these properties do not hold.
Also some possible candidates which would accompany joint measurability and
generalize these apparently useful properties are discussed.
 
19.  Approximate Joint Measurability of Spin Along Two Directions 
Teiko Heinosaari, Peter Stano, and Daniel Reitzner. International Journal of Quantum Information 6, 975 (2008); arXiv:0801.2712.
We study the existence of jointly measurable POVM approximations to two
noncommuting sharp spin observables. We compare two different ways to specify
optimal approximations.
 
20.  Coexistence of qubit effects 
Peter Stano, Daniel Reitzner, and Teiko Heinosaari. Phys. Rev. A 78, 012315 (2008); arXiv:0802.4248.
We characterize all coexistent pairs of qubit effects. This gives an
exhaustive description of all pairs of events allowed, in principle, to occur
in a single qubit measurement. The characterization consists of three disjoint
conditions which are easy to check for a given pair of effects. Known special
cases are shown to follow from our general characterization theorem.
 
21.  Control of electron spin and orbital resonance in quantum dots through spinorbit interactions 
Peter Stano and Jaroslav Fabian. Phys. Rev. B 77, 045310 (2008); arXiv:condmat/0611228.
Influence of resonant oscillating electromagnetic field on a single electron
in coupled lateral quantum dots in the presence of phononinduced relaxation
and decoherence is investigated. Using symmetry arguments it is shown that spin
and orbital resonance can be efficiently controlled by spinorbit interactions.
The control is possible due to the strong sensitivity of Rabi frequency to the
dot configuration (orientation of the dot and a static magnetic field) as a
result of the anisotropy of the spinorbit interactions. The so called easy
passage configuration is shown to be particularly suitable for magnetic
manipulation of spin qubits, ensuring long spin relaxation time and protecting
the spin qubit from electric field disturbances accompanying onchip
manipulations.
 
22.  Semiconductor Spintronics 
J. Fabian, A. MatosAbiague, C. Ertler, P. Stano, and I. Zutic. Acta Physica Slovaca 57, No.4&5, 565907 (2007); arXiv:0711.1461.
Spintronics refers commonly to phenomena in which the spin of electrons in a
solid state environment plays the determining role. In a more narrow sense
spintronics is an emerging research field of electronics: spintronics devices
are based on a spin control of electronics, or on an electrical and optical
control of spin or magnetism. This review presents selected themes of
semiconductor spintronics, introducing important concepts in spin transport,
spin injection, SilsbeeJohnson spincharge coupling, and spindependent
tunneling, as well as spin relaxation and spin dynamics. The most fundamental
spindependent nteraction in nonmagnetic semiconductors is spinorbit coupling.
Depending on the crystal symmetries of the material, as well as on the
structural properties of semiconductor based heterostructures, the spinorbit
coupling takes on different functional forms, giving a nice playground of
effective spinorbit Hamiltonians. The effective Hamiltonians for the most
relevant classes of materials and heterostructures are derived here from
realistic electronic band structure descriptions. Most semiconductor device
systems are still theoretical concepts, waiting for experimental
demonstrations. A review of selected proposed, and a few demonstrated devices
is presented, with detailed description of two important classes: magnetic
resonant tunnel structures and bipolar magnetic diodes and transistors. In most
cases the presentation is of tutorial style, introducing the essential
theoretical formalism at an accessible level, with casestudylike
illustrations of actual experimental results, as well as with brief reviews of
relevant recent achievements in the field.
 
23.  Orbital and spin relaxation in single and coupled quantum dots 
Peter Stano and Jaroslav Fabian. Phys. Rev. B 74, 045320 (2006); arXiv:condmat/0604633.
Phononinduced orbital and spin relaxation rates of single electron states in
lateral single and double quantum dots are obtained numerically for realistic
materials parameters. The rates are calculated as a function of magnetic field
and interdot coupling, at various field and quantum dot orientations. It is
found that orbital relaxation is due to deformation potential phonons at low
magnetic fields, while piezoelectric phonons dominate the relaxation at high
fields. Spin relaxation, which is dominated by piezoelectric phonons, in single
quantum dots is highly anisotropic due to the interplay of the BychkovRashba
and Dresselhaus spinorbit couplings. Orbital relaxation in double dots varies
strongly with the interdot coupling due to the cyclotron effects on the
tunneling energy. Spin relaxation in double dots has an additional anisotropy
due to anisotropic spin hot spots which otherwise cause giant enhancement of
the rate at useful magnetic fields and interdot couplings. Conditions for the
absence of the spin hot spots in inplane magnetic fields (easy passages) and
perpendicular magnetic fields (weak passages) are formulated analytically for
different growth directions of the underlying heterostructure. It is shown that
easy passages disappear (spin hot spots reappear) if the double dot system
loses symmetry by an xylike perturbation.
 
24.  Theory of phononinduced spin relaxation in laterally coupled quantum dots 
Peter Stano and Jaroslav Fabian. Phys. Rev. Lett. 96, 186602 (2006); arXiv:condmat/0512713.
Phononinduced spin relaxation in coupled lateral quantum dots in the
presence of spinorbit coupling is calculated. The calculation for single dots
is consistent with experiment. Spin relaxation in double dots at useful
interdot couplings is dominated by spin hot spots that are strongly
anisotropic. Spin hot spots are ineffective for a diagonal crystallographic
orientation of the dots with a transverse inplane field. This geometry is
proposed for spinbased quantum information processing.
 
25.  Spin properties of single electron states in coupled quantum dots 
Peter Stano and Jaroslav Fabian. Phys. Rev. B 72, 155410 (2005); arXiv:condmat/0506610.
Spin properties of single electron states in laterally coupled quantum dots
in the presence of a perpendicular magnetic field are studied by exact
numerical diagonalization. Dresselhaus (linear and cubic) and BychkovRashba
spinorbit couplings are included in a realistic model of confined dots based
on GaAs. Group theoretical classification of quantum states with and without
spin orbit coupling is provided. Spinorbit effects on the gfactor are rather
weak. It is shown that the frequency of coherent oscillations (tunneling
amplitude) in coupled dots is largely unaffected by spinorbit effects due to
symmetry requirements. The leading contributions to the frequency involves the
cubic term of the Dresselhaus coupling. Spinorbit coupling in the presence of
magnetic field leads to a spindependent tunneling amplitude, and thus to the
possibility of spin to charge conversion, namely spatial separation of spin by
coherent oscillations in a uniform magnetic field. It is also shown that spin
hot spots exist in coupled GaAs dots already at moderate magnetic fields, and
that spin hot spots at zero magnetic field are due to the cubic Dresselhaus
term only.
