arXiv subjects

Kirill Shtengel

Publications and source records attributed to Kirill Shtengel.

At least 37 records · Page 2Linked to original sources

arXiv2014

Reversible first-order transition in Pauli percolation

Percolation plays an important role in fields and phenomena as diverse as the study of social networks, the dynamics of epidemics, the robustness of electricity grids, conduction in disordered media, and geometric properties in statistical physics. We analyse a new percolation problem in which the first order nature of an equilibrium percolation transition can be established analytically and verified numerically. The rules for this site percolation model are physical and very simple, requiring only the introduction of a weight $W(n)=n+1$ for a cluster of size $n$. This establishes that a discontinuous percolation transition can occur with qualitatively more local interactions than in all currently considered examples of explosive percolation; and that, unlike these, it can be reversible. This greatly extends both the applicability of such percolation models in principle, and their reach in practice.

cond-mat.stat-mech↗

arXiv2014

Exotic circuit elements from zero-modes in hybrid superconductor/quantum Hall systems

Heterostructures formed by quantum Hall systems and superconductors have recently been shown to support widely coveted Majorana fermion zero-modes and still more exotic `parafermionic' generalizations. Here we establish that probing such zero-modes using quantum Hall edge states yields non-local transport signatures that pave the way towards a variety of novel circuit elements. In particular, we demonstrate quite generally that at low energies the zero-modes convert chirally moving quasiparticles into oppositely charged quasiholes propagating in the same direction---that is, they swap the sign of the chiral edge currents. One may then construct new and potentially useful circuit elements using this `perfect Andreev conversion' process, including superconducting current and voltage mirrors as well as transistors for fractional charge currents. Characterization of these circuit elements should provide striking evidence of the zero-mode physics.

cond-mat.str-el↗

arXiv2014

Adiabatic cooling of Majorana zero-modes in topological superconductors

We show that the presence of Majorana zero-modes in a one-dimensional topological superconductor can be detected by adiabatic cooling. This cooling effect results from an increase of the topological entropy associated with the ground state degeneracy due to such modes. Here, we consider an experimentally feasible topological superconductor: a strong spin-orbit coupled semiconducting quantum wire interfaced with an s-wave superconductor and subjected to a magnetic field. Numerical simulations of realistic experimental geometries indicate that the creation of Majorana zero-modes, which can be achieved by tuning of electronic gates or the external magnetic field, results in a measurable cooling effect. We also argue that this cooling effect results in an increase in the zero bias peak conductance.

cond-mat.mes-hall↗

arXiv2013

Thermopower and the Mott formula for a Majorana edge state

We study the thermoelectric effect between a conducting lead and a Majorana edge state. In the tunneling limit, we first use the Landuaer-Buttiker formalism to derive the Mott formula relating the thermopower and the differential conductance between a conducting lead and a superconductor. When the tunneling takes place between a conducting lead and a Majorana edge state, we show that a non-vanishing thermopower can exist. Combining measurements of the differential conductance and the voltage difference induced by the temperature difference between the conducting lead and the edge state, the Mott formula provides a unique way to infer the temperature of the Majorana edge state.

cond-mat.mes-hall↗

arXiv2013

Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure

Non-Abelian anyons promise to reveal spectacular features of quantum mechanics that could ultimately provide the foundation for a decoherence-free quantum computer. A key breakthrough in the pursuit of these exotic particles originated from Read and Green's observation that the Moore-Read quantum Hall state and a (relatively simple) two-dimensional p+ip superconductor both support so-called Ising non-Abelian anyons. Here we establish a similar correspondence between the Z_3 Read-Rezayi quantum Hall state and a novel two-dimensional superconductor in which charge-2e Cooper pairs are built from fractionalized quasiparticles. In particular, both phases harbor Fibonacci anyons that---unlike Ising anyons---allow for universal topological quantum computation solely through braiding. Using a variant of Teo and Kane's construction of non-Abelian phases from weakly coupled chains, we provide a blueprint for such a superconductor using Abelian quantum Hall states interlaced with an array of superconducting islands. Fibonacci anyons appear as neutral deconfined particles that lead to a two-fold ground-state degeneracy on a torus. In contrast to a p+ip superconductor, vortices do not yield additional particle types yet depending on non-universal energetics can serve as a trap for Fibonacci anyons. These results imply that one can, in principle, combine well-understood and widely available phases of matter to realize non-Abelian anyons with universal braid statistics. Numerous future directions are discussed, including speculations on alternative realizations with fewer experimental requirements.

cond-mat.str-el↗

arXiv2013

Spin Transfer of Quantum Information between Majorana Modes and a Resonator

We show that resonant coupling and entanglement between a mechanical resonator and majorana bound states can be achieved via spin currents in a 1D quantum wire with strong spin-orbit interactions. The bound states induced by vibrating and stationary magnets can hybridize thus resulting in spin-current induced $4π$-periodic torque, as a function of the relative field angle, acting on the resonator. We study the feasibility of detecting and manipulating majorana bound states with the use of magnetic resonance force microscopy techniques.

cond-mat.mes-hall↗

arXiv2012

Suppression of Hyperfine Dephasing by Spatial Exchange of Double Quantum Dots

We examine the logical qubit system of a pair of electron spins in double quantum dots. Each electron experiences a different hyperfine interaction with the local nuclei of the lattice, leading to a relative phase difference, and thus decoherence. Methods such as nuclei polarization, state narrowing, and spin-echo pulses have been proposed to delay decoherence. Instead we propose to suppress hyperfine dephasing by adiabatic rotation of the dots in real space, leading to the same average hyperfine interaction. We show that the additional effects due to the motion in the presence of spin-orbit coupling are still smaller than the hyperfine interaction, and result in an infidelity below 10^{-4} after ten decoupling cycles. We discuss a possible experimental setup and physical constraints for this proposal.

cond-mat.mes-hall↗

arXiv2012

Ettingshausen effect due to Majorana modes

The presence of Majorana zero-energy modes at vortex cores in a topological superconductor implies that each vortex carries an extra entropy $s_0$, given by $(k_{B}/2)\ln 2$, that is independent of temperature. By utilizing this special property of Majorana modes, the edges of a topological superconductor can be cooled (or heated) by the motion of the vortices across the edges. As vortices flow in the transverse direction with respect to an external imposed supercurrent, due to the Lorentz force, a thermoelectric effect analogous to the Ettingshausen effect is expected to occur between opposing edges. We propose an experiment to observe this thermoelectric effect, which could directly probe the intrinsic entropy of Majorana zero-energy modes.

cond-mat.mes-hall↗

arXiv2012

Exotic non-Abelian anyons from conventional fractional quantum Hall states

Non-Abelian anyons--particles whose exchange noncommutatively transforms a system's quantum state--are widely sought for the exotic fundamental physics they harbor as well as for quantum computing applications. There now exist numerous blueprints for stabilizing the simplest type of non-Abelian anyon, defects binding Majorana modes, by judiciously interfacing widely available materials. Following this line of attack, we introduce a device fabricated from conventional fractional quantum Hall states and s-wave superconductors that supports exotic non-Abelian anyons that bind `parafermions', which can be viewed as fractionalized Majorana fermions. We show that these modes can be experimentally identified (and distinguished from Majoranas) using Josephson measurements. We also provide a practical recipe for braiding parafermions and show that they give rise to non-Abelian statistics. Interestingly, braiding in our setup produces a richer set of topologically protected qubit operations when compared to the Majorana case. As a byproduct, we establish a new, experimentally realistic Majorana platform in weakly spin-orbit-coupled materials such as GaAs.

cond-mat.str-el↗

arXiv2011

Edge-Induced Qubit Polarization in Systems with Ising Anyons

We investigate the proposed experimental setup for measuring the topological charge in a an Ising anyon system by means of Fabry-Pérot interferometry with a chiral edge state. We show that such an interferometer has the unintended but not necessarily unwelcome effect of stabilizing the state of the system being measured (i.e., a topological qubit). We show further that interactions between the edge mode and the localized bulk quasiparticles can have the effect of polarizing the qubit, stabilizing its state. We discuss the these results in the context of recent interferometer experiments in the 5/2 fractional quantum Hall state, where the first of these effects is small, but the second may be relevant to the observed phenomena.

cond-mat.str-el↗

arXiv2011

Fermionic quantum dimer and fully-packed loop models on the square lattice

We consider fermionic fully-packed loop and quantum dimer models which serve as effective low-energy models for strongly correlated fermions on a checkerboard lattice at half and quarter filling, respectively. We identify a large number of fluctuationless states specific to each case, due to the fermionic statistics. We discuss the symmetries and conserved quantities of the system and show that for a class of fluctuating states in the half-filling case, the fermionic sign problem can be gauged away. This claim is supported by numerical evaluation of the low-lying states and can be understood by means of an algebraic construction. The elimination of the sign problem then allows us to analyze excitations at the Rokhsar-Kivelson point of the models using the relation to the height model and its excitations, within the single-mode approximation. We then discuss a mapping to a U(1) lattice gauge theory which relates the considered low-energy model to the compact quantum electrodynamics in 2+1 dimensions. Furthermore, we point out consequences and open questions in the light of these results.

cond-mat.str-el↗

arXiv2010

Improved phase gate reliability in systems with neutral Ising anyons

Recent proposals using heterostructures of superconducting and either topologically insulating or semiconducting layers have been put forth as possible platforms for topological quantum computation. These systems are predicted to contain Ising anyons and share the feature of having only neutral edge excitations. In this note, we show that these proposals can be combined with the recently proposed "sack geometry" for implementation of a phase gate in order to conduct robust universal quantum computation. In addition, we propose a general method for adjusting edge tunneling rates in such systems, which is necessary for the control of interferometric devices. The error rate for the phase gate in neutral Ising systems is parametrically smaller than for a similar geometry in which the edge modes carry charge: it goes as $T^3$ rather than $T$ at low temperatures. At zero temperature, the phase variance becomes constant at long times rather than carrying a logarithmic divergence.

cond-mat.mes-hall↗

arXiv2010

Implementing arbitrary phase gates with Ising anyons

Ising-type non-Abelian anyons are likely to occur in a number of physical systems, including quantum Hall systems, where recent experiments support their existence. In general, non-Abelian anyons may be utilized to provide a topologically error-protected medium for quantum information processing. However, the topologically protected operations that may be obtained by braiding and measuring topological charge of Ising anyons are precisely the Clifford gates, which are not computationally universal. The Clifford gate set can be made universal by supplementing it with single-qubit pi/8-phase gates. We propose a method of implementing arbitrary single qubit phase gates for Ising anyons by running a current of anyons with interfering paths around computational anyons. While the resulting phase gates are not topologically protected, they can be combined with "magic state distillation" to provide error-corrected pi/8-phase gates with a remarkably high threshold.

cond-mat.str-el↗

arXiv2010

Coulomb Blockade Doppelgangers in Quantum Hall States

In this paper, we ask the question: How well can Coulomb blockade experiments correctly identify and distinguish between different topological orders in quantum Hall states? We definitively find the answer to be: Quite poorly. In particular, we write the general expression for the spacing of resonance peaks in a simple form that explicitly displays its dependence on the conformal scaling dimensions of the systems' edge modes. This form makes transparent the general argument that the Coulomb blockade peak spacings do not provide a strongly indicative signature of the topological order of the system, since it is only weakly related to the braiding statistics. We bolster this general argument with examples for all the most physically relevant non-Abelian candidate states, demonstrating that they have Coulomb blockade doppelgangers -- candidate states at the same filling fraction with identical Coulomb blockade signatures, but dramatically different topological orders and braiding statistics.

cond-mat.str-el↗

arXiv2009

The non-Abelian Interferometer

We consider the tunneling current through a double point-contact Fabry-Perot interferometer such as used in recent experimental studies of the fractional quantum Hall plateau at filling fraction nu=5/2. We compare the predictions of several different models of the state of the electrons at this plateau: the Moore-Read, anti-Pfaffian, SU(2)_2 NAF, K=8 strong pairing, and (3,3,1) states. All of these predict the existence of charge e/4 quasiparticles, but the first three are non-Abelian while the last two are Abelian. We give explicit formulas for the scaling of charge e/2 and charge e/4 quasiparticle contributions to the current as a function of temperature, gate voltage and distance between the two point contacts for all three models. Based on these, we analyze several possible explanations of two phenomena reported for recent experiments by Willett et al., namely halving of the period of the observed resistance oscillations with rising temperature and alternation between the same two observed periods at low temperatures as the area of the interference loop is varied with a side gate. We conclude that the most likely explanation is that the observed alternation is due to switching between even and odd numbers of charge e/4 quasiparticles enclosed within the loop as a function of side gate voltage, which is a clear signature of the presence of non-Abelian anyons. However, there are important features of the data which do not have a simple explanation within this picture. We suggest further experiments which could help rule out some possible scenarios. We make the corresponding predictions for future tunneling and interference experiments at the other observed second Landau level fractional quantum Hall states.

cond-mat.mes-hall↗

arXiv2008

Lieb-Schultz-Mattis theorem for quasi-topological systems

In this paper we address the question of the existence of a spectral gap in a class of local Hamiltonians. These Hamiltonians have the following properties: their ground states are known exactly; all equal-time correlation functions of local operators are short-ranged; and correlation functions of certain non-local operators are critical. A variational argument shows gaplessness with $ω\propto k^2$ at critical points defined by the absence of certain terms in the Hamiltonian, which is remarkable because equal-time correlation functions of local operators remain short-ranged. We call such critical points, in which spatial and temporal scaling are radically different, quasi-topological. When these terms are present in the Hamiltonian, the models are in gapped topological phases which are of special interest in the context of topological quantum computation.

cond-mat.str-el↗

arXiv2008

Dzyaloshinskii-Moriya interactions in valence bond systems

We investigate the effect of Dzyaloshinskii-Moriya interactions on the low temperature magnetic susceptibility for a system whose low energy physics is dominated by short-range valence bonds (singlets). Our general perturbative approach is applied to specific models expected to be in this class, including the Shastry-Sutherland model of the spin-dimer compound SrCu_2(BO_3)_2 and the antiferromagnetic Heisenberg model of the recently discovered S=1/2 kagome compound ZnCu_3(OH)_6Cl_2. The central result is that a short-ranged valence bond phase, when perturbed with Dzyaloshinskii-Moriya interactions, will remain time-reversal symmetric in the absence of a magnetic field but the susceptibility will be nonzero in the T\to 0 limit. Applied to ZnCu_3(OH)_6Cl_2, this model provides an avenue for reconciling experimental results, such as the lack of magnetic order and lack of any sign of a spin gap, with known theoretical facts about the kagome Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2008

Local interactions and non-Abelian quantum loop gases

Two-dimensional quantum loop gases are elementary examples of topological ground states with Abelian or non-Abelian anyonic excitations. While Abelian loop gases appear as ground states of local, gapped Hamiltonians such as the toric code, we show that gapped non-Abelian loop gases require non-local interactions (or non-trivial inner products). Perturbing a local, gapless Hamiltonian with an anticipated ``non-Abelian'' ground-state wavefunction immediately drives the system into the Abelian phase, as can be seen by measuring the Hausdorff dimension of loops. Local quantum critical behavior is found in a loop gas in which all equal-time correlations of local operators decay exponentially.

cond-mat.stat-mech↗