arXiv subjects

Alessandro Romito

Publications and source records attributed to Alessandro Romito.

68 records · Page 4Linked to original sources

arXiv2012

Manipulating Majorana fermions using supercurrents

Topological insulator edges and spin-orbit-coupled quantum wires in proximity to s-wave super- conductors can be tuned through a topological quantum phase transition by a Zeeman field. Here we show that a supercurrent flowing in the s-wave superconductor also drives such a transition. We propose to use this new mechanism to generate and manipulate Majorana fermions that localize at domain walls between topological and nontopological regions of an edge or wire. In quantum wires, this method carries the added benefit that a supercurrent reduces the critical Zeeman field at which the topological phase appears.

cond-mat.mes-hall↗

arXiv2011

Probability distribution of Majorana end-state energies in disordered wires

One-dimensional topological superconductors harbor Majorana bound states at their ends. For superconducting wires of finite length L, these Majorana states combine into fermionic excitations with an energy $ε_0$ that is exponentially small in L. Weak disorder leaves the energy splitting exponentially small, but affects its typical value and causes large sample-to-sample fluctuations. We show that the probability distribution of $ε_0$ is log normal in the limit of large L, whereas the distribution of the lowest-lying bulk energy level $ε_1$ has an algebraic tail at small $ε_1$. Our findings have implications for the speed at which a topological quantum computer can be operated.

cond-mat.mes-hall↗

arXiv2011

Charge sensing amplification via weak values measurement

A protocol employing weak values (WVs) to obtain ultra sensitive amplification of weak signals in the context of a solid state setup is proposed. We consider an Aharonov-Bohm interferometer where both the orbital and the spin degrees of freedom are weakly affected by the presence of an external charge to be detected. The interplay between the spin and the orbital WVs leads to a significant amplification even in the presence of finite temperature, voltage, and external noise.

cond-mat.mes-hall↗

arXiv2010

Single-qubit lasing in the strong-coupling regime

Motivated by recent ``circuit QED'' experiments we study the lasing transition and spectral properties of single-qubit lasers. In the strong coupling, low-temperature regime quantum fluctuations dominate over thermal noise and strongly influence the linewidth of the laser. When the qubit and the resonator are detuned, amplitude and phase fluctuations of the radiation field are coupled, and the phase diffusion model, commonly used to describe conventional lasers, fails. We predict pronounced effects near the lasing transition, with an enhanced linewidth and non-exponential decay of the correlation functions. We cover a wide range of parameters by using two complementary approaches, one based on the Liouville equation in a Fock state basis, covering arbitrarily strong coupling but limited to low photon numbers, the other based on the coherent-state representation, covering large photon numbers but restricted to weak or intermediate coupling.

cond-mat.mes-hall↗

arXiv2009

Decoherence in a Cooper pair shuttle

We examine decoherence effects in the Josephson current of a Cooper pair shuttle. Dephasing due to gate voltage fluctuations can either suppress or enhance the critical current and also change its sign. The current noise spectrum displays a peak at the Josephson coupling energy and shows a phase dependence.

cond-mat.mes-hall↗

arXiv2009

Transport properties of a periodically driven superconducting single electron transistor

We discuss coherent transport of Cooper pairs through a Cooper pair shuttle. We analyze both the DC and AC Josephson effect in the two limiting cases where the charging energy $E_C$ is either much larger or much smaller than the Josephson coupling $E_J$. In the limit $E_J \ll E_C$ we present the detailed behavior of the critical current as a function of the damping rates and the dynamical phases. The AC effect in this regime is very sensitive to all dynamical scales present in the problem. The effect of fluctuations of the external periodic driving is discussed as well. In the opposite regime the system can be mapped onto the quantum kicked rotator, a classically chaotic system. We investigate the transport properties also in this regime showing that the underlying classical chaotic dynamics emerges as an incoherent transfer of Cooper pairs through the shuttle. For an appropriate choice of the parameters the Cooper pair shuttle can exhibit the phenomenon of dynamical localization. We discuss in details the properties of the localized regime as a function of the phase difference between the superconducting electrodes and the decoherence due to gate voltage fluctuations. Finally we point how dynamical localization is reflected in the noise properties of the shuttle.

cond-mat.mes-hall↗

arXiv2009

Full Counting Statistics of Cooper Pair Shuttling

The Cooper pair shuttle is a simple model system that combines features of coherent and incoherent transport. We evaluate the full counting statistics (FCS) of charge transfer via the shuttle in the incoherent regime. We describe two limiting cases when the FCS allows for classical interpretation. Generally, the classical interpretation fails yielding negative and imaginary "probabilities". This signals that superconducting coherence survives even in incoherent regime. We evaluate the current noise in some detail.

cond-mat.mes-hall↗

arXiv2009

Decoherence of Rabi oscillations of electronic spin states in a double quantum dot

We study the role of charge fluctuations in the decoherence of Rabi oscillations between spin states |$\uparrow \downarrow$>, |$\downarrow \uparrow$> of two electrons in a double dot structure. We consider the effects of fluctuations in energy and in the quantum state of the system, both in the classical and quantum limit. The role of state fluctuations is shown to be of leading order at sufficiently high temperature, applicable to actual experiments. At low temperature the low frequency energy fluctuations are the only dominant contribution.

cond-mat.mes-hall↗

arXiv2008

Weak values under uncertain conditions

We analyze the average of weak values over statistical ensembles of pre- and post-selected states. The protocol of weak values, proposed by Aharonov et al., is the result of a weak measurement conditional on the outcome of a subsequent strong (projective) measurement. Weak values can be beyond the range of eigenvalues of the measured observable and, in general, can be complex numbers. We show that averaging over ensembles of pre- and post-selected states reduces the weak value within the range of eigenvalues of the measured operator. We further show that the averaged result expressed in terms of pre- and post-selected density matrices, allows us to include the effect of decoherence.

cond-mat.mes-hall↗

arXiv2008

Tomography of many-body weak values: Mach-Zehnder interferometry

We propose and study a weak value (WV) protocol in the context of a solid state setup, specifically, an electronic Mach-Zehnder interferometer. This is the first specific proposal to measure both the real and imaginary part (i.e., complete tomography) of a WV. We also analyze the manifestation of many-body physics in the WV to be measured, including finite temperature and shot-noise-like contributions.

cond-mat.mes-hall↗

arXiv2008

Weak values of electron spin in a double quantum dot

We propose a protocol for a controlled experiment to measure a weak value of the electron's spin in a solid state device. The weak value is obtained by a two step procedure -- weak measurement followed by a strong one (post-selection), where the outcome of the first measurement is kept provided a second post-selected outcome occurs. The set-up consists of a double quantum dot and a weakly coupled quantum point contact to be used as a detector. Anomalously large values of the spin of a two electron system are predicted, as well as negative values of the total spin. We also show how to incorporate the adverse effect of decoherence into this procedure.

cond-mat.mes-hall↗

arXiv2004

Solid-State Quantum Communication With Josephson Arrays

Josephson junction arrays can be used as quantum channels to transfer quantum information between distant sites. In this work we discuss simple protocols to realize state transfer with high fidelity. The channels do not require complicate gating but use the natural dynamics of a properly designed array. We investigate the influence of static disorder both in the Josephson energies and in the coupling to the background gate charges, as well as the effect of dynamical noise. We also analyze the readout process, and its backaction on the state transfer.

quant-ph↗

arXiv2004

Chaotic dynamics in superconducting nanocircuits

The quantum kicked rotator can be realized in a periodically driven superconducting nanocircuit. A study of the fidelity allows the experimental investigation of exponential instability of quantum motion inside the Ehrenfest time scale, chaotic diffusion and quantum dynamical localization. The role of noise and the experimental setup to measure the fidelity is discussed as well.

cond-mat.supr-con↗

arXiv2002

b -> s gamma in a calculable model of Electroweak Symmetry Breaking

In a recently proposed extension of the Standard Model with a compact extra dimension of size R we compute the leading corrections, of relative order (m_t R)^2, to the Branching Ratio B -> X_s gamma. For 1/R = 370 \pm 80 GeV, the branching ratio is increased in a significant way relative to the SM, although not al level of being inconsistent with present measurements.

hep-ph↗