arXiv subjects

Takis Kontos

Publications and source records attributed to Takis Kontos.

16 recordsLinked to original sources

arXiv2025

Interferometric evidence of non-volatile anomalous phase shifts in exchange-spin-split Josephson supercurrent diodes

The recent realization of zero-field, polarity-reversible supercurrent rectification in proximity-magnetized Rashba-type Pt Josephson junctions (JJs) enables the development of superconducting logic circuits and cryogenic memory applications. Here, we demonstrate a non-volatile anomalous phase shift {\phi}_0 directly probed via superconducting quantum interferometry, providing phase-sensitive evidence of spontaneous time-reversal symmetry breaking in these Rashba-type systems. By replacing the Pt barrier with 5d or 4d element layers exhibiting different (para-)magnetic susceptibilities, spin-orbit coupling strengths, and electronic band structures, we elucidate the role of proximity effects in governing zero-field diode behavior. Ta (W) JJs exhibit zero-field diode efficiencies of ~17% (~5%) at 2 K, which are slightly (significantly) lower than those of Pt JJs. Notably, the diode polarity in Ta and W JJs is reversed relative to Pt JJs. Combined with the large zero-field diode efficiency (~15% at 2 K) observed in highly magnetic-susceptible Pd JJs, these results show that non-volatile {\phi}_0 and, consequently, zero-field diode performance can be tuned through proximity engineering of interfacial magnetic ordering and Rashba spin-orbit interaction.

cond-mat.supr-con↗

arXiv2019

Exponential suppression of bit-flips in a qubit encoded in an oscillator

A quantum system interacts with its environment, if ever so slightly, no matter how much care is put into isolating it. As a consequence, quantum bits (qubits) undergo errors, putting dauntingly difficult constraints on the hardware suitable for quantum computation. New strategies are emerging to circumvent this problem by encoding a qubit non-locally across the phase space of a physical system. Since most sources of decoherence are due to local fluctuations, the foundational promise is to exponentially suppress errors by increasing a measure of this non-locality. Prominent examples are topological qubits which delocalize quantum information over real space and where spatial extent measures non-locality. In this work, we encode a qubit in the field quadrature space of a superconducting resonator endowed with a special mechanism that dissipates photons in pairs. This process pins down two computational states to separate locations in phase space. As we increase this separation, we measure an exponential decrease of the bit-flip rate while only linearly increasing the phase-flip rate. Since bit-flips are continuously and autonomously corrected at the single qubit level, only phase-flips are left to be corrected via a one-dimensional quantum error correction code. This exponential scaling demonstrates that resonators with non-linear dissipation are promising building blocks for universal fault-tolerant quantum computation with drastically reduced hardware overhead.

quant-ph↗

arXiv2017

Cavity QED with hybrid nanocircuits: from atomic-like physics to condensed matter phenomena

Circuit QED techniques have been instrumental to manipulate and probe with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices where the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a new experimental playground to study the light-matter interaction in electronic circuits. Here, we present the experimental state of the art of Mesoscopic QED and its theoretical description. A first class of experiments focuses on the artificial atom limit, where some quasiparticles are trapped in nanocircuit bound states. In this limit, the Circuit QED techniques can be used to manipulate and probe electronic degrees of freedom such as confined charges, spins, or Andreev pairs. A second class of experiments consists in using cavity photons to reveal the dynamics of electron tunneling between a nanoconductor and fermionic reservoirs. For instance, the Kondo effect, the charge relaxation caused by grounded metallic contacts, and the photo-emission caused by voltage-biased reservoirs have been studied. The tunnel coupling between nanoconductors and fermionic reservoirs also enable one to obtain split Cooper pairs, or Majorana bound states. Cavity photons represent a qualitatively new tool to study these exotic condensed matter states.

cond-mat.mes-hall↗

arXiv2017

Direct cavity detection of Majorana pairs

No experiment could directly test the particle/antiparticle duality of Majorana fermions, so far. However, this property represents a necessary ingredient towards the realization of topological quantum computing schemes. Here, we show how to complete this task by using microwave techniques. The direct coupling between a pair of overlapping Majorana bound states and the electric field from a microwave cavity is extremely difficult to detect due to the self-adjoint character of Majorana fermions which forbids direct energy exchanges with the cavity. We show theoretically how this problem can be circumvented by using photo-assisted tunneling to fermionic reservoirs. The absence of direct microwave transition inside the Majorana pair in spite of the light-Majorana coupling would represent a smoking gun for the Majorana self-adjoint character.

cond-mat.mes-hall↗

arXiv2015

Electron-photon coupling in Mesoscopic Quantum Electrodynamics

Understanding the interaction between cavity photons and electronic nanocircuits is crucial for the development of Mesoscopic Quantum Electrodynamics (QED). One has to combine ingredients from atomic Cavity QED, like orbital degrees of freedom, with tunneling physics and strong cavity field inhomogeneities, specific to superconducting circuit QED. It is therefore necessary to introduce a formalism which bridges between these two domains. We develop a general method based on a photonic pseudo-potential to describe the electric coupling between electrons in a nanocircuit and cavity photons. In this picture, photons can induce simultaneously orbital energy shifts, tunneling, and local orbital transitions. We study in details the elementary example of a single quantum dot with a single normal metal reservoir, coupled to a cavity. Photon-induced tunneling terms lead to a non-universal relation between the cavity frequency pull and the damping pull. Our formalism can also be applied to multi quantum dot circuits, molecular circuits, quantum point contacts, metallic tunnel junctions, and superconducting nanostructures enclosing Andreev bound states or Majorana bound states, for instance.

cond-mat.mes-hall↗

arXiv2013

Squeezing light with Majorana fermions

Coupling a semiconducting nanowire to a microwave cavity provides a powerfull means to assess the presence or absence of isolated Majorana fermions in the nanowire. These exotic bound states can cause a significant cavity frequency shift but also a strong cavity nonlinearity leading for instance to light squeezing. The dependence of these effects on the nanowire gate voltages gives direct signatures of the unique properties of Majorana fermions, such as their self-adjoint character and their exponential confinement.

cond-mat.mes-hall↗

arXiv2012

Subradiant split Cooper pairs

We suggest a way to characterize the coherence of the split Cooper pairs emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying the radiative response of such a CPS inside a microwave cavity. The coherence of the split pairs manifests in a strongly nonmonotonic variation of the emitted radiation as a function of the parameters controlling the coupling of the CPS to the cavity. The idea to probe the coherence of the electronic states using the tools of Cavity Quantum Electrodynamics could be generalized to many other nanoscale circuits.

cond-mat.mes-hall↗

arXiv2011

Mesoscopic admittance of a double quantum dot

We calculate the mesoscopic admittance $G(ω)$ of a double quantum dot (DQD),which can be measured directly using microwave techniques. This quantity reveals spectroscopic information on the DQD and is also directly sensitive to a Pauli spin blockade effect. We then discuss the problem of a DQD coupled to a high quality photonic resonator. When the photon correlation functions can be developed along a random-phase-approximation-like scheme, the response of the resonator gives an access to $G(ω)$.

cond-mat.mes-hall↗

arXiv2010

A spin quantum bit with ferromagnetic contacts for circuit QED

We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nanoconductors. This allows to envision on-chip single spin manipulation and read-out using cavity QED techniques.

cond-mat.mes-hall↗

arXiv2009

Multi-terminal spin-dependent transport in ballistic carbon nanotubes

We study theoretically nonlocal spin transport in a ballistic carbon nanotube contacted to two ferromagnetic leads and two normal-metal leads. When the magnetizations of the two ferromagnets are changed from a parallel to an antiparallel configuration, the circuit shows a hysteretic behavior which is specific to the few-channel regime. In the coherent limit, the amplitude of the magnetic signals is strongly enhanced due to resonance effects occurring inside the nanotube. Our calculations pave the way for experiments on low-dimensional nonlocal spin transport, which should give results remarkably different from the experiments realized so far in the multichannel diffusive incoherent regime.

cond-mat.mes-hall↗

arXiv2008

Subnanosecond single electron source in the time-domain

We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy. The average single electron emission process is recorded with a 0.5 ns time resolution using a real-time fast acquisition card. Single electron signals are compared tosimulation based on scattering theory approach adapted for finite excitation energies.

cond-mat.mes-hall↗

arXiv2008

Single Carbon Nanotube Transistor at GHz Frequency

We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm of about 20 microSiemens is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/micrometer is typical of top gates on a conventional oxide with a dielectric constant equal to 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies . For our smallest devices, we find a large transit frequency equal to 50 GHz with no evidence of saturation in length dependence.

cond-mat.mes-hall↗

arXiv2007

An On-Demand Coherent Single Electron Source

We report on the electron analog of the single photon gun. On demand single electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission is triggered by application of a potential step which compensates the dot charging energy. Depending on the barrier transparency the quantum emission time ranges from 0.1 to 10 nanoseconds. The single electron source should prove useful for the implementation of quantum bits in ballistic conductors. Additionally periodic sequences of single electron emission and absorption generate a quantized AC-current.

cond-mat.mes-hall↗

arXiv2007

The relaxation time of a chiral quantum R-L circuit

We report on the GHz complex admittance of a chiral one dimensional ballistic conductor formed by edge states in the quantum Hall regime. The circuit consists of a wide Hall bar (the inductor L) in series with a tunable resistor (R) formed by a quantum point contact. Electron interactions between edges are screened by a pair of side gates. Conductance steps are observed on both real and imaginary parts of the admittance. Remarkably, the phase of the admittance is transmission-independent. This shows that the relaxation time of a chiral R-L circuit is resistance independent. A current and charge conserving scattering theory is presented that accounts for this observation with a relaxation time given by the electronic transit time in the c cuit.

cond-mat.mes-hall↗

arXiv2007

Nanospintronics with carbon nanotubes

One of the actual challenges of spintronics is the realization of a spin-transistor allowing to control spin transport through an electrostatic gate. In this review, we report on different experiments which demonstrate a gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyze spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

cond-mat.mes-hall↗

arXiv2005

Electric Field Control of Spin Transport

Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then positive. Common devices, such as the giant magneto-resistance sensor used in reading heads of hard disks, are based on this phenomenon. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realisation of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Such devices have, however, been elusive so far. We report here on a pronounced gate-field controlled MR in devices made from carbon nanotubes with ferromagnetic contacts. Both the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner. We emphasise that this spin-field effect is not restricted to carbon nanotubes but constitutes a generic effect which can in principle be exploited in all resonant tunneling devices.

cond-mat.mes-hall↗