arXiv subjects

U. Gennser

Publications and source records attributed to U. Gennser.

At least 37 records · Page 2Linked to original sources

arXiv2019

Macroscopic electron quantum coherence in a solid-state circuit

The quantum coherence of electronic quasiparticles underpins many of the emerging transport properties of conductors at small scales. Novel electronic implementations of quantum optics devices are now available with perspectives such as 'flying' qubit manipulations. However, electronic quantum interferences in conductors remained up to now limited to propagation paths shorter than $30\,μ$m, independently of the material. Here we demonstrate strong electronic quantum interferences after a propagation along two $0.1\,$mm long pathways in a circuit. Interferences of visibility as high as $80\%$ and $40\%$ are observed on electronic analogues of the Mach-Zehnder interferometer of, respectively, $24\,μ$m and $0.1\,$mm arm length, consistently corresponding to a $0.25\,$mm electronic phase coherence length. While such devices perform best in the integer quantum Hall regime at filling factor 2, the electronic interferences are restricted by the Coulomb interaction between copropagating edge channels. We overcome this limitation by closing the inner channel in micron-scale loops of frozen internal degrees of freedom, combined with a loop-closing strategy providing an essential isolation from the environment.

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arXiv2019

Thermoelectric Scanning Gate Interferometry on a Quantum Point Contact

We introduce a new scanning probe technique derived from scanning gate microscopy (SGM) in order to investigate thermoelectric transport in two-dimensional semiconductor devices. The thermoelectric scanning gate Microscopy (TSGM) consists in measuring the thermoelectric voltage induced by a temperature difference across a device, while scanning a polarized tip that locally changes the potential landscape. We apply this technique to perform interferometry of the thermoelectric transport in a quantum point contact (QPC). We observe an interference pattern both in SGM and TSGM images, and evidence large differences between the two signals in the low density regime of the QPC. In particular, a large phase jump appears in the interference fringes recorded by TSGM, which is not visible in SGM. We discuss this difference of sensitivity using a microscopic model of the experiment, based on the contribution from a resonant level inside or close to the QPC. This work demonstrates that combining scanning gate microscopy with thermoelectric measurements offers new information as compared to SGM, and provides a direct access to the derivative of the device transmission with respect to energy, both in amplitude and in phase.

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arXiv2019

Transferring the quantum state of electrons across a Fermi sea with Coulomb interaction

The Coulomb interaction generally limits the quantum propagation of electrons. However, it can also provide a mechanism to transfer their quantum state over larger distances. Here, we demonstrate such a form of teleportation, across a metallic island within which the electrons are trapped much longer than their quantum lifetime. This effect originates from the low temperature freezing of the island's charge $Q$ which, in the presence of a single connected electronic channel, enforces a one-to-one correspondence between incoming and outgoing electrons. Such high-fidelity quantum state imprinting is established between well-separated injection and emission locations, through two-path interferences in the integer quantum Hall regime. The added electron quantum phase of $2πQ/e$ can allow for strong and decoherence-free entanglement of propagating electrons, and notably of flying qubits.

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arXiv2018

Circuit Quantum Simulation of a Tomonaga-Luttinger Liquid with an Impurity

The Tomonaga-Luttinger liquid (TLL) concept is believed to generically describe the strongly-correlated physics of one-dimensional systems at low temperatures. A hallmark signature in 1D conductors is the quantum phase transition between metallic and insulating states induced by a single impurity. However, this transition impedes experimental explorations of real-world TLLs. Furthermore, its theoretical treatment, explaining the universal energy rescaling of the conductance at low temperatures, has so far been achieved exactly only for specific interaction strengths. Quantum simulation can provide a powerful workaround. Here, a hybrid metal-semiconductor dissipative quantum circuit is shown to implement the analogue of a TLL of adjustable electronic interactions comprising a single, fully tunable scattering impurity. Measurements reveal the renormalization group `beta-function' for the conductance that completely determines the TLL universal crossover to an insulating state upon cooling. Moreover, the characteristic scaling energy locating at a given temperature the position within this conductance renormalization flow is established over nine decades versus circuit parameters, and the out-of-equilibrium regime is explored. With the quantum simulator quality demonstrated from the precise parameter-free validation of existing and novel TLL predictions, quantum simulation is achieved in a strong sense, by elucidating interaction regimes which resist theoretical solutions.

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arXiv2018

Tunable Quantum Criticality and Super-ballistic Transport in a `Charge' Kondo Circuit

Quantum phase transitions are ubiquitous in many exotic behaviors of strongly-correlated materials. However the microscopic complexity impedes their quantitative understanding. Here, we observe thoroughly and comprehend the rich strongly-correlated physics in two profoundly dissimilar regimes of quantum criticality. With a circuit implementing a quantum simulator for the three-channel Kondo model, we reveal the universal scalings toward different low-temperature fixed points and along the multiple crossovers from quantum criticality. Notably, an unanticipated violation of the maximum conductance for ballistic free electrons is uncovered. The present charge pseudospin implementation of a Kondo impurity opens access to a broad variety of strongly-correlated phenomena.

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arXiv2018

Heat Coulomb Blockade of One Ballistic Channel

Quantum mechanics and Coulomb interaction dictate the behavior of small circuits. The thermal implications cover fundamental topics from quantum control of heat to quantum thermodynamics, with prospects of novel thermal machines and an ineluctably growing influence on nanocircuit engineering. Experimentally, the rare observations thus far include the universal thermal conductance quantum and heat interferometry. However, evidences for many-body thermal effects paving the way to markedly different heat and electrical behaviors in quantum circuits remain wanting. Here we report on the observation of the Coulomb blockade of electronic heat flow from a small metallic circuit node, beyond the widespread Wiedemann-Franz law paradigm. We demonstrate this thermal many-body phenomenon for perfect (ballistic) conduction channels to the node, where it amounts to the universal suppression of precisely one quantum of conductance for the transport of heat, but none for electricity. The inter-channel correlations that give rise to such selective heat current reduction emerge from local charge conservation, in the floating node over the full thermal frequency range ($\lesssim$temperature$\times k_\mathrm{B}/h$). This observation establishes the different nature of the quantum laws for thermal transport in nanocircuits.

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arXiv2017

Extracting single electron wavefunctions from a quantum electrical current

Quantum nanoelectronics has entered an era where quantum electrical currents are built from single to few on-demand elementary excitations. To date however, very limited tools have been implemented to characterize them. In this work, we present a quantum current analyzer able to extract single particle excitations present within a periodic quantum electrical current without any a priori hypothesis. Our analyzer combines two-particle interferometry and signal processing to extract the relevant electron and hole wavefunctions localized around each emission period and their quantum coherence from one emission period to the other. This quantum current analyzer opens new possibilities for the characterization and control of quantum electrical currents in nanoscale conductors and for investigations of entanglement in quantum electronics down to the single electron level.

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arXiv2016

Primary thermometry triad at 6 mK in mesoscopic circuits

Quantum physics emerge and develop as temperature is reduced. Although mesoscopic electrical circuits constitute an outstanding platform to explore quantum behavior, the challenge in cooling the electrons impedes their potential. The strong coupling of such micrometer-scale devices with the measurement lines, combined with the weak coupling to the substrate, makes them extremely difficult to thermalize below 10 mK and imposes in-situ thermometers. Here we demonstrate electronic quantum transport at 6 mK in micrometer-scale mesoscopic circuits. The thermometry methods are established by the comparison of three in-situ primary thermometers, each involving a different underlying physics. The employed combination of quantum shot noise, quantum back-action of a resistive circuit and conductance oscillations of a single-electron transistor covers a remarkably broad spectrum of mesoscopic phenomena. The experiment, performed in vacuum using a standard cryogen-free dilution refrigerator, paves the way toward the sub-millikelvin range with additional thermalization and refrigeration techniques.

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arXiv2016

Controlling charge quantization with quantum fluctuations

In 1909, Millikan showed that the charge of electrically isolated systems is quantized in units of the elementary electron charge e. Today, the persistence of charge quantization in small, weakly connected conductors allows for circuits where single electrons are manipulated, with applications in e.g. metrology, detectors and thermometry. However, quantum fluctuations progressively reduce the discreteness of charge as the connection strength is increased. Here we report on the full quantum control and characterization of charge quantization. By using semiconductor-based tunable elemental conduction channels to connect a micrometer-scale metallic island, the complete evolution is explored while scanning the entire range of connection strengths, from tunnel barrier to ballistic contact. We observe a robust scaling of charge quantization as the square root of the residual electron reflection probability across a quantum channel when approaching the ballistic critical point, which also applies beyond the regimes yet accessible to theory. At increased temperatures, the thermal fluctuations result in an exponential suppression of charge quantization as well as in a universal square root scaling, for arbitrary connection strengths, in agreement with expectations. Besides direct applications to improve single-electron functionalities and for the metal-semiconductor hybrids emerging in the quest toward topological quantum computing, the knowledge of the quantum laws of electricity will be essential for the quantum engineering of future nanoelectronic devices.

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arXiv2016

Electron phase shift at the zero-bias anomaly of quantum point contacts

The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.

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arXiv2016

Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states

Many-body correlations and macroscopic quantum behaviors are fascinating condensed matter problems. A powerful test-bed for the many-body concepts and methods is the Kondo model which entails the coupling of a quantum impurity to a continuum of states. It is central in highly correlated systems and can be explored with tunable nanostructures. Although Kondo physics is usually associated with the hybridization of itinerant electrons with microscopic magnetic moments, theory predicts that it can arise whenever degenerate quantum states are coupled to a continuum. Here we demonstrate the previously elusive `charge' Kondo effect in a hybrid metal-semiconductor implementation of a single-electron transistor, with a quantum pseudospin-1/2 constituted by two degenerate macroscopic charge states of a metallic island. In contrast to other Kondo nanostructures, each conduction channel connecting the island to an electrode constitutes a distinct and fully tunable Kondo channel, thereby providing an unprecedented access to the two-channel Kondo effect and a clear path to multi-channel Kondo physics. Using a weakly coupled probe, we reveal the renormalization flow, as temperature is reduced, of two Kondo channels competing to screen the charge pseudospin. This provides a direct view of how the predicted quantum phase transition develops across the symmetric quantum critical point. Detuning the pseudospin away from degeneracy, we demonstrate, on a fully characterized device, quantitative agreement with the predictions for the finite-temperature crossover from quantum criticality.

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arXiv2015

Disorder-induced stabilization of the quantum Hall ferromagnet

We report on an absolute measurement of the electronic spin polarization of the $ν=1$ integer quantum Hall state. The spin polarization is extracted in the vicinity of $ν=1$ (including at exactly $ν=1$) via resistive NMR experiments performed at different magnetic fields (electron densities), and Zeeman energy configurations. At the lowest magnetic fields, the polarization is found to be complete in a narrow region around $ν=1$. Increasing the magnetic field (electron density) induces a significant depolarization of the system, which we attribute to a transition between the quantum Hall ferromagnet and the Skyrmion glass phase theoretically expected as the ratio between Coulomb interactions and disorder is increased. These observations account for the fragility of the polarization previously observed in high mobility 2D electron gas, and experimentally demonstrate the existence of an optimal amount of disorder to stabilize the ferromagnetic state.

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arXiv2015

Robust quantum coherence above the Fermi sea

In this paper we present an experiment where we measured the quantum coherence of a quasiparticle injected at a well-defined energy above the Fermi sea into the edge states of the integer quantum Hall regime. Electrons are introduced in an electronic Mach-Zehnder interferometer after passing through a quantum dot that plays the role of an energy filter. Measurements show that above a threshold injection energy, the visibility of the quantum interferences is almost independent of the energy. This is true even for high energies, up to 130~$μ$eV, well above the thermal energy of the measured sample. This result is in strong contradiction with our theoretical predictions, which instead predict a continuous decrease of the interference visibility with increasing energy. This experiment raises serious questions concerning the understanding of excitations in the integer quantum Hall regime.

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arXiv2015

Commensurability resonances in two-dimensional magneto-electric lateral superlattices

Hybrid lateral superlattices composed of a square array of antidots and a periodic one-dimensional magnetic modulation are prepared in $\mathrm{Ga[Al]As}$ heterostructures. The two-dimensional electron gases exposed to these superlattices are characterized by magnetotransport experiments in vanishing average perpendicular magnetic fields. Despite the absence of closed orbits, the diagonal magnetoresistivity in the direction perpendicular to the magnetic modulation shows pronounced classical resonances. They are located at magnetic fields where snake trajectories exist which are quasi-commensurate with the antidot lattice. The diagonal magnetoresistivity in the direction of the magnetic modulation increases sharply above a threshold magnetic field and shows no fine structure. The experimental results are interpreted with the help of numerical simulations based on the semiclassical Kubo model.

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arXiv2015

Quantum limit of heat flow across a single electronic channel

Quantum physics predicts that there is a fundamental maximum heat conductance across a single transport channel, and that this thermal conductance quantum $G_Q$ is universal, independent of the type of particles carrying the heat. Such universality, combined with the relationship between heat and information, signals a general limit on information transfer. We report on the quantitative measurement of the quantum limited heat flow for Fermi particles across a single electronic channel, using noise thermometry. The demonstrated agreement with the predicted $G_Q$ establishes experimentally this basic building block of quantum thermal transport. The achieved accuracy of below 10\% opens access to many experiments involving the quantum manipulation of heat.

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arXiv2014

Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy

Quantum point contacts exhibit mysterious conductance anomalies in addition to well known conductance plateaus at multiples of 2e^2/h. These 0.7 and zero-bias anomalies have been intensively studied, but their microscopic origin in terms of many-body effects is still highly debated. Here we use the charged tip of a scanning gate microscope to tune in situ the electrostatic potential of the point contact. While sweeping the tip distance, we observe repetitive splittings of the zero-bias anomaly, correlated with simultaneous appearances of the 0.7 anomaly. We interpret this behaviour in terms of alternating equilibrium and non-equilibrium Kondo screenings of different spin states localized in the channel. These alternating Kondo effects point towards the presence of a Wigner crystal containing several charges with different parities. Indeed, simulations show that the electron density in the channel is low enough to reach one-dimensional Wigner crystallization over a size controlled by the tip position.

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arXiv2013

Tomonaga-Luttinger physics in electronic quantum circuits

In one-dimensional conductors, interactions result in correlated electronic systems. At low energy, a hallmark signature of the so-called Tomonaga-Luttinger liquids (TLL) is the universal conductance curve predicted in presence of an impurity. A seemingly different topic is the quantum laws of electricity, when distinct quantum conductors are assembled in a circuit. In particular, the conductances are suppressed at low energy, a phenomenon called dynamical Coulomb blockade (DCB). Here we investigate the conductance of mesoscopic circuits constituted by a short single-channel quantum conductor in series with a resistance, and demonstrate a proposed link to TLL physics. We reformulate and establish experimentally a recently derived phenomenological expression for the conductance using a wide range of circuits, including carbon nanotube data obtained elsewhere. By confronting both conductance data and phenomenological expression with the universal TLL curve, we demonstrate experimentally the predicted mapping between DCB and the transport across a TLL with an impurity.

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arXiv2012

Chargeless heat transport in the fractional quantum Hall regime

We demonstrate a direct approach to investigate heat transport in the fractional quantum Hall regime. At filling factor of 4/3, we inject power at quantum point contacts and detect the related heating from the activated current through a quantum dot. The experiment reveals a chargeless heat transport from a significant heating that occurs upstream of the power injection point, in the absence of a concomitant electrical current. By tuning in-situ the edge path, we show that the chargeless heat transport does not follow the reverse direction of the electrical current path along the edge. This unexpected heat conduction demonstrates a novel aspect, yet to be elucidated, of the physics in fractional quantum Hall systems.

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