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.
Coherent electron transfer from a localized state trapped in a quantum dot into a ballistic conductor, taking place in on-demand electron sources, in general may result in excitation of particle-hole pairs. We consider a simple model for these effects, involving a resonance level with time-dependent energy, and derive Floquet scattering matrix describing inelastic transitions of particles in the Fermi sea. We find that, as the resonance level is driven through the Fermi level, particle transfer may take place completely without particle-hole excitations for certain driving protocols. In partic…
We demonstrate the high fidelity splitting of electron pairs emitted on demand from a dynamic quantum dot by an electronic beam splitter. The fidelity of pair splitting is inferred from the coincidence of arrival in two detector paths probed by a measurement of the partitioning noise. The emission characteristic of the on-demand electron source is tunable from electrons being partitioned equally and independently to electron pairs being split with a fidelity of 90%. For low beam splitter transmittance we further find evidence of pair bunching violating statistical expectations for independent …
I. Introduction II. Electrons at the Fermi level III. Conductance quantization of a quantum point contact IV. Optical analogue of the conductance quantization V. Classical electron focusing VI. Electron focusing as a transmission problem VII. Coherent electron focusing (Experiment, Skipping orbits and magnetic edge states, Mode-interference and coherent electron focusing) VIII. Other mode-interference phenomena
Rapid coherent control of electron spin states is required for implementation of a spin-based quantum processor. We demonstrate coherent control of electronic spin states in a double quantum dot by sweeping an initially prepared spin singlet state through a singlet-triplet anti-crossing in the energy level spectrum. The anti-crossing serves as a beam splitter for the incoming spin singlet state. Consecutive crossings through the beam splitter, when performed within the spin dephasing time, result in coherent quantum oscillations between the singlet state and a triplet state. The all-electrical…
Coherent population trapping (CPT) refers to the steady-state trapping of population in a coherent superposition of two ground states which are coupled by coherent optical fields to an intermediate state in a three-level atomic system. Recently, CPT has been observed in an ensemble of donor bound spins in GaAs and in single nitrogen vacancy centers in diamond by using a fluorescence technique. Here we report the demonstration of CPT of an electron spin in a single quantum dot (QD) charged with one electron.
We demonstrate coherent control of cyclotron resonance (CR) in a two-dimensional electron gas (2DEG). We use a sequence of terahertz pulses to control the amplitude of CR oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the terahertz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using {\em single-particle} optical Bloch equations, showing no signatures of electron-electron interactions, …
The ability to manipulate coherently individual quantum objects organized in arrays is a prerequisite to any scalable quantum information platform. For electron spin qubits, it requires the fine tuning of large arrays of tunnel-coupled quantum dots. The cumulated efforts in linear dot arrays have permitted the recent realization of quantum simulators and multi-electron spin coherent manipulation. However, the two-dimensional scaling of such implementations remains undemonstrated while being compulsory to resolve complex quantum matter problems or process quantum information. Here, we demonstra…
Employing a real time effective action formalism we analyze electron transport and current fluctuations in comparatively short coherent conductors in the presence of electron-electron interactions. We demonstrate that, while Coulomb interaction tends to suppress electron transport, it may {\it strongly enhance} shot noise in scatterers with highly transparent conducting channels. This effect of excess noise is governed by the Coulomb gap observed in the current-voltage characteristics of such scatterers. We also analyze the frequency dispersion of higher current cumulants and emphasize a direc…
We have measured the low-temperature transport properties of a T-shaped quantum dot. Replicated oscillations superimposed on one-dimensional conductance steps are observed. These structures are consistent with electron phase-coherent length resonance effects in the ballistic regime. Using a simple model, we suggest that both one-dimensional and two-dimensional electron-electron scattering gives rise to electron phase-breaking in our system.
In analogy with quantum optics, short time correlations of the current fluctuations are used to characterize an on-demand electron source consisting of a quantum dot connected to a conductor via a tunable tunnel barrier. We observe a new fundamental noise for electrons associated with the quantum fluctuations of the electron emission time, which we call quantum jitter. In optimum operating conditions of the source, the noise reduces to the quantum jitter limit, which demonstrates single particle emission. Combined with the coherent manipulations of single electrons in a quantum conductor, this…
We develop a coherent beam splitter for single electrons driven through two tunnel-coupled quantum wires by surface acoustic waves (SAWs). The output current through each wire oscillates with gate voltages to tune the tunnel-coupling and potential difference between the wires. This oscillation is assigned to coherent electron tunneling motion that can be used to encode a flying qubit and is well reproduced by numerical calculations of time evolution of the SAW-driven single electrons. The oscillation visibility is currently limited to about 3%, but robust against decoherence, indicating that t…
We investigate coherent time-evolution of charge states (pseudo-spin qubit) in a semiconductor double quantum dot. This fully-tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.
We have investigated the transport characteristics of an electron pump consisting of an asymmetric double quantum dot at zero bias voltage which is subject to electromagnetic radiation. Depending on the energies of the intermediate states in the pumping cycle, electrons can be transferred through the dots incoherently via sequential tunneling or coherently via co-tunneling. The dc transport through the system can be controlled by the frequency of the applied radiation. We concentrate on resonant tunneling peaks in the pumping current that are interesting to observe experimentally.
We measure the energy distribution of electrons passing through a two-dimensional electron gas using a scanning probe microscope. We present direct spatial images of coherent electron wave flow from a quantum point contact formed in a GaAs/AlGaAs two-dimensional electron gas using a liquid He cooled SPM. A negative voltage is placed on the tip, which creates a small region of depleted electrons that backscatters electron waves. Oscillating the voltage on the tip and locking into this frequency gives the spatial derivative of electron flow perpendicular to the direction of current flow. We show…
We examine, using mixed classical-quantum electron-ion dynamics, electron transfer in a donor-acceptor-like molecular junction system based on polyacetylene. We identify two qualitatively-different transfer regimes: hopping and tunnelling. We discuss the criteria for achieving each one and for minimizing inelastic scattering and decoherence arising from the coupling to the ions, and we connect our main results to quantities derived from electron dynamics involving simpler, three-state model systems. We identify the requirements to have near-ballistic transfer.
We analyze current fluctuations in mesoscopic coherent conductors in the presence of electron-electron interactions. In a wide range of parameters we obtain explicit universal dependencies of the current noise on temperature, voltage and frequency. We demonstrate that Coulomb interaction decreases the Nyquist noise. In this case the interaction correction to the noise spectrum is governed by the combination $\sum_nT_n(T_n-1)$, where $T_n$ is the transmission of the $n$-th conducting mode. The effect of electron-electron interactions on the shot noise is more complicated. At sufficiently large …
We study thermoelectric transport through a coherent molecular conductor connected to two electron and two phonon baths using the nonequilibrium Green's function method. We focus on the mutual drag between electron and phonon transport as a result of `momentum' transfer, which happens only when there are at least two phonon degrees of freedom. After deriving expressions for the linear drag coefficients, obeying the Onsager relation, we further investigate their effect on nonequilibrium transport. We show that the drag effect is closely related to two other phenomena: (1) adiabatic charge pumpi…
A fundamental goal in the manipulation of quantum systems is the achievement of many coherent oscillations within the characteristic dephasing time T2*[1]. Most manipulations of electron spins in quantum dots have focused on the construction and control of two-state quantum systems, or qubits, in which each quantum dot is occupied by a single electron[2-7]. Here we perform quantum manipulations on a system with more electrons per quantum dot, in a double dot with three electrons. We demonstrate that tailored pulse sequences can be used to induce coherent rotations between 3-electron quantum st…
Understanding ultrafast coherent electron dynamics is necessary for application of a single-electron source to metrological standards, quantum information processing, including electron quantum optics, and quantum sensing. While the dynamics of an electron emitted from the source has been extensively studied, there is as yet no study of the dynamics inside the source. This is because the speed of the internal dynamics is typically higher than 100 GHz, beyond state-of-the-art experimental bandwidth. Here, we theoretically and experimentally demonstrate that the internal dynamics in a silicon si…
We study theoretically the coherent electron focusing in graphene nanoribbons. Using semiclassical and numerical tight binding calculations we show that perfect armchair edges give rise to equidistant peaks in the focusing spectrum. In the case of zigzag edges at low magnetic fields one can also observe focusing peaks but with increasing magnetic field a more complex interference structure emerges in the spectrum. This difference in the spectra can be observed even if the zigzag edge undergoes structural reconstruction. Therefore transverse electron focusing can help in the identification and …
We show that a current-carrying coherent electron conductor can be treated as effective bosonic energy reservoir involving different types of electron-hole pair excitation. For weak electron-boson coupling, hybrid energy transport between nonequilibrium electrons and bosons can be described by a Landauer-like formula. This allows for unified account of a variety of heat transport problems in hybrid electron-boson systems. As applications, we study the non-reciprocal heat transport between electrons and bosons, thermoelectric current from a cold-spot and electronic cooling of the bosons. Our un…
We report the first direct spectroscopic evidence for coherent electronic coupling between excitons and trions in atomically thin transition metal dichalcogenides, specifically monolayer MoSe2. Signatures of coupling appear as isolated cross-peaks in two-color pump-probe spectra, and the lineshape of the peaks reveals that the coupling originates from many-body interactions. Excellent agreement between the experiment and density matrix calculations suggests the formation of a correlated exciton-trion state due to their coupling.
The magnetic focusing of electrons has proven its utility in fundamental studies of electron transport. Here we report the direct imaging of magnetic focusing of electron waves, specifically in a two-dimensional electron gas (2DEG). We see the semicircular trajectories of electrons as they bounce along a boundary in the 2DEG, as well as fringes showing the coherent nature of the electron waves. Imaging flow in open systems is made possible by a cooled scanning probe microscope. Remarkable agreement between experiment and theory demonstrates our ability to see these trajectories and to use this…
A few electron double electrostatic lateral quantum dot can be transformed into a few electron triple quantum dot by applying a different combination of gate voltages. Quadruple points have been achieved at which all three dots are simultaneously on resonance. At these special points in the stability diagram four occupation configurations are possible. Both charge detection and transport experiments have been performed on this device. In this short paper we present data and confirm that transport is coherent by observing a Pi phase shift in magneto-conductance oscillations as one passes throug…
The correlated behavior of electrons determines the structure and optical properties of molecules, semiconductor and other systems. Valuable information on these correlations is provided by measuring the response to femtosecond laser pulses, which probe the very short time period during which the excited particles remain correlated. The interpretation of four-wave-mixing techniques, commonly used to study the energy levels and dynamics of many-electron systems, is complicated by many competing effects and overlapping resonances. Here we propose a coherent optical technique, specifically design…
We present a fully electronic analogue of coherent population trapping in quantum optics, based on destructive interference of single-electron tunneling between three quantum dots. A large bias voltage plays the role of the laser illumination. The trapped state is a coherent superposition of the electronic charge in two of these quantum dots, so it is destabilized as a result of decoherence by coupling to external charges. The resulting current I through the device depends on the ratio of the decoherence rate Gamma_phi and the tunneling rates. For Gamma_phi --> 0 one has simply I=e Gamma_phi. …
A single-electron transistor incorporated as part of a nanomechanical resonator represents an extreme limit of electron-phonon coupling. While it allows for fast and sensitive electromechanical measurements, it also introduces backaction forces from electron tunnelling which randomly perturb the mechanical state. Despite the stochastic nature of this backaction, under conditions of strong coupling it is predicted to create self-sustaining coherent mechanical oscillations. Here, we verify this prediction using time-resolved measurements of a vibrating carbon nanotube transistor. This electromec…
Using a recently-developed time-of-flight measurement technique with 1 ps time resolution and electron-energy spectroscopy, we developed a method to measure the longitudinal-optical-phonon emission rate of hot electrons travelling along a depleted edge of a quantum Hall bar. A comparison of the experimental results to a single-particle model implies that the main scattering mechanism involves a two-step process via intra-Landau-level transition. We show this scattering can be suppressed by controlling the edge potential profile, and a scattering length > 1 mm can be achieved, allowing the use …
The realization of integrated quantum circuits requires precise on-chip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and class…
In the quest for large-scale quantum computing, networked quantum computers offer a natural path towards scalability. Now that nearest neighbor entanglement has been demonstrated for electron spin qubits in semiconductors, on-chip long distance entanglement brings versatility to connect quantum core units. Here we realize the controlled and coherent transfer of a pair of entangled electron spins, and demonstrate their remote entanglement when separated by a distance of 6 microns. Driven by coherent spin rotations induced by the electron displacement, high-contrast spin quantum interferences ar…
Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite important experimental efforts to control the electron displacement over long distances, keeping the electron spin coherence after transfer remained up to now elusive. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 micrometers.…
We investigate phase coherent electronic transport in an open quantum system, which consists of quantum dots side-coupled to a nanowire. It is demonstrated that coherent switching can be characterized by adjusting the electronic energy. A comparative analysis of quantum coherence effects in side-coupled quantum-dot systems is presented. Our results demonstrate the relevance of electronic nanodevices based on coherent switching by appropriately detuning the side-coupled quantum dots that are located at variable distance along a wire.
We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on demand single spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several {\it spin pair}s can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on demand single spin em…
We evaluate the joint distributions of electron waiting times in coherent conductors described by scattering theory. Successive electron waiting times in a single-channel conductor are found to be correlated due to the fermionic statistics encoded in the many-body state. Our formalism allows us also to investigate the waiting times between charge transfer events in different outgoing channels. As an application we consider a quantum point contact in a chiral setup with one or both input channels biased by either a static or a time-dependent periodic voltage described by Floquet theory. The the…
We have studied the coherent spin dynamics in an oblique magnetic field of electrons localized on donors and placed in the middle of a single CdTe quantum well, by using a time-resolved optical technique: the photo-induced Faraday rotation. We showed that this dynamics is affected by a weak Overhauser field created via the hyperfine interaction of optically spin-polarized donor-bound electrons with the surrounding nuclear isotopes carrying non-zero spins. We have measured this nuclear field, which is on the order of a few mT and can reach a maximum experimental value of 9.4 mT. This value repr…
Manipulation of single spins is essential for spin-based quantum information processing. Electrical control instead of magnetic control is particularly appealing for this purpose, since electric fields are easy to generate locally on-chip. We experimentally realize coherent control of a single electron spin in a quantum dot using an oscillating electric field generated by a local gate. The electric field induces coherent transitions (Rabi oscillations) between spin-up and spin-down with pi/2 rotations as fast as ~55ns. Our analysis indicates that the electrically-induced spin transitions are m…
We experimentally demonstrate how coherent population trapping (CPT) for donor-bound electron spins in GaAs results in autonomous feedback that prepares stabilized states for the spin polarization of nuclei around the electrons. CPT was realized by excitation with two lasers to a bound-exciton state. Transmission studies of the spectral CPT feature on an ensemble of electrons directly reveal the statistical distribution of prepared nuclear spin states. Tuning the laser driving from blue to red detuned drives a transition from one to two stable states. Our results have importance for ongoing re…
The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measure…
We provide conclusive experimental evidence that zero bias anomaly in the differential resistance of magnetic tunnel junctions (MTJs) is due to electron-electron interaction (EEI), clarifying a long standing issue. Magnon effect that caused confusion is now excluded by measuring at low temperatures down to 0.2 K and with reduced AC measurement voltages down to 0.06 mV. The normalized change of conductance is proportional to $\ln{(eV/k_{B}T)}$, consistent with the Altshuler-Aronov theory of tunneling that describes the reduction of density of states due to EEI, but inconsistent with magnetic im…
We demonstrate fully three-dimensional and patterned localization of nitrogen-vacancy (NV) centers in diamond with coherence times in excess of 1 ms. Nitrogen δ-doping during CVD diamond growth vertically confines nitrogen to 4 nm while electron irradiation with a transmission electron microscope (TEM) laterally confines vacancies to less than 1 μm. We characterize the effects of electron energy and dose on NV formation. Importantly, our technique enables the formation of reliably high-quality NV centers inside diamond nanostructures, with applications in quantum information and sensing.
We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules may be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitate control of the device conductance in a transistor effect with high on/off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in …
Coherent coupling of defect spins with surrounding nuclei along with the endowment to read out the latter, are basic requirements for an application in quantum technologies. We show that negatively charged boron vacancies (VB-) in electron-irradiated hexagonal boron nitride (hBN) meet these prerequisites. We demonstrate Hahn-echo coherence of the VB- electron spin with a characteristic decay time Tcoh = 15 us, close to the theoretically predicted limit of 18 us for spin defects in hBN. Modulation in the MHz range superimposed on the Hahn-echo decay curve are shown to be induced by coherent cou…
Emergence of a classical particle trajectory concept from the full quantum description is a key feature of quantum mechanics. Recent progress of solid state on-demand sources has brought single-electron manipulation into the quantum regime, however, the quantum-to-classical crossover remains unprobed. Here we describe theoretically a mechanism for generating single-electron wave packets by tunneling from a driven localized state, and show how to tune the degree of quantumness. Applying our theory to existing on-demand sources, we demonstrate the feasibility of an experimental investigation of …
We present experimental observations and a study of quantum dynamics of strongly interacting electronic spins, at room temperature in the solid state. In a diamond substrate, a single nitrogen vacancy (NV) center coherently interacts with two adjacent S = 1/2 dark electron spins. We quantify NV-electron and electron-electron couplings via detailed spectroscopy, with good agreement to a model of strongly interacting spins. The electron-electron coupling enables an observation of coherent flip-flop dynamics between electronic spins in the solid state, which occur conditionally on the state of th…
We propose a source capable of on-demand emission of single electrons with a wave packet of controllable shape and phase. The source consists of a hybrid quantum system, relying on currently experimentally accessible components. We analyze in detail the emission of single electron time-bin qubits, which we characterize using the well known electronic Hong-Ou-Mandel (HOM) interferometry scheme. Specifically, we show that, by controlling the phase difference of two time-bin qubits, the Pauli peak, the electronic analogue of the well known optical HOM dip, can be continuously removed. The propose…
We review our recent proposals for the on-demand generation of entangled few-electron states using dynamic single-electron sources. The generation of entanglement can be traced back to the single-electron entanglement produced by quantum point contacts acting as electronic beam splitters. The coherent partitioning of a single electron leads to entanglement between the two outgoing arms of the quantum point contact. We describe our various approaches for generating and certifying entanglement in dynamic electronic conductors and we quantify the influence of detrimental effects such as finite el…
We propose a new transport mechanism through tunnel-coupled quantum dots based on the coherent population trapping effect. Coupling to an excited level by the coherent radiation of two microwaves can lead to an extremely narrow current antiresonance. The effect can be used to determine interdot dephasing rates and is a mechanism for a very sensitive, optically controlled current switch.
We show that an Aharonov-Bohm (AB) ring with asymmetric electron injection can act as a coherent detector of electron dephasing. The presence of a dephasing source in one of the two arms of a moderately-to-highly asymmetric ring changes the response of the system from total reflection to complete transmission while preserving the coherence of the electrons propagating from the ring, even for strong dephasing. We interpret this phenomenon as an implementation of an interaction-free measurement.
Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak-heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels.
We demonstrate that the [Yb(trensal)] molecule is a prototypical coupled electronic qubit-nuclear qudit system. The combination of noise-resilient nuclear degrees of freedom and large reduction of nutation time induced by electron-nuclear mixing enables coherent manipulation of this qudit by radio-frequency pulses. Moreover, the multi-level structure of the qudit is exploited to encode and operate a qubit with embedded basic quantum error correction.
We consider a single electron in a 1D quantum dot with a static slanting Zeeman field. By combining the spin and orbital degrees of freedom of the electron, an effective quantum two-level (qubit) system is defined. This pseudo-spin can be coherently manipulated by the voltage applied to the gate electrodes, without the need for an external time-dependent magnetic field or spin-orbit coupling. Single qubit rotations and the C-NOT operation can be realized. We estimated relaxation ($T_1$) and coherence ($T_{2}$) times, and the (tunable) quality factor. This scheme implies important experimental …
Recent experimental advances in the reconstruction of the Wigner function (WF) for electronic systems have led us to consider the possibility of employing this theoretical tool in the analysis of electron dynamics of uniaxially strained graphene. In this paper, we study the effect of strain on the WF of electrons in graphene under the interaction of a uniform magnetic field. This mechanical deformation modifies drastically the shape of the Wigner function of Landau and coherent states. The WF has a different behavior straining the material along the zigzag direction in comparison with the armc…
We propose a system where coherent thermal transport between two reservoirs in non-galvanic contact is modulated by independently tuning the electron-photon and the electron-phonon coupling. The scheme is based on two gate-controlled electrodes capacitively coupled through a dc-SQUID as intermediate phase-tunable resonator. Thereby the electron-photon interaction is modulated by controlling the flux threading the dc-SQUID and the impedance of the two reservoirs, while the electron-phonon coupling is tuned by controlling the charge carrier concentration in the electrodes. To quantitatively eval…
The distribution of electron waiting times is useful to characterize quantum transport in mesoscopic structures. Here we consider a generic quantum-coherent conductor consisting of a mesoscopic scatterer in a two-terminal setup. We extend earlier results for single-channel conductors to setups with several (possibly spin-degenerate) conduction channels and we discuss the effect of a finite electronic temperature. We present detailed investigations of the electron waiting times in a quantum point contact as well as in two mesoscopic interferometers with energy-dependent transmissions: a Fabry-P…
In high-purity n-type GaAs under strong magnetic field, we are able to isolate a lambda system composed of two Zeeman states of neutral-donor bound electrons and the lowest Zeeman state of bound excitons. When the two-photon detuning of this system is zero, we observe a pronounced dip in the excited-state photoluminescence indicating the creation of the coherent population-trapped state. Our data are consistent with a steady-state three-level density-matrix model. The observation of coherent population trapping in GaAs indicates that this and similar semiconductor systems could be used for var…
