Josephson-like Tunnel Resonance and Large Coulomb Drag in GaAs-Based Electron-Hole Bilayers.

Bilayers consisting of two-dimensional (2D) electron and hole gases separated by a 10 nm thick AlGaAs barrier are formed by charge accumulation in epitaxially grown GaAs. Both vertical and lateral electric transport are measured in the millikelvin temperature range. The conductivity between the layers shows a sharp tunnel resonance at a density of 1.1×10^{10} cm^{-2}, which is consistent with a Josephson-like enhanced tunnel conductance. The tunnel resonance disappears with increasing densities and the two 2D charge gases start to show 2D-Fermi-gas behavior. Interlayer interactions persist causing a positive drag voltage that is very large at small densities. The transition from the Josephson-like tunnel resonance to the Fermi-gas behavior is interpreted as a phase transition from an exciton gas in the Bose-Einstein-condensate state to a degenerate electron-hole Fermi gas.

Measurements of Phase Dynamics in Planar Josephson Junctions and SQUIDs.

We experimentally investigate the stochastic phase dynamics of planar Josephson junctions (JJs) and superconducting quantum interference devices (SQUIDs) defined in epitaxial InAs/Al heterostructures, and characterized by a large ratio of Josephson energy to charging energy. We observe a crossover from a regime of macroscopic quantum tunneling to one of phase diffusion as a function of temperature, where the transition temperature T^{*} is gate-tunable. The switching probability distributions are shown to be consistent with a small shunt capacitance and moderate damping, resulting in a switching current which is a small fraction of the critical current. Phase locking between two JJs leads to a difference in switching current between that of a JJ measured in isolation and that of the same JJ measured in an asymmetric SQUID loop. In the case of the loop, T^{*} is also tuned by a magnetic flux.

Gate induced quantum wires in GaAs/AlGaAs heterostructures by cleaved edge deposition.

Electric conductors with dimensions reduced to the nanometer scale are the prerequisite of the quantum devices upon which the future advanced electronics is expected to be based. In the past, the fabrication of one-dimensional (1D) wires has been a particular challenge because they have to be defect-free over their whole length, which can be several tens µm. Excellent 1D wires have been produced by cleaving semiconductors (GaAs, AlGaAs) in ultra high vacuum and overgrowing the pristine edge surface by molecular beam epitaxy (MBE)1,2. Unfortunately, this cleaved edge overgrowth (CEO) technique did not find wide-spread use because it requires a series of elaborate steps that are difficult to accomplish. In this Letter, we present a greatly simplified variation of this technique where the cleaving takes place in ambient air and the MBE overgrowth is replaced by a standard deposition process. Wires produced by this cleaved edge deposition (CED) technique have properties that are as least as good as the traditional CEO ones. Due to its simplicity, the CED technique offers a generally accessible way to produce 1D devices.

Marginal metallic state at a fractional filling of '8/5' and '4/3' of Landau levels in the GaAs/AlGaAs 2D electron system.

A metallic state with a vanishing activation gap, at a filling factor [Formula: see text] in the untilted specimen with [Formula: see text], and at [Formula: see text] at [Formula: see text] under a [Formula: see text] tilted magnetic field, is examined through a microwave photo-excited transport study of the GaAs/AlGaAs 2 dimensional electron system (2DES). The results presented here suggest, remarkably, that at the possible degeneracy point of states with different spin polarization, where the 8/5 or 4/3 FQHE vanish, there occurs a peculiar marginal metallic state that differs qualitatively from a quantum Hall insulating state and the usual quantum Hall metallic state. Such a marginal metallic state occurs most prominently at [Formula: see text], and at [Formula: see text] under tilt as mentioned above, over the interval [Formula: see text], that also includes the [Formula: see text] state, which appears perceptibly gapped in the first instance.

Spin Reversal of a Quantum Hall Ferromagnet at a Landau Level Crossing.

When Landau levels (LLs) become degenerate near the Fermi energy in the quantum Hall regime, interaction effects can drastically modify the electronic ground state. We study the quantum Hall ferromagnet formed in a two-dimensional hole gas around the LL filling factor ν=1 in the vicinity of a LL crossing in the heave-hole valence band. Cavity spectroscopy in the strong-coupling regime allows us to optically extract the spin polarization of the two-dimensional hole gas. By analyzing this polarization as a function of hole density and magnetic field, we observe a spin flip of the ferromagnet. Furthermore, the depolarization away from ν=1 accelerates close to the LL crossing. This is indicative of an increase in the size of skyrmion excitations as the effective Zeeman energy vanishes at the LL crossing.

Nagaoka ferromagnetism observed in a quantum dot plaquette.

Engineered, highly controllable quantum systems are promising simulators of emergent physics beyond the simulation capabilities of classical computers1. An important problem in many-body physics is itinerant magnetism, which originates purely from long-range interactions of free electrons and whose existence in real systems has been debated for decades2,3. Here we use a quantum simulator consisting of a four-electron-site square plaquette of quantum dots4 to demonstrate Nagaoka ferromagnetism5. This form of itinerant magnetism has been rigorously studied theoretically6-9 but has remained unattainable in experiments. We load the plaquette with three electrons and demonstrate the predicted emergence of spontaneous ferromagnetic correlations through pairwise measurements of spin. We find that the ferromagnetic ground state is remarkably robust to engineered disorder in the on-site potentials and we can induce a transition to the low-spin state by changing the plaquette topology to an open chain. This demonstration of Nagaoka ferromagnetism highlights that quantum simulators can be used to study physical phenomena that have not yet been observed in any experimental system. The work also constitutes an important step towards large-scale quantum dot simulators of correlated electron systems.

Study of narrow negative magnetoresistance effect in ultra-high mobility GaAs/AlGaAs 2DES under microwave photo-excitation.

The microwave-induced change in the narrow negative magnetoresistance effect that appears around zero magnetic field in high mobility GaAs/AlGaAs 2DES (≈107 cm2/Vs) is experimentally examined as a function of incident microwave power at a fixed bath temperature. The experimental results indicate that the narrow negative magnetoresistance effect exhibits substantially increased broadening with increasing microwave intensity. These magnetoresistance data were subjected to lineshape fits to extract possible variation of characteristic lengths with microwave intensity; the results suggest that characteristic lengths decrease by up to 50% upon increasing microwave power up to about 8 mW. We also examine the change in effective electron temperature, Te, due to the photo-excitation in the absence of a magnetic field. Combining these results suggests a correlation between electron heating and the observed change in the fit extracted characteristic lengths.

Virtual-photon-mediated spin-qubit-transmon coupling.

Spin qubits and superconducting qubits are among the promising candidates for realizing a solid state quantum computer. For the implementation of a hybrid architecture which can profit from the advantages of either approach, a coherent link is necessary that integrates and controllably couples both qubit types on the same chip over a distance that is several orders of magnitude longer than the physical size of the spin qubit. We realize such a link with a frequency-tunable high impedance SQUID array resonator. The spin qubit is a resonant exchange qubit hosted in a GaAs triple quantum dot. It can be operated at zero magnetic field, allowing it to coexist with superconducting qubits on the same chip. We spectroscopically observe coherent interaction between the resonant exchange qubit and a transmon qubit in both resonant and dispersive regimes, where the interaction is mediated either by real or virtual resonator photons.

Quantum dot thermometry at ultra-low temperature in a dilution refrigerator with a 4He immersion cell.

Experiments performed at a temperature of a few millikelvins require effective thermalization schemes, low-pass filtering of the measurement lines, and low-noise electronics. Here, we report on the modifications to a commercial dilution refrigerator with a base temperature of 3.5 mK that enable us to lower the electron temperature to 6.7 mK measured from the Coulomb peak width of a quantum dot gate-defined in an [Al]GaAs heteostructure. We present the design and implementation of a liquid 4He immersion cell tight against superleaks, implement an innovative wiring technology, and develop optimized transport measurement procedures.

Auger-spectroscopy in quantum Hall edge channels and the missing energy problem.

Quantum Hall edge channels offer an efficient and controllable platform to study quantum transport in one dimension. Such channels are a prospective tool for the efficient transfer of quantum information at the nanoscale, and play a vital role in exposing intriguing physics. Electric current along the edge carries energy and heat leading to inelastic scattering, which may impede coherent transport. Several experiments attempting to probe the concomitant energy redistribution along the edge reported energy loss via unknown mechanisms of inelastic scattering. Here we employ quantum dots to inject and extract electrons at specific energies, to spectrally analyse inelastic scattering inside quantum Hall edge channels. We show that the missing energy puzzle could be untangled by incorporating non-local Auger-like processes, in which energy is redistributed between spatially separate parts of the sample. Our theoretical analysis, accounting for the experimental results, challenges common-wisdom analyses which ignore such non-local decay channels.

Microwave-Cavity-Detected Spin Blockade in a Few-Electron Double Quantum Dot.

We investigate spin states of few electrons in a double quantum dot by coupling them to a magnetic field resilient NbTiN microwave resonator. The electric field of the resonator couples to the electric dipole moment of the charge states in the double dot. For a two-electron state the spin-triplet state has a vanishing electric dipole moment and can therefore be distinguished from the spin-singlet state. This way the charge dipole sensitivity of the resonator response is converted to a spin selectivity. We thereby investigate Pauli spin blockade known from transport experiments at finite source-drain bias. In addition we find an unconventional spin-blockade triggered by the absorption of resonator photons.

Coherent microwave-photon-mediated coupling between a semiconductor and a superconducting qubit.

Semiconductor qubits rely on the control of charge and spin degrees of freedom of electrons or holes confined in quantum dots. They constitute a promising approach to quantum information processing, complementary to superconducting qubits. Here, we demonstrate coherent coupling between a superconducting transmon qubit and a semiconductor double quantum dot (DQD) charge qubit mediated by virtual microwave photon excitations in a tunable high-impedance SQUID array resonator acting as a quantum bus. The transmon-charge qubit coherent coupling rate (~21 MHz) exceeds the linewidth of both the transmon (~0.8 MHz) and the DQD charge qubit (~2.7 MHz). By tuning the qubits into resonance for a controlled amount of time, we observe coherent oscillations between the constituents of this hybrid quantum system. These results enable a new class of experiments exploring the use of two-qubit interactions mediated by microwave photons to create entangled states between semiconductor and superconducting qubits.

All-Microwave Control and Dispersive Readout of Gate-Defined Quantum Dot Qubits in Circuit Quantum Electrodynamics.

Developing fast and accurate control and readout techniques is an important challenge in quantum information processing with semiconductor qubits. Here, we study the dynamics and the coherence properties of a GaAs/AlGaAs double quantum dot charge qubit strongly coupled to a frequency-tunable high-impedance resonator. We drive qubit transitions with synthesized microwave pulses and perform qubit readout through the state-dependent frequency shift imparted by the qubit on the dispersively coupled resonator. We perform Rabi oscillation, Ramsey fringe, energy relaxation, and Hahn-echo measurements and find significantly reduced decoherence rates down to γ_{2}/2π∼3 MHz corresponding to coherence times of up to T_{2}∼50 ns for charge states in gate-defined quantum dot qubits. We realize Rabi π pulses of width down to σ∼0.25 ns.

Cyclotron resonance in the high mobility GaAs/AlGaAs 2D electron system over the microwave, mm-wave, and terahertz- bands.

The reflected microwave power from the photo-excited high mobility GaAs/AlGaAs 2D device has been measured over the wide frequency band spanning from 30 to 330 GHz simultaneously along with diagonal magnetoresistance as a function of the magnetic field. Easily distinguishable resonances in the reflected power signal are observed at the same magnetic fields as a reduced amplitude in the Shubnikov-de Haas (SdH) oscillations of the diagonal magnetoresistance. The reflection resonances with concurrent amplitude reduction in SdH oscillations are correlated with cyclotron resonance induced by microwave, mm-wave, and terahertz photoexcitation. The magnetoplasma effect was also investigated. The results suggest a finite frequency zero-magnetic-field intercept, providing an estimate for the plasma frequency. The experimentally measured plasma frequency appears to be somewhat lower than the estimated plasma frequency for these Hall bars. The results, in sum, are consistent with an effective mass ratio of m*/m = 0.067, the standard value, even in these high mobility GaAs/AlGaAs devices, at very large filling factors. Preliminary findings from this article have been published as conference proceedings, see Kriisa, A., et al., J. of Phys. Conf. Ser. 864, 012057 (2017).

Floquet Spectroscopy of a Strongly Driven Quantum Dot Charge Qubit with a Microwave Resonator.

We experimentally investigate a strongly driven GaAs double quantum dot charge qubit weakly coupled to a superconducting microwave resonator. The Floquet states emerging from strong driving are probed by tracing the qubit-resonator resonance condition. In this way, we probe the resonance of a qubit that is driven in an adiabatic, a nonadiabatic, or an intermediate rate, showing distinct quantum features of multiphoton processes and a fringe pattern similar to Landau-Zener-Stückelberg interference. Our resonant detection scheme enables the investigation of novel features when the drive frequency is comparable to the resonator frequency. Models based on the adiabatic approximation, rotating wave approximation, and Floquet theory explain our experimental observations.

Signatures of transient Wannier-Stark localization in bulk gallium arsenide.

Many properties of solids result from the fact that in a periodic crystal structure, electronic wave functions are delocalized over many lattice sites. Electrons should become increasingly localized when a strong electric field is applied. So far, this Wannier-Stark regime has been reached only in artificial superlattices. Here we show that extremely transient bias over the few-femtosecond period of phase-stable mid-infrared pulses may localize electrons even in a bulk semiconductor like GaAs. The complicated band structure of a three-dimensional crystal leads to a strong blurring of field-dependent steps in the Wannier-Stark ladder. Only the central step emerges strongly in interband electro-absorption because its energetic position is dictated by the electronic structure at an atomic level and therefore insensitive to the external bias. In this way, we demonstrate an extreme state of matter with potential applications due to e.g., its giant optical non-linearity or extremely high chemical reactivity.

Coherent spin-photon coupling using a resonant exchange qubit.

Electron spins hold great promise for quantum computation because of their long coherence times. Long-distance coherent coupling of spins is a crucial step towards quantum information processing with spin qubits. One approach to realizing interactions between distant spin qubits is to use photons as carriers of quantum information. Here we demonstrate strong coupling between single microwave photons in a niobium titanium nitride high-impedance resonator and a three-electron spin qubit (also known as a resonant exchange qubit) in a gallium arsenide device consisting of three quantum dots. We observe the vacuum Rabi mode splitting of the resonance of the resonator, which is a signature of strong coupling; specifically, we observe a coherent coupling strength of about 31 megahertz and a qubit decoherence rate of about 20 megahertz. We can tune the decoherence electrostatically to obtain a minimal decoherence rate of around 10 megahertz for a coupling strength of around 23 megahertz. We directly measure the dependence of the qubit-photon coupling strength on the tunable electric dipole moment of the qubit using the 'AC Stark' effect. Our demonstration of strong qubit-photon coupling for a three-electron spin qubit is an important step towards coherent long-distance coupling of spin qubits.

Coherent backscattering in quasi-ballistic ultra-high mobility GaAs/AlGaAs 2DES.

A small and narrow negative-magnetoresistance (MR) effect that appears about null magnetic field over the interval -0.025 ≤ B ≤ 0.025 T in magnetotransport studies of the GaAs/AlGaAs 2D system with µ ≈ 107cm2/Vs is experimentally examined as a function of the sample temperature, T. The temperature dependent magnetoresistance data were fit using the Hikami et al. theory, without including the spin-orbit correction, to extract the inelastic length, li, which decreases rapidly with increasing temperature. It turns out that li < le, where le is the elastic length, for all T. Thus, we measured the single particle lifetime, τs, and the single particle mean free path ls = vFτs. A comparison between li and ls indicates that li > ls. The results suggest that the observed small and narrow magnetoresistance effect about null magnetic field could be a manifestation of coherent backscattering due to small angle scattering from remote ionized donors in the high mobility GaAs/AlGaAs 2DES.

Electron heating induced by an ac-bias current in the regime of Shubnikov-de Haas oscillation in the high mobility GaAs/Al x Ga1-x As two-dimensional electron system.

The magnetotransport properties of the high mobility GaAs/AlGaAs two-dimensional electron gas systems have been examined to determine the influence of the ac current bias on the carrier temperature. The changes in the line shape of Shubnikov-de Haas oscillations in the longitudinal magnetoresistance ([Formula: see text]) were followed as a function of the ac current bias in the temperature range of [Formula: see text] in order to determine the carrier heating effect due to the ac bias. The lineshape analysis of these oscillations indicates that the carrier temperature of the two-dimensional electron system is linearly proportional to the ac bias current.

B-periodic oscillations in the Hall-resistance induced by a dc-current-bias under combined microwave-excitation and dc-current bias in the GaAs/AlGaAs 2D system.

We report the observation of dc-current-bias-induced B-periodic Hall resistance oscillations and Hall plateaus in the GaAs/AlGaAs 2D system under combined microwave radiation- and dc bias excitation at liquid helium temperatures. The Hall resistance oscillations and plateaus appear together with concomitant oscillations also in the diagonal magnetoresistance. The periods of Hall and diagonal resistance oscillations are nearly identical, and source power (P) dependent measurements demonstrate sub-linear relationship of the oscillation amplitude with P over the span 0 < P ≤ 20 mW.