Measurements of the Fermi surface are a fundamental technique for determining the electrical and magnetic properties of solids. In two-dimensional (2D) systems, the area and diameter of the Fermi surface are typically measured using Shubnikov-de Haas oscillations and commensurability oscillations respectively. However, these techniques are unable to detect changes in the parity of the Fermi surface [i.e., when E(+k) not equal E(-k)]. Here, we show that transverse magnetic focusing can be used to detect such changes, because focusing only measures a well defined section of the Fermi surface and does not average over +k and -k. Furthermore, our results show that focusing is an order of magnitude more sensitive to changes in the Fermi surface than other 2D techniques. While we investigate a specific Fermi surface shift in this work, focusing could be used to investigate similar Fermi surface changes in other 2D systems.

We report the experimental observation of a spin-polarised conductance plateau at e ^2 / h in a clean one-dimensional (1D) quantum wire defined by back-gated, split gate devices on a GaAs/AlGaAs heterostructure in the absence of a magnetic field. The 1D devices were fabricated using standard lithography techniques consisting of split gates, and a custom-designed back gate allows for the modulation of carrier density within the 1D channel. The differential conductance shows regular quantised plateaus in units of 2 e ^2 / h as a function of back gate voltage, including the observation of the 0.7(2 e ^2 / h ) conductance anomaly. The 0.7 anomaly, on reducing the charge carrier concentration, gradually converts into a 0.5(2 e ^2 / h ) structure, indicating the lifting of spin degeneracy in the absence of a magnetic field. Our results suggest the potential of low-density 1D devices for applications in spintronics and quantum electronics.

Electronic Hong-Ou-Mandel (HOM) current noise interferometry has revealed anyonic statistics in Fractional Quantum Hall (FQH) states at ν = 1/3 and 2/5. However, hole-conjugate phases (½ < ν < 1), like ν = 2/3, host both charge and neutral edge modes, are disorder-sensitive, and pose challenges for interferometry. We present time-domain HOM and Photon-Assisted Shot Noise (PASN) measurements at ν = 2/3 to probe edge mode dynamics and tunneling charge. Using PASN's fractional Josephson relation, we measure an e/3 tunneling charge and show that DC shot noise overestimates charge below ~100 mK. PASN reveals damping of downstream charge modes due to limited propagation of upstream neutral modes beyond a micrometer-scale equilibration length. Time-resolved HOM measurements confirm picosecond pulse broadening. These results suggest revisiting the neutral mode status, highlight the limitations of HOM interferometry in hole-conjugate phases and provide a path to explore complex FQH states that host neutral or non-Abelian modes.

Next-generation communication systems require rapid and efficient control of terahertz (THz) signals to encode data streams. Graphene-based metamaterials emerge as a promising candidate for effective THz modulation as a result of graphene's large electrically controllable conductivity. However, a significant challenge arises from the inability of graphene to achieve full depletion at the Dirac point, limiting the modulation depth in most LC-resonant metamaterial modulators in transmission. To overcome this limitation, we exploit the destructive interference of Fresnel reflection components. Our study shows single-layer, solid-state graphene-based modulators operating in the terahertz range with several orders of magnitude modulation depth, validated through terahertz time-domain spectroscopy measurements. These findings underscore the potential of graphene-based metamaterials in advancing THz communication technologies.

A quantum point contact (QPC)-a constriction in a semiconducting two-dimensional electron system with a quantized conductance-is a building block of novel spintronic and topological electronic circuits. QPCs can also be used as readout electronics, charge sensors, or switches in quantum nanocircuits. A short and impurity-free constriction with superconducting contacts is a Cooper-pair QPC analogue known as a superconducting quantum point contact (SQPC). The technological development of such quantum devices has been prolonged due to the challenges of maintaining their geometrical requirement and nearmaterial and device engineering techniques and report on an innovative realization of nanoscale hybrid SQPC arrays with split gate technology in semiconducting 2D electron systems. We exploit the special gate tunability of the quantum wells, and demonstrate the first experimental observation of conductance quantization in hybrid InGaAs-Nb SQPCs. We observe reproducible quantized conductance at zero magnetic fields in multiple quantum nanodevices fabricated in a single chip and systematically investigate the quantum transport of SQPCs at low and high magnetic fields for their potential applications in quantum metrology, for extremely accurate voltage standards, and fault-tolerant quantum technologies.

We demonstrate a method to generate photons by injecting hot electrons into a pn junction within a GaAs/AlGaAs heterostructure. Hot electrons are generated by biasing across a mesoscopic potential in n-type region and travel toward p-type region through quantum Hall edge channel in the presence of magnetic field perpendicular to the substrate. The p-type region is created several microns away from the hot electron emitter by inducing interfacial charges using a surface gate. The energy relaxation of the hot electrons is suppressed by separating the orbitals before and after longitudinal-optical (LO) phonon emission. This technique enables the hot electrons to reach the p-type region and to recombine with induced holes followed by photon emissions. Hot electron-induced hole recombination is confirmed by a peak around 810nm in an optical spectrum that corresponds to excitonic recombination in a GaAs quantum well. An asymmetric structure observed in the optical spectrum as a function of the magnetic field originates from the chiral transport of the hot electrons in the Hall edge channel. We propose the combination of our technology and on-demand single-electron source would enable the development of an on-demand single photon source that is an essential building block to drive an optical quantum circuit and to transfer quantum information for a long distance.

The study of phonon coupling in doped semiconductors via electrical transport measurements is challenging due to unwanted temperature-induced effects such as dopant ionization and parallel conduction. Here, we study phonon scattering in 2D electrons and holes in the 1.6-92.5 K range without the use of extrinsic doping, where both acoustic and longitudinal optic (LO) phonons come into effect. We use undoped GaAs/ AlxGa1-xAs heterostructures and examine the temperature dependence of the sample resistivity, extracting phonon coupling constants and the LO activation energy. Our results are consistent with results obtained through approaches other than transport measurements and highlight the benefit of this approach for studying electron-phonon and hole-phonon coupling.

Surface-gated quantum dots (QDs) in semiconductor heterostructures represent a highly attractive platform for quantum computation and simulation. However, in this implementation, the barriers through which the QD is tunnel-coupled to source and drain reservoirs (or neighboring QDs) are usually non-rigid and capacitively influenced by the plunger-gate voltage ( V-P). In transport spectroscopy measurements, this leads to suppression of current and lifting of the Coulomb blockade for increasing negative and positive values of V-P, respectively. Consequently, the charge-occupancy of the QD can be tuned over a rather small range of V-P. By dynamically tuning the tunnel barriers to compensate for the capacitive effect of V-P, here we demonstrate a protocol that allows Coulomb blockade to be preserved over a remarkably large span of charge-occupancies, as demonstrated by clean Coulomb diamonds and well-resolved excited state features. The protocol will be highly beneficial for automated tuning and identification of the gate voltage space for optimal operation of QDs in large arrays required for a scalable spin quantum computing architecture.

Semiconductor quantum dots are promising candidates for the generation of nonclassical light. Coupling a quantum dot to a device capable of providing polarization-selective enhancement of optical transitions is highly beneficial for advanced functionalities such as efficient resonant driving schemes or applications based on optical cyclicity. Here, we demonstrate broadband polarization-selective enhancement by coupling a quantum dot emitting in the telecom O-band to an elliptical bullseye resonator. We report bright single-photon emission with a degree of linear polarization of 96%, Purcell factor of 3.9, and count rates up to 3 MHz. Furthermore, we present a measurement of two-photon interference without any external polarization filtering and demonstrate compatibility with compact Stirling cryocoolers by operating the device at temperatures up to 40 K. These results represent an important step towards practical integration of optimal quantum dot photon sources in deployment-ready setups.

Increasing the electric current from a single-electron source is a main challenge in an effort to establish the standard of the ampere defined by the fixed value of the elementary charge e and the operation frequency f. While the current scales with the frequency, due to an operation frequency limit for maintaining accurate single-electron transfer, parallelization of single-electron sources is expected to be a more practical solution to increase the generated electric current I=Nef, where N is the number of parallelized devices. One way to parallelize single-electron sources without increasing the complexity in device operation is to use a common gate. Such a scheme will require each device to have the same operation parameters for single-electron transfer. In order to investigate this possibility, we study the statistics for operation gate voltages using single-electron sources embedded in a multiplexer circuit. The multiplexer circuit allows us to measure 64 single-electron sources individually in a single cooldown. We also demonstrate the parallelization of three single-electron sources and observe the generated current enhanced by a factor of three.

A system in equilibrium keeps "exploring" nearby states in the phase space and consequently, fluctuations can contain information, that the mean value does not. However, such measurements involve a fairly complex interplay of effects arising in the device and measurement electronics, that are nontrivial to disentangle. In this paper, we briefly analyze some of these issues and show the relevance of a two-amplifier cross-correlation technique for semiconductors and thin films commonly encountered. We show that by using home-built very low-cost amplifiers, one can measure spectral densities as low as approximately 10 -18 -10 -19 V2 Hz-1. We apply this method to an ionic-liquid-gated Ga:ZnO channel and show that the glass transition of the ionic liquid brings about a change in the exponent of the low-frequency resistance fluctuations of the conducting channel. The variation is absent in ungated devices. Our analysis suggests that a log-normal distribution of the Debye-relaxation times of the fluctuations and an increased weight of the long-timescale relaxations can give a phenomenological but quantitative explanation of the observed change in the exponent.

Suppression of threading dislocations (TDs) in thin germanium (Ge) layers grown on silicon (Si) substrates has been critical for realizing high-performance Si-based optoelectronic and electronic devices. An advanced growth strategy is desired to minimize the TD density within a thin Ge buffer layer in Ge-on-Si systems. In this work, we investigate the impact of P dopants in 500-nm thin Ge layers, with doping concentrations from 1 to 50 × 1018 cm−3. The introduction of P dopants has efficiently promoted TD reduction, whose potential mechanism has been explored by comparing it to the well-established Sb-doped Ge-on-Si system. P and Sb dopants reveal different defect-suppression mechanisms in Ge-on-Si samples, inspiring a novel co-doping technique by exploiting the advantages of both dopants. The surface TDD of the Ge buffer has been further reduced by the co-doping technique to the order of 107 cm−2 with a thin Ge layer (of only 500 nm), which could provide a high-quality platform for high-performance Si-based semiconductor devices.

Single Electron Pump (SEP) devices that can deliver single electrons at high frequencies are fabricated using two different gate geometries. The pump quantum dot (QD) can be defined either using two finger gates where the pump operates in what is known as the conventional pumping regime (CPR), or alternatively, the QD can be designed with the use of a single finger gate and split gate, where the pump operates in the long pumping regime (LPR). Here we investigate pump-maps produced in both the CPR and LPR under the effects of a low magnetic field (1 T to -1 T) and observe both the evolution of the pump-map and pump accuracy. Measurements were carried out in a cryogen free dilution refrigeration system with the use of a superconducting magnetic. Clear discrepancies between the behaviour of Med current in the different regimes can be seen indicating a fundamental difference in the operation of the pumps in the two different pumping regimes.

Effective control of terahertz (THz) signals is crucial for next-generation communication systems to encode data. Graphene-based metamaterials offer a promising avenue for THz modulation, thanks to graphene's high, electrically controllable conductivity. However, a significant challenge arises from graphene's inability to achieve full depletion at the Dirac point, limiting the transmission modulation depth in most LC-resonant metamaterial modulators. To overcome this limitation, we take advantage of the destructive interference of Fresnel reflection components. We demonstrate a single-layer, solid-state modulator operating in the THz range, characterized through terahertz time-domain spectroscopy. By exciting the modulator from its substrate side, we achieve a large modulation depth exceeding 21 dB at 1.8THz with reconfiguration speeds in excess of 3 MHz.

A new quantum phenomenon, the in-plane photoelectric effect, has recently been discovered as a mechanism of far-infrared ( FIR) photoresponse generation in a two-dimensional electron gas (2DEG). This effect has shown promise for terahertz ( THz) detection due to its high photoconversion efficiency and a lack of an intrinsic response time limit. Initial detectors utilising the in-plane photoelectric effect, known as photoelectric tunable-step (PETS) detectors, have been developed and demonstrated to work as high-sensitivity FIR detectors. Here, we propose a PETS detector utilising a novel, broadband antenna adopted from a wide bow-tie geometry that minimises the area of 2DEG covered by the antenna. We demonstrate experimentally a large photoresponse to 2.0 THz radiation of an AlGaAs/GaAs heterojunction-based PETS detector with our novel antenna design. Under the same operating conditions, this detector shows much larger photocurrent and two-times improvement in rise time compared to an identical PETS detector fabricated simultaneously on the same chip but instead incorporating a bow-tie antenna. Our findings help facilitate the development of future high-speed, low-noise, ultra-sensitive FIR detector arrays.

We present an efficient machine learning based automated framework for the fast tuning of single-electron pump devices into current quantization regimes. It uses a sparse measurement approach based on an iterative active learning algorithm to take targeted measurements in the gate voltage parameter space. When compared to conventional parameter scans, our automated framework allows us to decrease the number of measurement points by about an order of magnitude. This corresponds to an eight-fold decrease in the time required to determine quantization errors, which are estimated via an exponential extrapolation of the first current plateau embedded into the algorithm. We show the robustness of the framework by characterizing 28 individual devices arranged in a GaAs/AlGaAs multiplexer array, which we use to identify a subset of devices suitable for parallel operation at communal gate voltages. The method opens up the possibility to efficiently scale the characterization of such multiplexed devices to a large number of pumps.

We present the first detailed study of the effect of a strong magnetic field on single-electron pumping in a device utilising a finger-gate split-gate configuration. In the quantum Hall regime, we demonstrate electron pumping from Landau levels in the leads, where the measurements exhibit pronounced oscillations in the lengths of the pumping plateaus with the magnetic field, reminiscent of Shubnikov-de Haas oscillations. This similarity indicates that the pumping process is dependent on the density of states of the 2D electron gas over a narrow energy window. Based on these observations, we develop a new theoretical description of the operation of single-electron pumps which for the first time allows for the determination of the physical parameters of the experiment; such as the capture energy of the electrons, the broadening of the quantised Landau levels in the leads, and the quantum lifetime of the electrons.

We report a THz detector that uses the in-plane photoelectric effect (IPPE) through the integration of a metamaterial with a dual-gated field-effect transistor (FET) array. In this study, we experimentally demonstrate coupling of the IPPE detection mechanism with a metamaterial antenna array, which results in substantial performance enhancement for THz detectors. We design, simulate, and optimise a brickwork array to efficiently confine incident radiation to the 2DEG layer, adapting these arrays to work as THz detectors. Under excitation with quantum cascade laser radiation at a frequency of 1.9 THz, our detector exhibits a photocurrent of 5.8 nA. This achievement surpasses the previously recorded maximum for single-antenna PETS detectors under identical experimental conditions, while simultaneously achieving significantly lower output impedance compared to any previously reported detector utilising the IPPE mechanism. This highly efficient metasurface-based detector with low output impedance holds the potential for developing high-throughput THz communication systems.

The mass production and the practical number of cryogenic quantum devices producible in a single chip are limited to the number of electrical contact pads and wiring of the cryostat or dilution refrigerator. It is, therefore, beneficial to contrast the measurements of hundreds of devices fabricated in a single chip in one cooldown process to promote the scalability, integrability, reliability, and reproducibility of quantum devices and to save evaluation time, cost and energy. Here, we use a cryogenic on-chip multiplexer architecture and investigate the statistics of the 0.7 anomaly observed on the first three plateaus of the quantized conductance of semiconductor quantum point contact (QPC) transistors. Our single chips contain 256 split gate field effect QPC transistors (QFET) each, with two 16-branch multiplexed source-drain and gate pads, allowing individual transistors to be selected, addressed and controlled through an electrostatic gate voltage process. A total of 1280 quantum transistors with nano-scale dimensions are patterned in 5 different chips of GaAs heterostructures. From the measurements of 571 functioning QPCs taken at temperatures T= 1.4 K and T= 40 mK, it is found that the spontaneous polarisation model and Kondo effect do not fit our results. Furthermore, some of the features in our data largely agreed with van Hove model with short-range interactions. Our approach provides further insight into the quantum mechanical properties and microscopic origin of the 0.7 anomaly in QPCs, paving the way for the development of semiconducting quantum circuits and integrated cryogenic electronics, for scalable quantum logic control, readout, synthesis, and processing applications.

We demonstrate a photoelectric tunable-step (PETS) terahertz (THz) detector with a symmetric dipole antenna based on a GaAs/AlGaAs heterojunction and use it as a model system to answer key questions for the design of any type of PETS detectors: the best antenna gap design for maximum response, the speed, and temperature dependence of the PETS response. Besides efficient high-speed detection of 1.9 THz radiation, we achieve operation up to 75 K, at 8-times higher temperatures than previously demonstrated. Our findings deepen the understanding of the in-plane photoelectric effect and pave the way to future high responsivity, fast PETS THz detectors operating at high temperatures.