Sergei Studenikin

Abstarct: The integer quantum Hall effect provides one of the most precise and robust manifestations of topological quantum transport. Yet, in semiconductor systems, its development to a fully spin-polarised state typically requires magnetic fields of several tesla and milli-Kelvin temperatures to overcome disorder, Landau-level broadening, and thermal smearing. Here we report a fully spin-polarised ν = 1 quantum Hall state in a two-dimensional (2D) hole gas hosted in a compressively strained germanium quantum well grown on silicon, stabilised at magnetic fields as low as 0.25 − 0.3 T and temperatures up to 1.5 K. This unusually low-field quantisation is enabled by an ultra-dilute carrier regime combined with extremely large Zeeman energy that approaches the cyclotron energy of free carriers over a wide density range, resulting in dominant spin splitting of Landau levels at sub-tesla fields to reach ν = 1 fully spin-polarized quantum limit. Magnetotransport and 2D Landau-fan measurements demonstrate that Landau-level broadening remains smaller than the spin splitting throughout the whole magnetic field range down to 0.25 mT, allowing robust quantisation of the Hall resistance at h / e ² . These results establish access to an experimentally unexplored regime of spin-polarised quantum Hall physics in a semiconductor platform and open new opportunities for exploring exchange-driven and correlated many-body states of dilute systems in small magnetic fields.

Scalable semiconductor quantum computing requires coherent coupling between non-neighboring qubits over distances far exceeding direct tunnel coupling. We introduce a paradigm of Quantum Phononic Links (QPLs), which are engineered phononic waveguides and cavities that mediate direct coherent interactions via strain field between spatially separated hole spin qubits hosted in compressively strained germanium on silicon. Exploiting the strong strain sensitivity and spin–orbit coupling of valence-band holes, phonons confined in QPLs couple directly to spin qubits via deformation potentials, without relying on piezoelectric transducers commonly used in surface acoustic waves technology. We show that phononic modes can be engineered to provide frequency-selective, coherent coupling between qubits separated from sub-micrometer to wafer-scale distances, while remaining compatible with CMOS fabrication. QPLs establish a material-native and scalable quantum bus for semiconductor spin qubits, opening a new route toward large-scale quantum processors and hybrid quantum networks.

We investigate resonant acoustic phonon scattering in the magnetoresistivity of an ultrahigh-mobility two-dimensional electron gas system subject to DC current in the temperature range 10 mK to 3.9 K. For a DC current density of ∼1.1 A/m, the induced carrier drift velocity v_{drift} becomes equal to the speed of sound s∼3 km/s. When v_{drift}≳s very strong resonant features with only weak temperature dependence are observed and identified as phonon-induced resistance oscillations at and above the "sound barrier." Their behavior contrasts with that in the subsonic regime (v_{drift}<s) where resonant acoustic phonon scattering is strongly suppressed when the temperature is reduced unless amplified with quasielastic inter-Landau-level scattering. Our observations are compared to recent theoretical predictions from which we can extract a dimensionless electron-phonon coupling constant of g^{2}=0.0016 for the strong nonlinear transport regime. We find evidence for a predicted oscillation phase change effect on traversing the "sound barrier." Crossing the "sound barrier" fundamentally alters the resulting phonon emission processes, and the applied magnetic field results in pronounced and sharp resonant phonon emission due to Landau level quantization.

In this paper, we explore alternative approaches to integrate acoustic wave functionalities into non-piezoelectric group IV materials, notably, germanium within germanium-on-silicon (GoS) heterostructures. This material system is promising as a spin qubit platform for scalable quantum computing architectures. We investigate the potential of surface acoustic waves (SAWs) inducing strain as an interaction mechanism that can be used for the manipulation of spin quantum states in lateral gated quantum dot (QD) devices. Using the Bir–Pikus formalism and k · p theory, we describe the theoretical basis for strain-induced modulation of the valence band in the GoS heterostructures. Our simulations demonstrate that the Rayleigh-type SAWs operating at GHz frequencies can create strain profiles that effectively modulate energy levels in quantum dots. Furthermore, replacing the piezoelectric material (AlN in this work) with a non-piezoelectric counterpart (Al2O3) preserves high-quality acoustic wave propagation while enabling purely mechanical coupling mechanisms, thus enhancing qubit coherence by reducing charge noise, which in piezoelectric materials stems from direct coupling between charge fluctuations and phonons. Proxy simulations with a double-plunger-gate system and floating potentials confirm the sensitivity of QD charge configuration energies to induced strain. These findings support SAW-driven detuning and align with recent predictions on strain-mediated spin–orbit coupling in hole spin qubits. This study highlights the promise of SAW-based strain engineering for scalable quantum control in CMOS-compatible platforms, paving the way for future experimental validation and integration with phononic crystals to advance mechanically enabled quantum architectures.

Spin exchange between confined holes and nuclei has been demonstrated for zero-dimensional quantum dots by optical techniques but has not been observed for gated planar structures. Here, enabled by strong spin-orbit interaction, and under microwave (MW) illumination, we report hyperfine interaction and dynamic polarization of the nuclei with confined heavy-holes in a GaAs/AlGaAs double quantum dot device. Distinct signatures of the resultant hyperfine field on the electron dipole spin resonance (EDSR) signal include: hysteresis on sweeping the magnetic (B-) field up and down with characteristics that are strongly dependent on both MW power and B-field sweep rate; free bidirectional dragging of the EDSR condition; stable locking on resonance on a timescale of hours; slow temporal change as the hyperfine field decays (T₁ nuclear decay time ~ 100 s); and oscillations in time commensurate with Larmor precession of the ⁷⁵As nuclei. We attain pumped nuclear (Overhauser) fields ~ 25 mT (~20% nuclear polarization).

The spin of a single hole confined in a planar gated double quantum dot system is manipulated at subharmonic frequencies of the electric dipole spin resonance. This is achieved by taking advantage of the intrinsic strong spin–orbit coupling that exists in the hole system. In our experiments, a single-hole spin is initially placed in one of the quantum dots, while the other dot is kept empty. The hole spin rotations are stimulated due to multiple anti-crossing events with empty spin levels of the other dot during the Landau–Zener–Stückelberg–Majorana interference. Using this technique, we are able to excite and detect spin-flip transitions occurring at high subharmonic frequencies, up to the 12th order.

Germanium (Ge) and silicon (Si), group IV semiconductors, as innovative epitaxial structures, continue to offer improved properties and enhanced parameters for applications in classical and quantum electronics as well as spintronics. The mobility of free carriers is one such parameter that defines the ultimate performance of electronic devices. The dependence of two-dimensional hole gas (2DHG) mobility on 2DHG density in an undoped gate controlled compressively strained Ge quantum well (QW) grown on Si (cs-GoS), with QW thicknesses of 15 and 30 nm, is studied and compared at T = 0.3 K. Experimental results show that 2DHG mobility is higher in the thinner QW. 2DHG mobilities of 3.1 × 106 and 1.55 × 106 cm2 V−1 s−1 are measured in 15 and 30 nm QWs, respectively, with the same carrier density of 0.9 × 1011 cm−2. The results are explained by the existing theory of carriers' scattering on background ionized impurities.

With the emergence of the quantum computing era, the spin physics of engineered semiconductor materials with large and tuneable effective g*-factor, which is a measure of the interaction between the magnetic field and the spin of the particle, has become of great interest because it offers new physics and engineering tools for spin’s manipulation and its addressable control. Here we suggest a semi-empirical method to determine out of plane effective g*-factor in high mobility 2D hole heterostructures. We experimentally study the electric-field tuneablity of effective g*-factor of holes in a strained germanium quantum well heterostructure. As a result of the material’s engineering, the g*-factor can be tuned in a large range from 13 to 24 that corresponds to the tuneable Zeeman spin splitting of heavy holes in the range from smaller, to equal, and to larger than the orbital Landau level quantization gap.

Sub-harmonics of electric dipole spin resonance (EDSR) mediated by Landau-Zener-St{ü}ckelberg-Majorana (LZSM) tunneling transitions are studied numerically and analytically in a Zeeman-split four level system with strong spin-orbit coupling that can be realized, for example, in a GaAs-based double quantum dot in a single-hole regime. The spin qubit is formed in one of the dots and the second dot is used as an auxiliary element to enhance functionality of the spin qubit. In particular, it is found that the spin rotation rate can be essentially enhanced due to the tunnel coupling with the auxiliary dot on both the main EDSR frequency and at its high sub-harmonics allowing the coherent spin $π$-rotations on a 10-ns time scale. Spin manipulation on high sub-harmonics is promising for new time-efficient schemes of the spin control and readout in qubit devices operating at high magnetic fields where the main harmonic is inaccessible due to hardware limitations.