High-power and broadband dual-comb sources in the terahertz frequency regime are much in demand for fast and high-resolution spectroscopies. The terahertz quantum cascade laser (QCL) is a powerful radiation source in the frequency range between 1 and 5 THz, which is an ideal candidate for frequency-comb and dual-comb generations in this frequency regime. In this work, we report on broadband on-chip dual-comb sources based on terahertz QCLs emitting at approximately 4.2 THz with a single plasmon waveguide configuration. Due to the fast intersubband transitions in QCLs, the dual-comb lines can be detected by employing the laser self-detection scheme. A finite-element analysis shows that a weak optical coupling between the two laser combs is sufficient for generating dual-comb lines via the nonlinear multiheterodyne frequency beatings. The noise-equivalent power of the QCL detector is also evaluated for different optical powers; furthermore, a microwave double injection is employed to stabilize the comb frequency and broaden the optical comb bandwidth. We find that even with weak radio-frequency powers, the optical comb bandwidth can be significantly broadened from 86 to 166 GHz. The double injection also allows us to investigate the carrier offset noise of QCL combs.

Heralding techniques are useful in quantum communication to circumvent losses without resorting to error-correction schemes or quantum repeaters. Such techniques are realized, for example, by monitoring for photon loss at the receiving end of the quantum channel while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a direct quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit quantum electrodynamics and is fully microwave controlled, making it an interesting candidate for future modular quantum-computing architectures.

We report on microwave optomechanics measurements performed on a nuclear adiabatic demagnetization cryostat, whose temperature is determined by accurate thermometry from below 500 $\mu \mathrm{K}$ to about 1 K. We describe a method for accessing the on-chip temperature, building on the blue-detuned parametric instability and a standard microwave setup. The capabilities and sensitivity of both the experimental arrangement and the developed technique are demonstrated with a very weakly coupled silicon-nitride doubly clamped beam mode of about 4 MHz and a niobium on-chip cavity resonating around 6 GHz. We report on an unstable intrinsic driving force in the coupled microwave-mechanical system acting on the mechanics that appears below typically 100 mK. The origin of this phenomenon remains unknown, and deserves theoretical input. It prevents us from performing reliable experiments below typically 10–30 mK; however no evidence of thermal decoupling is observed, and we propose that the same features should be present in all devices sharing the microwave technology, at different levels of strengths. We further demonstrate empirically how most of the unstable features can be annihilated, and speculate how the mechanism could be linked to atomic-scale two-level systems. The described microwave and microkelvin facility is part of the European Microkelvin Platform, and shall be used for further experiments within and below the millikelvin range.

We propose a theoretical scheme to achieve strong photon blockade via a single atom in a cavity. By utilizing the optical Stark shift, the dressed-state splitting between the higher and lower branches is enhanced, which results in a significant increase in the lower (higher) branch and a decrease in the higher (lower) branch for a negative (positive) Stark shift, and dominates the time evolution of the photon-number oscillations as well. Furthermore, two-photon excitation is suppressed via quantum interference for the optimal phase and Rabi frequency of an external microwave field. It is shown that the interplay between quantum interference and the enhanced vacuum Rabi splitting gives rise to a strong photon blockade that can be used to realize a high-quality single-photon source beyond the regime of strong atom-cavity coupling. In particular, by tuning the optical Stark shift, the second-order correlation function in our scheme can be made three orders of magnitude smaller than in the Jaynes-Cummings model, and, correspondingly, a large cavity photon number appears as well. Our proposal may imply exciting opportunities for potential applications in quantum networks and quantum information processing.

Two-dimensional materials-based van der Waals heterojunctions (vdWHs) are promising candidates for tunnel field-effect transistors (TFET) because of their atomically clean and electronically sharp junction interfaces and lattice mismatch-free characteristics. The tunneling behaviors of the TFETs greatly depend on the energy band alignments between different layers. By first-principles analyses, the band offsets in vdWHs are found to be very sensitive to the vdW interaction and temperature, especially for the type-III heterojunctions, which can be switched to the type-II configuration. This band-offset degradation is attributed to the interlayer charge transfer, generating built-in electric fields and potential differences, consistent with the plate condenser model. The charges are mainly transferred by two mechanisms: spontaneous and thermal excitation-assisted electron tunneling. The first one is caused by the electron wave function overlaps and energy difference of the valence band maximum and conduction band minimum, which belong to different materials, whereas the latter one is strongly correlated to the thermal excitations. The band-offset degradation is a general phenomenon in vdWHs, especially for those with a type-III band alignment. Therefore, attention should be paid in TFET investigations, since they are switched on by changing the band alignments to the type-III configuration.

Circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors yet is less studied in graphene due to the relatively weak light-matter interaction and is submerged by other nonlinear optical effects. We give experimental evidence of CPDE in vertically grown graphene (VGG) by terahertz (THz) emission spectroscopy. The emitted THz polarization states can be tuned to linear, left-handed, and right-handed elliptical polarizations by changing the helicity of the pump laser. Polarity reversal of the THz time-domain signal occurs with the opposite helicity of pump laser excitation due to the CPDE. Theory analysis suggests that both the linear photon drag effect and CPDE-induced transient photocurrents contribute to the THz emission from which the contribution weight of CPDE can be tuned by different elliptical states of the excitation light. The photon-helicity-dependent THz emission from VGG based on CPDE offers an alternative thought of graphene-based polarization sensitive THz sources for chiral analysis in the THz field.

Scanning nanoscale superconducting quantum interference devices (SQUIDs) are gaining interest as highly sensitive microscopic magnetic and thermal characterization tools of quantum and topological states of matter and devices. We introduce a technique of collimated differential-pressure magnetron sputtering for versatile self-aligned fabrication of SQUID-on-tip (SOT) nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point source, deposition. The technique provides access to a broad range of superconducting materials and alloys beyond the elemental superconductors employed in the existing thermal deposition methods, opening the route to greatly enhanced SOT characteristics and functionalities. Utilizing this method, we have developed molybdenum-rhenium (${\mathrm{Mo}}_{66}{\mathrm{Re}}_{34}$) SOT devices with sub-50-nm diameter, magnetic flux sensitivity of 1.2 $\mu {\mathrm{\Phi }}_0/\mathrm{H}{\mathrm{z}}^{1/2}$ up to 3 T at 4.2 K, and thermal sensitivity better than 4 $\mu \mathrm{K}/\mathrm{H}{\mathrm{z}}^{1/2}$ up to 5 T—about five times higher than any previous report—paving the way to nanoscale imaging of magnetic and spintronic phenomena and of dissipation mechanisms in previously inaccessible quantum states of matter.

We analyze the effects of time-dependent magnetic and rf fields on the spin dynamics of the Carr-Purcell-Meiboom-Gill sequence. The analysis is based on the decomposition of the magnetization into the eigenmodes of the propagator of a single refocusing cycle. For sufficiently slow changes in the external fields, the magnetization follows the changing eigenmodes adiabatically. This results in echo amplitudes that show regular modulations with time. Faster field changes can induce transitions between the eigenmodes. Such nonadiabatic behavior occurs preferentially at particular offsets of the Larmor frequency from the rf frequency where the eigenmodes become nearly degenerate. We introduce the instantaneous adiabaticity parameter $\mathcal{A}(t)$ that accurately predicts the crossover from the adiabatic to the nonadiabatic regime and allows the classification of field fluctuations. $\mathcal{A}(t)$ is determined solely by the properties of a single refocusing cycle under static conditions and the instantaneous value of the field offset and its temporal derivative. The analytical results are compared with numerical simulations.

The intercalation behavior of the ${\mathrm{Al}\mathrm{Cl}}_4$ molecule in graphite and its diffusion behavior within the galleries formed between two adjacent graphene layers are studied using density-functional approximations and ab initio molecular-dynamics simulations. We confirm that the ${\mathrm{Al}\mathrm{Cl}}_4$ prefers to stay in its tetrahedral geometry even after intercalation into graphite. More importantly, our molecular-dynamics simulations reveal an intriguing dynamical behavior of the resulting graphite intercalation compound: (i) the intercalated ${\mathrm{Al}\mathrm{Cl}}_4$ molecule additionally rotates around an axis perpendicular to the graphene planes while migrating inside the gallery, and (ii) the graphene planes of the host graphite slide over each other during the diffusion of the molecule. We find that the synergetic motion of the graphene planes significantly enhances the mobility of the ${\mathrm{Al}\mathrm{Cl}}_4$ in the host graphite, and such a cooperative behavior is a key factor behind the ultrahigh rate capability of the recently reported aluminum-ion battery (AIB) [Lin et al., Nature 520, 325 (2015)]. Furthermore, as a result of investigating the intercalation and diffusion behavior of the ${\mathrm{Al}\mathrm{F}}_4$ molecule in graphite, we propose to use ${\mathrm{Al}\mathrm{F}}_4^{-}$ as the charge-carrying anion for an $\mathrm{Al}$-graphite battery cell. We predict that an ${\mathrm{Al}\mathrm{F}}_4$-based AIB cell, if experimentally realizable, will have a larger cathode specific capacity, higher voltage, and higher rate capability.

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the six-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of (In,Ga)N quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight into the physics of modern light-emitting diodes.

Bendable signal transmission based on accelerating beams has become an increasingly interesting topic in recent years due to not only its fascinating propagation characteristics but also the potential to evade or resist obstructions within the transmission link. In this work, we propose a type of image-signal transmission based on nonconvex accelerating beams in contrast to convex accelerating beams such as Airy beams, which is expected to be more resilient to obstruction due to the special spectrum-position mapping characteristic. A constructed helical beam is used as the nonconvex accelerating beam for demonstration, and an image encoded in its angular spectrum is successfully retrieved after multiple self-bending without and with obstruction in the transmission link, which confirms the above idea and may open a new door for image-signal transmission in the fields of imaging, signal processing, and free-space communication.

The recently discovered phenomena of photomagnetic recording using femtosecond laser pulses in magnetic dielectrics opens up intriguing opportunities for ultrafast least-dissipative magnetic writing. Fundamental questions remain, however, about the processes, which limit the repetition rate of such writing. We report the magnetization dynamics and switching caused by a double-pulse excitation with various orientations of pump polarization. In particular, we study write and erase sequences, showing that mutual interference of the effects caused by two laser pulses can occur if the pulses are temporally separated by less than 60 ps. This imposes the fundamental limit for the repetition rate of writing (20 GHz), but this could be increased even further by strengthening the photoinduced magnetic anisotropy.

Photoelectron emission from solids, as driven by high-intensity lasers, offers a platform for the coherent control of electron motion on ultrashort spatiotemporal scales. By solving the time-dependent Schrödinger equation, we present an exact analytical solution for the nonlinear ultrafast electron emission from a dc-biased metal surface illuminated by two-color laser fields. We systematically examine the combined effects of a dc electric field and two-color laser fields. In addition to the remarkable tunability of electron emission processes due to interference from two-color laser fields, we find that a strong dc electric field not only opens up more tunneling emission channels, but also introduces intense modulation to the emission current. We find surprising results that strong current modulation (with respect to the phase difference of the two-color lasers) persists (>70%), even with a large dc bias (i.e., ratio of the electric fields for dc : fundamental laser : second-harmonic laser $\cong 1:0.5:0.07$). In the meantime, the average emission current level increases by about three orders of magnitude relative to the case of zero dc bias. Application of our model to time-resolved photoelectron spectroscopy is exemplified and shows that the dynamics of the n-photon excited states depends strongly on the applied dc field. Our study suggests a practical way to maintain a strong modulation to high current photoemission, by the addition of a large dc bias for two-color laser-induced electron emission.

Many advanced solar cell concepts propose surpassing the Shockley-Queisser limit by introducing multiple quasi-Fermi-level separations that are arranged in series and/or in parallel. Exceeding the Shockley-Queisser limit with any parallel arrangement involves intermediate states that deliver additional charge carriers at, ideally, the same electrochemical potential as the other elements in the parallel network. This can be thought of as voltage matching individual parallel components, and in intermediate-band materials is intricately linked to solar concentration and étendue mismatch between absorption and emission. Generally, to achieve voltage matching under suboptimal conditions, an additional degree of freedom in the absorption thresholds of the material through a carrier relaxation or ratchet step is required. We explain why the ideal ratchet step decreases with solar concentration and how it depends on the radiative efficiency and emission étendue of the individual transitions and provide illustrative examples. The ideal ratchet step at 1 sun is largely given by the étendue mismatch between incoming sunlight and emitted light, which results in a step on the order of 270 meV that increases logarithmically with the ratio of luminescence-extraction efficiencies of the transitions. For solar cell concepts that use Auger-type carrier-carrier interactions or molecular triplet states for energetic up-conversion or down-conversion, the ideal band-gap combinations and achievable efficiencies also depend on interaction rates. We show that Auger-assisted solar cells suffer more strongly from finite interaction rates than carrier-multiplication devices.

Quantum processors rely on classical electronic controllers to manipulate and read out the state of quantum bits (qubits). As the performance of the quantum processor improves, nonidealities in the classical controller can become the performance bottleneck for the whole quantum computer. To prevent such limitation, this paper presents a systematic study of the impact of the classical electrical signals on the qubit fidelity. All operations, i.e., single-qubit rotations, two-qubit gates, and readout, are considered, in the presence of errors in the control electronics, such as static, dynamic, systematic, and random errors. Although the presented study could be extended to any qubit technology, it currently focuses on single-electron spin qubits, because of several advantages, such as purely electrical control and long coherence times, and for their potential for large-scale integration. As a result of this study, detailed electrical specifications for the classical control electronics for a given qubit fidelity can be derived. We also discuss how qubit fidelity is affected by the limited performance of the general-purpose room-temperature equipment typically employed to control the few qubits available today. Ultimately, we show that tailor-made electronic controllers can achieve significantly lower power, cost, and size, as required to support the scaling up of quantum computers.

The phase state of a parametric resonator provides important information about the nonlinear damping and the ability to employ feedback to achieve self-sustained oscillations. We derive the minimum nonlinear damping value required to enable phase control of Duffing parametric resonators and confirm this using tunable nonlinear micromechanical devices with nonlinear damping values below and above the critical value. For softening resonators with nonlinear damping values beyond the critical value, we demonstrate parametric phase control to experimentally operate on the unstable branches of the parametric response. Parametric phase locking is an alternate method for forming a timing reference from a micromechanical resonator, provides a sensitive technique for measuring the nonlinear parameters, and enables future closed-loop measurements of the dynamics of driven single and coupled parametric resonators.

We present a numerical study of a microelectromechanical-system-based design of a fiber cavity integrated with an ion-trap system. Each fiber mirror is supported by a microactuator that controls the mirror’s position in three dimensions. The mechanical stability is investigated by a feasibility analysis, which shows that the actuator offers stable support of the fiber. The actuators move the fibers’ positions continuously with a stroke of more than $10\mu \mathrm{m}$, with mechanical resonance frequencies on the order of kilohertz. A calculation of the trapping potential shows that a separation between the ion and the fiber consistent with strong ion-cavity coupling is feasible. Our miniaturized ion-photon interface constitutes a viable approach to integrated hardware for quantum information.

We develop a concept for a traveling-wave parametric amplifier driven by a magnetic flux wave. The circuit consists of a serial array of symmetric dc superconducting quantum-interference devices coupled inductively to a separate superconducting $LC$ transmission line carrying the pump wave. The adjusted phase velocity of the pump flux wave of frequency ${\omega }_p$ ensures amplification of the signal (${\omega }_s$) and idler (${\omega }_i$) waves, with frequencies obeying the relation ${\omega }_s+{\omega }_i={\omega }_p$. The advantage of the proposed flux-driven linear circuit includes a large gain over a wide frequency range and overcoming of the pump depletion problem. Unlike conventional traveling-wave amplifiers, the signal and pump in the proposed circuit are applied to different ports, what can greatly simplify the whole measurement setup. The experimental parameters and characteristics of this amplifier are evaluated and show promise for applications in quantum-information single-photon circuits.

Ambient seismic noise contains valuable information about the structural properties of Earth’s subsurface. The superconducting gravimeter (SG) is the most suitable instrument for temporal gravity, but not for microseisms due to its limited bandwidth. We report an alternative type of vertical superconducting accelerometer with a bandwidth as high as 0.7 Hz. Its sensitivity is comparable with the current SG. The accelerometer consists of a superconducting mass-spring oscillator and a displacement sensor based on a superconducting quantum interference device. The oscillator is constructed by levitating a Meisnner-state proof mass using combined positive-stiffness and negative-stiffness superconducting coils. Its natural frequency can be adjusted by the coil currents over a large range. The enhanced sensitivity of displacement sensing compensates the sensitivity loss imposed by the broadened bandwidth. Experimental tests show that both the gravity variation and the microseism can be extracted from the measurement data. It would be quite meaningful to monitor the temporal gravity and structural evolution using such an accelerometer array in sensitive zones, for example, the crustal fault and volcano zones.

Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed multiferroic tunnel junctions (MFTJs), can be achieved not only by the magnetic alignment of two ferromagnets, but also by the electric polarization of the ferroelectric interlayer; this provides great opportunities for next-generation multistate memory devices. Here, we show that a ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3(\mathrm{LSMO})/{\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3(\mathrm{PZT})/\mathrm{Co}$ structured MFTJ device can exhibit multilevel resistance states in the presence of gradually reversed ferroelectric domains via tunneling electroresistance and tunneling magnetoresistance, respectively. Nonvolatile ferroelectric control in the MFTJ can be attributed to separate contributions that arise from two independent ferroelectric channels in the PZT interlayer with opposite polarization. Our study shows the dominant role of “mixed” ferroelectric states on achieving accumulative electrical modulation of multilevel resistance states in MFTJs; thus paving the way for multifunctional device applications.