Tunneling Current in a Double Quantum Dot Driven by Two-Mode Microwave Photons

We propose a frequency-tunable source to emit entangled microwave photons on the platform of a superconducting circuit, in which two superconducting transmission-line resonators are coupled via a capacitor and one resonator is inserted with a superconducting quantum interference device (SQUID) in the center. By pumping the circuit appropriately with an external coherent microwave signal through the SQUID, microwave photons are emitted in pairs out of the circuit. The entanglement between the two modes is demonstrated by numerically calculating the second-order coherence function and the logarithmic negativity of the output microwave signals. Due to the tunability of SQUID’s equivalent inductance, the frequencies of the entangled microwave photons can be tuned by an external flux bias in situ. Our proposal paves a new way for obtaining entangled frequency-tunable two-mode microwave photons.

A quantum transducer, when working as a microwave and optical entanglement generator, provides a practical way to coherently connect optical communication channels and microwave quantum processors. Recent experiments on a quantum transducer verifying entanglement between the microwave and optical photons show the promise of approaching that goal. While flying optical photons can be efficiently controlled or detected, the microwave photon needs to be stored in a cavity or converted to the excitation of a superconducting qubit for further quantum operations. However, it remains challenging to efficiently capture or detect a single microwave photon with an arbitrary time profile. This work focuses on this challenge in the setting of an entanglement-based quantum transducer and proposes a solution by shaping the optical pump pulse. By Schmidt decomposing the output entangled state, we show that the microwave-optical photon pair takes a specific temporal profile that is controlled by the optical pump. The microwave photon from the transducer can be nearly perfectly absorbed by a receiving cavity with tunable coupling and is ready to be converted to the excitation of superconducting qubits, enabling further quantum operations. Published by the American Physical Society 2024

Microwave photonics with superconducting quantum circuits

Modern electronic and optoelectronic devices are approaching nanometric dimensions where microscopic details cannot be treated in an effective way. Atomistic approaches become necessary for modelling structural, electronic and optical properties of such nanostructured devices. On the other hand, theoretical developments and numerical optimizations make device modelling approachable by atomistic methods. The purpose of this review is to report on microscopic theories to describe these nanostructured semiconductor devices. Empirical and density functional tight-binding as well as pseudopotential approaches are applied to the study of organic and inorganic semiconductor nanostructures and nanostructured devices. We show how these microscopic methods overcome the limitations imposed by the simplified approaches based on envelope function approximations and in the meantime keep the computational cost low. Typical calculations are shown for one-, two- and three-dimensional confined nanostructured devices, and comparisons with other approaches are outlined.

Elastic scattering is one of the useful approach to control the transmission behavior of microwave photons transporting in microwave quantum networks without energy consumption. Therefore, it has practical significance for the development of microwave quantum devices and the construction of multi-node microwave quantum networks. In view of the existence of the same device, specifically the transmission line embedded by a single Josephson junction, could be described by different circuit models (the series and parallel ones), in this paper we first theoretically analyze the transporting feature for the microwave photons being scattered by the different elastic scattering model, described by either the series or the parallel embedding models, generated by a single LC loop and a nonlinear Josephson junction device, respectively. The classical microwave transport theory predicts that, the series LC loop and the parallel LC loop lead to different microwave photon elastic scattering behaviors, i.e., the series LC circuit yields the resonant reflection and the parallel LC circuit leading alternatively to the resonant transmission. Recently, the transport properties of microwave photons scattered by a Josephson junction embedded in a transmission line had been discussed, and the results suggested that the Josephson junction embedded in the transmission line should be described by a series embedding circuit, which implies the resonant reflection. We argue here that, if the Josephson junction is embedded in parallel in the transmission line, the elastically scattered microwave photons should be transmitted by resonant transmission. In order to test which of the above two different embedding circuit models, yielding the completely different elastic scattering behaviors, is physically correct, we then fabricated such a device, i.e., a single Joseph junction device embedded in a transmission line is prepared, and measured its elastic scattering transmission coefficient at extremely low temperature. The results are consistent with the expected effects of the parallel embedding circuit model, but conflicted with the behaviors predicted by the series embedding circuit model in the literature. Based on the above theoretical and experimental analysis on the elastic scattering of a single Josephson junction device, we further propose a scheme to control the elastic scattering behavior of microwave photons by modulating a DC superconducting quantum interference device with a bypass current, which could be applied to the construction of a microwave quantum network based on elastic scattering node controls.

A tunneling system with a molecule between the leads was analyzed in the framework of the adiabatic approach. The adiabatic approach allowed us to consider effects of equilibrium ion positions changing and vibrational mode frequency modification due to the electrons tunneling through the molecule localized states. It was demonstrated that vibrational states become squeezed if the tunneling current flows through the molecule. The mean-square displacement of the molecule and the degree of squeezing depend on the system parameters and could be tuned by applied bias voltage. Obtained results demonstrate the possibility of molecule ions' effective ``freezing'' by the tunneling current.

In this paper, we study tunneling current properties through SiO2 gate oxides in Si metal-oxide-semiconductor field-effect transistors (MOSFETs) by applying a first principles method based on the density-functional theory and nonequilibrium Green’s function approach. We employed three structural models of SiO2 layers, which are β-quartz, β-cristobalite, and β-tridymite. As a result, we found that the β-cristobalite and β-tridymite models indicate similar tunneling current properties, while the β-quartz model predicts a substantially lower tunneling current. Further, the largest tunneling current is obtained for the β-tridymite SiO2 model, which is consistent with bandstructure parameters estimated for bulk SiO2 crystals. Therefore, electronic properties of bulk SiO2 crystals can still be important for tunneling current analysis in the nanoscale range of oxide thickness.

Recently, to improve the performance of an integrated metal-oxide-semiconductor (MOS) device, an attempt has been made in the industry to replace the amorphous oxide with a crystalline oxide. However, various characteristics caused by the difference between amorphous and crystalline oxide in the MOS structure have not been systematically investigated. Therefore, we demonstrate the difference in atomic interface structures, electronic structures, and tunneling properties concerning varied oxide phases in a representative system, Si/SiO2/Si structures, with sub-3 nm-thick silica from first-principles. We investigate two oxide phases of amorphous (a-) and crystalline (c-) SiO2 with and without H passivation at the interface. Si/a-SiO2 exhibits a smooth interface layer, whereas Si/c-SiO2 exhibits an abrupt interface layer, resulting in the thicker interface layer of Si/a-SiO2 than Si/c-SiO2. Thus for a given total silica thickness, the adequate tunneling-blocking thickness, where all the Si atoms form four Si–O bonds, is thinner in a-SiO2 than c-SiO2, originating more tunneling current through a-SiO2 than c-SiO2. However, the effects of dangling bonds at Si/c-SiO2 rather than Si/a-SiO2 on tunneling currents are crucial, particularly in valence bands. Furthermore, when the dangling bonds are excluded by H atoms at Si/c-SiO2, the tunneling current dramatically reduces, whereas the H-passivation effect on the tunneling blocking at Si/a-SiO2 is insignificant. Our study contributes systematic knowledge regarding oxide phases and interfaces to promote for high performance of MOS devices.

High-efficiency single-photon detection in the microwave domain is a key enabling technology for various quantum applications. However, the extremely low energy of microwave photons presents a fundamental challenge, preventing direct photon-to-charge conversion as achieved in optical systems using semiconductors. Here, we demonstrate continuous microwave photon detection with an efficiency approaching 70% in the single-photon regime. We use a hybrid system comprising a gate-defined double quantum dot (DQD) charge qubit in a gallium arsenide/aluminum gallium arsenide heterostructure, coupled to a high-impedance Josephson junction array cavity. We systematically optimize the hybrid architecture to maximize the detection efficiency by leveraging strong charge-photon coupling, tunable DQD tunnel rates, and the frequency tunability of both subsystems. The system efficiency is characterized over a frequency range of 3 to 5.2 gigahertz. Our results establish semiconductor-based cavity–quantum electrodynamics architectures as a scalable and versatile platform for efficient microwave photon detection, opening promising avenues for quantum microwave optics and quantum information technologies.

Towards nanoscale magnetic memory elements : fabrication and properties of sub - 100 nm magnetic tunnel junctions

Photoassisted scanning tunneling microscopy was used to investigate photoinduced currents in a dye-sensitized nanoporous ${\mathrm{TiO}}_{2}$ network in a locally resolved experiment. The light-induced tunneling current (LITC) was studied with respect to its dependence on the modulation frequency of the exciting light as well as on the externally applied bias. By this, two main contributions to the LITC were identified and assigned to both a tunneling current of photoelectrons from the ${\mathrm{TiO}}_{2}$ conduction band to the tip and a tunneling current driven by a photoinduced change of the voltage drop over the tunneling gap. Additionally, the observed frequency dependence of the LITC components is in agreement with the time scales expected for a hopping transport via localized energy states. Lateral variations in the LITC signal are found between aggregates of ${\mathrm{TiO}}_{2}$ particles, directly reflecting different electronic properties. These results might be important for further optimization of porous materials in applications such as dye-sensitized solar cells.

Microwave photons have become very important qubits in quantum communication as the first quantum satellite has been launched successfully. Therefore, it is a necessary and meaningful task for ensuring the high security and efficiency of microwave-based quantum communication in practice. Here, we present an original polarization entanglement purification protocol for nonlocal microwave photons based on the cross-Kerr effect in circuit quantum electrodynamics (QED). Our protocol can solve the problem that the purity of maximally entangled states used for constructing quantum channels will decrease due to decoherence from environment noise. This task is accomplished by means of the polarization parity-check quantum nondemolition (QND) detector, the bit-flipping operation, and the linear microwave elements. The QND detector is composed of several cross-Kerr effect systems which can be realized by coupling two superconducting transmission line resonators to a superconducting molecule with the N-type level structure. We give the applicable experimental parameters of QND measurement system in circuit QED and analyze the fidelities. Our protocol has good applications in long-distance quantum communication assisted by microwave photons in the future, such as satellite quantum communication.

We theoretically investigate charge transport through serial double quantum dots (SDQDs) with strong electron correlations using nonequilibrium Green's function techniques. In the linear response regime, we compute the charge stability diagram and analyze the Coulomb oscillatory tunneling current, revealing both thermal and nonthermal broadening effects on the current spectra in relation to two gate voltages. In the nonlinear response regime, we focus on tunneling currents in SDQDs under the Pauli spin blockade (PSB) scenario. We find that current rectification with negative differential conductance is significantly degraded as temperature increases, making it challenging to distinguish between the inter-site spin triplet and singlet states. Notably, we observe a robust reversed tunneling current that remains stable against temperature variations, provided the resonant channel in the PSB scenario is coupled to the states of the right (left) electrode, which is fully occupied (unoccupied) by particles. This characteristic provides valuable insights for designing transistors capable of operating over a wide temperature range.

During scanning of Si(111)7×7 the tunneling current has been determined at every tip position as a function of bias voltage keeping the distance between tip and sample at a constant value by stabilization of the tunneling current for only a small time interval. For positive (2 V) and negative (−2 V) stabilization voltage we have obtained two sets of current images. Our results show that the hitherto neglected energy-dependent transmission probability on the tunneling current and the intimate relation between electronic and topographic properties prevent a simple determination of the local density of states.

Quantum measurements disturb the quantum system being measured, and this is known as measurement-induced backaction. In this work, we consider a double quantum dot monitored by a nearby quantum point contact where the measurement-induced backaction plays an important role. Taking advantage of the quantum master equation approach, we calculate the tunnelling current, and propose a simple feedbackcontrol law to realize and stabilize the tunnelling current. Theoretical analysis and numerical simulations show that the feedback control law can make the current quickly convergent to the desired value.