Coupled Quantum Research Articles

Thermal Entanglement and Correlated Coherence in Two Coupled Double Quantum Dots Systems

AbstractIn this work, the thermal quantum correlations in two coupled double semiconductor charge qubits are investigated. This is carried out by deriving analytical expressions for both the thermal concurrence and the correlated coherence. The effects of the tunneling parameters, the Coulomb interaction, and the temperature on the thermal entanglement and on the correlated coherence are studied in detail. It is found that the Coulomb potential plays an important role in the thermal entanglement and in the correlated coherence of the system. The results also indicate that the Coulomb potential can be used for significant enhancement of the thermal entanglement and quantum coherence. One interesting aspect is that the correlated coherence capture all the thermal entanglement at low temperatures, that is, the local coherences are totally transferred to the thermal entanglement. Finally, the role played by thermal entanglement and the correlated coherence responsible for quantum correlations are focused on. It is shown that in all cases, the correlated coherence is more robust than the thermal entanglement so that quantum algorithms based only on correlated coherence may be more robust than those based on entanglement.

Coupled Cavity Mid-IR Quantum Cascade Lasers Fabricated by Dry Etching

In this work, two-section, coupled cavity, mid-IR quantum cascade lasers (QCLs) were characterized in terms of their tuning range and emission stability under operation towards potential application in detection systems. Devices were processed by inductively coupled plasma reactive ion etching (ICP-RIE) from InP-based heterostructure, designed for emission in the 9.x micrometer range. Single mode devices were demonstrated with a better than 20 dB side mode suppression ratio (SMRS). The fabrication method resulted in improved yield, as well as high repeatability of individual devices. Continuous, mode-hop-free tuning of emission wavelength was observed across ~4.5 cm−1 for the range of temperatures of the heat sink from 15 °C to 70 °C. Using the thermal perturbation in the lasing cavity, in conjunction with controlled hopping between coupled-cavity (CC) modes, we were able to accomplish tuning over the range of up to ~20 cm−1.

Effect of barrier width between GaAs/InGaAs/GaAs double coupled quantum wells on bipolar transport and terahertz radiation by hot carriers in lateral electric field

The dependences of the current and integral intraband terahertz electroluminescence intensity on the electric field in the n-InGaAs/GaAs heterostructures with asymmetric double tunnel-coupled quantum wells under the conditions of bipolar lateral transport are established to differ qualitatively for different interwell barrier widths. In the case of the thick (∼50 Å) barriers at electric fields less than those corresponding to the Gunn instability, high-frequency current oscillations are observed, and the emission intensity monotonously increases with increasing electric field strength over the entire studied range up to 3 kV/cm. At small barrier widths (∼30 Å), the current oscillations are practically absent, and the emission intensity substantially increases with the field strength between 1.5 and 2 kV/cm. The emission intensity increase is explained by the addition of direct electron and hole transitions between the size-quantization subbands to the indirect intrasubband transitions. The observed differences may be explained by the different relationships between the interband radiative recombination time of the minority charge carriers (holes) in the narrow wells and the hole tunneling time into the wide wells, dependent on the interwell barrier width.

Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System

A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit-QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.

Light-mediated strong coupling between a mechanical oscillator and atomic spins 1 meter apart.

Engineering strong interactions between quantum systems is essential for many phenomena of quantum physics and technology. Typically, strong coupling relies on short-range forces or on placing the systems in high-quality electromagnetic resonators, which restricts the range of the coupling to small distances. We used a free-space laser beam to strongly couple a collective atomic spin and a micromechanical membrane over a distance of 1 meter in a room-temperature environment. The coupling is highly tunable and allows the observation of normal-mode splitting, coherent energy exchange oscillations, two-mode thermal noise squeezing, and dissipative coupling. Our approach to engineering coherent long-distance interactions with light makes it possible to couple very different systems in a modular way, opening up a range of opportunities for quantum control and coherent feedback networks.

(Invited) Coupled Quantum Defects of DNA Functionalized Carbon Nanotubes

Single wall carbon nanotubes (SWCNTs) with sp3 defects created via covalent bonding of organic functional groups are rapidly emerging as a new material system for chemistry, physics, material science, engineering and quantum technologies. They are often referred to as “organic color centers” as they can mimic quantum mechanical properties of solid-state color centers.1 Their unique properties include highly enhanced molecularly wavelength-tunable fluorescence and room temperature single photon generation with 99% single photon purity at 1.55 µm telecommunication wavelength.2 To date, diazonium chemistry has been the chemists’ tool of the choice for creating sp3 defects sparsely distributed at random positions along the sidewall of SWCNTs. While such distribution is ideally suited for single photon generation, an ability to incorporate multiple coupled quantum defects in a well-defined spatial pattern is highly desirable for more sophisticated quantum information science applications involving entanglement. Recent discovery of covalently functionalizing SWCNTs with single-stranded DNA (ssDNA)3 open a possible path toward achieving this goal. In this process, SWCNTs form covalent bonds to the guanine nucleotides when aqueous suspensions of SWCNTs coated with ssDNA are exposed to singlet oxygen. As a result, creating a regular array of covalent defect with spacing defined by the ssDNA sequence become a possibility. To further explore this exciting possibility, we investigated intrinsic PL emission properties of ssDNA functionalized SWCNTs through low temperature single tube PL spectroscopy. In contrast to individual aryl functionalized SWCNTs that usually emit a single sharp isolated PL peaks, multiple PL peaks with their emission wavelengths varying from 1050 to 1300 nm were observed from individual ssDNA functinalized SWCNTs. We further conducted 2nd order photon correlation (g(2)) and cross-correlation experiments to investigate their quantum light emission properties. We observed photon anti-bunching behavior when an individual PL emission peak is spectrally isolated indicating that ssDNA functionalization creates a series of quantum defects along the length of SWCNTs. Interestingly, when the g(2) experiment is performed on combined emission of multiple (2—4) PL peaks, an incomplete antibunching with g(2)(0) values in the range of 0.2-0.4 is observed. Because these values are significantly smaller than g(2)(0) values estimated for 2-4 independent quantum emitters (0.5-0.75), we interpret that the series of defects created via ssDNA functionalization are coupled. Our photon cross-correlation experiment performed between two emission peaks of the group also reveal evidence of coupling. 1 Srinivasan, K. & Zheng, M. Nat. Photon. 11, 535 (2017). 2 Brozena, A.H., Kim, M., Powell, L.R. & Wang, Y. Nat. Rev. Chem. 3, 375-392 (2019). 3 Zheng, Y., Bachilo, S. M., Weisman, R. B. ACS Nano, 13, 8222 (2019)

Graphene-Interface with Electrogenic Bacterial Membrane: Electron Transport and Energetics

A synergistic, nanoscale electrical-interface with the membranes of exoelectrogenic microbes will have transformative impact on biological cell based electronic-devices. In this presentation, we will report a conformal graphenic interface on biocatalytic Geobacter sulfurreducens membrane that results in quantum-capacitance induced n-doping in graphene. This further enhances electron shuttling from the membrane to improve electron harvesting from the electrogenic membrane. The quantum coupling of reduced graphene oxide (rGO) with the connected protein-membrane channels leads to an additional electron density of 3.44 x 1012 cm-2 and an increase in the in-plane phonon vibration energies (G) of rGO by 5 cm-1. This n-doping enhances the electron transfer-rate from the cell membrane into the rGO improving the power density of a simplistic microbial fuel cell (MFC) by ~ 2 folds. The synergistic electron-harvesting and conformal membrane-interfacing of flexible 2D nanomaterials can lead to an evolution in the design of microbe-circuitry to power stand-alone nanodevices. Figure 1

Quantum simulation of quantum relativistic diffusion via quantum walks

Two models are first presented, of a one-dimensional discrete-time quantum walk (DTQW) with temporal noise on the internal degree of freedom (i.e., the coin): (i) a model with both a coin-flip and a phase-flip channel, and (ii) a model with random coin unitaries. It is then shown that both these models admit a common limit in the spacetime continuum, namely, a Lindblad equation with Dirac-fermion Hamiltonian part and, as Lindblad jumps, a chirality flip and a chirality-dependent phase flip, which are two of the three standard error channels for a two-level quantum system. This, as one may call it, Dirac Lindblad equation, provides a model of quantum relativistic spatial diffusion, which is evidenced both analytically and numerically. This model of spatial diffusion has the intriguing specificity of making sense only with original unitary models which are relativistic in the sense that they have chirality, on which the noise is introduced: the diffusion arises via the by-construction (quantum) coupling of chirality to the position. For a particle with vanishing mass, the model of quantum relativistic diffusion introduced in the present work, reduces to the well-known telegraph equation, which yields propagation at short times, diffusion at long times, and exhibits no quantumness. Finally, the results are extended to temporal noises which depend smoothly on position.

Topological and nontopological photonic states in two coupled circuit quantum electrodynamics chains

We investigate the topological and nontopological photonic states induced by the interchain coupling strength in a one-dimensional coupled circuit quantum electrodynamics chains system. In the case of strong coupling strength, we find that the original topologically nontrivial interface states of the system gradually disappear and finally become two nontopological impurity states. Meanwhile, two new topologically nontrivial interface states appear next to the two middle resonators. The processes can be clearly delineated by the energy eigenvalue spectrum and the state distributions of the system. Moreover, we present the phase diagram to illustrate the characteristics of phase transition of the system. Assisted by the lattice-based cavity input-output process, we can measure these photonic states, which are either localized or extended, in the steady state condition. Our scheme would be feasible for experimental implementation of topological and nontopological photonic states with the current physical system.

Defining the topological influencers and predictive principles to engineer the band structure of halide perovskites

Complex quantum coupling phenomena of halide perovskites are examined through ab-initio calculations and exact diagonalization of model Hamiltonians to formulate a set of fundamental guiding rules to engineer the bandgap through strain. The bandgap tuning in halides is crucial for photovoltaic applications and for establishing non-trivial electronic states. Using CsSnI$_3$ as the prototype material, we show that in the cubic phase, the bandgap reduces irrespective of the nature of strain. However, for the tetragonal phase, it reduces with tensile strain and increases with compressive strain, while the reverse is the case for the orthorhombic phase. The reduction can give rise to negative bandgap in the cubic and tetragonal phases leading to normal to topological insulator phase transition. Also, these halides tend to form a stability plateau in a space spanned by strain and octahedral rotation. In this plateau, with negligible cost to the total energy, the bandgap can be varied in a range of 1eV. Furthermore, we present a descriptor model for the perovskite to simulate their bandgap with strain and rotation. Analysis of band topology through model Hamiltonians led to the conceptualization of topological influencers that provide a quantitative measure of the contribution of each chemical bonding towards establishing a normal or topological insulator phase. On the technical aspect, we show that a four orbital based basis set (Sn-$\{s,p\}$ for CsSnI$_3$) is sufficient to construct the model Hamiltonian which can explain the electronic structure of each polymorph of halide perovskites.

Chiral quantum optics in photonic sawtooth lattices

Chiral quantum optics has become a burgeoning field due to its potential applications in quantum networks or quantum simulation of many-body physics. Current implementations are based on the interplay between local polarization and propagation direction of light in nanophotonic structures. In this manuscript, we propose an alternative platform based on coupling quantum emitters to a photonic \emph{sawtooth} lattice, a one-dimensional model with an effective flux per plaquette introduced by complex tunnelings. We study the dynamics emerging from such structured photonic bath and find the conditions to obtain quasi-perfect directional emission when the emitters are resonant with the band. In addition, we find that the photons in this bath can also mediate complex emitter-emitter interactions tunable in range and phase when the emitters transition frequencies lie within a band-gap. Since these effects do not rely on polarization they can be observed in platforms beyond nanophotonics such as matter-waves or circuit QED ones, of which we discuss a possible implementation.

Ligand engineering of colloid quantum dots and their application in all-inorganic tandem solar cells

How to effectively utilize the energy of the broad spectrum of sunlight is one of the basic problems in the research of tandem solar cells. Due to their size effect, quantum confinement effect and coupling effect, colloidal quantum dots (QDs) exhibit new physical properties that bulk materials don't possess. CdX (X = Se, S, etc.) and PbX (X = Se, S, etc.) QDs prepared by hot-injection methods have been widely studied in the areas of photovolitaic devices. However, the surfactants surrounding QDs seriously hinder the charge transport of QDs based solar cells. Therefore, how to fabricate high-performance tandem solar cells via ligands engineering has become a major challenge. In this paper, the latest progress of colloidal QDs in the research of all-inorganic tandem solar cells was summarized. Firstly, the improvement of QDs surface ligands and the optimization of ligands engineering were discussed, and the control of the physical properties of QDs films were realized. From the aspects of colloidal QDs, ligand engineering, and solar cell preparation, the future development direction of colloidal QDs solar cells was proposed, providing technical guidances for the preparation of low-cost and high-efficiency nanocrystalline solar cells. Ligands engineering can passivate the surface of the quantum dot film while maintaining its conductivity, and effectively improve the performance of tandem solar cell.

Quantum Acoustomechanics with a Micromagnet.

We show theoretically how to strongly couple the center-of-mass motion of a micromagnet in a harmonic potential to one of its acoustic phononic modes. The coupling is induced by a combination of an oscillating magnetic field gradient and a static homogeneous magnetic field. The former parametrically couples the center-of-mass motion to a magnonic mode while the latter tunes the magnonic mode in resonance with a given acoustic phononic mode. The magnetic fields can be adjusted to either cool the center-of-mass motion to the ground state or to enter into the strong quantum coupling regime. The center of mass can thus be used to probe and manipulate an acoustic mode, thereby opening new possibilities for out-of-equilibrium quantum mesoscopic physics. Our results hold for experimentally feasible parameters and apply to levitated micromagnets as well as micromagnets deposited on a clamped nanomechanical oscillator.

Stationary states of weak coupling limit-type Markov generators and quantum transport models

We prove that every stationary state in the annihilator of all Kraus operators of a weak coupling limit-type Markov generator consists of two pieces, one of them supported on the interaction-free subspace and the second one on its orthogonal complement. In particular, we apply the previous result to describe in detail the structure of a slightly modified quantum transport model due to Arefeva et al. (modified AKV’s model) studied first in [J. C. García et al., Entangled and dark stationary states of excitation energy transport models in many-particles systems and photosynthesis, Infin. Dimens. Anal. Quantum Probab. Relat. Top. 21(3) (2018), Article ID: 1850018, p. 21, doi:10.1142/S0219025718500182], in terms of generalized annihilation and creation operators.

Enhanced Entanglement in Hybrid Cavity Mediated by a Two-way Coupled Quantum Dot

AbstractWe investigate theoretically the entanglement in a hybrid Fabry-Perot cavity system. A membrane in the cavity acts as a mechanical resonator, and a two-level quantum dot is coupled to both the cavity mode and the mechanical resonator. The entanglements between the cavity field and the mechanical resonator, between the mechanical resonator and the quantum dot, as well as between the cavity field and the quantum dot are observed. The logarithmic negativities in the first two subsystems are much larger than those in the system without two-way coupled quantum dot, and the entanglements are robust against the thermal temperature (entanglements still exist in tens of Kelvin). We also find that without direct coupling between the cavity field and the mechanical resonator, one can till observe effective entanglement between them in our system. Our work is helpful and may have potential applications in the research of multipartite entanglement in physical system.

Unidirectional, Ultrafast, and Bright Spontaneous Emission Source Enabled By a Hybrid Plasmonic Nanoantenna

AbstractInefficient and wide‐angle emission as well as low emission rate of optical nanoemitters (such as quantum dots) have been strongly limiting their practical applications in next‐generation nanophotonic devices, such as nanoscale light‐emitting diodes (LEDs) and on‐chip single photon sources. Optical nanoantennas provide a promising way to deal with these challenges. Yet, there has been no solution that can overcome these drawbacks simultaneously on a single device. Here, a hybrid plasmonic nanoantenna consisting of a silver nanocube positioned at the center of a gold concentric‐ring structure is proposed, which can simultaneously enhance the emission directionality and decay rate of quantum dots embedded in the nanogap beneath the nanocube while maintaining high quantum efficiency. Coupling quantum dots to this nanoantenna can result in 60% of the emitted photons collected by the first lens with a numerical aperture (NA) of 0.5 and 21% for a NA of 0.12. The total emission intensity and decay rate are enhanced by 121‐fold and 424‐fold, respectively, compared with quantum dots on a glass substrate. A high quantum efficiency above 50% is obtained in simulation. This novel platform can be applied to enhance various types of optical nanoemitters and to develop high‐speed directional nano‐LEDs and single photon sources.

Enhanced and Polarization-Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

Here we report the fabrication and optical characterization of organic microcavities containing liquid-crystalline conjugated polymers (LCCPs): poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), poly(9,9-dioctylfluorene) (PFO) and poly(2,7-(9,9-dihexylfluorene)-co-bithiophene) (F6T2) aligned on top of a thin transparent Sulfuric Dye 1 (SD1) photoalignment layer. We extract the optical constants of the aligned films using variable angle spectroscopic ellipsometry and fabricate metallic microcavities in which the ultrastrong coupling regime is manifest both for the aligned and non-aligned LCCPs. Transition dipole moment alignment enables a systematic increase in the interaction strength, with unprecedented solid-state Rabi splitting energies up to 1.80 eV for F6T2, the first to reach the visible spectrum; with an optical gap of 2.79 eV this also gives the highest-to-date organic microcavity coupling ratio, 65$\%$. We also demonstrate that the coupling strength is polarization-dependent with bright polaritons photoluminescence for TE polarization parallel to the transition dipoles and either no emission or weakly coupled emission from the corresponding TM polarization. The use of uniaxally aligned organic microcavities with switchable coupling strength offers exciting prospects for direct observations of ultrastrong coupling signatures, quantum simulation, polaritonics and condensation related phenomena.

Coherent acoustic control of a single silicon vacancy spin in diamond

Phonons are considered to be universal quantum transducers due to their ability to couple to a wide variety of quantum systems. Among these systems, solid-state point defect spins are known for being long-lived optically accessible quantum memories. Recently, it has been shown that inversion-symmetric defects in diamond, such as the negatively charged silicon vacancy center (SiV), feature spin qubits that are highly susceptible to strain. Here, we leverage this strain response to achieve coherent and low-power acoustic control of a single SiV spin, and perform acoustically driven Ramsey interferometry of a single spin. Our results demonstrate an efficient method of spin control for these systems, offering a path towards strong spin-phonon coupling and phonon-mediated hybrid quantum systems.

Advancements in high refractive index media: from quantum coherence in atomic systems to deep sub-wavelength coupling in metamaterials [Invited

Refractive index enhancement is crucial in the field of lithography, imaging, optical communications, solar devices and many more. We present a review of advancements in the process of designing high refractive index metamaterials, starting from quantum coupling and photonic bandgap materials to metamaterials utilizing deep subwavelength coupling to achieve ever-high values of refractive index. A crisp and critical impression of each scheme are presented. The understanding of evolution of material design from intrinsic electronic states manipulation to meta-atoms design is not only fascinating but prerequisite to developing successful devices and applications.

Impact of QW coupling on the binding energy in InGaN/GaN under the effects of the size, the impurity and the internal composition

In the present paper, the binding energy of hydrogenic shallow-donor impurity in simple and double coupled quantum wells based on unstrained wurtzite (In,Ga)N/GaN is investigated. Considering the effective-mass and dielectric mismatches between the well and its surrounding matrix, the numerical calculations are performed within the framework of the parabolic band and the single band effective-mass approximations under the finite potential barrier using finite element method (FEM). According to our results, it appears that the main effect of the wells coupling is to enhance the binding energy. It is also obtained that the binding energy is strongly sensitive to the internal and external parameters and can be adjusted by the quantum well/barrier width, the impurity position and the internal Indium composition. Our results are in good agreement with the finding especially for those obtained by the variational approach.