Coherent Exchange Research Articles

Symmetry-protected collisions between strongly interacting photons.

Realizing robust quantum phenomena in strongly interacting systems is one of the central challenges in modern physical science. Approaches ranging from topological protection to quantum error correction are currently being explored across many different experimental platforms, including electrons in condensed-matter systems, trapped atoms and photons. Although photon-photon interactions are typically negligible in conventional optical media, strong interactions between individual photons have recently been engineered in several systems. Here, using coherent coupling between light and Rydberg excitations in an ultracold atomic gas, we demonstrate a controlled and coherent exchange collision between two photons that is accompanied by a π/2 phase shift. The effect is robust in that the value of the phase shift is determined by the interaction symmetry rather than the precise experimental parameters, and in that it occurs under conditions where photon absorption is minimal. The measured phase shift of 0.48(3)π is in excellent agreement with a theoretical model. These observations open a route to realizing robust single-photon switches and all-optical quantum logic gates, and to exploring novel quantum many-body phenomena with strongly interacting photons.

Vibration-assisted coherent excitation energy transfer in a detuned dimer**Project supported by the National Natural Science Foundation of China (Grant No. 11174233).

The important role of high-energy intramolecular vibrational modes for excitation energy transfer in the detuned photosynthetic systems is studied. Based on a basic dimer model which consists of two two-level systems (pigments) coupled to high-energy vibrational modes, we find that the high-energy intramolecular vibrational modes can enhance the energy transfer with new coherent transfer channels being opened when the phonon energy matches the detuning between the two pigments. As a result, the energy can be effectively transferred into the acceptor. The effective Hamiltonian is obtained to reveal the strong coherent energy exchange among the donor, the acceptor, and the high-energy intramolecular. A semi-classical explanation of the phonon-assisted mechanism is also shown.

Classical Stückelberg interferometry of a nanomechanical two-mode system

The transition from classical to quantum mechanics has intrigued scientists in the past and remains one of the most fundamental conceptual challenges in state-of-the-art physics. Beyond the quantum mechanical correspondence principle, quantum-classical analogies have attracted considerable interest. In this work, we present classical two-mode interference for a nanomechanical two-mode system, realizing classical St\"uckelberg interferometry. In the past, St\"uckelberg interferometry has been investigated exclusively in quantum mechanical two-level systems. Here, we experimentally demonstrate a classical analog of St\"uckelberg interferometry taking advantage of coherent energy exchange between two-strongly coupled, high quality factor nanomechanical resonator modes. Furthermore, we provide an exact theoretical solution for the double passage St\"uckelberg problem which reveals the analogy of the return probabilities in the quantum mechanical and the classical version of the problem. This result qualifies classical two-mode systems at large as a testbed for quantum mechanical interferometry.

Modified relaxation dynamics and coherent energy exchange in coupled vibration-cavity polaritons.

Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity. In this work, we report pump–probe infrared spectroscopy of the cavity-coupled C–O stretching band of W(CO)6 and the direct measurement of the lifetime of a vibration-cavity polariton. The upper polariton relaxes 10 times more quickly than the uncoupled vibrational mode. Tuning the polariton energy changes the polariton transient spectra and relaxation times. We also observe quantum beats, so-called vacuum Rabi oscillations, between the upper and lower vibration-cavity polaritons. In addition to establishing that coupling to an optical cavity modifies the energy-transfer dynamics of the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems.

Quantum melting of two-component Rydberg crystals

We investigate the quantum melting of one dimensional crystals that are realized in an atomic lattice in which ground state atoms are laser excited to two Rydberg states. We focus on a regime where both, intra- and inter-state density-density interactions as well as coherent exchange interactions contribute. We determine stable crystalline phases in the classical limit and explore their melting under quantum fluctuations introduced by the excitation laser as well as two-body exchange. We find that within a specific parameter range quantum fluctuations introduced by the laser can give rise to a devil's staircase structure which one might associate with transitions in the classical limit. The melting through exchange interactions is shown to also proceed in a step-like fashion, in case of small crystals, due to the proliferation of Rydberg spinwaves.

Demonstration of a Coherent Electronic Spin Cluster in Diamond.

An obstacle for spin-based quantum sensors is magnetic noise due to proximal spins. However, a cluster of such spins can become an asset, if it can be controlled. Here, we polarize and readout a cluster of three nitrogen electron spins coupled to a single nitrogen-vacancy spin in diamond. We further achieve sub-nm localization of the cluster spins. Finally, we demonstrate coherent spin exchange between the species by simultaneous dressing of the nitrogen-vacancy and the nitrogen states. These results establish the feasibility of environment-assisted sensing and quantum simulations with diamond spins.

Plasmon-Exciton Coupling Using DNA Templates.

Coherent energy exchange between plasmons and excitons is a phenomenon that arises in the strong coupling regime resulting in distinct hybrid states. The DNA-origami technique provides an ideal framework to custom-tune plasmon-exciton nanostructures. By employing this well controlled self-assembly process, we realized hybrid states by precisely positioning metallic nanoparticles in a defined spatial arrangement with fixed nanometer-sized interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60 nm while keeping their separation distance constant allowed us to precisely adjust the plasmon resonance of the structure to accurately match the energy frequency of a J-aggregate exciton. With this system we obtained strong plasmon-exciton coupling and studied far-field scattering at the single-structure level. The individual structures displayed normal mode splitting up to 170 meV. The plasmon tunability and the strong field confinement attained with nanodimers on DNA-origami renders an ideal tool to bottom-up assembly plasmon-exciton systems operating at room temperature.

Quantum simulation with a boson sampling circuit

In this work we study a system that consists of $2M$ matter qubits that interact through a boson sampling circuit, i.e., an $M$-port interferometer, embedded in two different architectures. We prove that, under the conditions required to derive a master equation, the qubits evolve according to effective bipartite XY spin Hamiltonians, with or without local and collective dissipation terms. This opens the door to the simulation of any bipartite spin or hard-core boson models and exploring dissipative phase transitions as the competition between coherent and incoherent exchange of excitations. We also show that in the purely dissipative regime this model has a large number of exact and approximate dark states, whose structure and decay rates can be estimated analytically. We finally argue that this system may be used for the adiabatic preparation of boson sampling states encoded in the matter qubits.

Non-Markovian discrete qubit dynamics

The study of open quantum systems is important for fundamental issues of quantum physics as well as for technological applications such as quantum information processing. The interaction of a quantum system with its environment is usually detrimental for the quantum properties of the system and leads to decoherence. However, sometimes a quantum control can lead to a coherent partial exchange of information between the system and the dynamics of the open system might become non-Markovian. In this article, we study experimentally discrete non-Markovian open quantum system dynamics. We implement a local control protocol using linear optics for controlling the information flow between the open system and the environment. We show how the transition from Markovian to non-Markovian dynamics can be controlled using only local operations for the open system.

Bidirectional conversion between microwave and light via ferromagnetic magnons

Coherent conversion of microwave and optical photons in the single-quantum level can significantly expand our ability to process signals in various fields. Efficient up-conversion of a feeble signal in the microwave domain to the optical domain will lead to quantum-noise-limited microwave amplifiers. Coherent exchange between optical photons and microwave photons will also be a stepping stone to realize long-distance quantum communication. Here we demonstrate bidirectional and coherent conversion between microwave and light using collective spin excitations in a ferromagnet. The converter consists of two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode called Kittel mode, where microwave photons and magnons in the respective modes are strongly coupled and hybridized. An itinerant microwave field and a travelling optical field can be coupled through the hybrid system, where the microwave field is coupled to the hybrid system through the cavity mode, while the optical field addresses the hybrid system through the Kittel mode via Faraday and inverse Faraday effects. The conversion efficiency is theoretically analyzed and experimentally evaluated. The possible schemes for improving the efficiency are also discussed.

Coherent Momentum Exchange above and within a Scots Pine Forest

Biorthogonal decomposition (BOD) is used to detect and study synchronous coherent structures occurring at multiple levels in the vertical momentum flux (u′w′) within and above a planted Scots pine forest during a 12-week continuous measurement period. In this study, the presented method allowed for the simultaneous detection and quantification of the number of coherent structures (N), their duration (D) and separation (S) at five measurement heights (z1–z5) covering the range z1/h = 0.11 to z5/h = 1.67, with h being the mean stand height at the measurement site. Results presented for five different exchange regimes (C1–C5) and for four different atmospheric stability conditions (stable, transition to stable, near-neutral, forced convection) demonstrate that during the measurement period, above-canopy momentum flux was only to a limited extent involved in the evolution of spatiotemporal momentum flux patterns found within the below-canopy space. Fully-coupled turbulent momentum exchange over the investigated height range occurred during 19% of all analyzed half-hourly datasets. Across the analyzed exchange regimes, the median contribution of strong sweeps and ejections to total momentum transfer above the canopy varied between 30% and 39% while covering 28%–32% of the time. In the below-canopy space, the contribution of coherent structures varied between 19% and 21% while covering the same amount of time. This suggests that momentum transfer through synchronous coherent structures is very efficient above the forest canopy, but attenuated in the below-canopy space. Since the majority of the presented results agrees well with the results from previous studies that analyzed coherent structures at single levels, the BOD is a promising tool for the consistent investigation of synchronous coherent structures at multiple measurement heights.

Counterfactual entanglement distribution using quantum dot spins

Counterfactual quantum communication allows distant users to share a message without any physical particles traveling through the quantum channel. Here, we present a counterfactual scheme to realize multiparty entanglement distribution. We first realize bipartite counterfactual entanglement distribution to create photon–electron entanglement by using interaction-free measurements and quantum dots. Then the atom–cavity systems are used to create many photon–photon entanglement pairs, which can connect photon–electron pairs with the help of an entanglement beam splitter. Consequently, the electron spins in distant optical microcavities are entangled by the coherent exchange of single flying photons. We also discuss the implementation issues to show that the presented scheme can be realized successfully in experiment. The numerical analysis about the efficiency of multiparty entanglement distribution indicates that there is a trade-off between the entanglement fidelity and the number of users.

Exciton-Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles.

Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.

Vector gain-guided dissipative solitons in a net normal dispersive fiber laser

We report on the experimental observation of spontaneous vector dissipative soliton train formation in a dispersion-managed cavity fiber laser with net normal cavity dispersion. The formation of the dissipative solitons is triggered by the nonlinearity-induced wave mixing and gain bandwidth limitation. So, they are named the gain-guided dissipative solitons to differentiate from those of the dissipative solitons formed in the mode-locked fiber lasers. The vector dissipative solitons are phase locked, and the coherent energy exchange between their two orthogonal polarization components is first revealed. Noise-like pulse emission and vector soliton rain effect are also experimentally observed.

Partnership between patients and health care professionals is an important determinant of patient satisfaction

Background: A five year single institution survey of patient experience was analysed according to organisational units and for disease- and treatment- based patient groups to detect potential predictors of patient satisfaction. Methods: The public and academic, acute care hospital was monitored during times of change in reimbursement and ownership (2010-2014) and exhibited a high level of overall patient satisfaction (score 3.5/4) which remained constant over the duration of the study. Out of 134,593 in-house patients during this period, 54,686 responded (40.6%) to the 18-question survey mailed a fortnight after hospital discharge. This patient satisfaction database was merged with the medical and administrative database in order to enable subgroup analysis and factorial analysis performed. Results: Despite a high level of overall patient satisfaction there existed remarkable heterogeneity within the hospital in patient satisfaction between different organisational units or patient groups. Patient socio-demographic factors remained stable among the groups and over time and made a significant but albeit small, contribution to patient satisfaction variance: organisational units (0.7%), age (0.22%), patient origin (0.12%), gender (0.1%), type of insurance (0.04%), migration status (0.03%) and destination (post-discharge location, 0.02%). Without a significant influence was comorbidity, admission status, or length of stay. Using factorial analysis the single most relevant factor was partnership which explained 25% of the variance of patient satisfaction. Partnership or “partnership between patients and health care professionals” included coherent information exchange, involvement in decision-making and treatment with dignity and respect. Other components influencing satisfaction were responsiveness, information, family involvement and pain control (accounting for 1%-8% variance). Conclusions: The structural heterogeneity of hospitals demands the analysis of patient satisfaction in subgroups and over periods of time (trend analysis). Furthermore, “partnership between patients and the health care professionals” appears to be a factor that offers potential for quality improvement initiatives.

Observation of Scaling in the Dynamics of a Strongly Quenched Quantum Gas

We report on the experimental observation of scaling in the time evolution following a sudden quench into the vicinity of a quantum critical point. The experimental system, a two-component Bose gas with coherent exchange between the constituents, allows for the necessary high level of control of parameters as well as the access to time-resolved spatial correlation functions. The theoretical analysis reveals that when quenching the system close to the critical point, the energy introduced by the quench leads to a short-time evolution exhibiting crossover reminiscent of the finite-temperature critical properties in the system's universality class. Observing the time evolution after a quench represents a paradigm shift in accessing and probing experimentally universal properties close to a quantum critical point and allows in a new way benchmarking of quantum many-body theory with experiments.

Enhancing light-harvesting power with coherent vibrational interactions: A quantum heat engine picture.

Recent evidence suggests that quantum effects may have functional importance in biological light-harvesting systems. Along with delocalized electronic excitations, it is now suspected that quantum coherent interactions with certain near-resonant vibrations may contribute to light-harvesting performance. However, the actual quantum advantage offered by such coherent vibrational interactions has not yet been established. We investigate a quantum design principle, whereby coherent exchange of single energy quanta between electronic and vibrational degrees of freedom can enhance a light-harvesting system's power above what is possible by thermal mechanisms alone. We present a prototype quantum heat engine which cleanly illustrates this quantum design principle and quantifies its quantum advantage using thermodynamic measures of performance. We also demonstrate the principle's relevance in parameter regimes connected to natural light-harvesting structures.

Twofold decoherence brought on by the phonon bath in a quantum-dot two-state system

Because of the inherent coupling, the influence from the phonon bath on the coherent energy exchange between a quantum-dot (QD) two-state system (TSS) and a resonant optical mode is a twofold action. One action is the influence on the construction of the coherence, and the other is on the constructed coherence. The latter has been extensively studied by the master-equation approach, whereas the former has been ignored so far. In the present paper, the influence from the phonon bath on the construction of the coherence is studied by using the small-polaron theory. It is found that in the presence of phonon coupling, the construction of the coherence depends on the state evolution of the phonon bath during the energy exchange. The coherence construction probability is calculated and found to be a decreasing function of both temperature and coupling strength. The spectrum including the twofold decoherence effect is calculated and the obtained result can help one to understand some peculiar observations in the photoluminescence spectrum of a QD TSS system, including the additional cavity line in the strong-coupling regime and the additional coherent scattering in the case of the Mollow triplet.

Optomechanical Entanglement Between an Ion and an Optical Cavity Field

We study an optomechanical system in which the mechanical motion of a single trapped ion is coupled to a cavity field for the realization of a strongly quantum correlated two-mode system. We show that for large pump intensities the steady state photon number exhibits bistable behaviour. We further analyze the occurrence of normal mode splitting (NMS) due to mixing of the fluctuations of the cavity field and the fluctuations of the ion motion which indicates a coherent energy exchange. We also find that in the parameter regime where NMS exists, the steady state of the system shows continuous variable entanglement. Such a two-mode optomechanical system can be used for the realization of continuous variable quantum information interfaces and networks.

High quality-factor Si/SiO(2)-InP hybrid micropillar cavities with submicrometer diameter for 1.55-μm telecommunication band.

We theoretically demonstrate high quality(Q)-factor micropillar cavities at 1.55-μm wavelength based on Si/SiO(2)-InP hybrid structure. An adiabatic design in distributed Bragg reflectors (DBRs) improves Q-factor for upto 3 orders of magnitude, while reducing the diameter to sub-micrometer. A moderate Q-factor of ~3000 and a Purcell factor of ~200 are realized by only 2 taper segments and fewer conventional DBR pairs, enabling single photon generation at GHz rate. As the taper segment number is increased, Q-factor can be boosted to ~10(5)-10(6), enabling coherent exchange between the emitter and the optical mode at 1.55 μm, which is applicable in quantum information networks.