Multiphoton Transitions Research Articles

Quantum tunneling effect on switching rates of bistable driven system

The influence of quantum tunneling effects on the bistability of nonlinear resonantly driven oscillators in the presence of external noise is analyzed by means of the generalized Fokker–Planck equation in quasi-energy space. Two time scales responsible for different stages of bistable system relaxation have been revealed. It was found that quantum tunneling effects could strongly change the switching rates between different stable states and considerably enhance occupation of the stable state with higher amplitude. It was also revealed that tunneling between nearly degenerate quasi-energy states and resonant multi-photon transitions between the genuine eigenstates of the nonlinear oscillator are just similar effects.

Anisotropy and polarization dependence of multiphoton charge carrier generation rate in diamond

Anisotropy of multiphoton carrier generation in monocrystalline IIa diamond and its dependence on the polarization state of excitation light (near-infrared few-cycle laser pulses) are investigated using photoluminescence and nonlinear absorption measurements. We observe anisotropy between multiphoton transition rates for light linearly polarized along the \ensuremath{\langle}100\ensuremath{\rangle} and \ensuremath{\langle}110\ensuremath{\rangle} crystallographic directions of diamond and a strong intensity-dependent decrease of the transition rate for circularly polarized light. Measured results are compared with numerical simulations using time-dependent density functional theory and with an analytical model assuming a parabolic two-band system and the Houston function as the time-dependent wave function of the valence and conduction bands.

Observation of parity-time symmetry breaking transitions in a dissipative Floquet system of ultracold atoms

Open physical systems with balanced loss and gain, described by non-Hermitian parity-time left( {{cal P}{cal T}} right) reflection symmetric Hamiltonians, exhibit a transition which could engender modes that exponentially decay or grow with time, and thus spontaneously breaks the {cal P}{cal T}-symmetry. Such {cal P}{cal T}-symmetry-breaking transitions have attracted many interests because of their extraordinary behaviors and functionalities absent in closed systems. Here we report on the observation of {cal P}{cal T}-symmetry-breaking transitions by engineering time-periodic dissipation and coupling, which are realized through state-dependent atom loss in an optical dipole trap of ultracold 6Li atoms. Comparing with a single transition appearing for static dissipation, the time-periodic counterpart undergoes {cal P}{cal T}-symmetry breaking and restoring transitions at vanishingly small dissipation strength in both single and multiphoton transition domains, revealing rich phase structures associated to a Floquet open system. The results enable ultracold atoms to be a versatile tool for studying {cal P}{cal T}-symmetric quantum systems.

Single-pulse-induced total population inversion between indirectly coupled quantum states

We propose a scheme to induce total population inversion between two indirectly coupled states of an atom. In contrast to well-known pulse shaping techniques, where the temporal intensity or phase of the laser field is modulated to maximize the population transfer, here we use a single unshaped Gaussian laser pulse of constant frequency to completely invert the population between the ground state and an excited state of the system, for which a direct transition is forbidden. In our numerical study, multiphoton transitions of atomic sodium are considered in the presence of dynamic Stark-shifts and a detailed analysis is given about the pulse duration and detuning dependence of the efficient population inversion between the 3s and 4s states.

Ensemble properties of charge carriers injected by an ultrashort laser pulse

The average effective mass of charge carriers produced by an intense ultrashort laser pulse in a transparent solid increases significantly as the excitation mechanism changes from multiphoton transitions to interband tunneling. We theoretically investigate this phenomenon for several dielectrics and semiconductors. For diamond as a representative dielectric, we present a detailed analysis of the laser-induced change of optical properties. When the concentration of free carriers is high, we find that the average effective mass controls not only the intraband charge-carrier transport but also the interband contributions to the optical response. We observe that the excitation-induced birefringence is particularly large for parameters where the plasma response compensates for the linear response of an unperturbed solid.

Entanglement between artificial atoms and photons of lossless cavities

We investigated the dynamics of atom-field entanglement for two natural or artificial two-level atoms interacting with a one-mode quantum electromagnetic field by means of multiphoton transitions in a lossless cavity. Tavis-Cummings model is used to describe the interaction of the atoms and real microwave coplanar cavity field. We carried out the mathematical modeling of the dynamics of the system under consideration for various initial states of the atomic subsystem and an intensive coherent field of the cavity. We showed that for small multiplicities, the atoms and the field, which were initially in a pure separable state, can return to this state during the evolution. We also found that for large multiplicities the atoms and the field are in the entangled atom-field state in the process of the system evolution with the exception of the initial time instant. These results can be used in the theory of quantum networks.

Multichannel dissociation of physisorbed chlorobenzene by KrF laser radiation

The photochemistry in adsorbed molecules is drastically different from that of gaseous ones. Seven channels of KrF laser-induced dissociation of chlorobenzene condensed on silica have been found using mass spectrometry. They include the multiple photon detachment of H and Cl atoms and the opening of the aromatic ring. A general approach is proposed for adlayer photolysis based on the excitation of molecules into repulsive electronic states when bonds are stretched. It consists of the multiphoton transition of molecules into high vibrational levels in the vibronic continuum, bond elongation, and stimulated photoemission. Secondary processes in desorbed fragments were also studied.

Multiphoton Raman transitions and Rabi oscillations in driven spin systems

In the framework of the non-secular perturbation theory based on the Bogoliubov averaging method, the coherent dynamics of multiphoton Raman transitions in a two-level spin system driven by an amplitude-modulated microwave field is studied. Closed-form expressions for the Rabi frequencies of these transitions have been obtained beyond the rotating wave approximation for the low-frequency driving component. It is shown that spin states dressed by the high-frequency component of the driving field are shifted due to the Bloch-Siegert-like effect caused by antiresonant interactions with the strong low-frequency driving. We predict that with increasing the order of the Raman transition the Rabi frequency decreases and the contribution of the Bloch-Siegert shift to this frequency becomes dominant. It is found that the amplitude and phase of the Rabi oscillations strongly depend on the initial phase of the low-frequency field as well as on detuning from multiphoton resonance. The recent experimental data for the second- and third-order Raman transitions observed for nitrogen-vacancy center in diamond [Z. Shu, et al., arXiv:1804. 10492] are well described in the frame of our approach. Our results provide new possibilities for coherent control of quantum systems.

Coping with attenuation of quantum correlations of two qubit systems in dissipative environments: multi-photon transitions

In this paper we investigate the dynamics of entanglement and quantum discord, as two measures of quantum correlation resources, between two effective two-level atoms interacting with a dissipative environment at zero temperature in the presence of Kerr medium and Stark shift where m-photon transitions are allowed. Also, dipole–dipole interaction between the two atoms is taken into account. The results show that, the entanglement and quantum discord between the two atoms, which have similar behaviour qualitatively, both can be protected by means of the adjustment of Stark shift, Kerr parameter and dipole–dipole interaction. In other words, the Stark shift besides Kerr medium effect always ensure a noticeable maintenance of entanglement for m-photon processes in the Markovian and non-Markovian regimes. In particular, we find that in the non-Markovian regime, as the strengths of Stark shift, the Kerr medium and dipole–dipole interaction are increased, the amount of entanglement, as well as quantum discord approach to their maximum value, i.e., quantum correlations are well-protected.

Construction of two-qubit logical gates by transmon qubits in a three-dimensional cavity**Project supported by the National Basic Research and Development Program of China (Grant No. 2011CBA00304), the National Natural Science Foundation of China (Grant Nos. 60836001 and 61174084), and the Tsinghua University Initiative Scientific Research Program, China (Grant No. 20131089314).

We report the implementation of qubit–qubit coupling in a three-dimensional (3D) cavity, using the exchange of virtual photons, to realize logical operations. We measure single photon and multi-photon transitions in this qubit–qubit coupling system and obtain its energy avoided-crossing spectrum. With ac-Stark effect, fast control of the qubits is achieved to tune the effective coupling on and off and the state-swap gate is successfully constructed. Moreover, using two-photon transition between the ground state and doubly excited states, a kind of two-photon Rabi-like oscillation is observed. A quarter period of this oscillation corresponds to the logical gate , which is used for generating Bell states. and are the foundations of future preparation of two-qubit Bell states and realization of CNOT gate.

Photon-assisted quantum state transfer and entanglement generation in spin chains

We propose a protocol for state transfer and entanglement generation between two distant spin qubits (sender and receiver) that have different energies. The two qubits are permanently coupled to a far off-resonant spin-chain, and the qubit of the sender is driven by an external field, which provides the energy required to bridge the energy gap between the sender and the receiver. State transfer and entanglement generation are achieved via virtual single-photon and multi-photon transitions to the eigenmodes of the channel.

Quantum-mechanical approach to the laser-assisted vacuum decay

The quantum-field-theoretical problem of the vacuum decay into electron-positron pairs induced by external force fields is mapped onto the framework of a quantum-mechanical scattering process. This mapping permits us to generalize the Hund conjecture, which relates the long-time pair-creation rate for a static and spatially localized electric field to the transmission coefficient, to general space-time dependent forces that can induce multiphoton transitions. This leads to conceptual as well as computational simplifications as the vacuum's decay rate can be obtained from the laser-assisted scattering of quantum-mechanical wave packets with the force field. Using this mapping we find an analytical expression for the pair-creation rate for the case where the laser's polarization direction is perpendicular to the supercritical static force field.

Broadband, electrically tunable third-harmonic generation in graphene.

Optical harmonic generation occurs when high intensity light (>1010 W m-2) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

Gate-tunable third-order nonlinear optical response of massless Dirac fermions in graphene

Materials with massless Dirac fermions can possess exceptionally strong and widely tunable optical nonlinearities. Experiments on graphene monolayer have indeed found very large third-order nonlinear responses, but the reported variation of the nonlinear optical coefficient by orders of magnitude is not yet understood. A large part of the difficulty is the lack of information on how doping or chemical potential affects the different nonlinear optical processes. Here we report the first experimental study, in corroboration with theory, on third harmonic generation (THG) and four-wave mixing (FWM) in graphene that has its chemical potential tuned by ion-gel gating. THG was seen to have enhanced by ~30 times when pristine graphene was heavily doped, while difference-frequency FWM appeared just the opposite. The latter was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant third-order nonlinearity. These truly amazing characteristics of graphene come from the possibility to gate-control the chemical potential, which selectively switches on and off one- and multi-photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphene-based nonlinear optoelectronic devices.

Modulation of multiphoton resonant high-order harmonic spectra driven by two frequency-comb fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11504288, 11374239, 11534008, and 91536115) and the Fundamental Research Funds for the Central Universities, China.

The frequency-comb structure in the extreme ultraviolet (XUV) and vacuum ultraviolet (VUV) regions can be realized by the high-order harmonic generation (HHG) process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental- and third-harmonics, respectively. The many-mode Floquet theorem (MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamental- and third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts (CEPs) fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields. We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation (HHG) spectra, driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths. It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.

Considerable improvement of entanglement swapping by considering multiphoton transitions via cavity quantum electrodynamics method

Recently we studied the effect of photon addition in the initial coherent field on the entanglement swapping which causes some improvements in the process [Soltani et al., Int. J. Mod. Phys. B 31, 1750198 (2017)]. In this paper, we investigate the influence of multiphoton transitions in the atom–field interaction based on the cavity quantum electrodynamics on the entanglement swapping and show its considerable constructive effect on this process. The presented model consists of two two-level atoms namely A1and A2and two distinct cavity fields F1and F2. Initially, the atoms are prepared in a maximally entangled state and the fields in the cavities are prepared in hybrid entangled state of number and coherent states, separately. Making the atom A2to interact with the field F1(via the generalized Jaynes–Cummings model which allows m-photon transitions between atomic levels in the emission and absorption processes) followed by their detection allows us to arrive at the entanglement swapping from the two atoms A1, A2and the two fields F1, F2to the atom–field A1–F2system. Then, we pay our attention to the time evolution of success probability of detecting processes and fidelity. Also, to determine the amount of entanglement of the generated entangled state in the swapping process, the linear entropy is evaluated and the effect of parameter [Formula: see text] concerning the multiphoton transitions on these quantities is investigated, numerically. It is observed that, by increasing the number of photons in the transition process, one may obtain considerable improvement in the relevant quantities of the entanglement swapping. In detail, the satisfactorily acceptable values 1 and 0.5 corresponding to success probability and fidelity are obtained for most of the times during observing of the above-mentioned procedure. We concluded that the presented formalism in this paper is much more advantageous than our presentation model in our earlier work mentioned above.

On-chip photonic transistor based on the spike synchronization in circuit QED

We consider the single photon transistor in coupled cavity system of resonators interacting with multilevel superconducting artificial atom simultaneously. Effective single mode transformation is used for the diagonalization of the Hamiltonian and impedance matching in terms of the normal modes. Storage and transmission of the incident field are described by the interactions between the cavities controlling the atomic transitions of lowest lying states. Rabi splitting of vacuum-induced multiphoton transitions is considered in input/output relations by the quadrature operators in the absence of the input field. Second-order coherence functions are employed to investigate the photon blockade and delocalization–localization transitions of cavity fields. Spontaneous virtual photon conversion into real photons is investigated in localized and oscillating regimes. Reflection and transmission of cavity output fields are investigated in the presence of the multilevel transitions. Accumulation and firing of the reflected and transmitted fields are used to investigate the synchronization of the bunching spike train of transmitted field and population imbalance of cavity fields. In the presence of single photon gate field, gain enhancement is explained for transmitted regime.

Simultaneous measurements of microwave photoresistance and cyclotron reflection in the multiphoton regime

We simultaneously measure photoresistance with electrical transport and plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multi-photon transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that higher order multi-photon features can be observed using higher carrier density samples.

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization.

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

多光子跃迁下the Number-Chaotic态光场与运动二能级原子相互作用系统的保真度

多光子跃迁下the Number-Chaotic态光场与运动二能级原子相互作用系统的保真度