Cavity Detuning Research Articles

Bright continuous-variable entanglement from the quantum optical dimer

By calculating correlation spectra of the output fields, we show theoretically that two evanescently coupled χ(2) second harmonic generators inside a Fabry–Perot cavity provide a tunable source of quadrature squeezed light, Einstein–Podolsky–Rosen correlations and quantum entanglement. Unlike systems using coupled downconverters, second harmonic generation has no oscillation threshhold, so that the entangled fields become macroscopically occupied as soon as the pumping fields are turned on. This system also gives two frequencies at which the entangled fields can have macroscopic intensity. We show how the entanglement properties can be controlled by adjusting the pumping, the coupling strengths and the cavity detunings.

Scheme for implementing perfect quantum dense coding with three-atom W-class state in cavity QED

We propose an experimentally feasible scheme of implementing perfect quantum dense coding with three-atom W-class state in cavity QED. In this scheme atoms interact simultaneously with a highly detuned cavity field and the cavity is only virtually excited, thus the scheme is insensitive to the cavity decay, which is very important in view of experiment. Moreover, we also propose a scheme of transmitting three bits of classical information by sending one qubit and one classical bit with 3-qubit W-class and GHZ states.

Macroscopic Superpositions and Entanglement of Mesoscopic Squeezed Vacuum States in Dissipative Cavity QED System

A scheme has been proposed for generating the macroscopic superpositions and the entanglement between the mesoscopic squeezed vacuum states by considering the two-photon interaction of N two-level atoms in a dissipative cavity with high quality factor assisted by a strong driving field. A number of multiparty entangled states between the atoms and the squeezed vacuum states, among the atoms, and among the squeezed vacuum states, can be prepared by virtue of a specific choice of the cavity detuning and the detuning of applied coherent field under the dissipation condition. The corresponding analytical expressions of the influence can be given. Moreover, we can also give a series of macroscopic entangled states between the usual coherent states and the squeezed vacuum states using the combination of the dissipative one-photon interaction Hamiltonian with the dissipative two-photon interaction Hamiltonian. We also discuss the experimental feasibility. Our scheme can be realized in the current techniques on the cavity QED.

Quantum Theory of Cavity-Assisted Sideband Cooling of Mechanical Motion

We present a quantum-mechanical theory of the cooling of a cantilever coupled via radiation pressure to an illuminated optical cavity. Applying the quantum noise approach to the fluctuations of the radiation pressure force, we derive the optomechanical cooling rate and the minimum achievable phonon number. We find that reaching the quantum limit of arbitrarily small phonon numbers requires going into the good-cavity (resolved phonon sideband) regime where the cavity linewidth is much smaller than the mechanical frequency and the corresponding cavity detuning. This is in contrast to the common assumption that the mechanical frequency and the cavity detuning should be comparable to the cavity damping.

Sideband Instability in the Compact THz Free-Electron Laser at KAERI

A frequency offset of the Sideband instability has been observed in a compact waveguide-mode terahertz (THz) free-electron laser (FEL). The spectra of the FEL pulses were measured by using a Fabry-Perot spectrometer having a resolution of 10−4 of the central wavelength at 2 − 3 THz range. The shift of the sideband was measured to be 0.5 − 1.2 μm, depending on the FEL wavelengths from 110 to 165 μm. An increase in the sideband shift for a longer wavelength could be explained by a change in the wave’s group velocity in a plane-parallel waveguide. Mode competition between the sidebands and the primary wave was observed by changing the cavity length of the FEL. We could decrease the number of modes and reduce the linewidth of the spectra by controlling the cavity detuning. We discuss the dependence of the complexity of the sideband instability on the FEL wavelengths and its gain characteristics.

Cascade atom in high-Q cavity: the spectrum for non-Markovian decay

The spontaneous emission spectrum for a three-level cascade configuration atom in a single mode high-Q cavity coupled to a zero temperature reservoir of continuum external modes is determined from the atom–cavity mode master equation using the quantum regression theorem. Initially the atom is in its upper state and the cavity mode empty of photons. Following Glauber, the spectrum is defined via the response of a detector atom. Spectra are calculated for the detector located inside the cavity (case A), outside the cavity end mirror (case B—end emission), or placed for emission out the side of the cavity (case C). The spectra for case A and case B are found to be essentially the same. In all the cases the predicted lineshapes are free of instrumental effects and only due to cavity decay. Spectra are presented for intermediate and strong coupling regime situations (where both atomic transitions are resonant with the cavity frequency), for cases of non-zero cavity detuning, and for cases where the two atomic transition frequencies differ. The spectral features for cases B(A) and C are qualitatively similar, with six spectral peaks for resonance cases and eight for detuned cases. These general features of the spectra can be understood via the dressed atom model. However, case B and C spectra differ in detail, with the latter exhibiting a deep spectral hole at the cavity frequency due to quantum interference effects.

Superconducting cavity control based on system model identification

A digital control system for the superconducting cavities for a linear accelerator is presented. The low level radio frequency system for the FLASH project in DESY is introduced. A field programmable gate array-based controller managed by MATLAB was developed to investigate the novel firmware implementation. An algebraic complex domain model is proposed for the system analysis. The calibration procedure of a signal path is considered for multi-channel control. For a given model structure, the input–output relation of the real plant with unknown parameters is applied. The over-determined matrix equation is created covering a long enough measurement range with the solution according to the least squares method. A base function approximation by a cubic B-spline set is applied to estimate the time-varying cavity detuning during the pulse. Control tables, feed-forward and set point, are determined for the required cavity performance, according to the recognized process. The feedback loop is tuned by fitting complex gain of the corrector unit according to the determined gain table. An adaptive control algorithm is applied for feed-forward and feedback modes. Experimental results including field measurement are presented for a cavity representative operation.

Mode hopping strongly affects observability of dynamical instability in optical parametric oscillators

Theoretical investigations of dynamical behavior in optical parametric oscillators (OPO) have generally assumed that the cavity detunings of the interacting fields are controllable parameters. However, OPOs are known to experience mode hops, where the system jumps to the mode of lowest cavity detuning. We note that this phenomenon significantly limits the range of accessible detunings and thus may prevent instabilities predicted to occur above a minimum detuning from being evidenced experimentally. As a simple example among a number of instability mechanisms possibly affected by this limitation, we discuss the Hopf bifurcation leading to periodic behavior in the monomode mean-field model of a triply resonant OPO and show that it probably can be observed only in very specific setups.

Output Phase Characteristics of a Nonlinear Regenerative Fiber Amplifier

A simple regenerative fiber amplifier with a nonlinear refractive index is described and analyzed. Using standard equations of operation, we model the circulating intensity within the resonant cavity and determine the nonlinear phase response to changes in this intensity over the entire range of initial cavity detuning. Simple stability criteria are established and described. It is shown that the presence of sufficient nonlinearity leads to stable regions of operation with consistent phase-intensity relationships. Based on these results, the phase of the amplifier output is predicted relative to the injected field for the entire range of initial detuning

Generation of multi-atom cluster states via an unconventional geometric phase shift in cavity QED

This paper proposes two schemes to generate the multi-atom cluster states. The first scheme is based on the interaction of atoms with a highly detuned cavity mode and a classical field, the second scheme is based on the interaction of atoms with a cavity mode, strongly driven by a resonant classical field.

Dynamics of polaritons resonantly created at the upper polariton branch

Abstract We have studied the polariton relaxation dynamics in a CdTe microcavity at low temperatures after resonant excitation into the upper polariton branch (UPB). Initially, we have set a negative exciton–cavity detuning, such that the energy difference between the two polariton branches coincides with that of an LO phonon. Our experimental results reveal a sublinear dependence of the integrated emission from the lower polariton branch (LPB) with excitation power. This evidences not only an inefficient LO phonon mediated relaxation from the UPB to the LPB but also a substantial inhibition of polariton relaxation along the LPB. After that, we have progressively reduced the negative detuning, approaching the exciton–cavity resonance. Under these conditions it is possible to observe a nonlinear emission arising from K ∼ 0 LPB-states similar to that observed after nonresonant excitation. Marked oscillations are present in the time evolution traces, with a period that does not depend on excitation power or detuning.

Light force fluctuations in a strongly coupled atom–cavity system

Between mirrors, the density of electromagnetic modes differs from the one in free space. This changes the radiation properties of an atom as well as the light forces acting on an atom. It has profound consequences in the strong-coupling regime of cavity quantum electrodynamics. For a single atom trapped inside the cavity, we investigate the atom–cavity system by scanning the frequency of a probe laser for various atom–cavity detunings. The avoided crossing between atom and cavity resonance is visible in the transmission of the cavity. It is also visible in the loss rate of the atom from the intracavity dipole trap. On the normal-mode resonances, the dominant contribution to the loss rate originates from dipole-force fluctuations which are dramatically enhanced in the cavity. This conclusion is supported by Monte Carlo simulations.

Rotating cavity solitons in semiconductor microresonators

We predict the existence of a ring-like localized solutions in wide aperture passive semiconductor quantum well microresonators. These dark ring cavity solitons exist for positive cavity detuning in a frequency range where nonlinear dispersive effects prevail against absorptive ones. As a rule, destabilization of these ring solitons leads to the formation of radial asymmetric rotating solitons in externally driven optical cavities.

Quantum dense coding using a peculiar tripartite entangled state

Following a recent proposal (Phys. Lett. A 346 (2005) 330) about quantum dense coding using a tripartite entangled GHZ state and W state, this paper proposes an experimentally feasible scheme for dense coding in cavity QED by using another peculiar tripartite entangled state. In the scheme the atoms interact simultaneously with a highly detuned cavity mode with the assistance of a classical field, the successful probability of dense coding scheme with this peculiar tripartite entangled state equals 1.

Determination of field amplitude and synchronous phase using the beam-induced signal in an unpowered superconducting cavity

At the Spallation Neutron Source superconducting linear accelerator, RF fields excited by a pulsed, high-intensity drifting beam in an unpowered superconducting cavity are measured with the cavity control circuit, in order to determine the synchronous phase and to calibrate the pickup probe of the cavity. Measurement errors arising from noise and from the excitation of other pass-band modes in the cavity are analyzed. The phase change due to beam acceleration as well as due to cavity detuning is computed with a model which is described in this paper. Simulation results show excellent agreement with measurements performed in drifting beam experiments. The measured cavity synchronous phase and accelerating gradient determined from the drifting beam measurements are in good agreement with those obtained from a time-of-flight based technique.

Wavelength Extension Effect on Lasing Characteristics of Highly-Strained GaInAs/GaAs Vertical-Cavity Surface-Emitting Lasers with Cavity Detuning

Cavity detuned highly-strained GaInAs/GaAs vertical cavity surface emitting lasers (VCSELs) were investigated. Devices emitting beyond 1.2 µm of the wavelength were realized by combining conventional quantum wells emitting at 1.18 µm and large wavelength detuned cavities. Various detuning conditions from -35 to 80 nm were compared for clarifying the detuning effect. The longest wavelength was 1.27 µm for an 80 nm detuned device. The highest operating temperature of a 55 nm detuned device was 140 °C under cw operation. It was found that approximately 50 nm detuning was necessary to reach the threshold gain level for a large detuned device and the condition was produced by self-heating. Relaxation resonance frequencies were characterized and over 2 GHz was observed even for a 1.27 µm VCSEL.

Radiation-pressure cooling and optomechanical instability of a micromirror

Recent table-top optical interferometry experiments and advances in gravitational-wave detectors have demonstrated the capability of optical interferometry to detect displacements with high sensitivity. Operation at higher powers will be crucial for further sensitivity enhancement, but dynamical effects caused by radiation pressure on the interferometer mirrors must be taken into account, and the appearance of optomechanical instabilities may jeopardize the stable operation of the next generation of interferometers. These instabilities are the result of a nonlinear coupling between the motion of the mirrors and the optical field, which modifies the effective dynamics of the mirror. Such 'optical spring' effects have already been demonstrated for the mechanical damping of an electromagnetic waveguide with a moving wall, the resonance frequency of a specially designed flexure oscillator, and the optomechanical instability of a silica microtoroidal resonator. Here we present an experiment where a micromechanical resonator is used as a mirror in a very high-finesse optical cavity, and its displacements are monitored with unprecedented sensitivity. By detuning the laser frequency with respect to the cavity resonance, we have observed a drastic cooling of the microresonator by intracavity radiation pressure, down to an effective temperature of 10 kelvin. For opposite detuning, efficient heating is observed, as well as a radiation-pressure-induced instability of the resonator. Further experimental progress and cryogenic operation may lead to the experimental observation of the quantum ground state of a micromechanical resonator, either by passive or active cooling techniques.

Experimental evidence for an optical spring

An optical spring effect has been observed in the motion of a Fabry-Perot cavity suspended in the Low-Frequency Facility, R and D experiment of the VIRGO Collaboration. The experimental setup consists of a 1-cm-long cavity hanging from a mechanical isolation system, conceived to suppress seismic noise transmission to the optical components of the interferometer. The observed radiation pressure effect corresponds to an optical stiffness k{sub opt} ranging between 2.5x10{sup 4} and 6.5x10{sup 4} N/m. This paper reports several measurements of the mirror relative displacement carried out in different working conditions. The measured error signal spectra show broad resonances at frequencies compatible with the optomechanical system. In other runs cavity detuning oscillations have been observed at subhertz frequencies. In these cases the power spectrum of the control loop error signal exhibited a broad resonance with superimposed uniformly spaced peaks. These observations, validated by a simple model, prove that the fluctuations of the optical spring effect can become so large as to exhibit nonlinear features.

Protocol for multi-party superdense coding by using multi-atom in cavity QED

We present a protocol for multi-party superdense coding by using multi-atom in cavity quantum electrodynamics (QED). It is shown that, with a highly detuned cavity mode and a strong driving field, the protocol is insensitive to both cavity decay and thermal field. It is even certain to identify GHZ states via detecting the atomic states. Therefore we can realize the quantum dense coding in a simple way in the multiparty system.

Beating quantum limits in an optomechanical sensor by cavity detuning

We study the quantum limits in an optomechanical sensor based on a detuned high-finesse cavity with a movable mirror. We show that the radiation pressure exerted on the mirror by the light in the detuned cavity induces a modification of the mirror dynamics and makes the mirror motion sensitive to the signal. This leads to an amplification of the signal by the mirror dynamics, and to an improvement of the sensor sensitivity beyond the standard quantum limit, up to an ultimate quantum limit only related to the mechanical dissipation of the mirror. This improvement is somewhat similar to the one predicted in detuned signal-recycled gravitational-wave interferometers, and makes a high-finesse cavity a model system to test these quantum effects.