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P. R. Berman

Publications and source records attributed to P. R. Berman.

At least 19 recordsLinked to original sources
arXiv2025

Field energy and angular momentum in spontaneous emission: A Schr\"odinger-picture approach

A Schr\"odinger-picture approach is used to calculate the field energy and angular momentum radiated by an atom undergoing spontaneous emission. The calculation is carried out using both the rotating-wave approximation (RWA) and Weisskopf-Wigner approximation (WWA). It is shown that a consistent application of the WWA leads to expressions for both the energy and angular momentum in the field that are finite for all times, in contrast to the results for a classical point dipole oscillator. Moreover, it is shown that the total angular momentum in the field is a sum of its spin and orbital components, again in contrast to the analogous results for a classical point dipole oscillator. Analytic expressions for the energy, spin angular momentum, and orbital angular momentum in the field are obtained for an atom undergoing spontaneous emission from a state having angular momentum H to a state having angular momentum G via an electric dipole transition. It is shown that the spin and orbital angular momenta in the field are equal, independent of the values of H and G. With a slight modification of the WWA, it can also be shown that the energy density in the field, the Poynting vector of the field, and the angular momentum flux of the field do not diverge at the origin.

quant-ph↗
arXiv2016

Atom interferometry in the presence of an external test mass

The influence of an external test mass on the phase of the signal of an atom interferometer is studied theoretically. Using traditional techniques in atom optics based on the density matrix equations in the Wigner representation, we are able to extract the various contributions to the phase of the signal associated with the classical motion of the atoms, the quantum correction to this motion resulting from atomic recoil that is produced when the atoms interact with Raman field pulses, and quantum corrections to the atomic motion that occur in the time between the Raman field pulses. By increasing the effective wave vector associated with the Raman field pulses using modified field parameters, we can increase the sensitivity of the signal to the point where the quantum corrections can be measured. The expressions that are derived can be evaluated numerically to isolate the contribution to the signal from an external test mass. The regions of validity of the exact and approximate expressions are determined.

physics.atom-ph↗
arXiv2015

Vertical dipole above a dielectric or metallic half-space - energy flow considerations

The emission pattern from a classical dipole located above and oriented perpendicular to a metallic or dielectric half space is calculated for a dipole driven at constant amplitude. This is a problem considered originally by Sommerfeld and analyzed subsequently by numerous authors. In contrast to most previous treatments, however, we focus on the energy flow in the metal or dielectric. It is shown that the radial Poynting vector in the metal points inwards when the frequency of the dipole is below the surface plasmon resonance frequency. In this case, energy actually flows of the interface at small radii. The Joule heating in the metal is also calculated and it is shown explicitly that Poynting's theorem holds for a cylindrical surface in the metal. When the metal is replaced by a dielectric having permittivity less than that of the medium in which the dipole is immersed, it is found that energy flows out of the interface for sufficiently large radii. In all cases it is assumed that the imaginary part of the permittivity of the metal or dielectric is much less than unity.

physics.class-ph↗
arXiv2013

Demonstration of quantum entanglement between a single electron spin confined to an InAs quantum dot and a photon

The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dot's excited state. We obtain a lower bound on the fidelity of entanglement of 0.59, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3 x 10^3 s^-1. This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network.

quant-ph↗
arXiv2009

Raman scheme for adjustable bandwidth quantum memory

We propose a scenario of quantum memory for light based on Raman scattering. The storage medium is a vapor and the different spectral components of the incoming signal are stored in different atomic velocity classes. One uses appropriate pulses to reverse the resulting Doppler phase shift and to regenerate the signal, without distortion, in the backward direction. The different stages of the protocol are detailed and the recovery efficiency is calculated in the semi-classical picture. Since the memory bandwidth is determined by the Raman transition Doppler width, it can be adjusted by changing the angle of the signal and control beams. The optical depth also depends on the beam angle. As a consequence the available optical depth can be optimized, depending on the needed bandwidth. The predicted recovery efficiency is close to 100% for large optical depth.

quant-ph↗
arXiv2008

A Single Charged Quantum Dot in a Strong Optical Field: Absorption, Gain, and the AC Stark Effect

We investigate a singly-charged quantum dot under a strong optical driving field by probing the system with a weak optical field. When the driving field is detuned from the trion transition, the probe absorption spectrum is shifted from the trion resonance as a consequence of the dynamic Stark effect. Simultaneously, a gain sideband is created, resulting from the coherent energy transfer between the optical fields through the quantum dot nonlinearity. As the pump detuning is moved from red to blue, we map out the anticrossing of these two spectral lines. The optical Bloch equations for a stationary two-level atom can be used to describe the numerous spectral features seen in this nano solid state system.

cond-mat.mes-hall↗
arXiv2006

Microscopic Theory of the Photon Recoil of an Atom in a Dielectric

An atom recoils when it undergoes spontaneous decay. In this paper we present a microscopic calculation of the recoil of a source atom imbedded in a dielectric medium. We find that the source atom recoils with the canonical photon momentum $n\hbar k_{0,}$ where $n$ is the index of refraction and $\hbar k_{0}$ is the photon momentum calculated at the source atom atomic frequency $ω_{0}$. We also show explicitly how the energy is conserved with the photon inside the medium.

quant-ph↗
arXiv2006

Quantum computing with magnetic atoms in optical lattices of reduced periodicity

We investigate the feasibility of combining Raman optical lattices with a quantum computing architecture based on lattice-confined magnetically interacting neutral atoms. A particular advantage of the standing Raman field lattices comes from reduced interatomic separations leading to increased interatomic interactions and improved multi-qubit gate performance. Specifically, we analyze a $J=3/2$ Zeeman system placed in $% σ_{+}-σ_{-}$ Raman fields which exhibit $λ/4$ periodicity. We find that the resulting CNOT gate operations times are in the order of millisecond. We also investigate motional and magnetic-field induced decoherences specific to the proposed architecture.

quant-ph↗
arXiv2006

Microscopic Theory of Spontaneous Decay in a Dielectric

The local field correction to the spontanous dacay rate of an impurity source atom imbedded in a disordered dielectric is calculated to second order in the dielectric density. The result is found to differ from predictions associated with both "virtual" and "real" cavity models of this decay process. However, if the contributions from two dielectric atoms at the same position are included, the virtual cavity result is reproduced.

quant-ph↗
arXiv2006

Cooling in reduced period optical lattices: non-zero Raman detuning

In a previous paper [Phys. Rev. A 72, 033415 (2005)], it was shown that sub-Doppler cooling occurs in a standing-wave Raman scheme (SWRS) that can lead to reduced period optical lattices. These calculations are extended to allow for non-zero detuning of the Raman transitions. New physical phenomena are encountered, including cooling to non-zero velocities, combinations of Sisyphus and "corkscrew" polarization cooling, and somewhat unusual origins of the friction force. The calculations are carried out in a semi-classical approximation and a dressed state picture is introduced to aid in the interpretation of the results.

physics.atom-ph↗
arXiv2006

Atomic entanglement generation with reduced decoherence via four-wave mixing

In most proposals for the generation of entanglement in large ensembles of atoms via projective measurements, the interaction with the vacuum is responsible for both the generation of the signal that is detected and the spin depolarization or decoherence. In consequence, one has to usually work in a regime where the information aquisition via detection is sufficiently slow (weak measurement regime) such as not to strongly disturb the system. We propose here a four-wave mixing scheme where, owing to the pumping of the atomic system into a dark state, the polarization of the ensemble is not critically affected by spontaneous emission, thus allowing one to work in a strong measurement regime.

quant-ph↗
arXiv2006

Cooperative spin decoherence and population transfer

An ensemble of multilevel atoms is a good candidate for a quantum information storage device. The information is encrypted in the collective ground state atomic coherence, which, in the absence of external excitation, is decoupled from the vacuum and therefore decoherence free. However, in the process of manipulation of atoms with light pulses (writing, reading), one inadvertently introduces a coupling to the environment, i.e. a source of decoherence. The dissipation process is often treated as an independent process for each atom in the ensemble, an approach which fails at large atomic optical depths where cooperative effects must be taken into account. In this paper, the cooperative behavior of spin decoherence and population transfer for a system of two, driven multilevel-atoms is studied. Not surprisingly, an enhancement in the decoherence rate is found, when the atoms are separated by a distance that is small compared to an optical wavelength; however, it is found that this rate increases even further for somewhat larger separations for atoms aligned along the direction of the driving field's propagation vector. A treatment of the cooperative modification of optical pumping rates and an effect of polarization swapping between atoms is also discussed, lending additional insight into the origin of the collective decay.

quant-ph↗
arXiv2005

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

cond-mat.mes-hall↗
arXiv2004

On the role of coupling in mode selective excitation using ultrafast pulse shaping in stimulated Raman spectroscopy

The coherence of two, coupled two-level systems, representing vibrational modes in a semiclassical model, is calculated in weak and strong fields for various coupling schemes and for different relative phases between initial state amplitudes. A relative phase equal to $π$ projects the system into a dark state. The selective excitation of one of the two, two-level systems is studied as a function of coupling strength and initial phases.

physics.chem-ph↗
arXiv2003

Autler-Townes spectroscopy of the $5S_{1/2}-5P_{3/2}-44D$ cascade of cold $^{85}$Rb atoms

We study nonlinear optical effects in the laser excitation of Rydberg states. $5S_{1/2}$ and $5P_{3/2}$ levels of $^{85}$Rb are strongly coupled by a strong laser field and probed by a weak laser tuned to the $5P_{3/2} - 44D$ Rydberg resonance. We observe high contrast Autler-Townes spectra which are dependent on the pump polarization, intensity and detuning. The observed behavior agrees with calculations, which include the effect of optical pumping.

quant-ph↗
arXiv2003

Spin squeezing via atom - cavity field coupling

Spin squeezing via atom-field interactions is considered within the context of the Tavis-Cummings model. An ensemble of N two-level atoms interacts with a quantized cavity field. For all the atoms initially in their ground states, it is shown that spin squeezing of both the atoms and the field can be achieved provided the initial state of the cavity field has coherence between number states differing by 2. Most of the discussion is restricted to the case of a cavity field initially in a coherent state, but initial squeezed states for the field are also discussed. Optimal conditions for obtaining squeezing are obtained. An analytic solution is found that is valid in the limit that the number of atoms is much greater than unity and is also much larger than the average number of photons, inititally in the coherent state of the cavity field. In this limit, the degree of spin squeezing increases with increasing a, even though the field more closely resembles a classical field for which no spin squeezing could be achieved.

quant-ph↗
arXiv2003

Theory of selective excitation in Stimulated Raman Scattering

A semiclassical model is used to investigate the possibility of selectively exciting one of two closely spaced, uncoupled Raman transitions. The duration of the intense pump pulse that creates the Raman coherence is shorter than the vibrational period of a molecule (impulsive regime of interaction). Pulse shapes are found that provide either enhancement or suppression of particular vibrational excitations.

physics.chem-ph↗