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arXiv:1204.5265 · Physical Review A

State-dependent atomic excitation by multiphoton pulses propagating along two spatial modes

Yimin Wang, Jiří Minář, Valerio Scarani

Published Aug 7, 2012• 11 citations
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Abstract

We investigate the dynamics of a single two-level atom, which interacts with pulses propagating in two spatial modes (right and left) and frequency continuum. Using Heisenberg equations of motion, we present the explicit analytical derivations and general formalisms for atomic excitation with two-spatial-mode multiphoton pulses in both the Fock and coherent states. Based on those formalisms, we show that perfect atomic excitation by a single-photon Fock state pulse can only be realized when it is rising exponentially shaped in the even mode, a balanced superposition of the two spatial modes. A single photon from a single spatial mode can only give half of the maximal atomic excitation probability. We also show that the maximum atomic excitation probability with multiphoton pulses in the even mode is a monotonic function of the average photon number for the coherent state, but not for the Fock states. Furthermore, we demonstrate that the atomic dynamics can be controlled by the initial relative phase between the two counterpropagating coherent state pulses incident on the atom, which is not the case with the two Fock state pulses.

DOI
10.1103/physreva.86.023811

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We investigate the dynamics of a single two-level atom, which interacts with pulses propagating in two spatial modes (right and left) and frequency continuum. Using Heisenberg equations of motion, we present the explicit analytical derivations and general formalisms for atomic excitation with two-spatial-mode multiphoton pulses in both the Fock and coherent states. Based on those formalisms, we show that perfect atomic excitation by a single-photon Fock state pulse can only be realized when it is rising exponentially shaped in the even mode, a balanced superposition of the two spatial modes. A single photon from a single spatial mode can only give half of the maximal atomic excitation probability. We also show that the maximum atomic excitation probability with multiphoton pulses in the even mode is a monotonic function of the average photon number for the coherent state, but not for the Fock states. Furthermore, we demonstrate that the atomic dynamics can be controlled by the initial relative phase between the two counterpropagating coherent state pulses incident on the atom, which is not the case with the two Fock state pulses.
We describe the interaction of two two-level atoms in free space with propagating modes of the quantized electromagnetic field, using the time-dependent Heisenberg-Langevin method. For single- photon pulses, we consider the effect of the pulse's spatial and temporal profiles on the atomic excitation. In particular, we find the ideal shape for a pulse to put exactly one excitation in any desired state of the bi-atomic system. Furthermore, we analyze the differences in the atomic dynamics between the cases of Fock state pulses and coherent state pulses.
Quantum information processing tasks have been implemented in various physical platforms. In this thesis, we consider two specific implementations: Part I presents a detailed study of the interaction of a single atom with a light pulse. Part II examines the case of a quantum controlled-phase gate in circuit quantum electrodynamics in the ultrastrong coupling regime. They are both actively pursued by many research groups. In Part I, we first present the framework for a single two-level atom interacting with the quantized electromagnetic field. The effect of temporal and spectral properties of the pulses on atomic excitation is studied for both Fock state and coherent state pulse. The detailed analysis of atomic excited state population is provided for single photon pulse with various temporal shapes. The work is then extended to a more general case with two spatial-modes and multi-photon pulses. The difference between Fock state and coherent state pulse on atomic excitation is further investigated. We find that at few photon levels, the maximum atomic excitation probability is a monotonic function of photon number for the coherent state pulse, but not for the Fock state pulse. Furthermore, we show that the atomic dynamics can be controlled by the initial relative phase between the two counter-propagating coherent state pulses incident on the atom, which is not the case with two Fock state pulses. The theoretical prediction is then compared with an experiment, where a single trapped 87Rb atom interacts with coherent state pulses engineered in temporal shape of rising exponential and rectangular form. Next, we propose a quantum memory setup based on a single two-level atom in a half cavity with a moving mirror. The efficiency of the quantum storage process is studied analytically and
State mapping between atoms and photons, and photon-photon interactions play an important role in scalable quantum information processing. We consider the interaction of a two-level atom with a quantized propagating pulse in free space and study the probability Pe(t){P}_{e}(t)Pe​(t) of finding the atom in the excited state at any time ttt. This probability is expected to depend on (i) the quantum state of the pulse field and (ii) the overlap between the pulse and the dipole pattern of the atomic spontaneous emission. We show that the second effect is captured by a single parameter $\ensuremath{\Lambda}\ensuremath{\in}[0,8\ensuremath{\pi}/3]$, obtained by weighting the dipole pattern with the numerical aperture. Then, Pe(t){P}_{e}(t)Pe​(t) can be obtained by solving time-dependent Heisenberg-Langevin equations. We provide detailed solutions for both single-photon Fock state and coherent states and for various temporal shapes of the pulses.
We study how the transient excitation probability of a two-level atom by a quantized field depends on the temporal profile of the incident pulse, in the presence of external losses, for both coherent and Fock states, and in two complementary limits: when the pulse contains only one photon (on average), and when the number of photons NNN is large. For the latter case we derive analytical expressions for the scaling of the excitation probability with NNN that can be easily evaluated for any pulse shape.
We investigate the interaction between a single atom and optical pulses in a coherent state with a controlled temporal envelope. In a comparison between a rising exponential and a square envelope, we show that the rising exponential envelope leads to a higher excitation probability for fixed low average photon numbers, in accordance with a time-reversed Weisskopf-Wigner model. We characterize the atomic transition dynamics for a wide range of the average photon numbers and are able to saturate the optical transition of a single atom with ≈50 photons in a pulse by a strong focusing technique.
We theoretically investigate quantum Rabi oscillations in a system consisting of a two-level atom (qubit) strongly coupled to a one-dimensional open waveguide. In contrast to conventional cavity quantum electrodynamics, the qubit interacts with a continuum of propagating modes, which gives rise to fundamentally different dynamical behavior. Within the rotating-wave approximation, we express the multimode Jaynes-Cummings Hamiltonian in terms of collective bosonic operators and show that the system possesses two integrals of motion, enabling an exact diagonalization of the Hamiltonian in the single-excitation subspace. We show that for a strongly interacting qubit-photon system, the dynamics are captured by a reduced two-level model. In this framework, each level is defined as the product of the atomic excited state and a specific field mode, which takes the form of a multiphoton Fock-like state. In this picture, the Rabi oscillations represent a collective phenomenon corresponding to oscillations between multiphoton states differing by a single photon. We then extend our analysis to multiphoton processes in which the initial field is a coherent state with a continuous spectrum. In this case, the Rabi frequency is shown to be sensitive to the spectral profile of the function that generates the coherent state.