445 research outputs found
Long-Range Tails in van der Waals Interactions of Excited-State and Ground-State Atoms
A quantum electrodynamic calculation of the interaction of an excited-state
atom with a ground-state atom is performed. For an excited reference state and
a lower-lying virtual state, the contribution to the interaction energy
naturally splits into a pole term, and a Wick-rotated term. The pole term is
shown to dominate in the long-range limit, altering the functional form of the
interaction from the retarded 1/R^7 Casimir-Polder form to a long-range
tail-provided by the Wick-rotated term-proportional to cos[2 (E_m-E_n) R/(hbar
c)]/R^2, where E_m < E_n is the energy of a virtual state, lower than the
reference state energy E_n, and R is the interatomic separation. General
expressions are obtained which can be applied to atomic reference states of
arbitrary angular symmetry. Careful treatment of the pole terms in the Feynman
prescription for the atomic polarizability is found to be crucial in obtaining
correct results.Comment: 13 pages; RevTe
Non-classical properties of the e.m. near field of an atom in spontaneous light emission
We use Glauber's correlation function function as well as the Green functions
formalism to investigate, in the case of a dipolar atomic transition, the
causal behaviour of the spontaneously emitted electromagnetic field. We also
examine the role played by the longitudinal electric field, which is not
described in terms of photonic (transverse) degrees of freedom. We predict the
existence of a genuinely quantum memory effect at the level of the near field
surrounding the atom, which keeps track of the past excitation and emission by
the atom
Long-range interactions of hydrogen atoms in excited states. III. nS-1S interactions for n >= 3
The long-range interaction of excited neutral atoms has a number of
interesting and surprising properties, such as the prevalence of long-range,
oscillatory tails, and the emergence of numerically large can der Waals C_6
coefficients. Furthermore, the energetically quasi-degenerate nP states require
special attention and lead to mathematical subtleties. Here, we analyze the
interaction of excited hydrogen atoms in nS states (3 <= n <= 12) with
ground-state hydrogen atoms, and find that the C_6 coefficients roughly grow
with the fourth power of the principal quantum number, and can reach values in
excess of 240,000 (in atomic units) for states with n = 12. The nonretarded van
der Waals result is relevant to the distance range R << a_0/alpha, where a_0 is
the Bohr radius and alpha is the fine-structure constant. The Casimir-Polder
range encompasses the interatomic distance range a_0/alpha << R << hbar c/L,
where L is the Lamb shift energy. In this range, the contribution of
quasi-degenerate excited nP states remains nonretarded and competes with the
1/R^2 and 1/R^4 tails of the pole terms which are generated by lower-lying mP
states with 2 <= m <= n-1, due to virtual resonant emission. The dominant pole
terms are also analyzed in the Lamb shift range R >> hbar c/L. The familiar
1/R^7 asymptotics from the usual Casimir-Polder theory is found to be
completely irrelevant for the analysis of excited-state interactions. The
calculations are carried out to high precision using computer algebra in order
to handle a large number of terms in intermediate steps of the calculation, for
highly excited states.Comment: 17 pages; RevTe
Virtual Resonant Emission and Oscillatory Long-Range Tails in van der Waals Interactions of Excited States: QED Treatment and Applications
We report on a quantum electrodynamic (QED) investigation of the interaction
between a ground state atom with another atom in an excited state. General
expressions, applicable to any atom, are indicated for the long-range tails
which are due to virtual resonant emission and absorption into and from vacuum
modes whose frequency equals the transition frequency to available lower-lying
atomic states. For identical atoms, one of which is in an excited state, we
also discuss the mixing term which depends on the symmetry of the two-atom wave
function (these evolve into either the gerade or the ungerade state for close
approach), and we include all nonresonant states in our rigorous QED treatment.
In order to illustrate the findings, we analyze the fine-structure resolved van
der Waals interaction for nD-1S hydrogen interactions with n=8,10,12 and find
surprisingly large numerical coefficients.Comment: 6 pages; RevTe
Conditions for anti-Zeno effect observation in free-space atomic radiative decay
Frequent measurements can modify the decay of an unstable quantum state with
respect to the free dynamics given by Fermi's golden rule. In a landmark
article, Nature 405, 546 (2000), Kofman and Kurizki concluded that in quantum
decay processes, acceleration of the decay by frequent measurements, called the
quantum anti-Zeno effect (AZE), appears to be ubiquitous, while its
counterpart, the quantum Zeno effect, is unattainable. However, up to now there
have been no experimental observations of the AZE for atomic radiative decay
(spontaneous emission) in free space. In this work, making use of analytical
results available for hydrogen-like atoms, we find that in free space, only
non-electric-dipolar transitions should present an observable AZE, revealing
that this effect is consequently much less ubiquitous than first predicted. We
then propose an experimental scheme for AZE observation, involving the electric
quadrupole transition between D 5/2 and S 1/2 in the heaviest alkali-earth ions
Ca + and Sr +. The proposed protocol is based on the STIRAP technique which
acts like a dephasing quasi-measurement
The onset time of Fermi's golden rule
Fermi's golden rule describes the decay dynamics of unstable quantum systems
coupled to a reservoir, and predicts a linear decay in time. Although it arises
at relatively short times, the Fermi regime does not take hold in the earliest
stages of the quantum dynamics. The standard criterion in the literature for
the onset time of the Fermi regime is , with
the frequency interval around the resonant transition frequency
of the system, over which the coupling to the reservoir does not
vary appreciably. In this work, this criterion is shown to be inappropriate in
general for broadband reservoirs, where the reservoir coupling spectrum takes
the form , and for which it is found
that for , the onset time of the Fermi regime is given by
where is the high-frequency cutoff of the reservoir.
Therefore, the onset of the Fermi regime can take place at times orders of
magnitude larger than those predicted by the standard criterion. This
phenomenon is shown to be related to the excitation of the off-resonant
frequencies of the reservoir at short times. For broadband reservoirs with
, and for narrowband reservoirs, it is shown that the standard
criterion is correct. Our findings revisit the conditions of applicability of
Fermi's golden rule and improve our understanding of the dynamics of unstable
quantum systems
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