Vacuum source-field correlations and advanced
waves in quantum optics
Adam Stokes
Photon Science Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom
January 17, 2018
The solution to the wave equation as a Cauchy
problem with prescribed fields at an initial time
t = 0 is purely retarded. Similarly, in the
quantum theory of radiation the specification of
Heisenberg picture photon annihilation and cre-
ation operators at time t > 0 in terms of opera-
tors at t = 0 automatically yields purely retarded
source-fields. However, we show that two-
time quantum correlations between the retarded
source-fields of a stationary dipole and the quan-
tum vacuum-field possess advanced wave-like
contributions. Despite their advanced nature,
these correlations are perfectly consistent with
Einstein causality. It is shown that while they do
not significantly contribute to photo-detection
amplitudes in the vacuum state, they do effect
the statistics of measurements involving the ra-
diative force experienced by a point charge in
the field of the dipole. Specifically, the disper-
sion in the charge’s momentum is found to in-
crease with time. This entails the possibility of
obtaining direct experimental evidence for the
existence of advanced waves in physical reality,
and provides yet another signature of the quan-
tum nature of the vacuum.
1 Introduction
Electrodynamics describes the production, propagation
and absorption of electromagnetic waves. The problem
most commonly encountered in classical electrodynam-
ics is that in which the source is specified at some ini-
tial time, and then the electromagnetic fields are sought
at later times. The unique solution to this problem is
fully retarded, which means that the electromagnetic
source-fields depend in a causal way on the state of the
source in the past [1, 12, 36]. Advanced source-fields,
which depend on the source in the future are obtained
when Maxwell’s equations are solved running into the
past [1, 31]. These solutions are not viewed as physical
within the context of an initial value problem.
It is well known that in quantum optics the com-
mutators of the free electromagnetic fields involve both
retarded and advanced green’s functions for the wave-
operator [6]. On the other hand the subject of advanced
waves in the presence of sources has received more lim-
ited attention in quantum theory. The reason for this
may be that the problem of finding the Heisenberg pic-
ture source-fields at time t > 0 in terms of Schr¨odinger
picture operators at time t = 0 is an initial value prob-
lem whose solution entails purely retarded source-fields.
However, due to differences between quantum and clas-
sical theories the complete absence of advanced contri-
butions in quantum optics is not immediate. The fluc-
tuating quantum vacuum offers one of the most striking
examples of such a difference. The theories also differ
in their treatment of electromagnetic correlations.
The quantum vacuum is used to interpret various
phenomena in quantum optics including spontaneous
emission [18], atomic energy-shifts [24, 35], and Casimir
forces [5]. Together with the laws of space and time
the quantum theory of the vacuum predicts intriguing
physical phenomena such as Hawking radiation asso-
ciated with black holes [11] and the Unruh-Davies ef-
fect whereby an accelerated detector in the vacuum
of Minkowski spacetime registers thermal excitations
[8, 34]. The Unruh-Davies effect and related effects [20]
can be understood in terms of correlations between the
quantum vacuum at timelike separated points in space-
time. Vacuum induced correlations across spacelike sep-
arations have also received attention [25].
In atomic and optical physics many effects attributed
to the quantum vacuum can often also be understood, at
least partially, in terms of radiation reaction [7, 13, 17].
Thus, identifying specific signatures associated with the
quantum vacuum remains of broad interest in quantum
optics. Recently, direct observation of vacuum fluctu-
ations has been achieved [19, 26, 27], which opens up
new possibilities for investigating the interplay between
vacuum fluctuations and the electromagnetic fields pro-
duced by charged sources.
In this paper we consider the fields produced by a
quantum dipole. We show that the presence of the
Accepted in Quantum 2017-12-15, click title to verify 1
arXiv:1710.06207v3 [quant-ph] 16 Jan 2018