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∆tll = t1l ’ t2l = T1 ’ T2 + , (10.88)
S 1 ’ S2
∆tss = t1s ’ t2s = T1 ’ T2 + , (10.89)
and two processes will not interfere if the di¬erence between their ∆ts is larger than
the two-photon coherence time „2 de¬ned by eqn (10.58). For example, eqns (10.86)
and (10.87) yield the di¬erence
∆L1 + ∆L2
∆tls ’ ∆tsl = „2 , (10.90)
where the ¬nal inequality follows from the condition (10.83) and the fact that „1 ∼ „2 .
The conclusion is that the processes l“s and s“l cannot interfere, since they lead to
di¬erent ¬nal states. Similar calculations show that l“s and s“l are distinguishable
from l“l and s“s; therefore, the only remaining possibility is interference between l“l
and s“s. In this case the di¬erence is
∆L1 ’ ∆L2
∆tll ’ ∆tss = , (10.91)
so that interference between these two processes can occur if the condition
|∆L1 ’ ∆L2 |
„2 (10.92)
is satis¬ed. The practical e¬ect of these conditions is that the interferometers must be
almost identical, and this is a source of experimental di¬culty.
When the condition (10.92) is satis¬ed, the ¬nal states reached by the short“short
and long“long paths are indistinguishable, so the corresponding amplitudes must be
added in order to calculate the coincidence probability, i.e.
P12 = |All + Ass |2 . (10.93)
The amplitudes for the two paths are
All = r1 t1 r2 t2 ei¦ll ,
Ass = r1 t1 r2 t2 ei¦ss ,

where (rj , tj ) and rj , tj are respectively the re¬‚ection and transmission coe¬cients
for the ¬rst and second beam splitter in the jth interferometer, and the phases ¦ll
¿¿¾ Experiments in linear optics

and ¦ss are the sums of the one-photon phases for each path. We will simplify this
calculation by assuming that all beam splitters are balanced and that the photon
frequencies are degenerate, i.e. ω1 = ω2 = ω0 = ωp /2. In this case the phases are
¦ll = ω0 (t1l + t2l ) = ω0 (T1 + T2 ) + (L1 + L2 ) ,
c (10.95)
¦ss = ω0 (t1s + t2s ) = ω0 (T1 + T2 ) + (S1 + S2 ) ,
and the coincidence probability is
P12 = cos2 , (10.96)
∆¦ = ¦ll ’ ¦ss = (∆L1 + ∆L2 ) . (10.97)
Now suppose that Bob and Alice initially choose the same optical delay for their
respective interferometers, i.e. they set ∆L1 = ∆L2 = ∆L, then
∆¦ ω0 ∆L
= ∆L = 2π , (10.98)
2 c »0
where »0 = 2πc/ω0 is the common wavelength of the two photons. If the delay ∆L
is arranged to be an integer number m of wavelengths, then ∆¦/2 = 2πm and P12
achieves the maximum value of unity. In other words, with these settings the behavior
of the photons at the ¬nal beam splitters are perfectly correlated, due to constructive
interference between the two probability amplitudes.
Next consider the situation in which Bob keeps his settings ¬xed, while Alice alters
her settings to ∆L1 = ∆L + δL, so that
∆¦ δL
= 2πm + π , (10.99)
2 »0
P12 = cos2 π . (10.100)
For the special choice δL = »0 /2, the coincidence probability vanishes, and the be-
havior of the photons at the ¬nal beam splitters are anti-correlated, due to complete
destructive interference of the probability amplitudes. This drastic change is brought
about by a very small adjustment of the optical delay in only one of the interferom-
eters. We should stress the fact that macroscopic physical events”the ¬ring of the
detectors”that are spatially separated by a large distance behave in a correlated or
anti-correlated way, by virtue of the settings made by Alice in only one of the inter-
In Chapter 19 we will see that these correlations-at-a-distance violate the Bell
inequalities that are satis¬ed by any so-called local realistic theory. We recall that a
theory is said to be local if no signals can propagate faster than light, and it is said to be
realistic if physical objects can be assumed to have de¬nite properties in the absence of
observation. Since the results of experiments with the Franson interferometer violate
Bell™s inequalities”while agreeing with the predictions of quantum theory”we can
conclude that the quantum theory of light is not a local realistic theory.
Single-photon interference revisited— ¿¿¿

Single-photon interference revisited—
The experimental techniques required for the Hong“Ou“Mandel demonstration of
two-photon interference”creation of entangled photon pairs by spontaneous down-
conversion (SDC), mixing at beam splitters, and coincidence detection”can also be
used in a beautiful demonstration of a remarkable property of single-photon interfer-
ence. In our discussion of Young™s two-pinhole interference in Section 10.1, we have
already remarked that any attempt to obtain which-path information destroys the
interference pattern. The usual thought experiments used to demonstrate this for the
two-pinhole con¬guration involve an actual interaction of the photon”either with
some piece of apparatus or with another particle”that can determine which pinhole
was used. The experiment to be described below goes even further, since the mere pos-
sibility of making such a determination destroys the interference pattern, even if the
measurements are not actually carried out and no direct interaction with the photons
occurs. This is a real experimental demonstration of Feynman™s rule that interference
can only occur between alternative processes if there is no way”even in principle”to
distinguish between them. In this situation, the complex amplitudes for the alterna-
tive processes must ¬rst be added to produce the total probability amplitude, and only
then is the probability for the ¬nal event calculated by taking the absolute square of
the total amplitude.

10.3.1 Mandel™s two-crystal experiment
In the two-crystal experiment of Mandel and his co-workers (Zou et al., 1991), shown in
Fig. 10.8, the beam from an argon laser, operating at an ultraviolet wavelength, falls on
the beam splitter BSp . This yields two coherent, parallel pump beams that enter into
two staggered nonlinear crystals, NL1 and NL2, where they can undergo spontaneous
down-conversion. The rate of production of photon pairs in the two crystals is so low
that at most a single photon pair exists inside the apparatus at any given instant. In

IFs Amp.
s1 Counter
V1 disc.
i1 s2
NL2 Coincidence
From argon laser i2
IFi disc.

Fig. 10.8 Spontaneous down-conversion (SDC) occurs in two crystals NL1 and NL2. The
two idler modes i1 and i2 from these two crystals are carefully aligned so that they coincide
on the face of detector Di . The dashed line in beam path i1 in front of crystal NL2 indicates a
possible position of a beam block, e.g. an opaque card. (Reproduced from Zou et al. (1991).)
¿¿ Experiments in linear optics

other words, we can assume that the simultaneous emission of two photon pairs, one
from each crystal, is so rare that it can be neglected.
The idler beams i1 and i2 , emitted from the crystals NL1 and NL2 respectively,
are carefully aligned so that their transverse Gaussian-mode beam pro¬les overlap as
exactly as possible on the face of the idler detector Di . Thus, when a click occurs in Di ,
it is impossible”even in principle”to know whether the detected photon originated
from the ¬rst or the second crystal. It therefore follows that it is also impossible”even
in principle”to know whether the twin signal wave packet, produced together with
the idler wave packet describing the detected photon, originated from the ¬rst crystal
as a signal wave packet in beam s1 , or from the second crystal as a signal wave packet
in beam s2 . The two processes resulting in the appearance of s1 or s2 are, therefore,
indistinguishable; and their amplitudes must be added before calculating the ¬nal
probability of a click at detector Ds .

10.3.2 Analysis of the experiment
The two indistinguishable Feynman processes are as follows. The ¬rst is the emission
of the signal wave packet by the ¬rst crystal into beam s1 , re¬‚ection by the mirror
M1 , re¬‚ection at the output beam splitter BSo , and detection by the detector Ds . This
is accompanied by the emission of a photon in the idler mode i1 that traverses the
crystal NL2”which is transparent at the idler wavelength”and falls on the detector
Di . The second process is the emission by the second crystal of a photon in the signal
wave packet s2 , transmission through the output beam splitter BSo , and detection by
the same detector Ds , accompanied by emission of a photon into the idler mode i2
which falls on Di . This experiment can be analyzed in two apparently di¬erent ways
that we consider below.

A Second-order interference
Let us suppose that the photon detections at Ds are registered in coincidence with
the photon detections at Di , and that the two idler beams are perfectly aligned. If a
click were to occur in Ds in coincidence with a click in Di , it would be impossible to
determine whether the signal“idler pair came from the ¬rst or the second crystal. In
this situation Feynman™s interference rule tells us that the probability amplitude A1
that the photon pair originates in crystal NL1 and the amplitude A2 of pair emission
by NL2 must be added to get the probability

|A1 + A2 |2 (10.101)

for a coincidence count. When the beam splitter BSo is slowly scanned by small trans-
lations in its transverse position, the signal path length of the ¬rst process is changed
relative to the signal path length of the second process. This in turn leads to a change
in the phase di¬erence between A1 and A2 ; therefore, the coincidence count rate would
exhibit interference fringes.
From Section 9.2.4 we know that the coincidence-counting rate for this experiment
is proportional to the second-order correlation function
(’) (+)
G(2) (xs , xi ; xs , xi ) = Tr ρin Es (xs ) Ei
(’) (+)
(xi ) Ei (xi ) Es (xs ) , (10.102)
Single-photon interference revisited— ¿¿

where ρin is the density operator describing the initial state of the photon pair produced
by down-conversion. The subscripts s and i respectively denote the polarizations of
the signal and idler modes. The variables xs and xi are de¬ned as xs = (rs , ts ) and
xi = (ri , ti ), where rs and ri are respectively the locations of the detectors Ds and Di ,
while ts and ti are the arrival times of the photons at the detectors. This description
of the experiment as a second-order interference e¬ect should not be confused with the
two-photon interference studied in Section 10.2.1. In the present experiment at most
one photon is incident on the beam splitter BSo during a coincidence-counting window;
therefore, the pairing phenomena associated with Bose statistics for two photons in
the same mode cannot occur.

B First-order interference
Since the state ρin involves two photons”the signal and the idler”the description in
terms of G(2) o¬ered in the previous section seems very natural. On the other hand,
in the ideal case in which there are no absorptive or scattering losses and the classical
modes for the two idler beams i1 and i2 are perfectly aligned, an idler wave packet will
fall on Di whenever a signal wave packet falls on Ds . In this situation, the detector Di is
actually super¬‚uous; the counting rate of detector Ds will exhibit interference whether
or not coincidence detection is actually employed. In this case the amplitudes A1 and
A2 refer to the processes in which the signal wave packet originates in the ¬rst or the
second crystal. The counting rate |A1 + A2 | at detector Ds will therefore exhibit the
same interference fringes as in the coincidence-counting experiment, even if the clicks
of detector Di are not recorded. In this case the interference can be characterized solely
by the ¬rst-order correlation function

G(1) (xs ; xs ) = Tr ρin Es (xs ) Es (xs ) .
(’) (+)

In the actual experiment, no coincidence detection was employed during the collection
of the data. The ¬rst-order interference pattern shown as trace A in Fig. 10.9 was
obtained from the signal counter Ds alone. In fact, the detector Di and the entire
coincidence-counting circuitry could have been removed from the apparatus without
altering the experimental results.

10.3.3 Bizarre aspects
The interference e¬ect displayed in Fig. 10.9 may appear strange at ¬rst sight, since the
signal wave packets s1 and s2 are emitted spontaneously and at random by two spatially
well-separated crystals. In other words, they appear to come from independent sources.
Under these circumstances one might expect that photons emitted into the two modes
s1 and s2 should have nothing to do with each other. Why then should they produce
interference e¬ects at all? The explanation is that the presence of at most one photon
in a signal wave packet during a given counting window, combined with the perfect
alignment of the two idler beams i1 and i2 , makes it impossible”even in principle”to
determine which crystal actually emitted the detected photon in the signal mode. This
is precisely the situation in which the Feynman rule (10.2) applies; consequently, the
amplitudes for the processes involving signal photons s1 or s2 must be added, and
interference is to be expected.
¿¿ Experiments in linear optics

Displacement of BSo in µm

Counting rate 4I (per second)

Phase in multiples of π

Fig. 10.9 Interference fringes of the signal photons detected by Ds , as the transverse position
of the ¬nal splitter BSo is scanned (see Fig. 10.8). Trace A is taken with a neutral 91%
transmission density ¬lter placed between the two crystals. Trace B is taken with the beam
path i1 blocked by an opaque card (i.e. a ˜beam block™). (Reproduced from Zou et al. (1991).)

Now let us examine what happens if the experimental con¬guration is altered
in such a way that which-path information becomes available in principle. For this
purpose we assign Alice to control the position of the beam splitter BSo and record
the counting rate at detector Ds , while Bob is put in charge of the entire idler arm,
including the detector Di . As part of an investigation of possible future modi¬cations of
the experiment, Bob inserts a neutral density ¬lter (an ideal absorber with amplitude
transmission coe¬cient t independent of frequency) between NL1 and NL2, as shown
by the line NDF in Fig. 10.8. Since the ¬lter interacts with the idler photons, but
does not interact with the signal photons in any way, Bob expects that he can carry
out this modi¬cation without any e¬ect on Alice™s measurements. In the extreme limit
t ≈ 0”i.e. the idler photon i1 is completely blocked, so that it will never arrive at
Di ”Bob is surprised when Alice excitedly reports that the interference pattern at Ds
has completely disappeared, as shown in trace B of Fig. 10.9.
Alice and Bob eventually arrive at an explanation of this truly bizarre result by
a strict application of the Feynman interference rules (10.1)“(10.3). They reason as
follows. With the i1 -beam block in place, suppose that there is a click at Ds but not at
Di . Under the assumption that both Ds and Di are ideal (100% e¬ective) detectors, it
then follows with certainty that no idler photon was emitted by NL2. Since the signal
and idler photons are emitted in pairs from the same crystal, it also follows that the
signal photon must have been emitted by NL1. Under the same circumstances, if there
are simultaneous clicks at Ds and Di , then it is equally certain that the signal photon
must have come from NL2. This means that Bob and Alice could obtain which-path
information by monitoring both counters. Therefore, in the new experimental con-
¬guration, it is in principle possible to determine which of the alternative processes
Tunneling time measurements— ¿¿

actually occurred. This is precisely the situation covered by rule (10.3), so the proba-
bility of a count at Ds is the sum of the probabilities for the two processes considered
separately; there is no interference. A truly amazing aspect of this situation is that the
interference pattern disappears even if the detector Di is not present. In fact”just as
before”the detector Di and the entire coincidence-counting circuitry could have been
removed from the apparatus without altering the experimental results. Thus the mere
possibility that which-path information could be gathered by inserting a beam block
is su¬cient to eliminate the interference e¬ect.
The phenomenon discussed above provides another example of the nonlocal char-
acter of quantum physics. Bob™s insertion or withdrawal of the beam blocker leads to
very di¬erent observations by Alice, who could be located at any distance from Bob.
This situation is an illustration of a typically Delphic remark made by Bohr in the
course of his dispute with Einstein (Bohr, 1935):
But even at this stage there is essentially the question of an in¬‚uence on the very
conditions which de¬ne the possible types of predictions regarding the future behavior
of the system.

With this hint, we can understand the e¬ect of Bob™s actions as setting the overall
conditions of the experiment, which produce the nonlocal e¬ects.
An interesting question which has not been addressed experimentally is the follow-
ing: How soon after a sudden blocking of beam path i1 does the interference pattern
disappear for the signal photons? Similarly, how soon after a sudden unblocking of
beam path i1 does the interference pattern reappear for the signal photons?

Tunneling time measurements—
Soon after its discovery, it was noticed that the Schr¨dinger equation possessed real,
exponentially damped solutions in classically forbidden regions of space, such as the
interior of a rectangular potential barrier for a particle with energy below the top of
the barrier. This phenomenon”which is called tunneling”is mathematically similar
to evanescent waves in classical electromagnetism.
The ¬rst observation of tunneling quickly led to the further discoveries of important
early examples, such as the ¬eld emission of electrons from the tips of cold, sharp
metallic needles, and Gamow™s explanation of the emission of alpha particles (helium
nuclei) from radioactive nuclei undergoing ± decay.
Recent examples of the applications of tunneling include the Esaki tunnel diode
(which allows the generation of high-frequency radio waves), Josephson tunneling be-
tween two superconductors separated by a thin oxide barrier (which allows the sensi-
tive detection of magnetic ¬elds in a S uperconducting QU antum I nterference D evice
(SQUID)), and the scanning tunneling microscope (which allows the observation of
individual atoms on surfaces).
In spite of numerous useful applications and technological advances based on tun-
neling, there remained for many decades after its early discovery a basic, unresolved
physics problem. How fast does a particle traverse the barrier during the tunneling
process? In the case of quantum optics, we can rephrase this question as follows: How
quickly does a photon pass through a tunnel barrier in order to reach the far side?
¿¿ Experiments in linear optics

First of all, it is essential to understand that this question is physically meaningless
in the absence of a concrete description of the method of measuring the transit time.
This principle of operationalism is an essential part of the scienti¬c method, but it is
especially crucial in the studies of phenomena in quantum mechanics, which are far re-
moved from everyday experience. A de¬nition of the operational procedure starts with
a careful description of an idealized thought experiment. Thought experiments were
especially important in the early days of quantum mechanics, and they are still very
important today as an aid for formulating physically meaningful questions. Many of
these thought experiments can then be turned into real experiments, as measurements
of the tunneling time illustrate.
Let us therefore ¬rst consider a thought experiment for measuring the tunneling
time of a photon. In Fig. 10.10, we show an experimental method which uses twin
photons γ1 and γ2 , born simultaneously by spontaneous down-conversion. Placing two
Geiger counters at equal distances from the crystal would lead”in the absence of any
tunnel barrier”to a pair of simultaneous clicks. Now suppose that a tunnel barrier
is inserted into the path of the upper photon γ1 . One might expect that this would
impede the propagation of γ1 , so that the click of the upper Geiger counter”placed
behind the barrier”would occur later than the click of the lower Geiger counter. The
surprising result of an experiment to be described below is that exactly the opposite
happens. The arrival of the tunneling photon γ1 is registered by a click of the upper
Geiger counter that occurs before the click signaling the arrival of the nontunneling
photon γ2 . In other words, the tunneling photon seems to have traversed the barrier
superluminally. However, for reasons to be given below, we shall see that there is no
operational way to use this superluminal tunneling phenomenon to send true signals
faster than the speed of light.
This particular thought experiment is not practical, since it would require the use
of Geiger counters with extremely fast response times, comparable to the femtosecond
time scales typical of tunneling. However, as we have seen earlier, the Hong“Ou“
Mandel two-photon interference e¬ect allows one to resolve the relative times of arrival
of two photons at a beam splitter to within fractions of a femtosecond. Hence, the

Fig. 10.10 Schematic of a thought experi-
ment to measure the tunneling time of the
photon. Spontaneous down-conversion gener-
ates twin photons γ1 and γ2 by absorption of a
photon from a UV pump laser. In the absence
of a tunnel barrier, the two photons travel the
UV laser
same distance to two Geiger counters placed γ0
equidistantly from the crystal, and two simul-
taneous clicks occur. A tunnel barrier (shaded
rectangle) is now inserted into the path of pho-
ton γ1 . The tunneling time is given by the time
crystal Geiger
di¬erence between the clicks of the two Geiger
Tunneling time measurements— ¿¿

impractical thought experiment can be turned into a realistic experiment by inserting
a tunnel barrier into one arm of a Hong“Ou“Mandel interferometer (Steinberg and
Chiao, 1995), as shown in Fig. 10.11.
The two arms of the interferometer are initially made equal in path length (per-
fectly balanced), so that there is a minimum”a Hong“Ou“Mandel (HOM) dip”in
the coincidence count rate. After the insertion of the tunnel barrier into the upper
arm of the interferometer, the mirror M1 must be slightly displaced in order to recover
the HOM dip. This procedure compensates for the extra delay”which can be either
positive or negative”introduced by the tunnel barrier. Measurements show that the

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