Photon beam generator

Optical waveguides – Having nonlinear property

Reexamination Certificate

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C359S330000

Reexamination Certificate

active

06421488

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a photon beam generating apparatus for generating two photon beams containing photons generated simultaneously and constituting a photon pair, and to a photon beam generating apparatus capable of determining the time of generation of the photons included in the photon beams. The photon beam generating apparatus according to the present invention can be used in a quantum cryptographic communications system, a quantum computation system, an analysis system, or a like system.
BACKGROUND ART
If photons constituting a pair can be produced simultaneously, the time and the position at which one of the paired photons is present can be determined through measurement of the other photon. In general, such a pair of photons having high temporal correlation is generated through generation of a parametric fluorescence pair.
A parametric fluorescence pair is constituted of two photons {overscore (h)}&ohgr;
i
and {overscore (h)}&ohgr;
s
produced when a photon having an energy of {overscore (h)}&ohgr;
0
enters a non-linear optical medium. In this case, {overscore (h)} represents a value obtained through division of Planck's constant h=6.62×10
−34
[j·s] by 2&pgr;. &ohgr;
s
, and &ohgr;
0
respectively represent the frequency of a signal beam, the frequency of an idler beam, and the frequency of an incident photon (“signal beam” and “idler beam” are conventional names representing respective photons in each photon pair). Also, according to the law of conservation of energy, the following relation is satisfied:
&ohgr;
0
=&ohgr;
i
+&ohgr;
s
.  (1)
In addition, the following relation with regard to conservation of momentum is satisfied:
{overscore (h)}k
0
={overscore (h)}k
i
+{overscore (h)}k
s
  (2)
where k
s
, k
i
, and k
0
respectively represent the frequency of the signal beam, the frequency of the idler beam, and the frequency of the incident photon beam. Equations (1) and (2) are called phase-matching conditions. In order to produce parametric fluorescence pairs, the phase-matching conditions must be satisfied within a medium having a sufficient non-linear constant.
FIG. 14
shows an example of a conventional technique utilizing parametric fluorescence pairs described in Sergienko et. al, Journal of Optical Society of America B, May 1995, Vol.
12
, No. 5, pp 859, “Experimental Evaluation of a Two-Photon Wave Packet in Type-II Parametric Downconversion.”
In
FIG. 14
, reference numeral
13
denotes an argon laser,
14
denotes an incident pump beam,
15
denotes a dispersion prism,
25
denotes a BBO crystal,
31
denotes a dispersion prism, and
32
denotes a parametric fluorescence beam. Although the experiment was conducted for measurement of time correlation between produced photons constituting a pair, in
FIG. 14
, portions other than the portion used for generation of photon beams are omitted for simplicity.
The argon laser
13
produces a single frequency UV laser beam (having a wavelength of 351.1 nm) serving as the incident pump beam
14
. The dispersion prism
15
is used for eliminating components other than the component having a wavelength of 351.1 nm from the beam generated by the argon laser. When the incident pump beam
14
enters the BBO crystal
25
, parametric fluorescence pairs are produced therein. In the experiment, the angle between the crystal axis of the BBO crystal
25
and the incident pump beam
14
is set at 49.2 degrees in order to satisfy a collinear condition. The collinear condition specifies that the wave number vector of the incident pump beam
14
is parallel with the wave number vectors of the produced fluorescence pairs. The details of the collinear condition will be described in greater detail in the embodiments of the present invention. Because the produced parametric fluorescence beams
32
travel along the axis of the incident pump beam
14
, the fluorescence beams
32
are separated from the incident beam
14
by the dispersion prism
31
before being used.
The collinear condition is used not only in the example described above but also in a wide range of experiments related to generation of parametric fluorescence pairs. The reason for this is as follows. When an optical system is constructed, the tilt angles and the positions of optical components are adjusted on the basis of observation of an image of a standard laser beam or an image of the standard laser beam reflected from a surface of each optical component. Generally, since the parametric fluorescence light is of extremely low intensity, a special device such as a cooled CCD must be employed in order to detect the position and the direction of propagation of the produced light. Thus, construction of an experimental system becomes difficult. However, under the collinear condition, a UV pump beam and generated fluorescence pairs propagate collinearly and in the same direction. Consequently, by setting a reference laser beam, which has a wavelength close to that of the fluorescence light, coaxial with the UV pump beam, construction of the experiment system becomes relatively easy.
However, generation of fluorescence pairs under the collinear condition has involved a major drawback. Under the collinear condition, the parametric fluorescence light is emitted over a wide angular range (6.5 degrees in the example). Accordingly, fluorescence pairs radiated in the same direction as the UV pump beam, which are used in the experiment, constitute only a portion of the fluorescence pairs which are actually produced. Consequently, in a conventional optical system:
1. it is difficult to convert the portion of the fluorescence pairs into a beam which has a circular or oval cross-section and which can be used easily;
2. it is difficult to cut out or select a pair of photons radiated in correlated directions; and
3. the quantity of the parametric fluorescence light per unit radiation angle is small.
The reasons for the cause of the above-mentioned difficulties and drawbacks will now be described in detail.
FIG. 15
shows results of calculation performed with regard to the radiation angles of parametric fluorescence pairs under the collinear condition, described in “Proposal for a Loophole-Free Bell Inequality Experiment,” Paul G. Kwiat, et. al., Physical Review A, Vol. 49, No. 5, (1994) pp 3209.
FIG. 15
is a plot showing radiation angles of fluorescence pairs with respect to the UV pump beam that enters the crystal in a direction perpendicular to the sheet of
FIG. 15
from the back thereof. The optical axis of the crystal is directed upward in FIG.
15
. Each hollow triangle indicates the radiation angle of an extraordinary-polarized fluorescence beam and each hollow circle indicates the radiation angle of an ordinary-polarized fluorescence beam. The hollow triangle and the hollow circle are both present at the origin, where the collinear condition is satisfied. As can be seen from the plot, under the collinear condition, the fluorescence pairs are radiated over a wide angular range. Accordingly, as has been described, in a case where only a portion of the fluorescence pairs radiated in the same direction as the UV pump beam is used, as in conventional experiments, the following difficulties arise.
1. As can be seen from
FIG. 15
, when fluorescence pairs radiated in the same direction as the UV pump beam are used, the fluorescence pairs are cut out as a portion of an arc as shown in the photographs of FIG.
8
. It is difficult to convert the thus-cut-out portion of the fluorescence pairs into a beam having a circular or an oval cross-section without involving a reduction in light intensity. Moreover, if the cut-out portion is converted as such by the use of a suitable pinhole, the number of useable fluorescence pairs decreases due to a loss produced at the pinhole.
2. Photons of a generated fluorescence pair are radiated to positions that are symmetric with respect to the origin in the plot of FIG.
15
. Accordingly, in order to obtain paired fluorescence beams each containin

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