Patent Abstract:
A communication system employs quantum entanglement by projecting photons through a nonlinear crystal. Some become parametrically down-converted into signal and idler photon pairs. The signal photons are projected to a receiver and the idler photons to a transmitter. The transmitter operator can alter the time width and a majority of the center wavelengths of the idler photons via a collapse event in the transmitter. Because of quantum entanglement, a corresponding change in the time width and center wavelengths of the signal photons as received at the receiver results. The purposeful causation of the collapse event or a lack of such purposeful causation can be used for binary communication. In addition, the sensing of an atmospheric condition may be performed by equating changes in received signal photon characteristics with changes in collapse conditions in the atmosphere.

Full Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to communications and, in particular, to a communications system employing the principle of quantum entanglement. 
   The constant desire for “greater bandwidth” reflects an ever increasing demand placed on modern communication systems to rapidly transfer large amounts of information from one place to another. Classical communication techniques have been quite effective in meeting this demand, but these techniques are now approaching their theoretical limits. 
   It is therefore considered desirable to explore non-traditional approaches to enhance communication. 
   SUMMARY 
   A non-traditional communications system utilizes the quantum mechanics principle of quantum entanglement. An example communication system employing quantum entanglement includes the steps of projecting a pulse of photons through a nonlinear crystal. Photons making up a portion of this projected pulse are each parametrically down-converted into a signal and idler quantum-entangled photon pair. This conversion results in a series of signal photons and a series of idler photons. Another portion of the projected pulse is not down-converted, resulting in a series of non down-converted pulse photons corresponding to the projected pulse. 
   The series of signal photons and series of non down-converted pulse photons are projected to a receiver. 
   The series of idler photons are projected to a transmitter. The transmitter contains a collapse condition wherein a time width of each of the idler photons is altered and wherein a majority of the center wavelengths of each of the idler photons is altered. Because of quantum entanglement, a change to an idler photon results in a corresponding change to a corresponding signal photon as received at the receiver. The transmitter also has a non-collapse condition wherein the time width and center wavelength of each of the idler photons is left unaltered and wherein the time width and center wavelength of each of the corresponding signal photons as received at said receiver are left unaltered. An example of such a collapse condition is a measurement of the frequency of the idler photons, however such a collapse condition may exist upon encountering certain atmospheric conditions such as atmospheric aerosols. 
   The receiver is used to provide detection of whether the signal photons corresponding to the projected pulse and as received at said receiver have been altered or not. This detection is enhanced by projecting the altered and unaltered signal photons through a nonlinear element that enhances the differences between the two types of signal photons. A cumulative time distribution of the series of signal photons as received at the receiver is then assessed for each pulse or for a number of pulses to determine whether the signal photons have been altered or not. 
   The purposeful causation of the collapse event or a lack of such purposeful causation can be used for binary communication. In addition, the sensing of an atmospheric condition may be performed by equating changes in received signal photon characteristics with changes in collapse conditions in the atmosphere. 
   Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary pulse source. 
       FIG. 2  shows an example transmitter. 
       FIG. 3  depicts a communications receiver. 
       FIG. 4  illustrates cumulative time distributions of signal photons in altered and unaltered states. 
       FIG. 5  shows an alternative receiver. 
   

   DESCRIPTION 
   A communications system to be further described herein includes a pulse source, a transmitter, and a receiver. The optical path length from the source to the receiver is made to be slightly greater than the optical path length from the source to the transmitter. 
   Referring now to  FIG. 1 , an example of pulse source  10  includes a laser  12 , a nonlinear crystal  14 , and a wavelength selective mirror  18 . 
   The laser is chosen to output pulses of photons. Suitable example pulses have full width at half maximum (FWHM) of approximately 200 femtoseconds. A suitable example laser pulse repetition frequency (PRF) is approximately 70 MHz. The pulse amplitude and pulse shape are highly stable from pulse to pulse. 
   Projected pulse  16  includes pulsed-laser photons. The laser pulse of photons are used as “pump” photons in nonlinear crystal (NLC)  14 . In NLC  14 , a portion of these “pump” photons may be parametrically down-converted into quantum entangled “signal” (S) and “idler” (I) pairs of photons. Another portion of these pump photons pass through NLC  14  in a non down-converted state (P). 
   Nonlinear crystal  14  is cut to allow parametric down-conversion via colinear, type I phase-matching, non-degenerate, however other cuts are possible. For example, colinear, type II, non-degenerate or degenerate; noncolinear, degenerate, and noncolinear, non-degenerate, type I or type II. The pump pulse photons (P) are assumed as being vertically polarized, and therefore any down-converted signal (S) and idler (I) photons produced will be horizontally polarized. Such signal photons will have a shorter wavelength than the idler photons. Under the type I phase-matching condition, the polarization orientation of the down-converted photons will be of a polarization that is orthogonal to the polarization of the non down-converted pump pulse photons. 
   An output from NLC ( 14 ) is incident on a wavelength selective mirror (WSM)  18  such as a dielectric mirror. The non down-converted pump pulse (P) photons and any signal (S) photons are totally reflected by WSM  18  and are sent to a receiver  20 , to be further described. The longer-wavelength idler (I) photons are transmitted through WSM  18  and are sent to a transmitter  22 , to be further described. 
   Both the signal photon (S) and idler (I) photon that are produced by the parametric down-conversion of a pump pulse photon in the nonlinear crystal NLC  14  have wide bandwidths. These wide bandwidths exist because the pump pulse photons have a wide bandwidth, (proportional to the reciprocal of the pulsewidth of the pump pulse), and because of the very large number of different frequency combinations of signal and idler photons allowed by energy conservation and phase matching. Since, quantum mechanically, the signal and idler photons represent a superposition of all allowed possibilities, the bandwidths of the signal and idler photons will be wider than the bandwidth of the pump pulse photons that produced them in NLC  14 . Consequently, the time width of both the signal photon and the idler photon will be shorter than the pulsewidth of the pump pulse. 
   For an assumed case of a 200 femtosecond (FWHM), Gaussian-shaped pump pulse  16  (center wavelength ˜390 nm), the time width of both the signal photon (center wavelength ˜683 nm) and the idler photon (center wavelength ˜909 nm) will be approximately 44 femtoseconds (FWHM). The time profile of both the signal photon and idler photon will be approximately Gaussian. 
   Different signal photons are created at different times. However, no inherent photon property distinguishes one signal photon from another signal photon (or one idler photon from another idler photon). As they are produced, each signal photon is identical to any other signal photon (same bandwidth, same time width, same center wavelength). Each idler photon is identical with any other idler photon, since (according to Quantum Mechanics) all allowed possibilities are present in superposition in each individual photon, and the possibilities that are allowed do not change from one photon to the next. 
   Referring to  FIG. 2 , an example transmitter  22  includes a moveable mirror (MM)  24 , a photon detector (PD)  26 , a spectrometer  28 , and a detector array  30 . Moveable mirror  24  can be inserted into or removed from idler photon beam path  32  coming from source  10  of  FIG. 1  and may be electro-optically switched. 
   If moveable mirror  24  is removed, a “collapse” condition beam path  34  results wherein idler photons (I) from source  10  will enter spectrometer  28 . In this condition spectrometer  28  and detector array  30  are used to make a precise measurement of the frequency of the idler photons. 
   If moveable mirror  24  is inserted into the beam path, the idler photons are reflected at mirror  24  and travel a non “collapse” condition path  36  to be incident on photon detector  26 . In this case, the idler photons are detected, but their energy is not measured. 
   Referring now to  FIG. 3 , example receiver  20  is to be described first in regard to its components and secondly in regard to its use. Receiver  20  includes three polarizing beam splitters ( 38 ,  40  and  42 ), a corner cube reflector  44 , three optical mirrors ( 46 ,  48 ,  50 ), a nonlinear dispersion element  52 , a forty-five degree polarization rotator  54 , a beam stop  56 , an optical Kerr cell (OKC)  58 , one (or more) color filters  60 , a photo-diode detector  62 , and two photon detectors  64  and  66 . 
   The non down-converted pump pulse (P) photons and the down-converted signal (S) photons from source  10  are incident on first polarizing beam splitter  38  at receiver  20 . Polarizing beam splitter  38  reflects the vertically polarized pump pulse (P) photons and transmits the horizontally polarized signal (S) photons. 
   The pump pulse (P) photons enter an adjustable “mirror delay channel” ( 68 ), that includes corner reflector  44  and mirror  46 . After reflecting from mirror  46 , the pump pulse (P) photons are incident on the forty-five degree polarization rotator  54 . The polarization rotator rotates the polarization direction of almost all of the pump pulse (P) photons to an angle that is forty-five degrees from vertical. 
   The pump pulse (P) photons are next incident on the second polarizing beam splitter  40 . This beam splitter has its transmission axis set forty-five degrees from vertical so that almost all of the pump pulse (P) photons are transmitted through this beam splitter. A small number of the pump pulse (P) photons that do not have their polarization direction at forty-five degrees from vertical are reflected by beam splitter  40  and are absorbed by beam stop  56 . 
   The pump pulse (P) photons that have been transmitted through beam splitter  40  are reflected by directing mirror  48  to optical Kerr cell (OKC)  58 . The pump pulse (P) photons are incident on the OKC at a small angle from the normal to be further explained. 
   After passing through polarizing beam splitter  38 , the signal (S) photons enter nonlinear dispersion element (DE)  52 . The DE is an appropriately cut piece of dispersing material, for example, SF6 glass, and serves to enhance differences between signal photons that have been altered by the communications process and those that have not been, as will be further described. 
   After exiting element  52 , the signal photons (S) are reflected by directing mirror  50  and have near normal incidence on OKC  58 . There is approximately a 5 degree difference in angle between the signal (S) photon direction and the pump pulse (P) direction at OKC  58 . By providing this divergence, versus an alignment of the pump pulse and signal photon directions, the signal to noise ratio of the receiver is improved, as pump pulse (P) photons are ultimately prevented from reaching photon detectors  64  and  66 . 
   Both the pump pulse (P) and signal (S) photons pass through the optical Kerr cell  58 . The intensity in the pump pulse is great enough that it alters the birefringent properties of the liquid in the Kerr cell. In the absence of the pump pulse, the liquid molecules are randomly oriented, and the liquid is optically isotropic: it does not change the polarization direction of light passing through the cell. 
   In the presence of the intense pump pulse, the liquid molecules in the Kerr cell are aligned in the direction of the polarization of the pump pulse (P) photons (which is forty-five degrees from vertical). This alignment of the liquid molecules causes the liquid to become optically birefringent. If the liquid is, for example, Carbon Disulfide, this birefringence remains through the duration of the pump pulse (P) and for approximately 1.8 picoseconds after the pump pulse (P) exits the Kerr cell. 
   Signal (S) photons that pass through the Kerr cell while it is optically birefringent have their polarization direction rotated from horizontal to vertical. Signal (S) photons that pass through the Kerr cell while it is optically isotropic are not affected, and their polarization direction remains horizontal. 
   After passing through optical Kerr cell  58 , the pump pulse (P) photons are incident on photo-diode detector  62 , which along with its accompanying electronics is used to count the number of pump pulses and to measure the intensity of the pump pulses. 
   After passing through OKC  58 , the signal (S) photons pass through one (or more) color filters  60 . The color filter(s) transmit the low frequency signal (S) photons but absorb any pump pulse (P) or other “out of bandwidth” photons that may have entered the signal (S) photon path. 
   The signal (S) photons next reach the last polarizing beam splitter  42 . Signal (S) photons that have had their polarization direction rotated from horizontal to vertical in the optical Kerr cell  58  are reflected at polarizing beam splitter  42  and are incident on vertical photon detector  64 . Signal (S) photons that passed through OKC  58  while its liquid was optically isotropic maintain their original, horizontal polarization direction; these photons pass through polarizing beam splitter  42  and reach horizontal photon detector  66 . The sensitive photon detectors, with their associated electronics, are capable of photon counting. 
   The absolute time difference between the arrival of the pump pulse (P) at optical Kerr cell  58  and the arrival of any signal (S) photons at the cell  58  is controlled by the position of corner reflector  44  in the pump pulse (P) path of receiver  20 . The arrival time difference can be adjusted by translating the corner reflector. 
   The “filtering” properties of the Kerr cell in conjunction with the last polarizing beam splitter, pass or do not pass signal photon information depending on the cumulative time distributions of these photons as they correspond to signal photons that have not been altered by a collapse event in the transmitter (binary “zero”), and to signal photons that have been altered by such an event (binary “one”) as will be further explained. 
   Referring to  FIGS. 2 and 3 , to send a binary “zero” from transmitter  22  to the receiver  20 , moveable mirror  24  of the transmitter is inserted into idler (I) photon beam path  32 . The idler (I) photons are reflected by mirror  24  and are detected by photon detector  26 . This detection does not measure the idler (I) photon energy. The properties of the idler (I) photons are not altered prior to their detection by photon detector  26 . Specifically, the center wavelength, bandwidth, and time width of each of the idler (I) photons are the same at the time of detection as they were when the photons were originally created in the down-conversion event in the nonlinear crystal of source  10 . 
   The detection of an idler (I) photon at transmitter  22  serves to “fix” the properties of its quantum-entangled partner, the signal (S) photon that is arriving at receiver  20 . Since the center wavelength, wide bandwidth, and short time width of each idler (I) photon were not changed prior to detection in transmitter  22 , each signal (S) photon that arrives at receiver  20  also has its original center wavelength, wide bandwidth, and short time width. 
   A slight uncertainty exists in the arrival time of a given signal (S) photon at the receiver. This is because the group velocity of the signal (S) photons is greater than the group velocity of the pump pulse (P) photons in the nonlinear crystal, and because the pump pulse has a non-zero time width. For the assumed case of a 200 femtosecond (FWHM) pump pulse and an 8 millimeter-long Beta Barium Borate (for example) nonlinear crystal, the arrival time uncertainty of a signal (S) photon at receiver  20  is slightly less than 2 picoseconds with respect to the arrival time of the pump pulse. It should be noted that other nonlinear crystal types besides Beta Barium Borate (BBO) are considered suitable, for example, a crystal of Potassium diHydrogen Phosphate or of Lithium Iodate are feasible. 
   Signal (S) photons that reach receiver  20  pass through polarizing beam splitter  38  and are then incident on nonlinear dispersion element  52  that has dispersive characteristics that enhance the differences between altered and unaltered signal photons. In this binary “zero” case, all signal (S) photons that arrive at nonlinear dispersion element  52  of receiver  20  have the same center wavelength. Consequently, the group velocity in the nonlinear dispersion element is the same for all of the signal (S) photons, and they all require the same amount of time, on average, to pass through element  52 . 
   Additionally, all signal (S) photons that arrive at receiver  20  have the same time width (˜44 femtoseconds, FWHM). Propagation through element  52  causes the time width of the signal (S) photons to increase. This increase is proportional to the inverse square of the initial time width. Assuming a total path length through SF6 glass of ˜1 meter, the very narrow initial time width of the signal (S) photons increases to ˜12.5 picoseconds (FWHM) after the nonlinear dispersion element. 
   Thus, with respect to the arrival time of the narrow pump pulse, any signal photons produced in the nonlinear crystal by that pump pulse arrive at the optical Kerr cell  58  with a Gaussian-shaped cumulative time distribution ˜14.5 picoseconds (FWHM), see distribution  70  of  FIG. 4 . 
   The liquid in the optical Kerr cell is somewhat dispersive. However, a suitable optical Kerr cell is only about 1 cm in length. Thus the dispersion due to the Kerr cell has only a minor effect on the time properties of the photons passing through it. 
   By adjusting the position of corner reflector  44  of receiver  20 , the arrival time of the pump pulse at cell  58  can be set so that most signal photons pass through OKC  58  before the pump pulse reaches it. Consequently, most of the signal photons representing the binary “zero” case pass through the optical Kerr cell while the cell&#39;s liquid is optically isotropic; their polarization direction remains horizontal. 
   Corner reflector  44  is set so that in the binary “zero” case (no energy measurement at transmitter  22  and hence no collapse event), almost all of the signal (S) photons that reach polarizing beam splitter  42  are horizontally polarized. Consequently, almost all of the signal (S) photons pass through polarizing beam splitter  42  and are detected at photon detector  66 . In the binary “zero” case, very few signal photons are detected at photon detector  64 . 
   To send a binary “one” from transmitter  22  to receiver  20 , moveable mirror  24  of transmitter  22  is removed from idler photon beam path  32 . Idler (I) photons that reach transmitter  22  enter spectrometer  28 . Spectrometer  28  and detector array  30  are used to precisely measure the frequency (energy) of each incident idler (I) photon. 
   Each idler (I) photon is (irreversibly) annihilated in a detection event in one of the elements of detector array  30  of transmitter  22 . Thus, spectrometer  28  and detector array  30  restrict each idler (I) photon to a narrow spectral region before it is detected. 
   Each idler (I) photon is quantum entangled with the signal (S) photon that was created with it in the down-conversion event in nonlinear crystal  14  of source  10  ( FIG. 1 ). The two photons are entangled both in energy and linear momentum (as well as other entangled parameters), because the signal, idler, and pump photons must obey energy conservation and momentum conservation (phase-matching). 
   A precise measurement of the idler (I) photon&#39;s frequency (or wavelength) places a constraint on the allowed signal-photon-frequencies. The precise measurement of the idler (I) photon frequency at transmitter  22  causes an instantaneous “collapse” of the signal (S) photon&#39;s bandwidth and, for a majority of the signal photons, an accompanying change in the signal (S) photon&#39;s center wavelength. A wide variance in the center wavelength of signal photons experiencing the collapse condition thus occurs. 
   The source to receiver and source to transmitter distances are set so that this “bandwidth collapse” occurs just before the signal photon reaches the receiver. 
   The degree to which the signal (S) photon&#39;s bandwidth is reduced depends on the original pump pulse bandwidth, on the resolution of the idler frequency measurement, and on the thickness of the nonlinear crystal. The new center wavelength (after the collapse) will be some value falling within the original, “uncollapsed” signal (S) photon bandwidth. The original Gaussian profile of the bandwidth acts as a probability density function (pdf) for the new center wavelength. 
   Since the precise measurement of the idler (I) photon&#39;s frequency causes the bandwidth of the signal (S) photon to decrease, the time width of the signal (S) photon must increase (due to Heisenberg Uncertainty). 
   For example, by using a 200 femtosecond (FWHM) pump pulse and an 8 millimeter-long BBO crystal, measurement of the idler (I) photon wavelength to within one Angstrom resolution causes the time width of its entangled partner signal (S) photon to increase to a value of approximately 1.4 picoseconds. The time profile of the “collapsed” signal (S) photon depends on the time width of the pump pulse and on the new center wavelength of the signal (S) photon. 
   The time required for a “collapsed” signal photon to propagate through the nonlinear dispersion element (DE) is determined by the photon&#39;s center wavelength and by its time width. The dominant factor impacting this time is the center wavelength, which determines the group velocity of the signal photon in the material of the DE. The initial time width of the signal (S) photon is a secondary factor that controls the amount by which the photon&#39;s time width spreads in traveling through nonlinear dispersion element  52 . From an initial value of ˜1.4 picoseconds in the binary “one” case, the signal (S) photon time width increases to ˜1.8 picoseconds, after passing through nonlinear dispersion element  52 . 
   As noted above, the dominant factor in determining the time required for a signal (S) photon to propagate through the nonlinear dispersion element is the photon&#39;s center wavelength. The group velocity in the nonlinear dispersion element  52  is a nonlinear function of the center wavelength. 
   In the binary “one” case, wherein signal photons are altered by the “collapse” event, the center wavelength changes from one signal photon to the next. Because of this, after passing through the nonlinear dispersion element  52 , each series of signal photons associated with a pump pulse arrive at the OKC within a slightly skewed, Gaussian-shaped cumulative time distribution ˜22 picoseconds (FWHM).  FIG. 4  shows an example distribution  72  representing signal photons of this binary “one” case. 
   Corner reflector  44  of receiver  20  is set so that, in the previously-described binary “zero” case, almost all signal photons pass through OKC  58  ahead of the pump pulse, while the cell liquid is optically isotropic, and their horizontal polarization direction is maintained. The binary “zero” case produces a much larger photon count rate at horizontal photon detector  66  than at vertical photon detector  64 . 
   In the binary “one” case, there is a greater overall cumulative “time spread” of the signal photons exiting nonlinear dispersion element  52  than exists in the binary “zero” case. Thus, there is a much larger probability that signal (S) photons will pass through OKC  58  at the same time as the pump pulse. Consequently, more signal (S) photons pass through the optical Kerr cell  58  while the cell liquid is birefringent. These signal (S) photons have their polarization direction rotated from horizontal to vertical, and they are subsequently reflected by polarizing beam splitter  42  and are detected at photon detector  64 . 
   In the binary “one” case, the photon count rate at photon detector  64  increases to well above the rate observed in the binary “zero” case. Additionally, the count rate at photon detector  66  in the binary “one” case decreases from the rate observed in the binary “zero” case, since the rate of production of signal and idler photon pairs (via parametric down-conversion in the nonlinear crystal) is the same in both the “zero” and “one” cases. 
   By observing the photon count rate at photon detector  64  versus the rate at photon detector  66 , an operator of the receiver can discern whether an operator at the transmitter is sending a binary “zero” or a binary “one”. 
   For sensing situations where there is not apriori information known regarding a specific transmitter, the “transmitter” becomes the media or atmosphere desired to be sensed. This media, which may be atomic, molecular, or of more dense composition, interacts with incident idler photons in an analogous manner to the spectrometer of the transmitter shown in  FIG. 2 . 
   Interaction of an idler photon with the media is equivalent to the binary “one” case described above wherein a collapse event is present. Non-interaction of an idler photon with the media is equivalent to the binary “zero” as described above. 
   Such sensing may be performed by adding a second adjustable “mirror delay channel” to the “front end” of the receiver. 
   Referring to  FIG. 5 , a modified receiver  20 ′ incorporating such a second mirror delay channel is shown. Such a second mirror delay channel  74  includes two optical mirrors ( 76 ,  78 ) and a corner reflector  80 . The remainder of the receiver is as described in  FIG. 3 . 
   The function of the additional mirror delay channel is to control the time at which signal photons and pump pulse photons reach first polarizing beam splitter  38  of receiver  20 ′. 
   If the time delay in the new mirror delay channel is set to a too small value, then signal and pump pulse photons arrive at and are detected in the receiver before any idler photons can reach the media to be sensed. This is equivalent to a binary “zero” condition described in regard to receiver  20  above in terms of detection of the signal photons at the receiver. 
   As the time delay in mirror delay channel  70  is increased, a point is reached where the optical path length from source  10  ( FIG. 1 ) to the media to be sensed is shorter than the optical path length from the source to the receiver. Idler photons will interact with the media to be sensed, before the signal and pump pulse photons reach polarizing beam splitter  38  of the receiver. This is equivalent to the binary “one” condition described in regard to receiver  20  above in terms of detection of the signal photons at the receiver. 
   In the sensor application, the system can determine whether the media to be sensed is present or absent, the amount of the media that is present—via the percentage of idler photons that interact with the media at a given distance, and the location of the media and its concentration as a function of position—via the translation of the additional corner reflector in the new mirror delay channel of the receiver. 
   Obviously, many modifications and variations of the invention are possible in light of the above description. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as has been specifically described.

Technology Classification (CPC): 7