Abstract:
A cross-ambiguity function generator (“CAF”) uses properties of quantum mechanics for computation purposes. The CAF has advantages over standard analog or digital CAF function generators, such as improved bandwidth. The CAF may be used for traditional geolocation or RADAR applications.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is related to U.S. Utility patent application Ser. No. 10/850,394 entitled “System and Method of Detecting Entangled Photons” to Kastella et al., which claims priority to U.S. Provisional Application Serial No. 60/472,731 entitled “System and Method of Detecting Entangled Photons” to Kastella et al., the disclosures of which are expressly incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to systems for and methods of calculating the cross-ambiguity function (“CAF”) using quantum mechanical properties.  
       BACKGROUND OF THE INVENTION  
       [0003]     The (narrow-band) CAF is generally given as  
           CAF   ⁡     (     τ   ,   δ     )       =       ∫   0   T     ⁢         s   1     ⁡     (   t   )       ⁢       s   2   *     ⁡     (     t   -   τ     )       ⁢     ⅇ           -   ⅈδ     *   t     -   τ     )       ⁢     ⅆ   t           ,       
 
 where s 1  and s 2  each represent a signal reading, each of which may arise from a single emitted signal or be a composite of several component signals possibly originating from different signal emitters. The signal readings s 1  and s 2  may be radio frequency (“RF”) or downconverted intermediate frequency (“IF”). In the above equation, the symbol τ is a time parameter, δ is a frequency parameter, the star symbol (“*”) represents the complex conjugate, and T represents the time interval over which the measurements are taken. The symbols τ and δ are used in the above equation to represent time and frequency shift, respectively, between component signals of s 1  and s 2  that originate from a common emitter. The parameter τ in the above equation is related to time difference on arrival (“TDOA”) and to receiver-dependant delays. The parameter δ in the above equation is related to frequency difference on arrival (“FDOA”) and to downconversion shifts. 
 
         [0004]     The representation of the CAF given above is for illustrative purposes and is not meant to be limiting. The CAF may take other forms, representations, or variants. By way of non-limiting example, one such form is a CAF that employs an additional term (e.g., β) for frequency-dependent Doppler shift. Such a form is particularly suited for broadband signals. Nevertheless, because such forms, representations, and variants are used to derive essentially the same information, the article “the” is used when referring to “CAF.” That is, any function that derives essentially the same information from essentially the same inputs is referred to herein as “the CAF.” 
         [0005]     The two dimensions τ and δ in the above equation define a plane, which is referred to as “the CAF plane.” Other representations of the CAF plane that do not use these specific symbols are also possible. The values of the CAF for specific values of τ and δ defines a surface over the plane, and peaks on this surface represent a signal source. By scanning the CAF plane, values of τ and δ for one or more signal emitters may be determined. The actual locations of the signal source(s) may be derived from this information. Thus, the CAF is used in RADAR processing and geolocation techniques. Using the above terminology, in RADAR a signal is transmitted, s 1  is received and s 2  is a copy of the transmitted signal. The received signal si includes signal components reflected from different objects, each of which will arrive at a different delay and different Doppler (frequency shift), which information is used to determine the range and (at least a component of the) speed of each object relative to the RADAR transmitter.  
         [0006]     The CAF is typically very computationally intensive to calculate, especially for broadband signals where a scale factor instead of, or in addition to, a frequency-shift term is used. Accordingly, standard analog or digital systems are relatively slow and expensive except for the narrowband case. Acousto-optical techniques have been proposed, but suffer from limited dynamic range and a very small TDOA search range. These and other drawbacks exist with current systems.  
         [0007]     Two photons quantum mechanically entangled together are referred to as an entangled-photon pair (also, “biphotons”). Traditionally, the two photons comprising an entangled-photon pair are called “signal” and “idler” photons. The designations “signal” and “idler” are arbitrary and may be used interchangeably. The photons in an entangled photon pair have a connection between their respective properties. Measuring properties of one photon of an entangled-photon pair determines properties of the other photon, even if the two photons are separated by a distance. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of an entangled-photon pair cannot be factored into a tensor product of two individual quantum states.  
       SUMMARY OF THE INVENTION  
       [0008]     According to an embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is provided. Electromagnetic signals are received. Entangled photons are generated. The entangled photons are modulated with information relating to the received electromagnetic signals to produce modulated photons. The modulated photons are detected, and detection information derived from the detecting is used to produce a cross ambiguity function value relating to the electromagnetic signals.  
         [0009]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The detecting may use a biphoton sensitive material. The detection information may comprise data representing indicia of biphoton absorption at a location along a magnetic field gradient. The detecting may be by way of detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization. The detecting may use an electronic coincidence counter. The detection information may include a number of biphoton detection events detected during a specified time period. The cross ambiguity function value may be related to a time offset and a frequency offset. A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of the entangled photons in proportion to the information relating to the received electromagnetic signals.  
         [0010]     According to another embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is presented. A first electromagnetic signal is received at a first location. A second electromagnetic signal is received at a second location. A pump laser produces pump laser photons. Entangled photons comprising signal photons and idler photons are generated from the pump laser photons. Either the signal photons or the idler photons are modulated with first information relating to the first electromagnetic signal to produce first modulated photons, and the other of the signal photons or the idler photons are modulated with second information relating to the second electromagnetic signal to produce second modulated photons. The first modulated photons and the second modulated photons are directed to an optical cavity. The first modulated photons and the second modulated photons are detected. Information derived from the detection is used to produce a cross ambiguity function value for the first electromagnetic signals and the second electromagnetic signals.  
         [0011]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The first information may include a first time parameter, a first frequency parameter, and an intermediate frequency signal corresponding to the first electromagnetic signal. The second information may include a second time parameter, a second frequency parameter, and an intermediate frequency signal corresponding to the second electromagnetic signal. A cross ambiguity function related to the first electromagnetic signal, the second electromagnetic signal, a time derived from the first time parameter and the second time parameter, and a frequency derived from the first frequency parameter and the second frequency parameter is calculated.  
         [0012]     A biphoton sensitive material may be used to detect a coincidence of the first modulated photons and the second modulated photons. The detection may include detecting indicia of biphoton absorption at a location along a magnetic field gradient. The detecting may include detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization. The detecting may use an electronic coincidence counter. The detection information may include a number of detection events during a specified time period. The cross ambiguity function value may be related to a time offset and a frequency offset.  
         [0013]     A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the first electromagnetic signals and the second electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of either the signal photons or the idler photons in proportion to the first information and rotating polarizations of the other of the signal photons or the idler photons in proportion to the second information. Producing the cross ambiguity function value may include calculating a square root and scaling.  
         [0014]     According to another embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is presented. Electromagnetic signals are received. Entangled photons are generated. The entangled photons are modulated with information relating to the received signals to produce modulated photons. The modulated photons are detected with a coincidence counter to produce a coincidence count. The coincidence count is used to produce a cross ambiguity function value for the electromagnetic signals.  
         [0015]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The cross ambiguity function value may be related to a time offset and a frequency offset. A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of the entangled photons in proportion to the information relating to the received signals to produce modulated photons. Producing the cross ambiguity function value may include calculating a square root and scaling.  
         [0016]     According to another embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is presented. First electromagnetic signals are received at a first location. Second electromagnetic signals are received at a second location. Pump laser photons are produced. Entangled photons comprising signal photons and idler photons are produced from the pump photons. Either the signal photons or the idler photons are modulated with first information relating to the first electromagnetic signal to produce first modulated photons and the other of the signal photons or the idler photons are modulated with second information relating to the second electromagnetic signal to produce second modulated photons. The first modulated photons and the second modulated photons are directed to an optical cavity. The first modulated photons and the second modulated photons are directed from the optical cavity to a coincidence counter to produce a coincidence count. The coincidence count is used to produce a cross ambiguity function value for the first electromagnetic signals and the second electromagnetic signals.  
         [0017]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The first information may include a first time parameter, a first frequency parameter, and an intermediate frequency signal corresponding to the first electromagnetic signal. The second information may include a second time parameter, a second frequency parameter, and an intermediate frequency corresponding to the second electromagnetic signal. A cross ambiguity function related to the first electromagnetic signals, the second electromagnetic signals, a time derived from the first time parameter and the second time parameter, and a frequency derived from the first frequency parameter and the second frequency parameter may be calculated. The cross ambiguity function value may be related to a time offset and a frequency offset. A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the first electromagnetic signals and the second electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of one of the signal photons or the idler photons in proportion to the first information and rotating polarizations of the other of the signal photons or the idler photons in proportion to the second information. Producing the cross ambiguity function value may include calculating a square root and scaling.  
         [0018]     According to another embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is presented. Electromagnetic signals are received. Entangled photons are generated. The entangled photons are modulated with information relating to signals received by the receiving to produce modulated photons. Absorption of the modulated photons by a biphoton sensitive material is detected. Detection information derived from the detecting is used to produce a cross ambiguity function value relating to the electromagnetic signals.  
         [0019]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The detection information may include data representing indicia of biphoton absorption at a location along a magnetic field gradient. The indicia of biphoton absorption may include at least one of fluorescence, phosphorescence, direct electron transfer, and ionization. The detection information may include a number of biphoton absorptions detected during a specified time period. The cross ambiguity function value may be related to a time offset and a frequency offset. A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of the entangled photons in proportion to the information. Producing the cross ambiguity function value may include calculating a square root and scaling.  
         [0020]     According to another embodiment of the present invention, a system for and method of calculating a cross-ambiguity function is presented. A first electromagnetic signal is received at a first location. A second electromagnetic signal is received at a second location. A pump laser produces pump laser photons. Entangled photons comprising signal photons and idler photons are produced from the pump photons. Either the signal photons or the idler photons are modulated with first information relating to the first electromagnetic signal to produce first modulated photons, and the other of the signal photons or the idler photons are modulated with second information relating to the second electromagnetic signal to produce second modulated photons. The first modulated photons and the second modulated photons are directed to an optical cavity containing a biphoton sensitive material. Biphoton absorption of the first modulated photons and the second modulated photons by the biphoton sensitive material is detected. Information relating to the detecting is used to produce a cross ambiguity function value for the first electromagnetic signals and the second electromagnetic signals.  
         [0021]     Various additional features of the above embodiment include the following. The cross ambiguity function value may correspond to a difference between two time values, a detuning between a pump laser frequency and a cavity resonance frequency, and a ratio of two scale parameters. The first information may include a first time parameter, a first frequency parameter, and an intermediate frequency signal corresponding to the first electromagnetic signal. The second information may include a second time parameter, a second frequency parameter, and an intermediate frequency signal corresponding to the second electromagnetic signal. A cross ambiguity function related to the first electromagnetic signal, the second electromagnetic signal, a time derived from the first time parameter and the second time parameter, and a frequency derived from the first frequency parameter and the second frequency parameter is calculated. The information relating to the detecting may include information of a location along a magnetic field gradient. The detecting may include detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization. The information relating to the detecting may include a number of biphoton absorptions detected during a specified time period. The cross ambiguity function value may be related to a time offset and a frequency offset. A second cross ambiguity function value related to a second time offset and a second frequency offset may be determined. A bandwidth of the first electromagnetic signals and the second electromagnetic signals may be on the order of, or exceed, one gigahertz. The modulating may include rotating polarizations of either the signal photons or the idler photons in proportion to the first information and rotating polarizations of the other of the signal photons or the idler photons in proportion to the second information. Producing the cross ambiguity function value may include calculating a square root and scaling.  
         [0022]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles and advantages of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a schematic diagram depicting using the CAF to geolocate signal emitters according to an embodiment of the present invention.  
         [0024]      FIG. 2  is a schematic diagram depicting an embodiment of a quantum CAF generator.  
         [0025]      FIG. 3  is a schematic diagram depicting a digital delay line embodiment.  
         [0026]      FIG. 4  is a schematic diagram depicting a rectangle function according to an embodiment of the present invention.  
         [0027]      FIG. 5  is a chart depicting signal bandwidth as a function of FDOA resolution for several cavities with different mirror reflectance according to various embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]      FIG. 1  is a schematic diagram depicting the use of the CAF to geolocate signal emitters  102 ,  104 ,  106 . Two sensors  110 ,  112  at different locations and moving at different velocities are used to detect emitters  102 ,  104 ,  106 . Sensors  110 ,  112  may be antennas, directional or otherwise. The signal from an emitter arrives at different times at sensors  110 ,  112 , depending on their distance from the emitter. The signals arrive Doppler-shifted differently at sensors  110 ,  112  depending on the relative motion between each sensor and the emitter. The signals (or their representations) from sensors  110 ,  112  are brought together using techniques known in the art. The CAF is then used to scan CAF plane  120  by testing multiple time difference of arrival (TDOA) and frequency difference of arrival (FDOA) points in CAF plane  120  according to the embodiment of  FIG. 1 . TDOA and FDOA parameters that produce a locally maximum CAF value (e.g.,  130 ,  132 ,  134 ) correspond to a signal emitter. The physical location of the emitter is then derived from those particular TDOA and FDOA values using techniques known in the art. By way of non-limiting example, standard computer hardware, firmware, software, or a combination thereof may be used to derive data representing a physical location from CAF values.  
         [0029]     The TDOA and FDOA themselves define surfaces in physical space. Assuming the emitter is on the surface of the Earth and stationary, the emitter must then lie at one of the points defined by intersection of these three surfaces. Other sensor arrangements may be used to locate a signal emitter without these assumptions. By way of non-limiting example, an embodiment of the present invention may use three sensors at three different locations to locate a signal emitter without requiring that it be located on the Earth&#39;s surface or be stationary.  
         [0030]     Each emitter  102 ,  104 ,  106  results in a peak in the CAF at the TDOA and FDOA corresponding to the location of the emitter. That is, TDOA and FDOA parameters corresponding to emitters  102 ,  104 ,  106  produces local maxima in CAF values. Each receiver receives a signal that is a composite of the three signals emitted from emitters  102 ,  104 ,  106 . Because the CAF peak is a function only of location for a given sensor geometry, the characteristics of the transmission (spread spectrum, frequency hopping, chirped, pulsed, CW, etc.) do not effect the location of the CAF peak in the CAF plane. Thus, a CAF may be used to locate multiple disguised-signal emitters.  
         [0031]      FIG. 2  depicts an embodiment of a quantum CAF generator. In this embodiment, the CAF is calculated as a function of the time difference of arrival, frequency difference of arrival, and a scale difference. Here “scale difference” refers to differences in time scaling between two signals, which is due to relative motion of sensors relative to the emitter. For narrowband signals, the effects of relative motion can be considered as a Doppler shift and accounted for in a frequency offset, without requiring scaling. For broadband signals, the scale difference accounts for the frequency-dependent Doppler shift, and the frequency offset is related to the FDOA, receiver-dependent biases (such as downconversion differences), and scaling.  
         [0032]     According to the embodiment of  FIG. 2 , a narrow tunable pump laser beam  200  is injected into a nonlinear crystal  204  (such as, by way of non-limiting example, beta barium borate or lithium niobate), where it undergoes parametric down conversion into signal photon beam  206  and idler photon beam  208 . Thus, each pump photon is split into an entangled photon pair consisting of a signal photon and an idler photon. Signal photon beam  206  and idler photon beam  208  are separated using polarizing beam splitter  210 . RF signals are received in a conventional manner (e.g., antennas) at two signal receivers and down converted to IF signals s 1 (t)  201  and s 2 (t)  202 , respectively. Each of s 1 (t)  201  and s 2 (t)  202  may be electronically delayed by digital delays  250 ,  252 , respectively. Signal photon beam  206  is delayed prior to modulation by a time dependent delay  212  to account for scaling. Idler photon beam  208  is delayed by an equivalent amount by time dependent delay  214  after modulation. Time dependent delays  212 ,  214  may be implemented using, by way of non-limiting example, materials that change refraction in response to electricity, mechanical movable mirrors, or slow light technology. Signal photon beam  206  and idler photon beam  208  are modulated with respective IF signals s 1 (t-τ 1 ) and s 2 (t-τ 2 ) as required by the particular embodiment of the present invention. The two beams are then combined using polarizing beam splitter  220  and injected into optical cavity  222 , which is tuned near the pump laser center frequency. The output from cavity  222  is split by polarizing beam splitter  224 , and the two signals are detected by detectors  226 ,  228  and correlated by means of coincidence counter  230 .  
         [0033]     Together, time dependent delays  212 ,  214  serve to introduce a scale term β. Time dependent delays  212 ,  214  may be set to cause the term β to take on any values less than one. For calculating CAF values for β greater than one, the embodiment of  FIG. 2  may be configured to swap the signal receivers that respectively process s 1 (t)  201  and s 2 (t)  202 .  
         [0034]     The embodiment of  FIG. 2  yields a coincidence count rate that is proportional to the magnitude squared of the CAF. This relation may be represented as, by way of non-limiting example:  
               〈         Φ   bi     ⁡     (         τ   1     -     τ   2       ,   δ   ,       β   1       β   2         )       -       Φ   bg     _       〉     ∝                ∫   0     Δ   ⁢           ⁢   T       ⁢         s   1     ⁡     (         β   1     ⁢   t     -     τ   1       )       ⁢       s   2     ⁡     (         β   2     ⁢   t     -     τ   2       )       ⁢     ⅇ       -   ⅈδ     ⁢           ⁢   t       ⁢     ⅆ   t              2     .             (   1   )             
 
         [0035]     In equation (1), τ=τ 2 −τ 1  is a time offset, δ is a frequency offset, ΔT is a time measurement interval, and the brackets           represent expected value. The symbol Φ bi  represents the biphoton count rate and the symbol Φ bg  represents the background count rate.  
         [0036]     The term β 1 /β 2  represents a scaling factor. More generally, the term β 1 /β 2  may be written as β, the ratio of scaling factors for the two signals. The delays  212 ,  214  in the embodiment of  FIG. 2 , for example, introduce a scaling factor β without requiring separate β 1  and β 2 . One representation of β in the embodiment of  FIG. 2  yields β 1 =β 1/2  and β 2 =β −1/2 .  
         [0037]     The embodiment of  FIG. 2  may be used to scan the CAF plane by testing values of τ=τ 2 −τ 1 , δ, and β=β 1 /β 2  and measuring biphoton and background count rates to derive the CAF value for these parameters. Standard computer hardware, software, firmware, or any combination thereof may be used to process coincidence count data in accordance with equation (1) and derive a CAF value. This value may be output, stored, and/or forwarded for additional processing. Such additional processing may include, by way of non-limiting example, storing the value together with other related CAF values, graphically displaying the value, removing noise, or employing a maximum-locating algorithm or circuit.  
         [0038]     The particular type of scanning depends on the embodiment and type of signal. For narrow-band signals, τ and δ are externally controlled to effect a scanning of the CAF plane. For broadband signals, scanning preferably occurs in β instead of δ, although δ still has to be set to account for receiver-dependent IF frequency differences. The frequency offset term δ is controlled by adjusting a cavity length or pump laser frequency as discussed further below.  
         [0039]      FIG. 3  depicts a digital delay line embodiment for delaying, by way of non-limiting example, the second IF signal by a given time offset, τ. The analog input signal s(t)  300  is sampled at a very high rate using an analog to digital converter (ADC)  302 , delayed in a digital buffer  304 , and then converted back into an analog signal using a digital to analog converter (DAC)  306 . Prior to conversion back to an analog signal, an arcsine function can be applied to the digital signal (via table lookup  308 ) to significantly reduce the higher order modulation terms. Finally, the DAC can be clocked at a different rate from the ADC to account for the scaling, β. When scaling is handled by the delay lines of  FIG. 3 , the time-dependent optical delays  212 ,  214  depicted in  FIG. 2  are not required. In general, an IF signal s(t)  300  entering the delay line produces a signal s(βt−τ)  310  exiting the delay line.  
         [0040]     In an embodiment of the present invention, the IF signals s 1 (t)  201 , s 2 (t)  202  of  FIG. 2  feed into the delay lines of  FIG. 3 , which, in turn, feed into the electro-optical modulators  216 ,  218 , respectively, of  FIG. 2 . Preferably, both get the benefit of the arcsine conversion. The time offset τ is then the difference between the delays τ 2 , τ 1  for the two inputs and can be either positive or negative. All the scaling can be handled on one input, or split between the two. By way of non-limiting example, the term β may be produced as a ratio of β 1  and β 2 , which are respectively introduced by a first and second delay on s 1  and s 2  to respectively model s 1 (β 1 t−τ 1 ) and s 2 (β 2 t−τ 2 ). When the β i  (for i=1,2) of an input stage is greater that one, some initial buffering is preferred to enable the DAC clock to run faster than the ADC clock without running out of data. Alternatively, since the scaling can be handled on either input, β i  for both inputs can be reduced together (maintaining the same ratio) until neither is greater than one. For purposes of exposition, β will be left out of the remaining derivations; β may readily be reintroduced where applicable.  
         [0041]     An analysis of the embodiment of  FIG. 2  follows. In the absence of the cavity and the modulators, the two-photon coincidence rate is proportional to (see Rubin, Klyshko, Shih and Sergienko, Phys. Rev. Vol. 50 No. 6, December 1994, pp 5122) the biphoton amplitude, which may be expressed as, by way of non-limiting example: 
 
 R   c   ∝&lt;ψ|E   1   (−)   E   2   (−)   E   2   (+)   E   1   (+) |ψ&gt;=|&lt;0 |E   2   (+)   E   1   (+)   |ψ&gt;|   2   =|A (τ 1 τ 2 )| 2 .   (2) 
 
         [0042]     In equation (2), τ i =T i =s i /c where T i  is the measurement time and s i  is the optical path length of the i-th photon for i=1,2. As shown in the above reference, the biphoton amplitude, A, can be written as, by way of non-limiting example:  
               A   ⁡     (       τ   1     ,     τ   2       )       =       η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢       Π   ⁡     (       τ   1     ⁢     τ   2       )       .               (   3   )             
 
 In equation (3), η 0  is a normalization constant, ω p  represents the pump frequency, and ω d  represents the difference between signal and idler frequencies. The term Π(τ) represents the rectangle function, which may be expressed as, by way of non-limiting example:  
               Π   ⁡     (   τ   )       =     {               ⁢       1   DL     ,                 ⁢       DL   &gt;   τ   &gt;   0     ,                     ⁢     0   ,                 ⁢     otherwise   .                       (   4   )             
 
 The symbol D represents the difference in the inverse group velocities of the ordinary and extraordinary rays in the crystal and L represents the length of the crystal. (For beta barium borate, D≈0.2 psec/mm.) The product DL determines the entanglement time. This probability amplitude can be interpreted as follows: If an idler photon is detected at time T 2 , then (for equal path lengths) the probability that the signal photon is detected at time T 1  goes to zero for T 1 &lt;T 2  or for T 1 &gt;T 2 +DL. (A representative non-limiting graph of the rectangle function is depicted in  FIG. 4 . The y-axis  410  represents coincidence probability, and the x-axis  420  represents difference in arrival time, scaled to DL.) 
 
         [0043]     The electro-optic modulators  216 ,  218  rotate the polarization of the signal photon beam  206  and idler photon beam  208  proportional to the IF input signals. Due to the polarization rotation, the second polarizing beam splitter  220  combines signal photon and idler photon beams with amplitudes that are equal to the sine of their respective rotation. More particularly, polarizing beam splitter  220  selectively passes e.g., vertical components of signal photon beam  206  and, e.g., horizontal components of idler photon beam  208  to cavity  222 . Polarizing beam splitter  220  thus trims the amplitude of the signal photon beam  206  and idler photon beam  208  in accordance with the rotational modulated information. The net effect is to impose a temporal variation on the amplitudes of both the signal and idler beams, resulting in a biphoton amplitude that may be represented as, by way of non-limiting example:  
                 A   sig     ⁡     (       τ   1     ,     τ   2     ,   τ     )       =       sin   ⁡     (       κ   1     ⁢       s   1     ⁡     (     τ   1     )         )       ⁢     sin   ⁡     (       κ   2     ⁢       s   2     ⁡     (       τ   2     -   τ     )         )       ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       w   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       w   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢       Π   ⁡     (       τ   1     -     τ   2       )       .               (   5   )             
 
 In equation (5), τ 1  and τ 2  are as defined above in reference to equation (2), and τ is a time offset as defined above in reference to equation (1). The terms κ 1  and κ 2  are set to limit the magnitude of the argument of the sine functions to less than π/2. This limitation prevents aliasing of the sine function. If the sine function is represented as a power series,  
                 sin   ⁡     (   x   )       =     x   -       x   3       3   !       +       x   5       5   !       -   …       ⁢           ,           (   6   )             
 
 then the signal and idler modulations can be written in terms of the IF signals plus higher-order terms. The higher order terms can be substantially suppressed by further reducing κ 1  and κ 2 . Otherwise, the higher order terms might interfere with the desired CAF. For purposes of exposition, we will assume suppression of the higher order terms and make use of Fourier transforms to write, by way of non-limiting example:  
                 A   sig     ⁡     (       τ   1     ,     τ   2     ,   τ     )       ≈       κ   1     ⁢       κ   2     ⁡     (       ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢     ⅇ     -     ⅈωτ   1         ⁢     ⅆ   ω         )       ⁢     (       ∫     -   ∞     ∞     ⁢         S   2     ⁡     (   ω   )       ⁢     ⅇ     -     ⅈω   ⁡     (       τ   2     -   τ     )           ⁢     ⅆ   ω         )     ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢       Π   ⁡     (       τ   1     -     τ   2       )       .               (   7   )             
 
 By independently modulating the signal photon beam and idler photon beam with two different (analog) signals, the biphoton probability amplitude is modulated by the product of the two signals. In general, this technique is useful for multiplying any two signals and may be implemented in signal processors other than CAF generators. 
 
         [0044]     Still in reference to  FIG. 2 , the signal and idler photons are recombined into a single beam by the second polarizing beam splitter  224  and directed to optical cavity  222 . Cavity  222  is characterized by the complex reflectance and transmittance coefficients of the two mirrors, r 1 , r 2 , t 1  and t 2 . When a biphoton encounters a mirror, the wave function for each component photon is split into a transmitted component and a reflected component. The output from cavity  222  consists of biphotons whose component photons have each completed some number of round trips through cavity  222 , each trip incurring an additional 2L c  of path length, where L c  is the length of cavity  222 . The total biphoton amplitude beyond cavity  222  can be written in terms of the modulated biphoton amplitude in the absence of cavity  222  as, by way of non-limiting example:  
                 A   cav     ⁡     (       τ   1     ,     τ   2     ,   τ     )       =       ∑     n   =   0     ∞     ⁢       ∑     m   =   0     ∞     ⁢       t   1   2     ⁢         t   2   2     ⁡     (       r   1     ⁢     r   2       )         n   +   m       ⁢         A   sig     ⁡     (         τ   1     -       2   ⁢     nL   c       c       ,       τ   2     -       2   ⁢   m   ⁢           ⁢     L   c       c       ,   τ     )       .                   (   8   )             
 
 Substituting for A sig  yields the following non-limiting expression for biphoton amplitude beyond cavity  222 :  
                 A   cav     ⁡     (       τ   1     ,     τ   2     ,   τ     )       =       κ   1     ⁢     κ   2     ⁢       ∑     n   =   0     ∞     ⁢       ∑     m   =   0     ∞     ⁢       t   1   2     ⁢         t   2   2     ⁡     (       r   1     ⁢     r   2       )         n   +   m       ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   p     2     ⁢     (       2   ⁢     (     n   +   m     )     ⁢     L   c       c     )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢       ⅇ       -   ⅈ     ⁢           ⁢       ω   p     2     ⁢     (       2   ⁢     (     n   -   m     )     ⁢     L   c       c     )         ·       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢         S   1     ⁡     (     ω   1     )       ⁢       S   2     ⁡     (     ω   2     )       ⁢     ⅇ     -     ⅈ   ⁡     [         ω   1     ⁡     (       τ   1     ⁢       2   ⁢     nL   c       c       )       +       ω   2     ⁡     (       τ   2     -     τ   ⁢       2   ⁢     nL   c       c         )         ]           ⁢     ⅆ     ω   2       ⁢     ⅆ     ω   1       ⁢     Π   ⁡     (       τ   1     -     τ   2     -       2   ⁢     (     n   -   m     )     ⁢     L   c       c       )                               (   9   )             
 
 For path lengths set such that 0&lt;τ 1 −τ 2 &lt;DL, A cav  goes to zero for all terms where m≠n. Keeping only the diagonal (m=n) elements, A cav  may be expressed as, by way of non-limiting example:  
                 A   cav     ⁡     (       τ   1     ,     τ   2     ,   τ     )       =         κ   1     ⁢     κ   2     ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢         t   1   2     ⁢     t   2   2       DL     ⁢       ∑     n   =   0     ∞     ⁢     [         (       r   1     ⁢     r   2       )       2   ⁢   n       ⁢     ⅇ     -       ⅈω   p     ⁡     (       2   ⁢     nL   c       c     )           ⁢       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢         S   1     ⁡     (     ω   1     )       ⁢       S   2     ⁡     (     ω   2     )       ⁢     ⅇ     -     ⅈ   ⁡     [         ω   1     ⁢     τ   1       +       ω   2     ⁡     (       τ   2     -   τ     )       -       (       ω   1     +     ω   2       )     ⁢       2   ⁢     nL   c       c         ]           ⁢     ⅆ     ω   2       ⁢     ⅆ     ω   1               ]         =       κ   1     ⁢     κ   2     ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢           ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢           ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢     ⅇ     ⅈ   ⁡     (       2   ⁢     φ   1       +     2   ⁢     φ   2         )         ⁢         T   1     ⁢     T   2       DL     ⁢       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢             S   1     ⁡     (     ω   1     )       ⁢       S   2     ⁡     (     ω   2     )       ⁢     ⅇ     -     ⅈ   ⁡     [         ω   1     ⁢     τ   1       +       ω   2     ⁢     τ   2       -       ω   2     ⁢   τ       ]               [     1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (             ω   p     -     (       ω   1     +     ω   2       )       c     ⁢   2   ⁢     L   c       -     2   ⁢     ϕ   1       -     2   ⁢     ϕ   2         )               ]       ⁢     ⅆ     ω   2       ⁢     ⅆ     ω   1                         (   10   )             
 
 In equation (10), the terms φ i  represent phase shift picked up by the photons as a result of the transmittance t i  of the mirrors for i=1,2. Similarly, the terms φ i  represent phase shift picked up by the photons as a result of the reflectance r i  of the mirrors for i=1,2. The terms T i  represent the intensity transmittances corresponding to complex transmittance coefficients t i  and the terms R i  represent the intensity reflectances corresponding to complex reflectance coefficients r i  for i=1,2. With a change of variables (ω 1 =ω, ω 2 =ν−ω) this expression may be written as, by way of non-limiting example:  
                 A   cav     ⁡     (       τ   1     ,     τ   2     ,   τ     )       =       κ   1     ⁢     κ   2     ⁢     η   0     ⁢     ⅇ     ⅈ   ⁢       ω   p     2     ⁢     (       τ   1     +     τ   2       )         ⁢     ⅇ       -   ⅈ     ⁢       ω   d     2     ⁢     (       τ   1     -     τ   2       )         ⁢     ⅇ     ⅈ   ⁡     (       2   ⁢     φ   1       +     2   ⁢     φ   2         )         ⁢         T   1     ⁢     T   2       DL     ⁢       ∫     -   ∞     ∞     ⁢       ⅇ       -   ⅈ     ⁢           ⁢   v   ⁢           ⁢     τ   2         ⁢         ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2     ⁡     (     v   -   ω     )       ⁢     ⅇ           -     ⅈ   [   ω   )       ⁢     (       τ   1     -     τ   2       )       -       (     v   -   ω     )     ⁢   τ       ]       ⁢           ⁢     ⅆ   ω           [     1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (             ω   p     -   v     c     ⁢   2   ⁢     L   c       -     2   ⁢     ϕ   1       -     2   ⁢     ϕ   2         )               ]       ⁢           ⁢     ⅆ   v                   (   11   )             
 
         [0045]     The denominator has a minimum magnitude when the modulated biphoton is resonant with the cavity, which occurs when, by way of non-limiting example:  
                       ω   p     -   v     c     ⁢     L   c       -     ϕ   1     -     ϕ   2       =       q   ⁢           ⁢   π     =           ω   res     c     ⁢     L   c       -     ϕ   1     -     ϕ   2                 (   12   )             
 
 In equation (12), q may be any integer. Cavity biphoton resonance is a consequence of the frequency entanglement which requires that the sum of the frequencies of the signal and idler photons of a biphoton pair equal the frequency of the pump beam, even though the signal and idler beams are themselves rather broad in frequency. When the pump beam frequency is detuned from the biphoton resonance frequency, the biphoton resonance condition is only met for pairs of signals that differ in frequency by the same amount as the detuning. Thus the detuning between pump and cavity effectively select the frequency offset. 
 
         [0046]     If cavity  222  is made sufficiently short, the spacing between resonant modes (the free spectral range) can be made to exceed the bandwidth of the modulating signals. In this case, we can take ω res  to be the biphoton resonant frequency closest to the pump frequency. This may be expressed as, by way of non-limiting example:  
               ω   res     =           π   ⁢           ⁢   c       L   c       ⁡     [       nint   ⁡     (       1   π     ⁢     (           ω   p     c     ⁢     L   c       -     ϕ   1     -     ϕ   2       )       )       +         ϕ   1     +     ϕ   2       π       ]       .             (   13   )             
 
 The minimum denominator occurs when ν is the difference between the pump frequency and the biphoton resonance frequency (ν=ω p −ω res ). Note that it is the difference between the pump frequency and the biphoton resonance frequency that determines the frequency offset (ν or δ), so that either the pump frequency or the cavity length can be changed to select the frequency offset, which ever is the most convenient. 
 
         [0047]     The probability that the two components of the biphoton arrive within a very short interval (˜DL) is equal to the magnitude squared of the biphoton probability amplitude integrated over this short time, which may be expressed as, by way of non-limiting example:  
                       P   cav     ⁡     (       τ   1     ,   τ   ,     ω   res       )       =       ⁢       ∫       τ   1     -   DL       τ   1       ⁢                A   cov     ⁡     (       τ   1     ,     τ   2     ,   τ     )            2     ⁢           ⁢     ⅆ     τ   2                       =       ⁢         (         κ   1     ⁢     κ   2     ⁢     η   0     ⁢     T   1     ⁢     T   2       DL     )     2     ⁢       ∫       τ   1     -   DL       τ   1       ⁢                  ∫     -   ∞     ∞     ⁢       ⅇ       -   ⅈ     ⁢           ⁢   v   ⁢           ⁢     τ   2         ⁢         ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2     ⁡     (     v   -   ω     )       ⁢     ⅇ     -     ⅈ   ⁡     [       ω   ⁡     (       τ   1     -     τ   2       )       -       (     v   -   ω     )     ⁢   τ       ]           ⁢           ⁢     ⅆ   ω           1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (           ω   p     -     ω   res     -   v     c     ⁢   2   ⁢     L   c       )                 ⁢           ⁢     ⅆ   v              ⁢             2     ⁢     ⅆ     τ   2                         =       ⁢         (         κ   1     ⁢     κ   2     ⁢     η   0     ⁢     T   1     ⁢     T   2       DL     )     2     ⁢       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢       ⅇ       -     ⅈ   ⁡     (     v   -     v   ′       )         ⁢     r   1         ⁢               ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2     ⁡     (     v   -   ω     )       ⁢           ⁢     ⅇ       i   ⁡     (     v   -   ω     )       ⁢     (     r   +     DL   2       )                                   S   1   *     ⁡     (     ω   ′     )       ⁢       S   2   *     ⁡     (       v   ′     -     ω   ′       )       ⁢     ⅇ       -     i   ⁡     (       v   ′     -     ω   ′       )         ⁢     (     τ   +     DL   2       )                     sin   ⁢           ⁢     c   (       DL     2   ⁢   π       ⁢     (     v   -   ω   -     v   ′     +     ω   ′       )     ⁢     ⅆ   ω     ⁢           ⁢     ⅆ     ω   ′                                 (     1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     i   ⁡     (           ω   p     -     ω   res     -   v     c     ⁢   2   ⁢     L   c       )               )               (     1   -       R   1     ⁢     R   2     ⁢     ⅇ     i   ⁡     (           ω   p     -     ω   res     -     v   ′       c     ⁢   2   ⁢     L   c       )             )             ⁢           ⁢     ⅆ   v     ⁢           ⁢     ⅆ     v   ′                             (   14   )             
 
 If the probability of coincidence is averaged over the time ΔT, in the limit as ΔT goes to infinity, the leftmost exponential term can be replaced with a delta function (δ(ν−ν′). This replacement is appropriate as a close approximation. With τ&gt;&gt;DL and recognizing that the sinc function is essentially equal to one (since DL is on the order of picoseconds while the frequencies are only GHz) we get, by way of non-limiting example:  
                         P   cav     ⁡     (     τ   ,     ω   res       )       _     =       ⁢       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0     ⁢     T   1     ⁢     T   2       )     2     DL     ⁢       ∫     -   ∞     ∞     ⁢               (       ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2     ⁡     (     v   -   ω     )       ⁢     ⅇ     -   ⅈωπ       ⁢           ⁢     ⅆ   ω         )               (       ∫     -   ∞     ∞     ⁢         S   1   *     ⁡     (     ω   ′     )       ⁢       S   2   *     ⁡     (     v   -     ω   ′       )       ⁢     ⅇ       ⅈω   ′     ⁢   τ       ⁢           ⁢     ⅆ     ω   ′           )                    1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (           ω   p     -     ω   res     -   v     c     ⁢   2   ⁢     L   c       )                    2       ⁢           ⁢     ⅆ   v                       =       ⁢       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0     ⁢     T   1     ⁢     T   2       )     2     DL     ⁢       ∫     -   ∞     ∞     ⁢                  ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -   v     )       ⁢     ⅇ     -   ⅈωτ       ⁢           ⁢     ⅆ   ω              2              1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (           ω   p     -     ω   res     -   v     c     ⁢   2   ⁢     L   c       )                    2       ⁢           ⁢     ⅆ   v                         (   15   )             
 
         [0048]     For a very high Q cavity, the denominator becomes very small when ν≈ω p −ω res . Evaluating the numerator at this FDOA, pulling it out of the integral and evaluating the remaining integral only over the IF signal bandwidth, yields, by way of non-limiting example:  
                         P   cav     ⁡     (     τ   ,     ω   res       )       _     ≈       ⁢       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0     ⁢     T   1     ⁢     T   2       )     2     DL     ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (         ω   ⁢             p     -     ω   res       )       )       ⁢     ⅇ     -   ⅈωτ       ⁢     ⅆ   ω              2                       ⁢       ∫       ω   p     -     ω   res     -     Ω   2           ω   p     -     ω   res     +     Ω   2         ⁢       1            1   -       R   1     ⁢     R   2     ⁢     ⅇ     -     ⅈ   ⁡     (           ω   p     -     ω   res     -   v     c     ⁢   2   ⁢     L   c       )                    2       ⁢           ⁢     ⅆ   v                     ≈       ⁢       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0       )     2     DL     ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )       ⁢     ⅇ     -   ⅈωτ       ⁢           ⁢     ⅆ   ω              2                       ⁢       (         (       T   1     ⁢     T   2       )     2       1   -       R   1   2     ⁢     R   2   2           )     ⁡     [     Ω   +     2   ⁢     c     L   c       ⁢       tan     -   1       ⁡     (         R   1     ⁢     R   2     ⁢     sin   ⁡     (       Ω   ⁢           ⁢     L   c       c     )           1   -       R   1     ⁢     R     2   ⁢               ⁢     cos   ⁡     (       Ω   ⁢           ⁢     L   c       c     )             )           ]                     (   16   )             
 
         [0049]     An additional simplification is possible when the IF signal bandwidth Ω is much greater than the biphoton resonance width and less than the free spectral range. This condition may be expressed by way of non-limiting example as:  
                 c     2   ⁢     L   c         ⁢     (     2   -     R   1     -     R   2       )     ⁢     &lt;&lt;   Ω       &lt;         π   ⁢           ⁢   c       L   c       .             (   17   )             
 
 When the condition represented in equation (17) obtains, the bracketed term in equation (16) is approximately equal to the free spectral range, and the average probability of biphoton coincidence may be expressed as, by way of non-limiting example:  
                   P   cav     ⁡     (     τ   ,     ω   res       )       _     ≈       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0       )     2     DL     ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )       ⁢     ⅇ     -   ⅈωτ       ⁢           ⁢     ⅆ   ω              2     ⁢     (         (       T   1     ⁢     T   2       )     2       1   -       R   1   2     ⁢     R   2   2           )     ⁢         π   ⁢           ⁢   c       L   c       .               (   18   )             
 
         [0050]     Finally, for a lossless cavity with identical mirrors (which simplifies the exposition but is not meant to be limiting), the average probability of biphoton coincidence may be expressed as, by way of non-limiting example:  
                   P   cav     ⁡     (     τ   ,     ω   res       )       _     ≈       1     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0       )     2     DL     ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )       ⁢     ⅇ     -   ⅈωτ       ⁢           ⁢     ⅆ   ω              2     ⁢     (         (     1   -   R     )     3         (     1   +   R     )     ⁢     (     1   +     R   2       )         )     ⁢         π   ⁢           ⁢   c       L   c       .               (   19   )             
 
 The scale factor β may be inserted in equation (19) in analogy with equation (1). In particular, the arguments of S 1  and S 2  may be divided by β 1  and β 2 , respectively. Assuming by way of non-limiting example a very narrow pump beam, the frequency resolution Δω of the CAF may be given by the width of the biphoton resonance. This relation may be expressed as, by way of non-limiting example:  
             Δω   =       c     L   c       ⁢       (     1   -   R     )     .               (   20   )             
 
         [0051]      FIG. 5  depicts IF signal bandwidth  510  as a function of FDOA resolution  520  for several cavities with different mirror reflectance. The ratio of the FDOA resolution to the IF signal bandwidth may be derived from equation (20) and represented as, by way of non-limiting example:  
                 Δ   ⁢           ⁢   ω     Ω     &gt;         (     1   -   R     )     π     .             (   21   )               
         [0052]     This ratio indicates the granularity of the measurements possible within the IF bandwidth. The resolution may, however, be degraded somewhat due to finite averaging time and signal-to-noise issues.  
         [0053]     The time resolution of the CAF is related to the signal bandwidth, and the signal-to-noise ratio. The signal-to-noise ratio (“SNR”) is calculated presently. The probability of detection for a single photon (either signal or idler) is given as, by way of non-limiting example:  
                   P   j     _     =       1     Δ   ⁢           ⁢   T       ⁢                κ   j     ⁢       s   j     ⁡     (   t   )              2     _     ⁢       ∫         ω   p     2     ⁢       Ω   j     2             ω   p     2     +       Ω   j     2         ⁢       I   j     ⁡     (   ω   )             ⁢     
     ⁢           ⁢           T   1     ⁢     T   2                1   -           R   1     ⁢     R   2         ⁢     ⅇ     ⅈ   ⁢           ⁢     (       2   ⁢           ⁢     L   c     ⁢     ω   c       +     ϕ   1     +     ϕ   2       )                  2       ⁢           ⁢       ⅆ   ω     .               (   22   )             
 
 In equation (22), the term I(ω) represents the spectral distribution of the signal and idler photons, and the index j indicates either signal or idler. If the length of the cavity is such that q of equation (12) is odd, then the signal and idler beams are centered on a transmission null. If the free spectral range of the cavity is made greater than the width of the spectral distribution of the beams (Ω j ), then for a very high Q cavity this can be approximated as, by way of non-limiting example:  
                   P   j     _     ≈       1     Δ   ⁢           ⁢   T       ⁢                κ   j     ⁢       s   j     ⁡     (   t   )              2     _     ⁢     4   π     ⁢         T   1     ⁢     T   2           (     1   +         R   1     ⁢     R   2           )     2           ⁢     
     ⁢           =       4     Δ   ⁢           ⁢   T   ⁢           ⁢   π       ⁢                κ   j     ⁢       s   j     ⁡     (   t   )              2     _     ⁢           (     1   -   R     )     2         (     1   +   R     )     2       .               (   23   )             
 
 Equation (23) assumes for purposes of exposition and by way of non-limiting example that the mirrors are identical and lossless. The signal coincidence count rate can be written as, by way of non-limiting example:  
                 Φ   cc_bi     =         Φ   bi     ⁢       P   cav     _       =       Φ     Δ   ⁢           ⁢   T       ⁢         (       κ   1     ⁢     κ   2     ⁢     η   0       )     2       D   ⁢           ⁢   L             ⁢     
     ⁢           ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )       ⁢     ⅇ       -   ⅈ     ⁢           ⁢   ω   ⁢           ⁢   τ       ⁢           ⁢     ⅆ   ω              2     ⁢     
     ⁢           ⁢       (         (     1   -   R     )     3         (     1   +   R     )     ⁢     (     1   +     R   2       )         )     ⁢         π   ⁢           ⁢   c       L   c       .               (   24   )             
 
 Factors of β may be inserted into the arguments of S 1  and S 2  here in analogy with equation (1). The accidental coincidence rate can be written as, by way of non-limiting example:  
               Φ   cc_bg     =         Φ   bi   2     ⁢         P   s     ⁢     P   i       _     ⁢     T   c       =           (     4   ⁢     Φ   bi     ⁢     κ   1     ⁢     κ   2       )     2         (     Δ   ⁢           ⁢   T   ⁢           ⁢   π     )     2       ⁢                  s   1     ⁡     (   t   )            2     ⁢              s   2     ⁡     (   t   )            2       _     ⁢         (     1   -   R     )     4         (     1   +   R     )     4       ⁢       T   c     .                 (   25   )             
 
         [0054]     In equation (25), T c  represents the coincidence interval. Recognizing that η 0   2 =DL in general, the background-limited signal-to-noise may be represented as, by way of non-limiting example:  
               SNR   bg     =         Φ   cc_bi         2   ⁢           ⁢   B   ⁢           ⁢     Φ   cc_bg           =            ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )                       (   26   )                           ⁢       ⅇ       -   ⅈω     ⁢           ⁢   τ       ⁢           ⁢     ⅆ   ω            2     ⁢         κ   1     ⁢     κ   2     ⁢     π   2     ⁢   c       4   ⁢     L   c     ⁢       2   ⁢           ⁢   B   ⁢           ⁢     T   c     ⁢                  s   1     ⁡     (   t   )            2     ⁢              s   2     ⁡     (   t   )            2       _             ⁢       (     1   -     R   2       )       (     1   +     R   2       )                             
 
 In equation (26), B is the bandwidth (inverse integration time) of the coincident counter. The photon noise limited signal-to-noise may be represented as, by way of non-limiting example:  
               SNR   shot     =         Φ   bi         2   ⁢           ⁢   B   ⁢           ⁢     Φ   bg           =       κ   1     ⁢     κ   2     ⁢           Φ   ⁢           ⁢   π   ⁢           ⁢   c       2   ⁢           ⁢   B   ⁢           ⁢   Δ   ⁢           ⁢     TL   c         ⁢         (     1   -   R     )     3         (     1   +   R     )     ⁢     (     1   +     R   2       )                         (   27   )                       ⁢              ∫     -   ∞     ∞     ⁢         S   1     ⁡     (   ω   )       ⁢       S   2   *     ⁡     (     ω   -     (       ω   p     -     ω   res       )       )       ⁢     ⅇ       -   ⅈω     ⁢           ⁢   τ       ⁢           ⁢     ⅆ   ω              .                           
 
 The total SNR may be represented as, by way of non-limiting example:  
             SNR   =           SNR   bg     ⁢     SNR   shot             SNR   bg   2     +     SNR   shot   2           .             (   28   )             
 
         [0055]     In some embodiments of the present invention, the IF signals entering the electro-optical modulators are conditioned by the delay lines of  FIG. 3 , and no separate delay lines for τ 1 , τ 2  as depicted in  FIG. 2  or delay lines of (1−β)t/2 in the optical pathways as depicted in  FIG. 2  are required. In such embodiments, the signal and idler beams are respectively modulated with IF signals s 1 (β 1 t−τ 1 ) and s 2 (β t t−τ 2 ).  
         [0056]     Note that in some embodiments of the present invention, the only limitation on bandwidth is the speed of the ADC, DAC, and EOM components of  FIGS. 2 and 3 . These components preferably operate at twice the frequency of the bandwidth under consideration.  
         [0057]     In some embodiments of the present invention, the delay line of  FIG. 3  may be used to temporarily store the received signals and clock them out repeatedly at a much faster rate to modulate the photon beams while changing any, or a combination of, time, frequency, and scale parameters. In this way, multiple CAF values can be computed for each signal snapshot. The output rate can be an order of magnitude faster than the input. During such temporary storage, more data on the received signals may be gathered.  
         [0058]     In some embodiments of the present invention, different ways of modulating the signal and idler photon beams are contemplated. Such techniques include, by way of non-limiting example, a Mach-Zender modulator, acousto-optic modulator, or other type of modulator. Other parameters may be modulated instead of or in addition to polarization. Such parameters include, by way of non-limiting example, intensity and frequency.  
         [0059]     Entangled photons may be produced according to a variety of methods. By way of non-limiting example, entangled photons may be produced according to types I or II parametric down-conversion. Furthermore, any nonlinear crystal, not limited to beta barium borate or lithium niobate, may be used. Other ways to produce entangled photons include: excited gasses, materials without inversion symmetry, and generally any properly phase-matched medium. Entangled photon production consistent with this disclosure is not limited to using any particular non-linear crystal. Furthermore, the entangled photons are not limited to any particular wavelength or frequency. Biphotons whose constituent signal and idler photons are orthogonally polarized may be used as well as biphotons whose constituent signal and idler photons are polarized in parallel.  
         [0060]     In some embodiments of the present invention, the cavity and coincidence counter are replaced with a cell containing a biphoton sensitive material (“BSM”), such as, by way of non-limiting example, rubidium-87 ( 87 Rb). Such a substance typically has a two-photon absorption line near the pump frequency that fluoresces following absorption of a biphoton pair. A magnetic field can be used to detune the absorption line from the pump frequency in order to measure the FDOA. Detectors along the cell measure the fluorescence. A magnetic field gradient results in detuning that is a function of the distance along the cell. Signals with different FDOAs would then fluoresce at different locations within the cell. Essentially, the magnetic field gradient replaces, or supplements, the detuning between the cavity and the pump laser. Thus, detecting fluorescence in a BSM cell at particular locations along a magnetic field gradient indicates particular FDOAs. In such embodiments, CAF values for multiple FDOAs could be measured at once. In embodiments that employ a BSM, the magnetic field gradient reduces or eliminates the need to scan frequency difference parameters. In BSM embodiments, the number of biphoton absorptions detected during a specified time period (e.g., as τ 1  and/or τ 2  are changed) is used to derive TDOA information in analogy with embodiments that employ a coincidence counter.  
         [0061]     In some embodiments of the present invention that employ a BSM cell, indicia other than fluorescence may be used to detect entangled photon absorption. By way of non-limiting example, entangled-photon absorption may result in fluorescence, phosphorescence, direct electron transfer, or ionization of the absorbing material. Detecting fluorescence, phosphorescence, direct electron transfer, or ionization may be used to detect entangled-photon absorption. Also by way of non-limiting example, avalanche photodiodes, photo multiplier tubes (PMT), or other devices may be used to detect the fluorophotons, ionization, direct electron transfer, or other absorption indicia at particular locations in the BSM cell.  
         [0062]     Scanning the CAF plane may be accomplished in various ways in embodiments of the present invention. Embodiments with multiple DACs, cavities, and detectors could be used to simultaneously compute many points on a CAF plane. In some embodiments, these techniques obviate the need for scanning entirely for the broadband case. For the narrow-band case, multiple delay lines and cavities with the gradient-dependent BSM efficiently scans the CAF plane.  
         [0063]     The following considerations are with regard to calibration. It may be preferable in some embodiments to use collimated white light to adjust for equal path lengths, since the short coherence length yields only a few fringes. In some embodiments of the present invention, one modulator is driven with a frequency equal to half the free spectral range, which results in transmission through the cavity if the cavity is tuned to be resonant to the pump but not resonant to signal and idler (e.g., if 2L is an odd number of pump wavelengths, where L is the cavity length). Scanning the modulation frequency is a way to determine where the cavity is tuned.  
         [0064]     Some embodiments of the present invention may be useful for bi-static RADAR. Locating objects using the reflection of GPS signals by correlating reflection (multipath) with a direct path from a satellite is possible. TDOA plus ephemeris for multiple satellites yields location. FDOA is useful for identifying specific satellites (e.g., GPS satellites).  
         [0065]     In some embodiments of the present invention, parallel quantum CAF function generators are possible. Such generators allow simultaneous processing of multiple CAF elements. This may be accomplished by way of multiple ratios of scaling factors and time offsets (β and τ, respectively), and can be generated with a single analog-to-digital converter and multiple digital-to-analog converters.  
         [0066]     In some embodiments of the present invention, the residual signal photon beam and idler photon beam components that are not directed to cavity  222  by polarizing beam splitter  220  in the embodiment of  FIG. 2  are directed to a second cavity and coincidence counter. Such a combination allows for additional QCAF processing. The residual orthogonal components sent to the second cavity are proportional to one minus the components that are directed to cavity  222 . By injecting these components into the second cavity (e.g., one identical to cavity  222 ), a useful signal results (e.g., identical to the signal produced by coincidence counter  230 ). Such a second cavity preferably has a detuning that is sufficiently far away from zero or any of the component IF frequencies of s 1 (t)  201  or s 2 (t)  202  to avoid undesirable resonance peaks. Two FDOAs may be evaluated at once if the second cavity is detuned from the pump beam differently from cavity  222 .  
         [0067]     In some embodiments of the present invention, the delay lines of  FIG. 2  need not include differential clock rate capability. In such embodiments, tuning the pump laser or adjusting the cavity length alone are sufficient for adjusting the Doppler factor or scaling factor. These embodiments are particularly useful for detecting narrowband RF signals.  
         [0068]     In some embodiments of the present invention, time dependent optical delays may be inserted before and after the electro-optical modulators to account for scaling. For short optical delays, optical modulators, which change the index of refraction in response to voltage, can be used. Other ways to introduce optical delays include lengths of optical fiber, which may be switched in and out of the path by way of optical switches.  
         [0069]     The equations contained in this disclosure are illustrative and representative and are not meant to be limiting. Alternate equations may be used to represent the same phenomena described by any given equation. In particular, the equations disclosed herein may be modified by adding error-correction terms, higher-order terms, or otherwise accounting for physical inaccuracies, using different names for constants or variables, or using different expressions. Other modifications, substitutions, replacements, or alterations of the equations may be performed. Further, the symbols, variables, and parameters in each equation or formula are to be interpreted for that specific equation or formula. That is, each symbol, variable, and parameter is to be interpreted with respect to the equation or formula in which it appears. The same symbol may be used to represent different quantities in different equations or formulas in the present disclosure.  
         [0070]     While the foregoing description includes details and specificities, it should be understood that such details and specificities have been included for the purposes of explanation only, and are not to be interpreted as limitations of the present invention. Many modifications to the embodiments described above can be made without departing from the spirit and scope of the invention, as it is intended to be encompassed by claims and their legal equivalents.