Abstract:
A system for and method of detecting and characterizing materials using entangled photons is presented. The material may be at a great distances from the detector and may be biological material, complex organic compounds, or inorganic chemicals. The disclosed system and method provide advantages over traditional techniques in that they are largely impervious to atmospheric reduction of probing radiation and in that less probing radiation is required. The reduced probe energy requirement allows for detecting and characterizing sensitive material with significantly reduced material bleaching compared with traditional techniques.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority to U.S. Utility patent application Ser. No. 10/850,394, to Kastella et al., entitled “System and Method of Detecting Entangled Photons,” filed on May 21, 2004, and benefit of U.S. Provisional Patent Application No. 60/555,675, to Freeling et al., entitled “Entangled-Photon Spectroscopy for Stand-Off Detection and Characterization,” filed on Mar. 24, 2004, the disclosures of which are expressly incorporated by reference herein in their entireties. 
     
    
     GOVERNMENT INTERESTS  
       [0002]     To the extent that this invention was made with government support under contract number-F33615-99-D-1501, delivery order  0009 , awarded by the U.S. Air Force, the government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The invention relates to ascertaining properties of a material using entangled photons. In particular, the invention relates to directing entangled photons to a material, possibly located at a distance from the source of entangled photons, in order to ascertain properties of the material by detecting evidence of entangled photon absorption by the material.  
         [0005]     2. Discussion of Background Information  
         [0006]     Photons are quanta of electromagnetic energy. Multiple photons may be entangled or not entangled. Photons that are not entangled together (i.e., random photons) exist as independent entities. In contrast, entangled photons have a connection between their respective properties.  
         [0007]     Two photons entangled together are referred to as an entangled-photon pair (also, “biphotons”). Traditionally, photons comprising an entangled-photon pair are called “signal” and “idler” photons. Measuring properties of one photon of an entangled-photon pair determines results of measurements of corresponding properties of the other photon, even if the two entangled 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 product of two individual quantum states.  
         [0008]     In general, more than two photons may be entangled together. More than two photons entangled together are referred to as “multiply-entangled” photons. Measuring properties of one or more photons in a set of multiply-entangled photons restricts properties of the rest of the photons in the set by constraining measurement outcomes. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of a set of n&gt;2 multiply-entangled photons cannot be factored into a product of n separate states. The term “entangled photons” refers to both biphotons and multiply-entangled photons.  
         [0009]     General techniques for ascertaining spectroscopic properties of materials are known. Such techniques typically rely on directing non-entangled (random) photons at a material, which absorbs the photons and emits fluorescence. In general, these techniques rely on varying the frequency of the non-entangled photons. By comparing incident photon energy with the energy of resulting fluorophotons, the absorbing material may be crudely characterized.  
       SUMMARY OF THE INVENTION  
       [0010]     According to an embodiment of the present invention, a system for and method of determining spectroscopic properties of a material are presented. The method comprises producing entangled photons having at least one known entangled photon parameter and directing the entangled photons to a material. Fluorescence resulting from entangled photon absorption by the material is observed, and spectroscopic properties of the material are deduced from the at least one known entangled photon parameter and the fluorescence observations. The spectroscopic properties of the material are deduced from a relationship between absorption characteristics determined by the fluorescence observations and the known entangled photon parameters.  
         [0011]     Various optional and preferable features and advantages of embodiments of the present invention include the following. Materials that may be detected and characterized include organic materials, biological materials, and inorganic materials. The probing radiation may be selected to be easily transmitted by the atmosphere. A material may be detected and characterized at a great distance from the invention embodiment. Low-energy probing radiation may be used, which results in reduced material bleaching when compared to traditional techniques.  
         [0012]     Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of certain embodiments of the present invention, in which like numerals represent like elements throughout the several views of the drawings, and wherein:  
         [0014]      FIG. 1  is a schematic diagram of an embodiment of the present invention;  
         [0015]      FIG. 2  is a schematic diagram of an adjustable nonlinear crystal according to an embodiment of the present invention;  
         [0016]      FIG. 3  is a schematic diagram of an adjustable delay according to an embodiment of the present invention;  
         [0017]      FIG. 4  is a plot of entangled-photon absorption in hydrogen as a function of interbeam delay and entanglement time according to an embodiment of the present invention;  
         [0018]      FIG. 5  is a plot of entangled-photon absorption as a function of interbeam delay and entanglement time for rubidium according to an embodiment of the present invention;  
         [0019]      FIG. 6  is a schematic diagram illustrating energy levels in an atom or molecule undergoing entangled two-photon absorption according to an embodiment of the present invention;  
         [0020]      FIG. 7  is a plot of typical biological material absorption and fluorescence curves together with an atmospheric transmittance curve according to an embodiment of the present invention;  
         [0021]      FIG. 8  is a plot of required power as a function of range for both entangled and random photons according to an embodiment of the present invention;  
         [0022]      FIG. 9  is a schematic diagram of biphoton propagation according to an embodiment of the present invention; and  
         [0023]      FIG. 10  is a plot of beta barium borate (BBO) production of biphotons with differing constituent photon frequencies as a function of BBO angle. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, the description taken with the drawings provides a fundamental understanding of the present invention, making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.  
         [0025]      FIG. 1  is a schematic diagram of a stand-off detection and characterization embodiment  100  according to the present invention. Laser  105  produces an ultraviolet (UV) light pump beam  110  with a wavelength of, by way of non-limiting example, 389 nanometers (nm). Pump beam  110  is directed to adjustable nonlinear crystal  115 . Adjustable nonlinear crystal  115  is preferably constructed of beta barium borate (BBO), although other nonlinear crystal materials may be used. Adjustable nonlinear crystal  115  converts pump beam  110  into entangled-photon pairs  120  via type-II parametric downconversion. Entangled-photon pairs  120  produced by adjustable-nonlinear crystal  115  comprise signal photons  130  and idler photons  135 . Signal photons  130  are polarized at 90° to idler photons  135 . Entangled-photon pairs  120  are directed to polarizing beam splitter (PBS)  125 , which separates signal photons  130  from idler photons  155  according to polarization. Signal photons  130  are directed to sample material  145 , and idler photons are directed to adjustable delay  140 . Adjustable delay  140  includes movable mirror set  150 . Idler photons  135  leaving adjustable delay  140  are directed to sample material  145 , where they join signal photons  130 . Sample material  145  may absorb signal photons  130  and idler photons  135  by way of entangled-photon absorption. Fluorophotons  165  released by sample material  145  in response to entangled-photon absorption are gathered by collector  170  and directed to detector  175 . Preferably, collector  170  has a relatively large aperture. Band-pass, high-pass, or low-pass filtering may be incorporated into detector  175  to select particular fluorophotons and suppress background noise. Detector  175  is preferably a high-efficiency low-noise single-photon detector, such as, by way of non-limiting example, a photo-multiplier tube or an avalanche photodiode. Detector  175  is placed in the image plane to detect fluorophotons  165  and feed corresponding data to a computer (not shown). Such data may comprise, by way of non-limiting example, one or more electrical signals indicative of detection.  
         [0026]     Stand-off detection and characterization embodiment  100  is used to analyze spectroscopic properties of sample material  145 . More particularly, by manipulating parameters associated with entangled photons and monitoring whether sample material  145  fluoresces in response to being bombarded by these photons, stand-off detection and characterization embodiment  100  ascertains certain spectroscopic properties of sample material  145 . As discussed below in reference to  FIGS. 2 and 3 , entangled-photon parameters that may be usefully manipulated in embodiments of the present invention include entanglement time and interbeam delay. Standard electronic hardware or software is used to gather relevant entangled-photon parameter information and fluorescence observations and thereby compute derived spectroscopic information. Specifics of this derivation and the types of spectroscopic information that may be determined are presented in detail below in reference to  FIG. 6 .  
         [0027]      FIG. 2  is a schematic diagram of an exemplary adjustable nonlinear crystal  200  according to an embodiment of the present invention. Adjustable nonlinear crystal  200  may be used in the embodiment of  FIG. 1  in the capacity of adjustable nonlinear crystal  115 . Adjustable nonlinear crystal  200  includes two wedge-shaped halves  215 ,  220 . Nonlinear crystal half  215  is fixed relative to incoming photon beam  205 . Nonlinear crystal half  215  abuts nonlinear crystal half  220  along their respective hypotenuse faces  260 ,  250  with an index-matching fluid disposed there between. Nonlinear crystal half  215  is translatable with respect to nonlinear crystal half  220 . Accordingly, adjustable nonlinear crystal  200  is capable of being configured so as to have different effective widths. Incoming photon beam  205  preferably enters adjustable nonlinear crystal  200  perpendicular to entrance face  230  of nonlinear crystal half  215 . Entangled photons  210  exit adjustable nonlinear crystal through exit face  240  of nonlinear crystal half  220 . Entrance face  230  and exit face  240  of nonlinear crystal halves  215 ,  220 , respectively, are preferably parallel, regardless of the translation positions of nonlinear crystals halves  215 ,  220  with respect to each-other. The effective thickness of adjustable nonlinear crystal  200  may be ascertained using, by way of non-limiting example, interferometry. Reporting logic capable of determining the effective thickness of adjustable nonlinear crystal  200  is operatively coupled with a computer.  
         [0028]     Adjustable nonlinear crystal  200  is used to produce entangled photons with a known entanglement time. Entanglement time is an entangled photon parameter that depends on the thickness of the nonlinear crystal that produces the entangled photons. Typically denoted T e , entanglement time may be computed as, by way of non-limiting example, T e =DL/2, where D is the difference in inverse group velocities of ordinary and extraordinary rays leaving the nonlinear crystal, and L is the length of the nonlinear crystal. By way of non-limiting example, for BBO, the parameter D may be approximated as D≈0.2 psec/mm, where “psec” denotes picoseconds. Because the effective width of adjustable nonlinear crystal  200  is manipulable, adjustable nonlinear crystal  200  selects the entanglement time of produced entangled photons. Thus, setting various translation positions of adjustable nonlinear crystal halves  215 ,  220  allows for production of entangled photons with various corresponding entanglement times. The configuration for selecting entanglement time illustrated in  FIG. 2  is not meant to be limiting; other techniques and configurations are also contemplated.  
         [0029]      FIG. 3  is a schematic diagram of an adjustable delay device  300  according to an embodiment of the present invention. Adjustable delay  300  may be used in the embodiment of  FIG. 1  in the capacity of adjustable delay  140 . Adjustable delay  300  includes a first mirror set  310  and a second mirror set  320 . Second mirror set  320  is translatable with respect to first mirror set  310 . By manipulating the distance between first mirror set  310  and second mirror set  320 , the optical path length traveled by photons entering adjustable delay  300  may accordingly be manipulated. In operation, a photon beam entering adjustable delay  300  encounters first mirror set  310  and is diverted to second mirror set  320 . Second mirror set  320  reflects the photon beam back to first mirror set  310 , which returns the photon beam to the line defined by the entering photon beam. The increased distance provided by adjustable delay may be monitored using, by way of non-limiting example, interferometry. The particular arrangement for delaying photons disclosed by  FIG. 3  is not meant to be limiting; other apparatuses and configurations for delaying photons are also contemplated.  
         [0030]     Adjustable delay  300  is used to select an interbeam delay parameter of entangled photons. More particularly, when used in an embodiment of the present invention, adjustable delay  300  interposes a temporal delay between constituent photons of an entangled photon pair. This delay, generally denoted by τ, is referred to as the “interbeam delay” associated with the entangled photons. That is, the interbeam delay of an entangled photon pair is the delay induced by lengthening the optical path of one of the signal or idler photon relative to the optical path length of the other of the signal or idler photon.  
         [0031]      FIG. 4  is a plot of entangled-photon absorption by hydrogen as a function of interbeam delay and entanglement time according to an embodiment of the present invention. More particularly,  FIG. 4  depicts entangled photon absorption  410  measured in decibels (dB) as a function of entanglement time T e    420  and interbeam delay τ  430 . As is apparent from  FIG. 4 , entangled photon absorption is periodic in each of entanglement time and interbeam delay. Additionally, entangled photon absorption is essentially nonexistent, as represented by portion  440 , whenever the interbeam delay  430  is greater than the entanglement time  420 . Thus, embodiments of the present invention generally ensure that entanglement time  420  is greater than interbeam delay  430 .  
         [0032]     Embodiments of the present invention generally seek to manipulate entanglement time and interbeam delay so as to maximize entangled photon absorption by the sample material.  FIG. 4  illustrates that in the case of hydrogen, there are many possible combinations of entanglement time and interbeam delay that maximize entangled photon absorption, e.g.,  450 . The entanglement time and interbeam delay of a set of entangled photons are typically manipulated as disclosed above in reference to  FIGS. 2 and 3 , respectively. Maximizing entangled photon absorption generally maximizes detectible fluorescence, which is in turn used to derive spectroscopic properties of the sample material as described in detail below in reference to  FIG. 6 . Note however, that either or both of entanglement time and interbeam delay may be manipulated according to embodiments of the present invention. Note also that entangled-photon absorption need not be maximized for embodiments of the present invention to successfully operate. That is, a less-than-maximal amount of entangled-photon absorption suffices in some embodiments of the present invention.  
         [0033]      FIG. 5  is a plot of entangled two-photon absorption (ETPA) as a function of interbeam delay and entanglement time for rubidium-87 ( 87 Rb) according to an embodiment of the present invention. More particularly,  FIG. 5  depicts the magnitude squared of the quantum mechanical ETPA matrix element Me divided by a scaling factor 2D for  87 Rb as a function of entanglement time and interbeam delay. In general, an ETPA matrix element is indicative of the ability of a material to absorb entangled-photon pairs. As illustrated by  FIG. 5 , there are several combinations of entanglement time and interbeam delay that maximize entangled photon absorption, e.g., local maxima  540 .  
         [0034]      FIG. 6  is a schematic diagram illustrating energy levels in a sample material undergoing ETPA according to an embodiment of the present invention. Each solid horizontal line segment  610 ,  615 ,  625 ,  630  represents an electron energy level in the material. Initial energy state ε i    610  represents the energy level of the material before ETPA. Final energy state ε f    615  represents the post-ETPA energy level of the material, just before it fluoresces. The entangled photons absorbed by the material as represented in  FIG. 6  originate from a down-conversion process in a nonlinear crystal driven by a pump laser of frequency cop. Thus, the sum of the frequencies of the signal photon and idler photon in such an entangled photon pair is ω p . For degenerate signal and idler photons, dotted line  620  represents an energy level corresponding to one-half of op. The final energy level ε f    615  minus the initial energy level ε i  equals the total energy of the entangled photon pair, ω p .  
         [0035]     Immediately after absorbing the entangled photons, the material&#39;s electronic state makes virtual transitions through energy levels  625 ,  630 , which are higher than the total energy of the entangled photons. Such energy levels are denoted by ε j  and referred to as virtual states. An embodiment of the present invention is capable of discerning properties of the material, including the material&#39;s identity, from spectroscopic information related to such virtual states. One such distinguishing property, discussed further below, is an energy difference between such virtual states ε j  and the ground state ε i  of the material.  
         [0036]     Toward explaining how an embodiment of the present invention derives spectroscopic information, an ETPA cross-section theory is disclosed presently. An ETPA cross-section tells how likely a material is to absorb entangled-photon pairs. The larger the ETPA cross-section, the more strongly a material absorbs entangled-photon pairs. The ETPA cross-section of material as a function of entanglement time and interbeam delay may be represented as, by way of non-limiting example:  
                 σ   e     ⁡     (       T   e     ,   τ     )       =         πω   p   2       16   ⁢     A   e     ⁢     T   e         ⁢     δ   ⁡     (       ɛ   f     -     ɛ   i     -     ω   p       )       ⁢       s   ⁡     (       T   e     ,   τ     )       .               (   1   )             
 
 In equation (1), σ e  represents the ETPA cross section, T e  represents the entanglement time, τ represents the interbeam delay, ω p  represents the pump laser frequency, and A e  represents the entanglement area. Entanglement area is a quantity associated with the region in a nonlinear crystal in which a single photon gives rise to two entangled photons. The entanglement area may be approximated as, by way of non-limiting example: A e ≈λ s λ i /[sin(θ s ) sin(θ i )], where λ s , λ i  are the wavelengths of the signal and idler photon, respectively, and θ s , θ i  are the opening angles of the signal photon and idler photon beams, respectively. The term s(T e , τ) in equation (1) contains spectroscopic information. This term may be written as, by way of non-limiting example:  
               s   ⁡     (       T   e     ,   τ     )       =                ∑   j     ⁢       A   j     ⁡     (     2   -     ⅇ     -       ⅈΔ   j     ⁡     (       T   e     -   τ     )           -     ⅇ     -       ⅈΔ   j     ⁡     (       T   e     +   τ     )             )              2     .             (   2   )             
 
 In equation (2), the symbol A j  is defined as, by way of non-limiting example, the ratio of the dipole transition matrix element D j  to the energy difference  
         Δ   j     =         ɛ   .     j     -     ɛ   i     -       ω   p     2           
 
 for the transition |ε i           →|δ j           →|ε f            as illustrated by  FIG. 6 . By way of illustrative, non-limiting example, for equation (2), line widths are assumed to be small (T e κ j &lt;&lt;1). Averaging equation (2) over several entanglement times yields, by way of non-limiting example:  
                 s   _     ⁡     (   τ   )       =       4   ⁢       ∑     j   ,   k       ⁢       A   j   *     ⁢     A   k           +     2   ⁢       ∑   j     ⁢              A   j          2     ⁢       (     1   +     cos   ⁡     [     2   ⁢     Δ   j     ⁢   τ     ]         )     .                     (   3   )             
 
 The symbols of equation (3) are as described above in reference to equations (1) and (2). The symbol * denotes Hermitian conjugation. Equation (3) is periodic in τ with a period of 2Δ j . Fourier transforming equation (3) with respect to τ yields, by way of non-limiting example:  
                           s   _     ^     0     ⁡     (   ω   )       ≡       ⁢     ∫         s   _     ⁡     (   τ   )       ⁢     ⅇ   ⅈωτ     ⁢     ⅆ   τ                     =       ⁢         (       4   ⁢       ∑     j   ,   k       ⁢       A   j   *     ⁢     A   k           +     2   ⁢       ∑   j     ⁢            A   j          2           )     ⁢     ∫       ⅇ   ⅈωτ     ⁢     ⅆ   τ           +     2   ⁢       ∑   j     ⁢            A   j          2                           ⁢     ∫       (     cos   ⁡     [     2   ⁢     Δ   j     ⁢   τ     ]       )     ⁢     ⅇ   ⅈωτ     ⁢     ⅆ   τ                     =       ⁢         (       4   ⁢       ∑     j   ,   k       ⁢       A   j   *     ⁢     A   k           +     2   ⁢       ∑   j     ⁢            A   j          2           )     ⁢     δ   ⁡     (   ω   )         +                     ⁢       ∑   j     ⁢              A   j          2     ⁢       (       δ   ⁡     (     ω   +     2   ⁢     Δ   j         )       +     δ   ⁡     (     ω   -     2   ⁢     Δ   j         )         )     .                       (   4   )             
 
 Equation (4) reveals a spectrum with a peak at zero frequency and peaks symmetrically located at ±2Δ j . Thus, embodiments of the present invention may determine Δ j  by locating peaks in a plot of equation (4). The amplitudes of the symmetric peaks are represented by  
             D   j   *     ⁢     D   j         Δ   j   2       .       
 
 The transition matrix elements, D j , are of the form D j ≡         ε f |d|ε j                     ε j |d|ε i           , with d≡{right arrow over (p)}·ê the electric dipole operator for the transitions |ε j           →|ε f           and |ε i           →|ε j           , where {right arrow over (p)} is the momentum operator and ê a unit vector. 
 
         [0037]     Thus, an embodiment of the present invention, such as that depicted above in reference to  FIG. 1 , determines spectroscopic information of a sample material by exploiting properties of equations (1)-(4). First, the embodiment directs entangled photons having known entangled-photon parameter values at the sample material. The parameters may be any, or a combination of, entanglement time, interbeam delay, and polarization. The values of these parameters may be scanned in a discrete or continuous fashion. If the embodiment is directed to detecting a particular material, the parameter values may have a limited domain. Second, the embodiment gathers data relating to observed fluorescence and the parameters of the associated incident entangled photons. The embodiment then constructs a data structure representation of observed fluorescence as a function of entangled-photon parameters. This function corresponds with equation (2) above. This, and the other functions subsequently described, are preferably stored as conventional data structures.  
         [0038]     Third, if appropriate, values of entanglement time are averaged out, leaving a function of a single variable (e.g., interbeam delay). The resulting function corresponds with equation (3) above. Fourth, this function is Fourier transformed to yield a function whose domain is energy (equivalently, frequency). This function corresponds with equation (4) above and represents a spectral analysis. This function may be stored and used to identify and characterize the material. Alternately, or in addition, by inspecting a plot of this function for peaks, embodiments of the present invention ascertain energy difference values Δ j . These values comprise spectroscopic information that is valuable in its own right. In particular, embodiments of the present invention may use these values to identify and characterize the sample material. Fifth, embodiments of the present invention may further process or analyze the spectral analysis function or the functions built from observed data and corresponding to equations (2) and (3) to derive yet more spectroscopic information. Such information includes the material&#39;s energy level difference ε j −ε i . Because the energy difference Δ j  is a function of cop, which is known, as well as ε j −ε i , embodiments of the present invention may determine this latter quantity once Δ j  is known. Sixth, the spectroscopic information obtained by observation is compared with pre-stored spectroscopic information in order to identify and characterize the material. Any of the spectroscopic information discussed herein may be used for such a comparison.  
         [0039]     Embodiments of the present invention are capable of identifying materials by comparing observed properties with properties stored in a pre-formed database. The database information may be gathered by performing laboratory analysis of materials of interest. Such a database acts as a storehouse of material “fingerprints,” which may be used to identify an unknown sample based on its observed spectroscopic properties. A match between stored properties and observed properties yields a material identification. Such a database preferably includes spectral analysis functions, energy differences Δ j , and energy level differences ε j −ε i , for a variety of materials. These data may further include shape information of fluorescence-response curves. More particularly, the database preferably includes representations of Δ j , ε j −ε i , and other spectroscopic properties, as functions of entanglement time, interbeam delay, and other entangled-photon parameters. Such representations are preferably stored as conventional data structures. These representations may be compared with material responses to multiple entangled-photon bombardments. Such comparisons allow for matching “shapes” of various response curves, instead of matching single point responses.  
         [0040]     Another quantity that embodiments of the present invention may ascertain is the magnitude of transition matrix element D j . This can be seen by computing the second derivative of equation (3) with respect to τ prior to Fourier transforming. The second derivative of equation (3) may be represented as, by way of non-limiting example:  
                   ∂   2     ⁢       s   _     ⁡     (   τ   )           ∂     τ   2         =       -   8     ⁢       ∑   j     ⁢       D   j   *     ⁢     D   j     ⁢       cos   ⁡     [     2   ⁢     Δ   j     ⁢   τ     ]       .                   (   5   )             
 
 Equation (5) is then Fourier transformed to yield, by way of non-limiting example:  
                           s   _     ^     1     ⁡     (   ω   )       ≡       ⁢     ∫           ∂   2     ⁢       s   _     ⁡     (   τ   )           ∂     τ   2         ⁢     ⅇ   ⅈωτ     ⁢     ⅆ   τ                     =       ⁢       -   8     ⁢       ∑   j     ⁢       D   j   *     ⁢     D   j     ⁢     ∫       cos   ⁡     [     2   ⁢     Δ   j     ⁢   τ     ]       ⁢     ⅇ   ⅈωτ     ⁢     ⅆ   τ                           =       ⁢       -   4     ⁢       ∑   j     ⁢       D   j   *     ⁢         D   j     ⁡     (       δ   ⁡     (     ω   +     2   ⁢     Δ   j         )       +     δ   ⁡     (     ω   -     2   ⁢     Δ   j         )         )       .                         (   6   )             
 
 Equation (6) contains transition matrix magnitude information. The graph of equation (6) represents spectral information, has no d.c. (zero frequency) component, and possesses symmetrically-located peaks. The amplitude of each peak is proportional to the magnitude of the transition matrix element D j  for the given Δ j . Accordingly, transition matrix magnitude information is also preferably included in the material “fingerprint” database (e.g., transition matrix magnitude as a function of one or more entangled-photon parameters). Such information provides additional data that may be used to identify a sample. 
 
         [0041]     Note that embodiments of the present invention are capable of ascertaining sample material spectroscopic information using only a single pump laser frequency. This is in contrast with conventional spectroscopy in which the frequency of probing radiation varies and intermediate state properties correlate only with probe radiation frequency. Embodiments of the present invention may ascertain a plethora of spectroscopic material information by using a single pump frequency and varying one or more entangled-photon parameter(s). Such parameters may include entanglement time and interbeam delay, as discussed above, and also preferably include others, such as constituent photon polarization and relative energy distribution between signal and idler photons (or between multiple photons for multiply-entangled photon embodiments). Nevertheless, in some embodiments of the present invention, pump laser frequency may also be varied.  
         [0042]      FIG. 7  is a plot of typical biological material absorption and fluorescence curves together with an atmospheric transmittance curve according to an embodiment of the present invention. The x-axis  740  represents photon wavelength measured in nm. Curve  710 , with a peak at approximately 240 nm, represents photon absorption probability for biological material as a function of photon wavelength. Thus, for curve  710 , the y-axis  750  represents absorption probability. Curve  720 , with a peak at about 360 nm, represents biological material fluorescence emissions. For curve  720 , the y-axis  750  represents fluorescence probability according to fluorophoton wavelength. Curve  730  represents atmospheric transmittance as a function of photon wavelength. Thus, for curve  730 , the y-axis  750  represents atmospheric transmittance.  
         [0043]      FIG. 7  also illustrates the limitations of traditional spectroscopic techniques for long-range biological material identification. As depicted in  FIG. 7 , biological materials typically have an excitation band in the UV spectral range (e.g., 220-300 nm). This band generally corresponds with the absorption band of aromatic amino acids found in biological organisms, such as tryptophan, phenolalanine and tyrosine. However, atmospheric transmittance for this spectral range is very low, and for single photons, biological material absorption and atmospheric transmission windows do not in general overlap. Consequently, traditional techniques for long-range standoff detection and characterization of biological materials are limited because the atmosphere absorbs nearly all of the UV light before it reaches the biological material. This phenomenon makes non-entangled photon spectroscopic techniques for detection of UV-induced biological material fluorescence at long ranges impossible.  
         [0044]     In contrast, embodiments of the present invention overcome these obstacles. By using two entangled photons instead of a single photon to excite biological materials, standoff detection and characterization range is greatly improved. In particular, instead of UV photons, embodiments of the present invention use entangled-photon pairs produced from single UV photons for biological or other material detection and characterization. Such entangled-photon pairs may be produced from UV photons such that their individual wavelengths are both in the visible light range. This is a consequence of conservation of energy, which states that the sum of the constituent entangled photon&#39;s energies must equal the energy of the photon from which they were derived. By way of non-limiting example, a 255 nm UV photon may be used to produce an entangled photon pair consisting of a 550 nm photon  760  and 475 nm photon  770 , both of which are readily transmitted by the atmosphere.  
         [0045]     Moreover, the atmosphere is much more transparent to visible photons than it is to UV photons. Such entangled-photon pairs produced from UV photons are in general readily absorbed by biological materials. Thus, compared with random UV photons, entangled-photon pairs derived from UV photons travel farther in the atmosphere while retaining the ability to excite biological materials to fluorescence. In this manner, entangled photons having favorable characteristics for atmospheric transmission may be produced and used to characterize materials. Embodiments of the present invention may be used to identify and characterize materials at ranges of less than one kilometer, several kilometers, several tens of kilometers, or several hundreds of kilometers. The ability to identify and characterize materials at such ranges is particularly useful when the material under analysis may be hazardous.  
         [0046]     Other advantages of using entangled-photon pairs also exist. For example, entangled photons produced from a high-energy (e.g., UV) photon are safer than a single high-energy photon. UV light is dangerous to the human eye, while entangled photons may be produced from a UV photon such that both constituent photons are harmless to the human eye. Additionally, lower-energy photons are less detectable, which allows for covert material detection and characterization where required.  
         [0047]     An embodiment of the present invention may select entangled photons with constituent photons&#39; frequencies particularly suited for travel through the atmosphere with little loss. By way of non-limiting example,  FIG. 7  depicts several transmittance peaks, e.g.,  780 ,  790 ,  795 . Such an embodiment may generate entangled photons whose constituent photons&#39; frequencies lie in the domain of one or more of such peaks. Entangled photons consisting of such constituent photons are thus specially suited for atmospheric transmittance. Atmospheric transmittance may be measured for each situation, and the entangled photons selected accordingly, to allow for remote material identification and characterization using photons particularly suited for transmittance according to local atmospheric characteristics.  
         [0048]     Thus, entangled-photon pairs may be used to excite materials at distances far greater than those available from conventional techniques. Spectroscopic information may accordingly be gained using embodiments of the present invention at distances that were previously intractable. Note that the techniques disclosed herein are not limited to use on biological materials; any material, whether organic or inorganic, may be analyzed.  
         [0049]      FIG. 8  is a plot of required power as a function of range for both entangled and random photons according to an embodiment of the present invention. In particular, the x-axis  810  represents range in kilometers, and the y-axis  820  represents power in arbitrary units (thus the y-axis represents orders of magnitude of power). Curve  840  represents the power required to transmit random photons through the atmosphere. Curve  830  represents the power required to transmit entangled photons through the atmosphere. As is apparent from  FIG. 8 , appropriately-configured entangled photons require less power to transmit through the atmosphere up to ranges on the order of 1000 kilometers. Thus, for long-range applications, entangled photons are superior to random photons.  
         [0050]     Entangled-photon-production power requirements are analyzed presently. The number of received photons generally depends on at least incident flux, dwell time T d  (the time spent bombarding the material at, and detecting fluorophotons from, a particular location), the number of target molecules N i , the fluorescence efficiency E f  (typically a few percent) and the collection efficiency E c . Incorporating these factors, the number of collected photons N c  may be represented as, by way of non-limiting example: 
 
 N   c   =T   d   N   t   R   TPA   E   f   E   c .  (7) 
 
 In equation (7), R TPA  represents a two-photon absorption rate, which may be either random or entangled. Assume for purposes of illustration and by way of non-limiting example that the material to be detected and characterized is in the form of a cloud. Given the depth l of the cloud, the area Ab of the beam at the cloud, and the density ρ of the cloud, we have N t =lA b ρ. Assuming, for the purpose of illustration, perfect detection modules (actual efficiencies are about 0.5), for a collection aperture of area D 2 , the collection efficiency at range R may be estimated as, by way of non-limiting example: 
 
E c ˜3D 2 /4πR 2 ˜D 2 /R 2 .  (8) 
 
 The beam spot area for biphotons of wavelength λ at range R may be estimated at, by way of non-limiting example: 
 
A b ˜(λR/D) 2 .  (9) 
 
 Combining equations (7), (8), and (9) yields:  
                     N   c     =       ⁢       T   d     ⁢     lA   b     ⁢   ρ   ⁢           ⁢     R   TPA     ⁢     E   f     ⁢     E   c                   =       ⁢       T   d     ⁢       l   ⁡     (       λ   ⁢           ⁢   R     D     )       2     ⁢   ρ   ⁢           ⁢     R   TPA     ⁢           E   f     ⁡     (     D   R     )       2     .                   =       ⁢       T   d     ⁢   l   ⁢           ⁢     λ   2     ⁢   ρ   ⁢           ⁢     R   TPA     ⁢     E   f                     (   10   )             
 
 The two-photon production rate may be estimated from equation (10) as, by way of non-limiting example:  
               R   TPA     =         N   c         T   d     ⁢   l   ⁢           ⁢     λ   2     ⁢   ρ   ⁢           ⁢     E   f         .             (   11   )             
 
 By selecting a value of N c  that corresponds with a desired detection level, it is possible to estimate how much output power is needed to achieve that detection level as a function of range. The transmitted power P is assumed by way of non-limiting example to be un-pulsed. The energy of each transmitted photon is E λ =hc/λ=hε f /2 and the flux density may be written as, by way of non-limiting example:  
             ϕ   =         P   ⁢           ⁢   λ         A   b     ⁢   h   ⁢           ⁢   c       =         P   ⁢           ⁢   λ   ⁢           ⁢     D   2         h   ⁢           ⁢   c   ⁢           ⁢     λ   2     ⁢     R   2         =         P   ⁢           ⁢     D   2         h   ⁢           ⁢   c   ⁢           ⁢   λ   ⁢           ⁢     R   2         .                 (   12   )             
 
 The entanglement area of biphotons as initially generated may be transformed by subsequent optics. The entanglement area at the absorbing molecule thus depends on the distance from the sensor to the target and corresponds with the size of the beam spot at the absorbing molecule. Thus, the power requirement may be estimated as, by way of non-limiting example:  
                   P     ETPA   -     A   b         ~         R   TPA     ⁢   h   ⁢           ⁢   c   ⁢           ⁢   λ   ⁢           ⁢     R   2             σ   _     e     ⁢     D   2           ⁢       A   b       A   e         =           R   TPA     ⁢   h   ⁢           ⁢   c   ⁢           ⁢     λ   3     ⁢           ⁢     R   4             σ   _     e     ⁢     A   e     ⁢     D   4         .             (   13   )             
 
 In equation (13), {overscore (σ e )} represents the ETPA cross-section, R TPA  represents the entangled two-photon absorption rate, and P ETPA-Ab  represents the power incident on the material required to achieve the selected N c  value. In contrast, the power required for equivalent random two-photon absorption may be written as, by way of non-limiting example:  
               P   RTPA     =         h   ⁢           ⁢   c   ⁢           ⁢   λ   ⁢           ⁢     R   2       D     ⁢           R   TPA           σ   _     e     ⁢     A   e     ⁢     T   e           .               (   14   )             
 
 In equation (14), R TPA  represents the random two-photon absorption rate. 
 
         [0051]      FIG. 9  is a schematic diagram of biphoton propagation according to an embodiment of the present invention. Pump beam  910  is directed to nonlinear crystal  915 , which produces entangled photon beam  920 . At the single-photon level, pump beam photon  925  is converted into signal photon  930  and idler photon  935 . These individual photons  940  represent coherent wave functions, whereas beams  945  are generally incoherent. The beam waist  950  is generally indicative of entanglement area. Lens  955  focuses divergent entangled photon beam  920  into convergent entangled photon beam  960 . Convergent entangled photon beam  960  arrives at image plane  965 , which is the location at which signal photon  930  and idler photon  935  are absorbed.  
         [0052]     The path of each photon is individually analyzed as follows. Signal photon  930  originates at coordinate {right arrow over (r 2 )} from the face of nonlinear crystal  915  and is transformed according to, by way of non-limiting example: 
 
 I ( {right arrow over (r     2     )})=∫   d{right arrow over (r     1     )}   I ( {right arrow over (r     1     )})|   h ( {right arrow over (r     1     )},   {right arrow over (r     2     )})|   2 .  5) 
 
 In equation (15), 1 represents the signal photon beam intensity at the exit face of nonlinear crystal  915 , {right arrow over (r 1 )} represents the coordinate in image plane  965  of signal photon  930 , and h represents the system transfer function for lens  955 . An equation analogous to equation (15) obtains for incoherent transformation of idler photon  935 . The coherent wave function of the entangled photon pair is transformed as, by way of non-limiting example: 
 
Ψ( {right arrow over (r     3     )},   {right arrow over (r     4     )})=∫   d{right arrow over (r     1     )}∫   d{right arrow over (r     2     )}Ψ(   {right arrow over (r     1     )},   {right arrow over (r     2     )})   h ( {right arrow over (r     1     )},   {right arrow over (r     3     )})   h ( {right arrow over (r     2     )},   {right arrow over (r     4     )}).   (16) 
 
 In equation (16), the parameters are the same as those of equation (15), with the following additions: Ψ represents the biphoton coherent wave function, {right arrow over (r 3 )} represents the coordinate of idler photon  935  at image plane  965 , and {right arrow over (r 4 )} represents the coordinate of signal photon  930  at image plane  965 . The entanglement area for the photons depicted in  FIG. 9  may be computed as, by way of non-limiting example, A e =πr e   2 , where r e  is the solution to |Ψ(r 3 , r 4 −r e )| 2 =1/e. 
 
         [0053]      FIG. 10  is a plot of BBO production of biphotons with differing constituent photon frequencies as a function of BBO angle. The x-axis  1020  represents BBO crystal angle in degrees, and the y-axis represents photomultiplier tube (PMT) readings. The plot illustrates that constituent photons of entangled-photon pairs are available in a variety of wavelengths and may be selected according to BBO crystal angle. As represented by curves 1030 and 1040, 760 nm idler photons and 780 nm signal photons are available at approximately 39.6° and 40.4°, respectively. As represented by curves 1050 and 1060, 780 nm idler photons and 760 nm signal photons are available at approximately 40.5° and 41.5°, respectively.  
         [0054]     In one embodiment the present invention requires less probing radiation energy compared with conventional techniques using similar levels of eye-safe radiation. This has the benefit of offering material identification and characterization with reduced material bleaching. In contrast with traditional photon sources, which provide Poisson-distributed photons that arrive randomly and independently, an entangled-photon source according to an embodiment of the present invention provides photons entangled in space and time. Such photons may be configured to arrive simultaneously at a material rather than randomly and independently, thus necessitating lower intensity radiation while achieving a similar degree of absorption. Another advantage of using entangled photons is seen by examining absorption as a function of photon flux. In general, random two-photon absorption by a material for non-entangled photons is proportional to the square of the incident radiation flux. In contrast, entangled-photon absorption by a material is generally linearly proportional to the incident radiation flux. Thus, under low-flux conditions (under the critical flux, i.e., the flux at which random two-photon absorption and ETPA rates are equal), less flux is required of entangled photons when compared with non-entangled photons to achieve the same level of absorption. As a result, less energy is required to probe a material. Material samples that would be bleached by radiation required for conventional techniques may be safely identified and characterized without such bleaching according to an embodiment of the present invention.  
         [0055]     Many different materials may be identified and characterized in embodiments of the present invention. Examples of biological materials that may be detected and characterized at long ranges include, by way of non-limiting example, nerve gas (such as sarin), spores (such as anthrax), bacteria, viruses, and organic chemicals. Inorganic materials that may be detected and characterized include, by way of non-limiting example, potentially radioactive materials (such as cobalt, cesium, radium, plutonium, or uranium), chlorine gas, and other materials with tactical significance. Materials may be solid, liquid, gas, aerosolized, or colloids. An embodiment of the present invention may be configured to detect chemical, biological, or potentially nuclear material dispersion (e.g., a so-called “dirty bomb”) over large ranges, such as in the upper atmosphere or on the grounds of a government compound.  
         [0056]     Embodiments of the present invention may have various configurations of entangled-photon sources and fluorescence detectors. By way of non-limiting example, the entangled-photon source may be at the same physical location as the fluorescence detector, or may be at a different physical location. Either or both of the entangled-photon source and florescence detector may be fixed or mobile, on the ground, in the air, or orbiting the Earth. Either or both of the entangled-photon source and florescence detector may be located on a vehicle, such as a terrestrial motor vehicle or aircraft. Either or both of the entangled-photon source and florescence detector may be located in close proximity to the material to be identified, such as in the same room. For laboratory settings, an embodiment of the present invention may be a single desktop unit. In some embodiments, entangled-photon pairs are sent over optical fiber, and subsequent fluorophoton emissions are detected and analyzed as described herein.  
         [0057]     Biphotons of any frequency may be used in embodiments of the present invention. That is, entangled photons may be produced from light of any frequency. In particular, the present invention is not limited to using entangled photons produced from UV light to ascertain spectroscopic properties. Entangled photons may be produced from photons of any frequency consistent with the present invention.  
         [0058]     The various calculations, comparisons, and judgments required during operation in order to ascertain spectroscopic properties and identify and characterize materials according to embodiments of the present invention may be accomplished by conventional computer hardware or software. These calculations are preferably performed automatically during the normal course of operation of embodiments of the present invention. By way of non-limiting example, the computations and comparisons associated with equations (1)-(6) used to identify and characterize materials may be performed by appropriately programmed or configured standard computer hardware or software.  
         [0059]     Note that the terms “signal” and “idler” may be used interchangeably. More particularly, as used herein, no distinction is drawn between signal photons and idler photons.  
         [0060]     By way of elaborating on earlier definitions, entanglement time is a quantity associated with the spread in phase differences between signal photons and associated idler photons. That is, entanglement time relates to the collection of differences in phase between signal photons and associated idler photons produced by an entangled-photon source (e.g., a non-linear crystal). Entanglement time may be, by way of non-limiting example, considered as the average time difference between when ordinary and extraordinary rays leave a nonlinear crystal. Ordinary rays leaving a nonlinear crystal are typically associated with signal photons, and extraordinary rays leaving a nonlinear crystal are typically associated with idler photons. By way of non-limiting example, entanglement time is a function of the length l of a non-linear crystal used to produce the entangled photons, and may be described as T e =l(n o -n e )/2c, where n o , n e  are indices of refraction associated with ordinary and extraordinary crystal directions, respectively. By way of non-limiting example, entanglement times on the order of T e =5×10 −13  seconds are possible with a crystal length of 5 mm. These parameters yield an entanglement distance (the distance that light can travel during the entanglement time) of 0.15 mm.  
         [0061]     Alternate embodiments of the present invention may calculate entanglement area A e  according to the following, which describes a non-limiting exemplary technique for such computation. It is typically possible to calculate fourth-order correlation height and width coefficients. In the direction defined by the x-axis, and for a given signal photon at location x s , the idler photon in location x i  will generally arrive within Δx of x s  (i.e., x i =x s ±Δx). Similarly, in the z-axis direction, and for a given signal photon at location z s , the idler photon in location z i  will arrive within Δz of z s  (i.e., z i =z s ±Δz). The quantities Δx and Δz may generally be computed using fourth-order correlation theory. The entanglement area A e  may be derived as the product of Δx and Δz (i.e., A e =ΔxΔz).  
         [0062]     Entangled photons with selected entanglement times may be produced according to a variety of techniques. Instead of adjustable nonlinear crystal  200  of  FIG. 2 , embodiments of the present invention may use two right parallelepiped nonlinear crystals that are closely spaced apart to produce entangled photons with selected entanglement times. In such embodiments, adjusting the distance between the two nonlinear crystals controls the entanglement time of entangled-photon pairs produced by the crystals. In alternate embodiments of the present invention, a single wedge-shaped nonlinear crystal may be used to produce entangled photons with selected entanglement times. By moving such a crystal relative to the pump laser such that the width through which the pump laser beam passes varies, entangled photons with different entanglement times may be produced. In other alternate embodiments of the present invention, entanglement times of entangled-photon pairs may be selected by spectral filtering. In such embodiments, constituent photons of entangled-photon pairs are selected by choosing the appropriate angle (as described above in reference to  FIG. 10 ) or by optical filtering. Each photon&#39;s angle at exiting the crystal depends on the photon&#39;s propagation speed through the nonlinear crystal. Because, as discussed above, entanglement time may be considered as the average time difference between when ordinary and extraordinary rays leave the nonlinear crystal, by selecting the angle at which photons appear, embodiments of the present invention effectively select the entanglement time as well.  
         [0063]     Other types of entangled photons and techniques for producing them within the scope of the present invention include the following. Those of ordinary skill in the art are capable of producing entangled-photon pairs, triples, etc. By way of non-limiting example, entangled photons may be produced according to types I or II parametric down-conversion. That is, 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. For type-I downconversion, signal photons may be separated from idler photons (and recombined with idler photons) using dichroic glass. For both types of downconversion, signal photons and idler photos may be selected as they exit the biphoton source by providing apertures at the appropriate angles. Any non-central symmetric nonlinear crystal, not limited to BBO, may be used. Other ways to produce entangled photons include: excited gasses, materials without inversion symmetry, and generally any properly phase-matched medium. Furthermore, the entangled photons are not limited to any particular wavelength or frequency.  
         [0064]     In embodiments of the present invention, evidence of entangled-photon absorption may be of a variety of forms. 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, or other devices may be used to detect the fluorophotons, ionization, direct electron transfer, or other absorption indicia.  
         [0065]     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 disclosed herein. In particular, the equations disclosed herein may be modified by adding error-correction terms, higher-order terms, or otherwise accounting for inaccuracies, using different names for constants or variables, using different expressions, or accounting for propagation of light through different media. Other modifications, substitutions, replacements, or alterations of the equations may be performed.  
         [0066]     The particular optical manipulation devices depicted herein are illustrative and representative and are not meant to be limiting. By way of non-limiting example, apertures, filters, lenses, and particular lasers disclosed herein may be replaced with devices known to those of ordinary skill in the art.  
         [0067]     Alternate embodiments of the present invention may delay one photon in various ways. By way of non-limiting example, a length of optical fiber may be inserted into the path of one or both-photons. Alternately, sets of mirrors may be used to increase the path length of one or both photons. Other techniques for delaying one or more photons may also be used.  
         [0068]     Note that this disclosure follows standard physics notational conventions. By way of non-limiting example, in some places Planck&#39;s constant h (and h) and the speed of light c may both considered to be one (1) for the purpose of calculations. This convention allows, inter alia, for common units for frequency and energy, as well as common units for time and distance (e.g., temporal delays may be considered as spatial lengths and vice versa). This notational convention is accounted for after calculations have been performed in order to deduce correct units for application purposes. This disclosure also uses Dirac bracket notation (e.g., |ψ i           ), known to those of ordinary skill in the art, to denote quantum states.  
         [0069]     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to certain embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of associated claims, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses.