Abstract:
A single-photon source (SPS) ( 10 ) adapted to output single-photons (P 3 ) at telecommunication wavelengths is disclosed. The SPS includes a color-centered diamond-nanocrystal (CCDN) single-photon source (SPS) ( 20 ) adapted to emit input photons (P 1 ) having a wavelength A 1  that lies outside of the main telecommunication wavelength bands. A non-linear optical medium ( 50 ) pumped using pump photons (P 2 ) of wavelength A 2  receives the input photons and optically downconverts them to output photons (P 3 ) having a wavelength λ 3 &gt;λ 1  wherein λ 3  is within a telecommunication wavelength band. An optical filter ( 60 ) arranged downstream of the non-linear optical medium substantially blocks the pump photons (P 2 ) while allowing for the transmission of the output photons. A QKD system that uses the SPS source of the present invention is also disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to single-photon sources, and in particular to a diamond nanocrystal single-photon source having a wavelength converter. 
       BACKGROUND ART 
       [0002]    Single-photon light sources are finding increasing use for a variety of applications, including quantum computing and quantum communications. Most present-day quantum communication applications rely on weak coherent pulses (WCPs) formed by attenuating multi-photon light pulses so that the WCPs have, on average, less than one photon per pulse. However, this implies that, on average, some WCPs will have more than one photon per pulse, which diminishes the quantum security or quantum computing efficacy provided by true single-photon pulses. Accordingly, true single-photon light sources are often preferred, and in fact have been shown to provide greater transmission distance for quantum communication systems as compared to WCP-based systems. 
         [0003]    A number of different types of single-photon sources have been developed based on the emission properties of single molecules, atoms, color centers, and semiconductor structures, such as quantum dots. Of these different single-photon sources, diamond nanocrystals having a “color center,” such as nitrogen vacancy (“NV”) or a nickel center (NE8), offer several key advantages for quantum communication and quantum computing applications. 
         [0004]    One key advantage is that a color-centered diamond nanocrystal can emit single photons at room temperature. Another key advantage is that single-photon emission from a color-centered diamond nanocrystal avoids problems associated with the single-photon having to travel through a high-refractive-index material, which interferes with the clean transmission of the single photon. This is because color-centered diamond nanocrystals are sufficiently small so that refraction effects are insubstantial. Further, the small size of color-centered diamond nanocrystals (e.g., 10 to 100 nm) means that only a small volume of material needs to be pumped with a pump light source. This results in only very small amounts of background light from the pump light source. Other advantages include a low multi-photon probability and long coherence time. 
         [0005]    Despite these advantages, a major problem with color-centered diamond nanocrystals as single-photon sources is the limited wavelength choices of the emitted photons, which is governed by the atomic-level structure of the color centers. This limits the suitability of color-centered diamond nanocrystals as single-photon sources for optical-fiber-based quantum computation and quantum telecommunication applications, such as quantum key distribution (QKD) and quantum memory devices, which operate best at the known telecommunication wavelengths. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the invention is a single-photon source. The source includes a color-centered diamond-nanocrystal (CCDN) single-photon source (SPS) adapted to emit input photons of wavelength λ 1 . A non-linear optical medium is arranged to receive the input photons. A pump light source is in optical communication with the non-linear optical medium and is adapted to generate pump photons having a wavelength λ 2  that pump the non-linear optical medium so as allow the non-linear optical medium to optically downconvert said first photons passing through the non-linear optical medium to form output photons having a wavelength λ 3  longer than wavelength λ 1 . An optical filter is arranged downstream of the non-linear optical medium and is adapted to substantially block the pump photons and to substantially transmit said output photons. 
         [0007]    Another aspect of the invention is a method of generating single photons. The method includes generating input photons having a wavelength λ 1  using a color-center diamond nanocrystal (CCDN) single-photon source. The method also includes inputting the input photons into a non-linear optical material that is pumped so as to downconvert the input photons. The method further includes forming from the downconvert input photons output photons having an output wavelength λ 3 . 
         [0008]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. 
         [0010]    Whenever possible, the same reference numbers or letters are used throughout the drawings to refer to the same or like parts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is schematic diagram of an example embodiment of the color-centered diamond nanocrystal (CCDN) single-photon source (SPS) according present invention; 
           [0012]      FIG. 2  is a detailed schematic diagram of the CCDN SPS of  FIG. 1 ; and 
           [0013]      FIG. 3  is a detailed schematic diagram of an example non-linear optical medium of the CCDN SPS of  FIG. 1 ; and 
           [0014]      FIG. 4  is a schematic diagram of a QKD system that employs the CCDN SPS of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]      FIG. 1  is schematic diagram of an example embodiment of a single-photon source (SPS)  10  according to the present invention. SPS  10  includes an optical axis A 1 . Arranged along optical axis A 1  is a color-centered (e.g., NV or NE8) diamond nanocrystal (CCDN) SPS  20  that generates single photons P 1  having a wavelength λ 1 . Single photons P 1  are referred to herein as “input photons” for reasons that will become apparent from the discussion below. In an example embodiment, input photons P 1  from the NV center have a wavelength λ 1 ˜637 nm. 
         [0016]    SPS  10  further includes a pump light source  30  arranged along a second optical axis A 2  that intersects optical axis A 1 . Pump light source  30  emits pump light (photons) P 2  at a wavelength λ 2 . In an example embodiment, λ 2 ˜1080 nm. Other pump wavelengths may be used depending on the input photon wavelength λ 1  and the output photon wavelength λ 3 , as explained below. In an example embodiment, pump light source  30  is or includes a Nd:YAG laser, a GaAs laser diode, an InGaAsP laser diode, or the like. 
         [0017]    SPS  10  includes at the intersection of axes A 1  and A 2  a multiplexing element  40  that multiplexes input photons P 1  and pump photons P 2  so that they travel in the same direction along optical axis A 1 . 
         [0018]    SPS  10  further includes along optical axis A 1  and optically downstream of multiplexing element  40  a non-linear optical medium  50 , such as a non-linear bulk crystal or a periodically poled waveguide (including an optical fiber waveguide). Non-linear optical medium  50  is adapted to be pumped by photons P 2  and perform frequency downconversion on photons P 1  that are inputted into the non-linear optical medium-hence the use of the phrase “input photons” for photons P 1 . Non-linear optical medium  50  is adapted to perform downconversion on input photons P 1  and generate downconverted output photons P 3  having a wavelength λ 3 . Described herein is a downconversion interaction based on three-wave mixing, but other conversion schemes, such as a four-wave mixing conversion scheme, can be used as well. 
         [0019]    In an example embodiment, SPS  10  also includes a temperature control unit  52  in thermal communication with non-linear optical medium  50  to control the temperature of the non-linear optical medium. In an example embodiment, a temperature sensor  54  is also provided in thermal communication with the non-linear optical medium to measure its temperature and provide a corresponding temperature signal ST. 
         [0020]    When pumping non-linear optical medium  50  with pump photons P 2 , some pump photons travel all the way through the non-linear optical medium and exit the other side. Accordingly, SPS  10  also includes a filter  60  adapted to substantially filter out the pump photons of wavelength λ 2  so that substantially only downconverted output photons P 3  of wavelength λ 3  are emitted by SPS  10  as an output beam B. 
         [0021]    SPS  10  also includes a controller  70  operably coupled to CCDN SPS  20 , to pump light source  30 , and to temperature control unit  52 . Controller  70  is adapted (e.g., programmed) to coordinate and controls the operation of these elements via respective control signals S 20 , S 30  and S 52  to control the overall operation of SPS  10 . For example, controller  70  synchronizes the operation of pump light source  30  so that it pumps non-linear optical medium  50  prior to input photons P 1  arriving at the non-linear optical medium. Controller  70  is also adapted to receive temperature signal ST from temperature sensor  54  and process this signal so as to control the temperature of non-linear optical medium  50  via control signal S 52 . 
         [0022]      FIG. 2  is a detailed schematic diagram of an example embodiment of a CCDN SPS  20  of  FIG. 1  that follows the work of Jean-Francois Roch et al., as described in the article www.physique.ens-chachan.fr/franges_photon/single_photon_source.htm (hereinafter, “the Roch article”), which article is incorporated by reference herein. In the description of CCDN SPS  20  associated with  FIG. 2 , both light rays and photons are used for the sake of convenience to describe and show the various light (photon) paths. With reference to  FIG. 2 , CCDN SPS  20  includes a pump light source  100  that generates pump light (photons) P 4  of λ 4 . In an example embodiment, λ 4 =1008 nm for NV color centers 
         [0023]    CCDN SPS  20  further includes a dichroic mirror  104  arranged along optical axis A 1  in the optical path of pump photons P 4 . Dichroic mirror  104  is adapted to reflect pump photons P 4  so that they travel along optical axis A 1  to a scanning mirror  106 , which serves to fold optical axis A 1 . Dichroic mirror  104  is also designed to pass light of wavelength λ 1 . A high-numerical-aperture (NA) object lens  110  is arranged along the folded optical axis A 1  so as to receive pump light P 4  from scanning mirror  106 . 
         [0024]    SPS  20  includes a movable stage  114  that supports a substrate  120  that includes color-centered diamond nanocrystals  130  formed therein or thereupon as described in the Roch article. 
         [0025]    The pulsed pump light P 4  is focused by objective lens  110  onto the particular color-centered diamond nanocrystals  130  as determined by the position of movable stage  114  and scanning mirror  106 . The energy in the pump light pulses is selected to ensure that the defect center in the irradiated nanocrystal  130  is pumped efficiently. In an example embodiment, single photons P 1  having a wavelength λ 1  centered at about 637 nm are then emitted by NV color-centered diamond nanocrystal  130  at a rate proportional to the repetition rate of pump light source  110 . Likewise, single photons P 1  having a wavelength λ 1  centered about 800 nm are emitted by NE8 color-centered diamond nanocrystal  130  at a rate proportional to the repetition rate of pump light source  110 . Single photons P 1  are collected by objective lens  110 , reflected by scanning mirror  106  and then pass through dichroic mirror  104 . Single photons P 1  then travel through a filter  120  that substantially blocks pump photons P 4  of wavelength λ 4 , thereby becoming “input photons” of wavelength λ 1 . 
         [0026]    As discussed above, controller  70  is adapted to coordinate and control the operation of SPS  20  via control signals S 20  that travel to pump light source  100 , movable stage  114 , and scanning mirror  106 . 
         [0027]      FIG. 3  is a close up schematic diagram of an example embodiment of non-linear optical medium  50  that is or otherwise includes a periodically poled (PPL) waveguide  56 , such as formed from lithium niobate (PPLN). PPLN waveguides suitable for use in the present invention are commercially available from a number of vendors such as HC Photonics, Inc., and Thorlabs, Inc. 
         [0028]      FIG. 3  also shows an example embodiment of multiplexer  40  that includes a dichroic mirror  42  adapted to pass light of wavelength λ 1  from SPS source  20  traveling along optical axis A 1 , and to reflect pump light of wavelength λ 2  that initially travels along optical axis A 2  so that it travels along optical axis A 1  toward non-linear optical medium  50 . 
         [0029]    In an example embodiment, pump wavelength λ 2  is selected according to the relationship 1/λ 2 =(1/λ 1 )−(1/λ 3 ). In an example embodiment, output wavelength λ 3  Of SPS source  10  is within one of the known telecommunication wavelength bands, such as in the O-band, E-band, S-band, C-band , L-band or U-band. In a specific example embodiment, λ 3  is one of the minimum optical fiber attenuation wavelengths of 1550 nm or 1310 nm. 
         [0030]    Table 1 below summarizes the different wavelengths for an NV CCDN SPS source  20  and a NE8 CCDN SPS source for λ 3 =1550 nm and 1310 nm. 
         [0000]                                                            TABLE 1                   Wavelength Table                λ 1     λ 2     λ 3                              NV   637 nm   1080 nm   1550 nm           NV   690 nm   1244 nm   1550 nm           NE8   800 nm   1653 nm   1550 nm           NV   637 nm   1310 nm   1310 nm           NV   690 nm   1458 nm   1310 nm           NE8   800 nm   2055 nm   1310 nm                        
QKD System with CCDN SPS
 
         [0031]      FIG. 4  is a schematic diagram of a generalized QKD system  200  that includes CCDN SPS  10 . QKD system includes a first QKD station ALICE and a second QKD station BOB optically coupled by an optical fiber link FL. ALICE includes as a light source CCDN SPS  10  as described above. Alice also includes a modulator MA (e.g., a phase or polarization modulator) optically coupled to CCDN SPS  10  as well as to optical fiber link FL. 
         [0032]    ALICE also includes a controller CA adapted to coordinate the operation of CCDN SPS  10  to emit output photons P 3  in response to a control signal SO. Controller CA also times the operation of modulator MA via a modulator control signal SMA to modulate the output photons based on randomly selecting a modulation from a set of basis modulations according to the particular QKD protocol. For the sake of convenience, this process is referred to herein as selective random modulation. The result is the formation of once-modulated quantum signals P 3 ′ that enter optical fiber link FL and travel over to BOB. 
         [0033]    BOB includes a modulator MB (again, a phase or polarization modulator) optically coupled to optical fiber link FL, and a single-photon-detector (SPD) unit DB optically coupled to the modulator. BOB also includes a controller CB adapted to time the activation of modulator MB via a modulator control signal SMB to the arrival of once-modulated quantum signal P 3 ′ to form twice-modulated quantum signal P 3 ″. The modulation at BOB, like that at ALICE, is also based on selective random modulation. Controller CB also gates SPD unit DB via a detector gating signal SG to the expected arrival time of the twice-modulated quantum signal. SPD unit DB detects the twice-modulated signal and is adapted to discern the overall imparted phase (e.g., via constructive or destructive interference as detected in respective SPDs in the SPD unit) and provides the result to controller CB via a detector measurement signal SDB. 
         [0034]    Controllers CA and CB are adapted to communicate with one another (e.g., over optical fiber link FL or a separate public communication link PCL) to synchronize the overall operation of QKD system  200 , and to perform the QKD procedures. The QKD procedures generally include (publicly) comparing the modulations (i.e., basis and bit values associated with the selective random modulation) to establish a raw key, performing sifting to arrive at a sifted key, performing error correction to arrive at an error-corrected key, and performing privacy amplification to arrive at a privacy-amplified key, as described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag (2001), in Chapter 2, which Chapter is incorporated by reference herein. 
         [0035]    QKD system  200  has the advantage that CCDN SPS source  10  provides a reliable, on-demand source of single-photons at a wavelength λ 3  suitable for use for long-distance QKD, such as λ 3 =1310 nm or 1550 nm. 
         [0036]    It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.