Abstract:
A robust, quickly tunable narrow-linewidth entangled photon source system based on Spontaneous Parametric Down Conversion (SPDC) of the pump light in periodically polled LiNbO 3  (PPLN) waveguides. The photon source provides narrow-linewidth, entangled output photons having a wavelength in the telecom C-Band wavelength. To tailor the output spectrum of the output photons, the PPLN waveguide is arranged between two end waveguides having LiNbO3-embedded Bragg gratings, thereby forming a tunable Fabry-Perot cavity. The resulting narrow output linewidth of the output photons makes the system desirable for use in a long-distance quantum key distribution (QKD) system.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to sources of single photons, and particularly to tunable, compact single-photon sources and quantum key distribution (QKD) systems using same. 
     BACKGROUND OF THE INVENTION 
     QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits introduced errors that reveal her presence. 
     The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett (“the Bennett Patent”), which patent is incorporated by reference herein, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), which article is incorporated herein by reference. The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. 
     Entanglement-based Quantum Communication (QC) and, in particular, QKD, is one of the most promising applications of Quantum Information Science (QIS). A number of technical challenges, however, must be overcome before QKD becomes commercially practical. One of these challenges includes developing high-photon-flux entanglement sources for telecommunication wavelengths (e.g., 1550 nm). Because of the low attenuation of 1550-nm photons in optical fibers, they are natural information carriers for long-distance communication links. 
     Because quantum communication is based on single photon exchange and detection, it is very important to minimize the number of “unintended” photons—that is to say, the photons that do not participate in the information exchange. Therefore, deploying QKD system in an existing optical fiber communication infrastructure populated by classical communication channels requires very aggressive spectral filtering to minimize the background noise and optimize the signal-to-noise ratio (SNR) at the receiver. 
     For optimal quantum communication system performance, the width and the shape of the receiver bandpass filter must match the transmitted signal. Narrowing the transmitter spectrum decreases the receiver spectral bandwidth. Since the received noise is proportional to receiver spectral bandwidth, using narrow-line transmitters results in higher SNR, which extends the quantum channel distance budget. 
     Spontaneous parametric processes naturally produce relatively broadband emission spectra, typically on the order of a few nm to few tens of nm. A broad signal spectrum can cause the signals to spread due to dispersion in fiber-optics-based QKD system. Hence, additional filtering is required. One way to perform such filtering is to use an external bandpass filter at the transmitter output. Unfortunately, this decreases the photon flux and overall link efficiency, which reduces the performance of the quantum communication system. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a robust, quickly tunable narrow-linewidth entangled photon source system based on Spontaneous Parametric Down Conversion (SPDC) of the pump light in periodically polled LiNbO3 (PPLN) waveguides. In an example embodiment, the photon source system is operated in a synchronous (i.e.—pulsed) regime at the telecom C-Band wavelength (1529 to 1563 nm). In order to tailor the output spectrum, the PPLN waveguide is arranged between two end waveguides having LiNbO3-embedded Bragg gratings, thereby forming a tunable Fabry-Perot cavity. The resulting narrow output linewidth of the entangled photons makes the system desirable for use in a long-distance Quantum Key Distribution (QKD) system. The system is also desirable for use in entanglement-based QKD systems in hybrid optical networks where quantum channels are wavelength-multiplexed with classical (i.e., non-single-photon) channels. By varying the applied electric field in the end waveguides, the spectrum of single/entangled photons can be rapidly tuned, making the system appropriate for use in QKD systems and reconfigurable QKD networks. 
     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. 
     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 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the photon source system of the present invention; 
         FIG. 2  is a plot of the reflected light intensity (arbitrary units) vs. wavelength (nm) for the Fabry-Perot cavity formed by the three waveguides of the photon source system of  FIG. 1 ; 
         FIG. 3  is a plot of the wavelength (λ) vs. the applied electric field (V/cm) for the electrodes in the end waveguides, illustrating the shift in cavity resonant wavelength by changing the media refractive index via an electro-optic effect; and 
         FIG. 4  is a schematic diagram of a QKD system that utilizes the photon source system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention includes a tunable narrow-linewidth entangled photon source system (hereinafter, simply “photon source system” or “system”) based on Spontaneous Parametric Down Conversion (SPDC) of the pump light in periodically polled lithium niobate (or LiNbO 3 ) (PPLN) waveguides.  FIG. 1  is a schematic diagram of an example embodiment of a photon source system  10  in the form of a composite waveguide. Photon source  10  includes an entanglement-generating PPLN waveguide  20  that includes a guiding layer  22  having formed therein a Bragg grating  24 . Waveguide  20  has opposite ends  26 A and  26 B. 
     It is noted here that that in recent years, PPLN waveguides have become commercially available at reasonable prices from a number of vendors (e.g., HC Photonics, Inc., and Thorlabs, Inc.). 
     System  10  also includes two end waveguides  30 A and  30 B in the form of tunable Bragg filters/reflectors arranged at respective ends  26 A and  26 B of waveguide  20 , so that waveguide  20  is sandwiched therebetween. Waveguides  30 A and  30 B each include respective waveguide layers  32 A and  32 B and respective Bragg gratings  34 A and  34 B formed therein. Waveguides  30 A and  30 B each also includes electrode pairs  50 A,  52 A and  50 B,  52 B arranged on respective sides of waveguide layers  32 A and  32 B so that the waveguide layers can be subjected to an electric field, as explained below. For the end waveguides  30 A and  30 B, suitable waveguides are LiNbO3-embedded tunable Bragg gratings such as those recently been developed by SWET Optics, GmbH of Germany. Waveguide  20 , in combination with the surrounding end waveguides  30 A and  30 B, form a Fabry-Perot cavity having an adjustable cavity resonant wavelength. 
     Electrodes  50 A and  52 A are electrically coupled to a voltage source  60 A, while electrodes  50 B and  52 B are electrically coupled to a voltage source  60 B. 
     In an example embodiment, system  10  also includes a temperature control element  70  in thermal communication with waveguide  20  to control the temperature of the waveguide. A temperature sensor  78  is also provided to measure the temperature of waveguide  20  and provide a temperature signals ST. 
     System  10  also includes a pump light source  90  adapted to emit pump light  92 , e.g., at a wavelength of 775 nm. 
     System  10  further includes a controller  100  electrically coupled to pump light source  90 , voltage sources  60 A and  60 B, temperature control element  70 , and temperature sensor  78 . In an example embodiment, controller  100  is a microprocessor, or a computer that includes a microprocessor, wherein the controller is programmed with instructions to carry out the method of operation of the system as described below. In an example embodiment, the microprocessor is or includes a field-programmable gate array (FPGA). The instructions in controller  100  can be implemented either in hardware or software (e.g., an FPGA or central processing unit (CPU)), and can exist in a variety of forms both active and inactive. For example, they can exist as one or more software programs comprised of program instructions in source code, object code, executable code or other formats. Any of the above formats can be embodied on a computer-readable medium, which include storage devices and signals, in compressed or uncompressed form. Exemplary computer-readable storage devices include conventional computer system RAM (random access memory), ROM (read only memory), EPROM (erasable, programmable ROM), EEPROM (electrically erasable, programmable ROM), flash memory and magnetic, optical disks or tapes. 
     Method of Operation 
     With continuing reference to  FIG. 1 , in the operation of photon source system  10 , controller  100  sends a pump signal SP to pump light source  90 , which in response thereto generates pump light  92  at a desired wavelength. In an example embodiment, pump light source  90 —and thus photon source system  100 —operates in synchronous (i.e.—pulsed) regime, preferably at an output wavelength in the telecommunication C-Band, namely, between 1529 nm and 1563 nm. 
     In an example embodiment, the wavelength of pump light  92  is 775 nm. In a more general example embodiment, the pump light has a wavelength that is half that of one of a desired telecommunication wavelengths, such as those mentioned above. Pump light  92  serves to generate entangled photons P 1  and P 2  (dashed arrows) within guiding layer  22  of waveguide  20  via a SPDC process. Photons P 1  and P 2  are outputted from end waveguide  30 B as output light (output photons)  102 . 
     Controller  100  also receives temperature signal ST from temperature sensor  78  and in response thereto controls the temperature of waveguide  20  via temperature control element  70  and a temperature control signal STC provided thereto. 
     Meanwhile, controller  100  sends voltage control signals SA and SB to voltage controllers  60 A and  60 B. In response thereto, voltage controllers  60 A and  60 B provide voltage signals SVA and SVB (not shown) to their corresponding electrodes  50 A,  52 A and  50 B,  52 B so as to create first and second electrical potentials between the two sets of electrodes, which in turn generates respective electric fields within waveguide layers  32 A and  32 B. The electrical fields serve to change the media index of refraction and shift the cavity resonant wavelength of the Fabry-Perot cavity formed by waveguides  30 A,  20  and  30 B. Thus, system  10  exploits the change of the grating transfer function via applied electric fields in waveguides  30 A and  30 B to tune the resonant cavity wavelength and, correspondingly, the output spectrum of the entangled output photons P 1  and P 2 . 
     Entangled output photons P 1  and P 2  have a relatively narrow-line output spectrum as compared to entangled photons typically generated through SPDC mechanisms. According to Bragg&#39;s law, the grating reflection peaks when the light wavelength (λ), grating period (Λ), and the material refractive index (n) satisfy the condition λ B =2Λn, where λ B  is the Bragg wavelength. The reflected spectrum bandwidth δλ=λ B -λ 0  is defined by the length of the grating (T) and its period: (δλ/λ B )=(Λ/T). Thus, the grating acts as an optical bandpass filter, or a mirror with a wavelength-dependent reflectivity. The dependence of the reflected light intensity on the wavelength is often referred to as the “transfer function” of the grating. The present invention exploits the ability to change the transfer function using an applied electric field in the end waveguides  30 A and  30 B to provide a narrowly tuned output spectrum for the outputted entangled photons P 1  and P 2 . 
     When the Bragg grating is embedded in an electro-optical material such as LiNbO 3 , the average refractive index of the media depends on the applied electric field (E). In this case, the Bragg-selected wavelength can be adjusted by changing the material refractive index with the external electric field.  FIGS. 2 and 3  illustrate the Bragg wavelength dependence on the applied electric field. 
     To the best of the inventor&#39;s knowledge, all other commercially available tunable Bragg gratings use thermo-mechanical material expansion to change the grating period and, consequently, the Bragg wavelength. However, the LiNBO 3  gratings employed in the present invention are simple and have no moving parts. This allows for photon source system  10  to be tuned much quicker than prior art systems, i.e., faster than 1 nm/s, which is orders of magnitude quicker than prior art systems. 
     The resulting narrow-output linewidth of entangled photons P 1  and P 2  makes system  10  a good light source for performing long-distance QKD. It is also useful for entanglement-based QKD deployment in hybrid optical networks where quantum channels are wavelength multiplexed with classical (i.e., non-single-photon) channels. By varying the applied electric field in end waveguides  30 A and  30 B, the spectrum of the output photons can be rapidly tuned, which is a useful property for reconfigurable QKD networks. Note that for QKD applications, one or both of the outputted photons P 1  and P 2  and be used. 
     QKD System with Photon Source System 
       FIG. 4  is a schematic diagram of a QKD system  200  that includes a first QKD station Alice and a second QKD station Bob optically coupled via an optical fiber link FL. Alice includes photon source system  10  as described above. QKD system  200  is, for example, as described in the above-mentioned Bennett Patent, or in U.S. Pat. No. 7,102,121 to LaGasse, which patent is also incorporated by reference herein. 
     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.