Article
Noncollinear and nondegenerate polarizationentangled photon generation via concurrent typeI parametric downconversion in PPLN
Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States
Optics Express (Impact Factor: 3.49). 11/2006; 14(21):1006072. DOI: 10.1364/OE.14.010060 Source: PubMed
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 "Earlier, periodically poled lithium niobate (PPLN) waveguide structures were suggested for producing spontaneous parametric downconversion [30] and the conditions required for generating counterpropagating entangled photons from an unguided pump field were established [31]. Furthermore, the generation of noncollinear and nondegenerate polarizationentangled photons via concurrent TypeI parametric downconversion was demonstrated in a PPLN crystal [32]. The use of lithium niobate photonic circuits has a number of merits: 1) the properties of the material are wellunderstood since it has low loss and has long been the basis of integratedoptics technology [33], [34, Chap. "
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ABSTRACT: We consider the design of photonic circuits that make use of Ti:LiNbO3 diffused channel waveguides to generate photons with various combinations of modal, spectral, and polarization entanglement. Downconverted photon pairs are generated via spontaneous parametric downconversion (SPDC) in a twomode waveguide (TMW). We study a class of photonic circuits comprising: 1) a nonlinear periodically poled TMW structure; 2) a set of singlemode waveguide (SMW) and TMWbased couplers arranged in such a way that they suitably separate the three photons comprising the SPDC process; and, for some applications, 3) a holographic Bragg grating that acts as a dichroic reflector. The first circuit produces two frequencydegenerate downconverted photons, each with even spatial parity, in two separate SMWs. Changing the parameters of the elements allows this same circuit to produce two nondegenerate downconverted photons that are entangled in frequency or simultaneously entangled in frequency and polarization. The second photonic circuit is designed to produce modal entanglement by distinguishing the photons on the basis of their frequencies. A modified version of this circuit can be used to generate photons that are doubly entangled in mode number and polarization. The third photonic circuit is designed to manage dispersion by converting modal, spectral, and polarization entanglement into path entanglement. 
 "Moreover, lithium niobate offers a number of ancillary advantages: 1) its properties are wellunderstood since it is the basis of integratedoptics technology [16]; 2) circuit elements, such as twomode waveguides and polarizationsensitive modeseparation structures, have low loss [2]; 3) it exhibits an electrooptic effect that can modify the refractive index at rates up to tens of GHz and is polarizationsensitive [17, Sec. 20.1D]; and 4) periodic poling of the secondorder nonlinear optical coefficient is straightforward so that phasematched parametric interactions [18] [19], such as SPDC and the generation of entangledphoton pairs [20] [21], can be readily achieved. Moreover, consistency between simulation and experimental measurement has been demonstrated in a whole host of configurations [22] [23] [24] [25] [26]. "
Article: Modal and polarization qubits in Ti:LiNbO_3 photonic circuits for a universal quantum logic gate
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ABSTRACT: Lithium niobate photonic circuits have the salutary property of permitting the generation, transmission, and processing of photons to be accommodated on a single chip. Compact photonic circuits such as these, with multiple components integrated on a single chip, are crucial for efficiently implementing quantum information processing schemes.We present a set of basic transformations that are useful for manipulating modal qubits in Ti:LiNbO(3) photonic quantum circuits. These include the mode analyzer, a device that separates the even and odd components of a state into two separate spatial paths; the mode rotator, which rotates the state by an angle in mode space; and modal Pauli spin operators that effect related operations. We also describe the design of a deterministic, twoqubit, singlephoton, CNOT gate, a key element in certain sets of universal quantum logic gates. It is implemented as a Ti:LiNbO(3) photonic quantum circuit in which the polarization and mode number of a single photon serve as the control and target qubits, respectively. It is shown that the effects of dispersion in the CNOT circuit can be mitigated by augmenting it with an additional path. The performance of all of these components are confirmed by numerical simulations. The implementation of these transformations relies on selective and controllable power coupling among single and twomode waveguides, as well as the polarization sensitivity of the Pockels coefficients in LiNbO(3). 
Article: Generation of fibercoupled, nondegenerate, polarizationentangled photons for quantum communication
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ABSTRACT: Thesis (S.B.)Massachusetts Institute of Technology, Dept. of Physics, 2009. Page 42 blank. Includes bibliographical references. The production of polarizationentangled photon pairs from spontaneous parametric downconversion (SPDC) enables many applications of quantum information processing. In this thesis, we use type0 phasematched downconversion of pump photons from a 532 nm continuouswave laser to generate 798 nm signal and 1.6 tim idler photon pairs in periodicallypoled, congruent lithium niobate (PPLN). Difference frequency generation of 798 nm is used for characterizing PPLN, including phase matching bandwidth and effective nonlinear coefficient. Optimal focusing for generating a single spatial mode SPDC output allows efficient coupling of signal and idler photons. Through coincidence counting, our source's spectral brightness is measured to be 3.6 x 105 Hz/mW/nm detected pairs/s/mW of pump power per nm of output photon bandwidth with an idler conditional detection efficiency of 1.6%. This work is a significant first step toward realizing a highflux source of nondegenerate polarizationentangled photons. by Bhaskar Mookerji. S.B.