Spontaneous Four-wave Mixing Research Articles

Narrowband biphotons with polarization-frequency-coupled entanglement

We demonstrate the generation of narrowband biphotons with polarization-frequency coupled hy- perentanglement from spontaneous four-wave mixing in cold atoms. The coupling between polariza- tion and frequency is realized through a frequency shifter and linear optics. When the polarization- frequency degrees of freedom are decoupled, it is robust to create polarization and frequency Bell states, confirmed by the polarization quantum-state tomography and the two-photon temporal quan- tum beating. Making use of the polarization-frequency coupling to transfer polarization phase retard to the entangled frequency modes, we produce a frequency Bell state with tunable phase difference between its two bases.

Generation of hyper-entanglement in polarization/energy-time and discrete-frequency/energy-time in optical fibers.

In this paper, a generation scheme for telecom band hyper-entanglement is proposed and demonstrated based on the vector spontaneous four wave mixing (SFWM) processes in optical fibers. Two kinds of two-photon states are generated, one is hyper-entangled in the degree of freedoms (DOFs) of energy-time and polarization, the other is hyper-entangled in DOFs of energy-time and discrete-frequency. Experiments of Franson-type interference, two-photon interference under non-orthogonal polarization bases and spatial quantum beating are realized to demonstrate the entanglement in energy-time, polarization and frequency, respectively. This scheme provides a simple way to realize telecom band hyper-entanglement, which has potential for large geographic-scale applications of quantum communication and quantum information over optical fibers.

Coherence time limit of the biphotons generated in a dense cold atom cloud.

Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.

Intermodal and cross-polarization four-wave mixing in large-core hybrid photonic crystal fibers.

Degenerate four-wave mixing is considered in large mode area hybrid photonic crystal fibers, combining photonic bandgap guidance and index guidance. Co- and orthogonally polarized pump, signal and idler fields are considered numerically by calculating the parametric gain and experimentally by spontaneous degenerate four-wave mixing. Intermodal and birefringence assisted intramodal phase matching is observed. Good agreement between calculations and experimental observations is obtained. Intermodal four-wave mixing is achieved experimentally with a conversion efficiency of 17%.

Generation of hyper-entanglement on polarization and energy-time based on a silicon micro-ring cavity.

In this paper, hyper-entanglement on polarization and energy-time is generated based on a silicon micro-ring cavity. The silicon micro-ring cavity is placed in a fiber loop connected by a polarization beam splitter. Photon pairs are generated by the spontaneous four wave mixing (SFWM) in the cavity bi-directionally. The two photon states of photon pairs propagate along the two directions of the fiber loop and are superposed in the polarization beam splitter with orthogonal polarizations, leading to the polarization entanglement generation. On the other hand, the energy-time entanglement is an intrinsic property of photon pairs generated by the SFWM, which maintains in the process of the state superposition. The property of polarization entanglement is demonstrated by the two photon interferences under two non-orthogonal polarization bases. The property of energy-time entanglement is demonstrated by the Franson type interference under two non-orthogonal phase bases. The raw visibilities of all the measured interference fringes are higher than 1/2, the bench mark for violation of the Bell inequality. It indicates that silicon micro-ring cavity is a promising candidate to realize high performance hyper-entanglement generation.

Polarization switch of four-wave mixing in large mode area hybrid photonic crystal fibers.

Degenerate spontaneous four-wave mixing is considered in a large mode area hybrid photonic crystal fiber. Numerical and experimental results show birefringence assisted four-wave mixing for a certain polarization state of the pump field. The parametric gain can be turned on and off by switching the polarization state of the pump field between the two principal axis of the hybrid photonic crystal fiber.

Measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference.

We propose and demonstrate an approach to measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference. Through six sets of two-photon interference measurements projected onto different polarization subspaces, we can reconstruct the amplitude and phase functions of the biphoton temporal waveform. For the first time, we apply this technique to experimentally determine the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms.

Spontaneous Four-Wave Mixing in an Irregular Nanofiber

The features of biphotons states generated via spontaneous four-wave mixing in nanofibers with a variable cross-section are studied. The spectral amplitude of the biphoton field is calculated and the effects of interference and phase modulation of the biphoton field in such structures is discussed.

Spontaneous Parametric Down-Conversion and Dynamical Casimir Effect for Surface Plasmon Polaritons and Guided Waves

The features of biphotons states generated via spontaneous four-wave mixing in nanofibers with a variable cross-section are studied. The spectral amplitude of the biphoton field is calculated and the effects of interference and phase modulation of the biphoton field in such structures is discussed.

Physical Layer Impairment Model for CO-OFDM System Considering Amplified Spontaneous Emission and Four-wave Mixing

AbstractIt’s known that different physical impairments can occur in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Not only linear impairments but also nonlinear impairments can cause significant signal degradations. It is important to analyze the physical layer impairment (PLI) model which should consider these impairments. In this paper, a new PLI model is developed, where amplified spontaneous emission (ASE) categorized to linear impairments and four-wave mixing (FWM) categorized to nonlinear impairments are regarded as the main noise sources. The performance of developed PLI model in terms of Q factor versus number of subcarriers and Q factor versus fiber length are extensively simulated. Numerical results show that when the OFDM signal passes through fiber link, the noise power expansion occurs. This expansion which is related to the fiber length, number of subcarriers as well as input power, can cause higher degradation in OFDM signal. Through precise analysis, optimum parameters of the transmission system can be chosen. Simulations prove that Q factor of CO-OFDM, whose number of subcarriers is 128, fiber length is 90 km and input power is −2 dBm, is up to abound 10 dB. In addition, in order to verify the developed PLI model, comparisons of the situation with and without considering FWM effects have also been made respectively. Results show that the performance of the model conforms to the actual conditions.

Effects of time ordering in quantum nonlinear optics

We study time-ordering corrections to the description of spontaneous parametric down-conversion (SPDC), four-wave mixing (SFWM), and frequency conversion using the Magnus expansion. Analytic approximations to the evolution operator that are unitary are obtained. They are Gaussian preserving, and allow us to understand order-by-order the effects of time ordering. We show that the corrections due to time ordering vanish exactly if the phase-matching function is sufficiently broad. The calculation of the effects of time ordering on the joint spectral amplitude of the photons generated in SPDC and SFWM are reduced to quadrature.

Competition between spontaneous parametric four-wave mixing and fluorescence in Pr3+:YSO

We report the competition between the spontaneous parametric four-wave mixing and second- or fourth-order fluorescence (FL) signals in Pr3+:Y2SiO5 crystal. By changing the powers of controlling fields and blocking different fields, the competitive two signals can be identified through the dressing effect. Meanwhile, the delay of fourth-order FL in the time domain results from splitting the energy level. Such results can find potential applications in optical information storage and processing on a photonic chip.

In situ fine tailoring of group velocity dispersion in optical microfibers via nanocoatings.

We experimentally demonstrate a convenient technique for in situ fine group velocity dispersion (GVD) tailoring in optical microfibers via dielectric nanocoatings. This was elaborated by successively depositing poly-dimethylsiloxane (PDMS) nanocoatings around a 1.2 μm-diameter optical microfiber with a modified dip-coating method. In situ dispersion measurements showed that the GVD was tailored by 55 ps/nm•km at 1580 nm, and the zero-dispersion wavelength (ZDW) was red shifted by 30 nm. Numerical simulations showed that GVD tailoring in optical microfibers could bring signal (idler) tuning in spontaneous four-wave mixing (FWM) and spectral bandwidth expanding in supercontinuum (SC) generation, implying that this in situ fine GVD tailoring technique would offer optical microfibers with many new opportunities for applications in nonlinear optics.

Dressed four-wave mixing second-order Talbot effect

We theoretically demonstrate second-order Talbot effect (SOTE) based on entangled photon pairs. The photon pairs are generated from the spontaneous parametric four-wave mixing (SPFWM) process in a cold atomic medium and can be taken as the imaging light in order to realize coincidence recording. A strong standing wave is used to create the electromagnetically induced grating in the entangled photon pairs channels. By changing the frequency detuning of the standing wave or the other optical fields participating in the process, we can manipulate the contrast of the second-order Talbot image. We use the second-order correlation function and the dressed-state picture to explain the SOTE occurring in the SPFWM process. Moreover, we demonstrate the scheme for SOTE based on the spatially correlated twin beams generated from the SPFWM process with injection. This scheme provides a convenient detection proposal for the SOTE at the cost of the image contrast. Compared to the previous self-imaging schemes, the present schemes have the characteristic of controllable image contrast and of nonlocal imaging, and thus, they might broaden their applications in imaging techniques and find applications in quantum lithography.

Seeded spontaneous parametric four-wave mixing and fluorescence of Pr3+:YSO

We demonstrate the seeded spontaneous parametric four-wave mixing (SP-FWM) process associated with the fluorescence signals of Pr3+:YSO both theoretically and experimentally. For the first time, the dressed SP-FWM processes are observed by self-dressing and/or external-dressing in this crystal. Various possible amplification processes are discussed by selectively seeding various multi-wave mixing (MWM) signals. Two contributions of the dressing effect and the dipole–dipole interaction to the linewidths of the SP-FWM signal and the conjugate signal of the seeding MWM signal associated with the fluorescence signal have been distinguished. In addition, the Autler–Townes-like (or Autler–Townes) splitting effect of the fluorescence spectrum induced by itself (or external) field and the mutual interaction between two fluorescence signals are investigated which are well explained using the presented theoretical model. Such results can find potential applications in all-optical communication and optical information processing on a photonic chip.

Multidressed suppression and enhancement of spontaneous parametric four-wave-mixing processes

For the first time to our knowledge, a dressed spontaneous parametric four-wave mixing (SP-FWM) process and fluorescence (FL) signals in hot alkali atom vapor are demonstrated in both theory and experiment. First, by scanning pumping fields, Autler–Townes (AT) splitting in SP-FWM and two-photon suppression dip in FL are observed. Then, by scanning coupling fields, single-, double-, and triple-dressed effect on those signals is demonstrated, such as suppression and/or enhancement of SP-FWM signal and suppressed two-photon emission peak of FL. The interaction effect between dressing fields on SP-FWM and FL signals is also observed. Such investigations can find potential applications in multichannel narrowband long-distance communication.

Photon pairs with coherence time exceeding 1 μs

The generation of nonclassical photon pairs with a long coherence time is key for applications that range from fundamental to quantum communication and metrology. Spontaneous four-wave mixing with electromagnetically induced transparency has been demonstrated as one of the most efficient methods; however, narrowing the bandwidth and producing photon pairs with a temporal length beyond 1 μs remains a technical challenge due to noise considerations and the need for cold atoms with a high optical depth (OD). In this work, we demonstrate the generation of narrowband photon pairs with a controllable coherence time up to 1.72 μs in a laser-cooled atomic ensemble with an OD as high as 130. At such a high OD, we find that the pump laser field spatial profile has a significant effect on the time–frequency entangled two-photon waveform. We also confirm the quantum particle nature of heralded narrowband single photons generated from this source.

Engineering frequency-time quantum correlation of narrow-band biphotons from cold atoms.

The nonclassical photon pair, generated via a parametric process, is naturally endowed with a specific form of frequency-time quantum correlations. Here, we report complete control of frequency-time quantum correlations of narrow-band biphotons generated via spontaneous four-wave mixing in a cold atomic ensemble. We have experimentally confirmed the generation of frequency-anticorrelated, frequency-correlated, and frequency-uncorrelated narrow-band biphoton states, as well as verifying the strong nonclassicality of the correlations. Our work opens up new possibilities for engineering narrow-band entangled photons for various quantum optical and quantum information applications.

Subnatural-linewidth polarization-entangled photon pairs with controllable temporal length.

We demonstrate an efficient experimental scheme for producing polarization-entangled photon pairs from spontaneous four-wave mixing (SFWM) in a laser-cooled (85)Rb atomic ensemble, with a bandwidth (as low as 0.8 MHz) much narrower than the rubidium atomic natural linewidth. By stabilizing the relative phase between the two SFWM paths in a Mach-Zehnder interferometer configuration, we are able to produce all four Bell states. These subnatural-linewidth photon pairs with polarization entanglement are ideal quantum information carriers for connecting remote atomic quantum nodes via efficient light-matter interaction in a photon-atom quantum network.

Configurable spatiotemporal properties in a photon-pair source based on spontaneous four-wave mixing with multiple transverse modes.

We present an experimental and theoretical study of photon pairs generated by spontaneous four-wave mixing (SFWM), based on birefringent phasematching, in a fiber that supports more than one transverse mode. We present SFWM spectra, obtained through single-channel and coincidence photon counting, which exhibit multiple peaks shown here to be the result of multiple SFWM processes associated with different combinations of transverse modes for the pump, signal, and idler waves.