Yun-Kun Jiang

Detection of Sub-Shot-Noise Spatial Correlation in High-Gain Parametric Down Conversion

Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.

Quantum Storage of Orbital Angular Momentum Entanglement in an Atomic Ensemble

Constructing a quantum memory for a photonic entanglement is vital for realizing quantum communication and network. Because of the inherent infinite dimension of orbital angular momentum (OAM), the photons OAM has the potential for encoding a photon in a high-dimensional space, enabling the realization of high channel capacity communication. Photons entangled in orthogonal polarizations or optical paths had been stored in a different system, but there have been no reports on the storage of a photon pair entangled in OAM space. Here, we report the first experimental realization of storing an entangled OAM state through the Raman protocol in a cold atomic ensemble. We reconstruct the density matrix of an OAM entangled state with a fidelity of 90.3%±0.8% and obtain the Clauser-Horne-Shimony-Holt inequality parameter S of 2.41±0.06 after a programed storage time. All results clearly show the preservation of entanglement during the storage.

Orbital angular momentum light frequency conversion and interference with quasi-phase matching crystals

Light with helical phase structures, carrying quantized orbital angular momentum (OAM), has many applications in both classical and quantum optics, such as high-capacity optical communications and quantum information processing. Frequency conversion is a basic technique to expand the frequency range of the fundamental light. The frequency conversion of OAM-carrying light gives rise to new physics and applications such as up-conversion detection of images and generation of high dimensional OAM entanglements. Quasi-phase matching (QPM) nonlinear crystals are good candidates for frequency conversion, particularly due to their high-valued effective nonlinear coefficients and no walk-off effect. Here we report the first experimental second-harmonic generation (SHG) of an OAM-carried light with a QPM crystal, where a UV light with OAM of 100 ℏ is generated. OAM conservation is verified using a specially designed interferometer. With a pump beam carrying an OAM superposition of opposite sign, we observe interesting interference phenomena in the SHG light; specifically, a photonics gear-like structure is obtained that gives direct evidence of OAM conservation, which will be very useful for ultra-sensitive angular measurements. Besides, we also develop a theory to reveal the underlying physics of the phenomena. The methods and theoretical analysis shown here are also applicable to other frequency conversion processes, such as sum frequency generation and difference-frequency generation, and may also be generalized to the quantum regime for single photons.

The generation of polarization-entangled photon pairs using periodically poled lithium niobate waveguides in a fibre loop

We report the generation of polarization-entangled photon pairs at all telecom band using two PPLN waveguides in a fiber loop. The visibility is about 92% in our measurement. Phase is stable for the entangled state.

Generation of light with controllable spatial patterns via the sum frequency in quasi-phase matching crystals.

Light beams with extraordinary spatial structures, such as the Airy beam (AB), the Bessel-Gaussian beam (BGB) and the Laguerre-Gaussian beam (LGB), are widely studied and applied in many optical scenarios. We report on preparation of light beams with controllable spatial structures through sum frequency generation (SFG) using two Gaussian pump beams in a quasi-phase matching (QPM) crystal. The spatial structures, including multi-ring-like BGB, donut-like LGB, and super-Gaussian-like beams, can be controlled periodically via crystal phase mismatching by tuning the pump frequency or crystal temperature. This phenomenon has not been reported or discussed previously. Additionally, we present numerical simulations of the phenomenon, which agree very well with the experimental observations. Our findings give further insight into the SFG process in QPM crystals, provide a new way to generate light with unusual spatial structures, and may find applications in the fields of laser optics, all-optical switching, and optical manipulation and trapping.

Hypothesis testing for an entangled state produced by spontaneous parametric down-conversion

Generation and characterization of entanglement are crucial tasks in quantum information processing. A hypothesis testing scheme for entanglement has been formulated. Three designs were proposed to test the entangled photon states created by the spontaneous parametric down conversion. The time allocations between the measurement vectors were designed to consider the anisotropic deviation of the generated photon states from the maximally entangled states. The designs were evaluated in terms of the

Toward high-dimensional-state quantum memory in a cold atomic ensemble

p

An ultra-broadband continuously-tunable polarization-entangled photon-pair source covering the C+L telecom bands based on a single type-II PPKTP crystal

value based on the observed data. It has been experimentally demonstrated that the optimal time allocation between the coincidence and anticoincidence measurement vectors improves the entanglement test. A further improvement is also experimentally demonstrated by optimizing the time allocation between the anticoincidence vectors. Analysis on the data obtained in the experiment verified the advantage of the entanglement test designed by the optimal time allocation.

Experimental verification of fault tolerant quantum key distribution protocol.

The reversible transfer of the quantum information between a photon, an information carrier, and a quantum memory with high fidelity and reliability is the prerequisite for realizing a long-distance quantum communication and a quantum network. Encoding photons into higher-dimensional states could significantly increase their information-carrying capability and network capacity. Moreover, the large-alphabet quantum key distribution affords a more secure flux of information. Quantum memories have been realized in different physical systems, such as atomic ensembles and solid systems etc., but to date, all quantum memories only realize the storage and retrieval of the photons lived in a two-dimensional space spanned for example by orthogonal polarizations, therefore only a quantum bit could be stored there. Here, we report on the first experimental realization of a quantum memory storing a photon lived in a three-dimensional space spanned by orbital angular momentums via electromagnetically induced transparency in a cold atomic ensemble. We reconstruct the storage process density matrix with the fidelity of 85.3% by the aid of a 4-f imaging system experimentally. The ability to store a high-dimensional quantum state with high fidelity is very promising for building a high-dimensional quantum network.

Indirect precise angular control using four-wave mixing

The first experimental preparation is reported of an ultra-broadband continuously-tunable highly polarization-entangled photon-pair source via spontaneous parametric down-conversion in a single type-II phase-matched bulk periodically-poled KTiOPO4 (PPKTP) crystal. A tuning band of more than 60 nm for the down-converted photons is achieved experimentally, which covers the C+L telecom bands. The photon pair generation rate is about 1.63 × 104 (s mW nm)−1. The calculated S parameter values of CHSH inequality between 2.60 ± 0.04 (minimum) and 2.72 ± 0.07 (maximum) over the whole tunable range clearly demonstrate high entanglement of the source. In combination with the dense wavelength-division multiplexing (DWDM) technique, our source could be used to enhance the transmission capacity of a communication channel in the field of quantum communication.