Kwang-Kyoon Park

Time-bin entangled photon pairs from spontaneous parametric down-conversion pumped by a cw multi-mode diode laser.

Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.

Einstein-Podolsky-Rosen Entanglement of Narrow-Band Photons from Cold Atoms

Einstein-Podolsky-Rosen (EPR) entanglement introduced in 1935 deals with two particles that are entangled in their positions and momenta. Here we report the first experimental demonstration of EPR position-momentum entanglement of narrow-band photon pairs generated from cold atoms. By using two-photon quantum ghost imaging and ghost interference, we demonstrate explicitly that the narrow-band photon pairs violate the separability criterion, confirming EPR entanglement. We further demonstrate continuous variable EPR steering for positions and momenta of the two photons. Our new source of EPR-entangled narrow-band photons is expected to play an essential role in spatially multiplexed quantum information processing, such as, storage of quantum correlated images, quantum interface involving hyperentangled photons, etc.

Coherent and dynamic beam splitting based on light storage in cold atoms.

We demonstrate a coherent and dynamic beam splitter based on light storage in cold atoms. An input weak laser pulse is first stored in a cold atom ensemble via electromagnetically-induced transparency (EIT). A set of counter-propagating control fields, applied at a later time, retrieves the stored pulse into two output spatial modes. The high visibility interference between the two output pulses clearly demonstrates that the beam splitting process is coherent. Furthermore, by manipulating the control lasers, it is possible to dynamically control the storage time, the power splitting ratio, the relative phase, and the optical frequencies of the output pulses. With further improvements, the active beam splitter demonstrated in this work might have applications in photonic photonic quantum information and in all-optical information processing.

Time-bin entangled photon pairs from spontaneous parametric down-conversion pumped by a cw multi-mode diode laser

The Franson interferometer consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and the idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit. It is often used to prepare time-bin entanglement of two photons in the two SPDC pumping regimes: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally.

Preservation of transverse spatial coherence of an optical pulse in atomic vapor quantum memory

We report preservation of transverse spatial coherence of an optical pulse stored in atomic vapor quantum memory. Using Young-type spatial interference, it is clearly demonstrated that the atomic vapor quantum memory preserves transverse spatial coherence.

Generation of time-bin entangled photon pairs utilizing coherence revival property of a CW multi-mode laser

We report another regime for generation of time-bin entangled photon pairs and demonstrate the scheme experimentally. In our scheme, the photon pairs are pumped by a cw multi-mode laser having coherence revival property.

Experimental investigation of transverse spatial coherence of an optical pulse in atomic vapor quantum memory

We experimentally investigate the transverse spatial coherence of an optical pulse stored in atomic vapor quantum memory. Using Young-type spatial interference, it is demonstrated that the atomic vapor quantum memory preserves transverse spatial coherence.

Manipulation of frequency-time quantum correlation of narrow-band photon pairs

A specific form of frequency-time quantum correlations is naturally inherent in the nonclassical photon pairs generated via a parametric process. Here, complete manipulation of frequency-time quantum correlations of narrowband biphotons is reported.

Entangling photons using a continuous-wave multi-mode diode laser

Entangled photons represent an essential component of future quantum information technologies. When encoding quantum information, a particular degree of freedom is required for a single photon (e.g., polarization, path, or time-bin). In quantum communication, the time-bin mode (the time of arrival) of a single photon is especially important. Time-bin photonic qubits (quantum bits) are robust against a variety of decoherence effects that occur during long-distance fiber transmission.1 The biggest source of decoherence that occurs in transmissions such as these is polarization-mode dispersion. Because the qubit is encoded in the time-of-arrival degree of freedom, the time-bin qubit is resistant to this dispersion. Time-bin modes are constructed by binning the arrival times of single photons at a detector: see Figure 1(a). Arrival times can be assigned to a specific time-bin mode (e.g., 1, 2, 3, : : : , d ), and d time-bin modes can construct a d -dimensional Hilbert space for the encoding of quantum information. The time-bin modes for a pair of single photons can be entangled. When two photons fall within the same time-bin mode, the possibilities of their being at a specific time-bin mode are quantum-superposed, resulting in a time-bin-entangled photon pair. We generated time-bin-entangled photons using a process known as spontaneous parametric down-conversion (SPDC).2, 3 In this process, a nonlinear crystal is pumped with a laser diode, resulting in the creation of two photons with combined energies and momenta equal to those of the originating photon. Conventionally, the generation of time-bin-entangled photons placed strict requirements on pump laser operation, necessitating the use of either an ultrafast mode-locked laser or a single-mode Figure 1. (a) Time-bin modes of a single photon. A single photon may be found in one of the three time-bin modes, which are chosen carefully to fully accommodate the temporal width of a single photon. (b) Timebin-entangled signal-idler (s and i) photons. The three time-bin modes are quantum-superposed.