Bell-state preparation using pulsed nondegenerate two-photon entanglement

Abstract

We report a novel Bell state preparation experiment. High-purity Bell states are prepared by using femtosecond pulse pumped nondegenerate collinear spontaneous parametric down-conversion. The use of femtosecond pump pulse does not result in reduction of quantum interference visibility in our scheme in which post-selection of amplitudes and other traditional mechanisms, such as, using thin nonlinear crystals or narrow-band spectral filters are not used. Another distinct feature of this scheme is that the pump, the signal, and the idler wavelengths are all distinguishable, which is very useful for quantum communications.PACS Number: 03.67. Hk, 03.65.Bz, 42.50.Dv Preparation and measurement of the Bell states are two important issues in modern quantum optics, especially for quantum communications, quantum teleportation, etc [1]. For photons, such states can be realized by using the entangled photon pairs generated in spontaneous parametric down-conversion (SPDC). By making appropriate local operations on the SPDC photon pairs, one can prepare all four Bell states.The polarization Bell states, for photons, can be written aswhere the subscripts 1 and 2 refer to two different photons, photon 1 and photon 2, respectively, and they can be arbitrarily far apart from each other. |X and |Y form the orthogonal basis for the polarization states of a photon, for example, it can be horizontal ( |H ) and vertical (|V ) polarization state, as well as |45 • and |−45 • , respectively. This means that the quantum interference should be independent of the choice of the bases. Such an experiment was first performed by Shih and Alley in which non-collinear type-I SPDC and a beamsplitter were used to prepare a Bell state [2], but it is very difficult to align such a system. Collinear type-II SPDC is thus developed [3]. There is, however, a common problem: the entangled photon pairs have 50% chances of leaving at the same output ports of the beamsplitter. Therefore, the state prepared after the beamsplitter may not be considered as a Bell state without amplitude postselection [4]. Only when one considers the coincidence contributing terms by throwing away two out of four amplitudes (post-selection of 50% of the amplitudes), the state is then said to be a Bell state. This problem is later solved by using non-collinear type-II SPDC or using two non-collinear type-I SPDC [5]. * Email: yokim@umbc.edu † Permanent address: Department of Physics, Moscow State University, Moscow, 119899, Russia.In the cw pumped SPDC, entangled photon pairs occur randomly since the process is "spontaneous", so whereabouts of the photon pair is completely uncertain within the coherence length of the pump laser beam. This huge time uncertainty makes it difficult for applications such as generation of multi-photon entangled state, quantum teleportation, etc, as interactions between entangled photon pairs generated from different sources are required. This difficulty was thought to be solved by using a femtosecond pulse laser as a pump. Unfortunately, femtosecond...

Biphoton interference with a quantum dot entangled light source

“…The visibility of the measured biphoton interferogram compared to that for single photon detection demonstrates enhanced phase resolution of the interferometer with a biphoton source [7]. Note that the exciton and biexciton photons need not have the same wavelength in order to observe biphoton interference [24], as it is the total energy of the two-photon state that defines the de Broglie wavelength. This is the usual case for quantum dots.…”

Generation of quantum‐entangled twin photons by waveguide nonlinear‐optic devices

“…An arrangement consisting of cascaded perpendicularly oriented two nondegenerate collinear Type-I BPM SPDC crystals combined with a dichroic beam splitter for converting |H, H and |V, V state to 1/2{|H; H +e iϕ |V; V } state was proposed, and used for demonstration of Bell state preparation by femtosecond pulse pumping [35].…”

“…Photon pairs with polarization correlation were generated by the cascaded optical transitions from an initial state to a final state, both of zero angular momentum, via an intermediate state, and used for the first experimental test of the quantum theory and demonstration of the nonlocality of the twin photons [25]. Then, a variety of methods of twin-photon generation with higher efficiency and simpler setup were developed based upon the spontaneous parametric downconversion (parametric florescence) in second-order nonlinear-optic (NLO) crystals [26][27][28][29][30][31][32][33][34][35]. These methods have been used widely in different quantum optics and QIT experiments.…”

“…At present, the major technique to generate quantumentangled photons is spontaneous parametric downconversion (SPDC) of pump laser photons in second-order NLO crystals [26][27][28][29][30][31][32][33][34][35]. Recently there has been remarkable progress in NLO devices for optical wavelength conversion through development of the quasi-phase-matched (QPM) NLO devices using periodically domain-inverted structures and waveguide structures [48].…”

Scalable scheme for entangling multiple ququarts using linear optical elements

“…Here we consider three different types of beam splitters, an ordinary 50/50 beam splitter (BS), a polarizing beam splitter (PBS), and a dichroic beam splitter (DBS). Since Shih-Alley and Hong-Ou-Mandel type quantum interference is used to generate entangled states of the two ququarts, it is necessary that the SPDC process is pumped by a ultrafast laser [24,29,30]. The delay τ adjusts the overlap of the two ququarts at the beam splitter, hence controlling temporal distinguishability.…”