Entangled and hyper-entangled states of light are valuable tools of quantum information protocols. Here, we discuss entanglement generation in quantum dot systems and its extension to hyper entanglement. We review the current results and give a perspective for possible improvement.
Single semiconductor quantum dots, due to their discrete energy structure, form single photon and twin photon sources that are characterized by a well-defined frequency of the emitted photons and inherently sub-Poissonian statistics. The single photons are generated through a recombination of an electron-hole pair formed by an electron from the conduction band and a hole from the valence band. When excited to the biexciton state quantum dots can provide pairs of photons emitted in a cascade. It has been shown that this biexciton-exciton cascade can deliver entangled pairs of photons. To achieve a deterministic generation of photon pairs from a quantum dot system one requires exciting it using a two-photon resonant excitation of the biexciton. Particularly, an efficient and coherent excitation of the biexciton requires the elimination of the single exciton probability amplitude in the excitation pulse and reaching the lowest possible degree of dephasing caused by the laser excitation. These two conditions impose contradictory demands on the excitation pulse-length and its intensity. We addressed this problem from a point of view that does not include interaction of the quantum dot with the semiconductor environment. We found an optimized operation regime for the system under consideration and provide guidelines on how to extend this study to other similar systems. In particular, our study shows that an optimal excitation process requires a trade-off between the biexciton binding energy and the excitation laser pulse length.
T. Jennewein, J. P. Bourgoin, B. Higgins, C. Holloway, E. Meyer-Scott, C. Erven, B. Heim, Z. Yan, H. Hübel, G. Weihs, E. Choi, I. D'Souza, D. Hudson, R. Laflamme
Satellites offer the means to extend quantum communication and quantum key distribution towards global distances. We will outline the proposed QEYSSat mission proposal, which involves a quantum receiver onboard a satellite that measures quantum signals sent up from the ground. We present recent studies on the expected performance for quantum links from ground to space. Further studies include the demonstration of high-loss quantum transmission, and analyzing the effects of a fluctuating optical link on quantum signals and how these fluctuations can actually be exploited to improve the link performance.
We investigate an extended version of the Bragg reflection waveguide (BRW) with air gaps as one of the layers. This design has the potential of drastically simplifying the epitaxial structure for integrated nonlinear optical elements at the expense of more complicated structuring. This approach would afford much more flexibility for designing and varying BRW structures. Here, we discuss an extension of the established theory for BRW slabs and report our results of applying Marcatili's method for rectangular waveguides to the BRW case. With this analytic approach we can estimate the effective index of the modes orders of magnitudes faster than with full numerical techniques, such as finite-difference time-domain (FDTD) or finite elements. Initial results are mixed; while phase-matched designs have been found, they currently have no significant advantage over other schemes.
We demonstrate efficient photon pair generation for quantum communication using an all-semiconductor approach. In an AlGaAs Bragg-reflection waveguide we employ spontaneous parametric down-conversion to produce photon pairs at telecommunication wavelengths. The various phase-matching solutions present in our device can be used to create timebin or polarization entanglement. This approach can to lead to a fully integrated photon pair source with the pump laser, active and passive optical devices all on a single semiconductor chip.
We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD)
system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links
to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link
and the other photon detected locally next to the source is described. The system performs the complete QKD
protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of
communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER)
of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality
experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m
between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell
parameter of 2.51 ± 0.11.
We have constructed an entanglement based quantum key distribution system that links three buildings, covering
a largest distance of 1575 m. The photons are transmitted via telescopes through free space. In this paper, we
give a detailed description of our system and the protocol that we implemented. We analyze system components
and design considerations. Some preliminary results of a one-link experiment are presented.
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