Quantum networking tools with single atoms and single photons

In this project we use single trapped ions and single photons to develop and investigate tools for quantum optical information technology; in particular, we aim at implementing basic building blocks for a quantum network.

In quantum mechanics, the measurement of a state generally modifies ("projects") this state: once measured, the original state may be lost, such that one cannot measure all the properties of a quantum state at the same time. Therefore, when quantum states such as the properties of individual photons are used for transmitting quantum information, any operation between their emission and their final reception has to happen without measuring them. In particular, this is true for intermediate storage of the quantum state. This makes photon-atom interfaces necessary which convert the quantum state coherently between the two systems.

Our experimental setup consists of two linear Paul traps mounted in separate vacuum chambers, in which we trap single or few calcium ions (40Ca+). The ions are laser-cooled, and we use in-vacuum high numerical aperture optics (HALOs) to efficiently interface the single ions with light at different wavelengths [1].


We also operate a spontaneous parametric down-conversion (SPDC) source of entangled photon pairs. The source is narrowband and tunable for efficient interaction with one of the transitions in the Ca+ ion [2,3].

Recent work
With two ions, one in each of the two traps, we have realized two-photon interference of single photons emitted by the distant ions, both in continuous [1] and in pulsed [4] excitation mode. This is a necessary step for implementing a probabilistic protocol that entangles two distant ions [5,6].
With one single ion, we have demonstrated the creation of bandwidth-tunable single photons at high rate, approaching the Fourier limit within a factor of two [4]. Single photons from a single ion were also shown to exhibit increased anti-bunching if specific combinations of polarisations of the exciting lasers and the detected photons are employed [7]. Controlled generation of single photons is of general importance in quantum communication.
Using one ion and the photon source, we have furthermore established for the first time the interaction of a down-conversion based entangled photon pair source and a single trapped ion [8]. In a subsequent experiment we observed correlations between the detection of a photon from the photon source and the absorption of its partner photon by an ion [9]. This experiment has milestone character, as it connects for the first time two outstanding physical systems in quantum information science: single ions, which are paradigmatic in quantum information processing, and SPDC photon pairs, which have the same fundamental role in quantum communication.

In future experiments we will continue obtaining control over absorption and emission processes at the level of individual particles. In general, we pursue the implementation of quantum optical experiments that lead to a better understanding and control of the involved processes and thereby to the development of new tools for quantum optical information technologies.

[1]  Gerber et al., New Journal of Physics, 2009, 11, 013032
[2]  Haase et al., Optics Letters, 2009, 34, 55
[3]  Piro et al., J. Phys. B, 2009, 42, 114002
[4]  Almendros et al., Phys. Rev. Lett., 2009, 103, 213601
[5]  Simon et al., Phys. Rev. Lett., 2003, 91, 110405
[6]  Moehring et al., Nature, 2009, 449, 68
[7]  Rohde et al., J. Opt. Soc. Am. B, 2010, 27, A81
[8]  Schuck et al., Phys. Rev. A, 2010, 81, 011802
[9]  Piro et al., Nature Physics, 2010, 7, 17