University of Oxford (UOXF)
University of Sussex (UoS)
Université de Franche-Comté (UFC)
Université de Bourgogne (UBT)
Foundation of Theoretical and Computational Physics and Astrophysics – University of Sofia (TCPA)
QuTools GmbH (QuT)
QuBig GmbH (QuB)
Université Technologique de Troyes (UTT)
One of our major goals is to design a large-scale quantum network employing single atoms, ions and photons as information carriers. This needs efficient interfacing of light with matter. One widely studied method is to use high-finesse optical cavities with discrete frequency modes and very narrow line widths. These include fibre-tip optical cavities for optimal optical access, compatibility with micro traps for atoms and ions, and the direct coupling of optical cavity modes and waveguides (in connection to WP4 where waveguides at the nanoscale are considered).
In ion trap quantum computing, qubits are stored in chains of ions, individually addressable by lasers [Nature 453, 1008 (2008)]. Conditional logic mediated through the excitation of collective phonon modes and logic gates have been demonstrated; highly entangled states were produced. Promising alternative approaches are photon-mediated atom-atom or ion-ion interactions [Nature 431, 1075 (2004)], where the particles couple to the cavity field. Transferring the quantum state of a stationary qubit onto a flying photonic qubit enables the creation of quantum networks in which smaller quantum processors are combined and thus create larger computing structures. Although the first step has been shown [Nature 485, 482 (2012), Nat. Phot. 7, 219 (2013)], a high-fidelity and high-speed ion-photon interface is still beyond current experiments. Recent progress in CQED with neutral atoms lead to the demonstration of highly efficient single photon sources, of atom-photon entanglement [Science 317, 488 (2007)] and to the transfer of quantum states between remote atoms [Nature 484, 195 (2012)].
With the help of recently developed fibre-tip cavities, the tight integration of atom/ion-photon interfaces has become possible. We will investigate novel schemes to combine fibre-tip cavities with trapping structures for atoms and ions and to implement schemes to faithfully control and exploit the interaction of single atomic particles with a single mode of an optical cavity. UoS develops key technologies to control the interaction of trapped ions with individual photons in optical fibre-tip cavities. UoS stores and laser-cools calcium ions in miniature radio-frequency traps. The linear strings in these traps may contain from a single to several tens of ions, with a position uncertainty below 10 nm [Phys. Rev. Lett. 116, 223001 (2016)]. UoS has developed a set-up to laser machine fibres to form mirrors for high-finesse cavities of long length and low astigmatism, a prerequisite for the efficient coupling of photons into and out of the cavity through the fibres. UoS mitigates the mode-coupling losses with several new mode-matching techniques. UOXF is going to explore a complementary cavity design, which incorporates micro-mirrors on arbitrarily shaped standard substrates, produced by ion-beam milling.
With the development of versatile optical trapping methods such as optical tweezers, and the possibility for cavity-controlled interactions, atoms have emerged as an excellent system for quantum information storage and processing. Emission and re-absorption of photons from arbitrary quantum states by means of strongly coupled atom-cavity systems, time reversal of these processes and mapping between atomic and photonic quantum states under full quantum control are the physical background for this WP [Contemp. Phys. 51, 289 (2010)]. The development of highly robust schemes to exploit the cavity-mediated interaction of atoms/ions and photons for quantum computation and communication is therefore crucial. Recently, TCPA and UOXF jointly developed a coherent control scheme [New J. Phys. 12, 063024 (2010); ibid 13, 103036 (2011)], which allows initialising time-bin encoded quantum bits in an arbitrary state superposition, including full phase control [New J. Phys. 15, 053007 (2013)]. In combination with fully-controlled arrays of atoms in optical tweezers [New J. Phys. 14, 73051 (2012)], these methods will allow the simultaneous deterministic generation of individual photons in arbitrary modes, and thus deliver the resources needed for fully scalable linear-optical quantum computing or one-way quantum computing based on entangled cluster states. Furthermore UOXF has taken first successful steps towards hybrid quantum processing with single atoms and cavity photons in integrated photonic chips [Phys. Rev. Lett. 117, 023602 (2016)].
Additionally, the use of high-dimensional entangled states is a key enabler for high-capacity quantum information processing [Nat. Phys., 4, 282 (2008)]. High visibility quantum interference and high integration are essential for quantum photonic applications [J. Opt. 18, 104001 (2016)]. Time-energy entangled photon pairs at telecommunication wavelengths allow implementation of high visibility experiments and are especially well suited for integration with the current fiber optic infrastructure. UFC and AU will develop a comb frequency twin photon source based on high Q whispering gallery mode (WGM) resonator and implement a manipulation of a high-dimensional entangled states in frequency domain using their recent method [New J. Phys. 14, 043015, (2012)].
We will develop technologies to interface ions and atoms with photons, exploiting their strong interaction in high-finesse optical cavities, and explore new ideas to design efficient matter-light interfaces based on trapped atoms or ions. We also implement a stand-alone frequency comb entangled source and a system using photonic components to manipulate high dimensional entangled states in frequency domain. The activities in this WP are experimental, theoretical and industrial, involving the main activity of four ESRs. A key element is the strong interaction with the other WPs. Groups from the ion (UoS) and atom trapping (UOXF) communities will closely collaborate and jointly develop and disseminate new methods. Since all involved partners have the same aims, the ESRs will benefit from an intense exchange. The fact that TCPA and UBT are strongly involved in theoretical work highly relevant for the experimental programme of this WP provides a unique advantage. ESRs from TCPA, UBT, and UTT will support the experimental activities at UOXF, UoS and UFC.