Photon Conversion for Trapped-ion Quantum Networks (PhiNet)
In this project we investigate a way to interface light with the quantum states of trapped atomic ions. Our approach is to exploit a nonlinear optical process to change the frequency of single photons emitted and absorbed by trapped ions, from their natural values to those that are optimal for quantum networking. This frequency-shifter could act as a universal adapter for accessing and distributing the quantum states of trapped ions: between different traps and other quantum systems. Our experimental goals are to use such an adapter to observe entanglement between matter (an ion) and light (a photon) that has travelled over tens of kilometers, and to enable new ion-hybrid quantum systems. Our experiments combine single photon non-linear optics, atomic physics, cavity quantum electrodynamics (cavity QED) and quantum information science.
Why change the photons emitted by trapped ions?
Trapped atomic ions provide an excellent system with which to investigate and engineer quantum phenomena . Photons, which are emitted and absorbed by trapped ions, can be used to interface with and distribute the quantum states of the ions. Photons could thus be used to establish quantum links between remote networks of trapped ions, or between ions and other examples of quantum matter . Developing such quantum networks further would open up new directions in fields of quantum metrology, communication and computation: for new scientific tools, technology and for fundamental science . Indeed, ions in traps a few meters apart have been quantum linked (entangled) via the exchange of single photons at their natural frequency , and recently the first coupling between an ion and a quantum dot has been observed . Extending these ideas further opens up many new research challenges, two of which we aim to tackle in our work:
The photon absorption problem: The photons readily emitted by trapped ions are typically at frequencies that are strongly absorbed in optical fiber and the atmosphere. This limits the distance that such photons can distribute quantum correlations and information.
The photon mismatch problem: Different examples of quantum matter (such as ion, atoms or quantum dots) typically emit and absorb photons at specific and incompatible frequencies. This represents an obstacle to interfacing different kinds of systems at a quantum level.
A possible solution to these problems is to develop a tool that allows the frequency of photons emitted by ions to be changed, whilst preserving their quantum properties. Such a tool would allow the photons emitted by trapped ions to be shifted to the points of minimal absorption in optical waveguides and to values directly compatible with other quantum systems.
A photonic quantum adapter for trapped ions. A non-linear process allows the frequency of single photons to be tuned over a broad range of values: from those with minimal transmission loss, to those compatible with other kinds of quantum matter.
How can the frequency of a single photon be changed?
Our approach is to exploit the process of difference frequency generation (DFG) mediated by a crystal with a nonlinear response to light.
The process of difference frequency generation mediated by a material with a nonlinear response to light. Coupling the photon and pump laser field into waveguides integrated into the nonlinear material, could enable near unit conversion efficiency. In our lab we explore both ridge & buried waveguide approaches.
A single photon (frequency f1) enters the crystal, overlapped with a strong classical ‘pump’ laser field (frequency fpump). Ideally, the single photon emerges from the crystal with a new frequency f2, that is reduced by that of the pump (f2 = f1 - fpump). Overall, this process could be used to convert high energy (frequency) photons emitted by trapped ions, to lower energies that are more useful sending over long distance and talking to other quantum systems. The process should be fully reversible, allowing photon energies to be increased again then for reabsorbtion by quantum matter.
There has already been a lot of theoretical and experiment work on single photon frequency conversion in other contexts (see below). The new challenges for us are to apply this technique to the kinds of photons emitted by ions and to do so with high efficiency and low noise. Furthermore, all the spectral, temporal and quantum properties of the photons (other than the central frequency) should be preserved through the conversion process. This will require new levels of control over both the trapped-ion and the conversion process.
We will exploit the emerging technology of waveguide-integrated non-linear crystals. Here, micron scale waveguides are integrated into the crystal, defined by either a localised dopant, or geometrically by milling out ridges.
Our main experimental goals
We aim to learn as much as possible about the photon conversion technique and its potential to open up new ways to interface with trapped ions. We have two main experimental goals over the next five years. First, we aim to observe entanglement between an ion and a frequency-converted 1550 nm photon. 1550 nm is the wavelength with minimal loss in optic fiber, used by the global telecommunications industry. As such, 1550 nm (or other such ‘telecom bands’) would make an ideal frequency standard for interfacing quantum systems and building quantum networks. Second, we aim to observe entanglement between an ion and a photon that has been distributed over tens of kilometers of optical fiber. Distributed entanglement represents a fundamental resource for new science and technology.
Our experimental systems
There are two parts to the experimental systems in our lab, which are currently being set up. The first part is the nonlinear conversion system in which waveguide chips are combined with precision optics for in- and out-coupling, high-power mid infrared pump lasers, narrowband optical filtering stages and single photon detectors.
Our second experimental system is a trapped-ion single photon source. Our source will combine an ion trap with an integrated optical cavity to achieve a near-deterministic and on-demand source of 854 nm photons that entangled with the electronic state of a calcium ion. Such a system exploits cavity quantum electrodynamic effects to stimulate photon emission into the cavity mode with high probability . This system will generate photons for the frequency conversion project and will be one of three CQED-enhanced ion traps in Innsbruck that form a three node light-matter quantum network (project 'SciNet'). In such a network the trapped calcium ions offer long-lived quantum memories and deterministic quantum logic.
The trapped-ion single photon source that we are developing: a linear-Paul ion trap (gold electrodes) with an integrated optical cavity (blue mirrors). The ion, or string of ions, is at the centre of the cavity mode (red). An ultraviolet laser pulse (not shown) triggers the emission of a single photon from an ion into the cavity mode, which then exits through a pre-determined cavity mirror with high probability. Our system design is based on an established Innsbruck design  and optimised for highly-efficienct photon extraction.
Team working on the photon conversion project, December 2015. (MM, JS, MK, BL)
Martin Meraner (Intern)
Josef Schupp (PhD)
Muir Kumph (Postdoc)
Ben Lanyon (Project Leader)
We work with the group of Prof. Martin Fejer at Stanford University (weblink), who are world leaders in building and developing the technology of waveguide-integrated photon conversion chips. Prof Fejer, and Senior Scientist Dr. Carsten Langrock are working closely with us to develop waveguide technology for trapped-ion photon conversion.
We are not the only ones!
The ideas behind, and first experimental demonstrations of, single photon frequency conversion are several decades old. Only in the last few years, research on the application of photon conversion to precisely controllable quantum matter has begun. For example, photons from both a quantum dot  and a cold atomic ensemble  have been down-converted to the telecom bands. As such, converting photons emitted by trapped-ions to the telecom bands would open up the possibility of ion-dot and ion-ensemble hybrid system, via a telecom link.
We are aware of at least two other teams working on experimental photon conversion for trapped ions. The group of Christoph Becher in Saarbrucken, who have done pioneering work on conversion for quantum dots and diamond color centers, are working in this direction. The group of Prof. Jungsang Kim (Duke University) and collaborators are also working on a two-step process to link transitions in both Yb+ to telecom .
This project is funded by the START programme of the Austrian Science Fund
(Fonds zur Förderung der wissenschaftlichen Forschung, FWF)
Our team also receives support and funding from:
The Institute for Quantum Optics
and Quantum Information (IQOQI)
|The Army Research Laboratory, of the USA|