## 605. WE-Heraeus-Seminar on MACROSCOPIC ENTANGLEMENT
Physikzentrum Bad Honnef The programm is available here.
- Ulrik L. Andersen
- Maria Chekhova
- Christoph Marquardt
Seminar contents and program: ## Collected PresentationsA collection of presenatitions given at the seminar can be found here. ## Scientific backgroundEntanglement is one of the most striking features of the quantum world [Ein35]. Entanglement between two or more subsystems of a composite system means that a subsystem, taken separately, has its properties very uncertain but strongly correlated with the ones of the other subsystems. Entanglement is the enabling resource for extremely fast quantum computation, unconditionally secure communication and sensing with unprecedented resolution. Furthermore, entanglement is believed to be a fundamental resource that might govern the formation of space-time and of new phases of matter [Ami08]. Entanglement has often been seen as an exclusive prerogative of systems of a few particles but to understand and exploit the full potential of entanglement it must be engineered and controlled in much larger and more complex systems [Ved14]. Possibly the strongest feature of quantum entanglement is the ability to formulate and violate Bell's inequalities. They are derived under the assumption that the system can be described by certain parameters that are realistic (i.e., exist a priori, before any experiment) and local (i.e., do not influence each other instantly at a distance). This concept of local realism inevitably fails, through the violation of certain Bell's inequalities. At present, the existence of entanglement and the violation of Bell's inequalities is experimentally verified for most microscopic quantum systems. The existing loopholes for local realism still remain but loophole-free Bell tests are coming soon. Meanwhile, the existence of entanglement for large (macroscopic) objects [Bru06], and the possibility to observe it, is still debated [Pen96, Leg02, Arn14]. It is the idea of entanglement between a microscopic particle and a macroscopic object that led Schrödinger to his famous `cat paradox' [Sch35]. Recently, various quantum systems were considered as candidates for the role of an observable `Schrödinger cat'. The pioneering experiments with relatively large ensembles of microwave photons [Har13] and ions [Win13] were carried out by the groups of Haroche and Weinland and eventually honored by the Nobel Prize of 2012. This alone illustrates the importance of the `quest' for macroscopic entanglement. Later, macroscopic superpositions were observed for large ensembles of atoms [Est08, Gro10], molecules, for superconducting circuits, macroscopic currents in SQUIDs and other material systems [Arn14]. An interesting aspect of these experiments is the ability to quantify the macroscopicity of a quantum system [Fro14]. Whereas the number of particles involved can often be verified it is not necessarily straightforward how to associate “quantumness” to a collection of particles. There is much discussion about the possibility to observe entanglement for macroscopic (bright) states of light. It is worth noting here that while for macroscopic material objects even interference can be considered as an interesting quantum phenomenon, in the case of light it is purely classical, and only entanglement, or even violation of Bell's inequalities, can witness truly quantum behavior. On the other hand, it is especially interesting to look for entanglement for macroscopic light beams because the number of photons in a light beam is a measure of its efficiency for interactions, both with the matter and with light. As an example of `Schrödinger cat states' in optics, superpositions of macroscopic coherent states have been discussed for several decades. They are now obtained in many labs by subtracting a photon from weakly squeezed vacuum [Our06], but this technique only provides weak states, which are therefore called `Schrödinger kittens'. `Adult' (bright) Schrödinger cat states are too fragile to decoherence and hardly possible to observe. For this reason, alternative ways for entangling bright light beams with single photons or with each other have been recently proposed and partly implemented [Bru13, Lvo13]. It is unknown whether entanglement can exist on a macroscopic scale as envisaged by Schrödinger’s cat experiment. It has for example been conjectured that gravity, experienced by large objects, might cause a degradation of entanglement, rendering the object in a classical state [Pen96]. In order to test this claim one could prepare a massive object, like a mechanical oscillator [Cha11, Leh13], in an entangled state and monitor its evolution in time [Mar03] or using alternative approached [Pik12, Bah14]. Such table-top experiments will also enable the exploration of the interface between quantum physics and gravity, which hitherto has been investigated only with large-scale facilities [Cam14]. This might open a new route to fundamental tests of quantum gravity. All these diverse fields share similar goals and difficulties when approaching the extreme regime of macroscopicity in quantum mechanics. When Schrödinger and Einstein introduced their thoughts, they thought about “Gedankenexperiments”. Today, we are in a position where these experiments could actually become a reality. ## AimsThe workshop aims to bring together experts from various fields, ranging from solid state physics and atomic physics to quantum optics, and discuss the main results and challenges in the description, classification and experimental generation of macroscopic entangled states. The main experts of the diverse communities will be invited, including theoretical and experimental ones. The seminar is intended to provide a good introduction for newcomers interested in bringing aspects of macroscopic entanglement to their fields. Thus the seminar will also be a good opportunity for PhD students and young post-docs to get introduced to this exciting field. Bringing together an international group of young researchers will further initiate mutually beneficial exchanges between these groups and can be expected to stimulate stronger collaborations in the field. ## Invited speakers**Markus Aspelmeyer**(University of Vienna)**Marco Bellini**(Istituto Nazionale di Ottica – CNR, Firenze)**John Bollinger****Dirk Bouwmeester**(University of California, Santa Barbara and University of Leiden)**Warwick Bowen**(University of Queensland, Brisbane)**Caslav Brukner**(University of Vienna)**Michel Brune**(Laboratoire Kastler Brossel, Paris)**Ivette Fuentes**(University of Nottingham and University of Vienna)**Florian Fröwis**(University of Geneva)**Farit Khalili**(Lomonosov Moscow State University)**Chan U Lei**(California Institute of Technology)**Morgan Mitchell**(ICFO, Barcelona)**Markus Oberthaler**(University of Heidelberg)**Eugene Polzik**(Niels Bohr Institute - University of Copenhagen)**Margaret Reid**(Swinburne University of Technology, Melbourne)**Pedram Roushan**(University of California, Santa Barbara)**Roman Schnabel**(University of Hamburg)**Fabio Sciarrino**(Sapienza University of Rome)**Vlatko Vedral**(University of Oxford and National University of Singapore)**Chen Wang**(Yale University, New Haven)**Eva Weig**(University of Konstanz)**Marek Zukowski**(University of Gdansk)
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