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Physikzentrum Bad Honnef

January 17th - January 22nd 2016.

The programm is available here.


  • Ulrik L. Andersen
  • Maria Chekhova
  • Christoph Marquardt

Max Planck Institute
for the Science of Light
Günther-Scharowsky-Str.1/Building 24
D-91058 Erlangen
Tel.: +49(0)9131-6877-101
Fax: +49(0)9131-6877-109
email: weh605@mpl.mpg.de
web:  http://indico.mpl.mpg.de/event/2/

Seminar contents and program:

Collected Presentations

A collection of presenatitions given at the seminar can be found here

Scientific background

 Entanglement 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.


The 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


[Ami08] L. Amico, R. Fazio, A. Osterloh, and V. Vedral, Entanglement in many-body systems, Rev. Mod. Phys. 80, 517 (2008).

[Arn14] M. Arndt and K. Hornberger, Testing the limits of quantum mechanical superposition, Nature Physics 10, 271 (2014).

[Bah14] M. Bahrami, M. Paternostro, A. Bassi, and H. Ulbricht, Proposal for a noninterferometric Test of Collapse Models in Optomechanical Systems, Phys. Rev. Lett. 112, 210404 (2014).

[Bru06] Č. Brukner, V. Vedral, and A. Zeilinger, Crucial role of quantum entanglement in bulk properties of solids, Phys. Rev. A 73, 012110 (2006).

[Bru13] N. Bruno et al., Displacement of entanglement back and forth between the micro and macro domains. Nature Physics 9, 545 (2013).

[Cam14] G. Amelino-Camelia, Gravity in quantum mechanics, Nature Physics, 10, 254 (2014).

[Cha11] J. Chan et al., Laser cooling of a nanomechanical oscillator into its quantum ground state Nature 478, 89 (2011).

[Del08] S. Deléglise et al., Reconstruction of non-classical cavity field states with snapshots of their decoherence, Nature 455, 510 (2008).

[Ein35] Einstein et al, Can quantum-mechanical description of physical reality be considered complete?, Phys. Rev. 47, 777 (1935).

[Est08] J. Esteve, , C. Gross, , A. Weller, S. Giovanazzi, and M.K. Oberthaler, Squeezing and entanglement in a Bose-Einstein condensate. Nature 455, 1216 (2008).

[Fro14] F. Fröwis, N. Sangouard, N. Gisin, Linking Measures for Macroscopic Quantum States via Photon-Spin Mapping, arxiv:1405.0051

[Gho14] R. Ghobadi, S. Kumar, B. Pepper, D. Bouwmeester, A.I. Lvovsky, and C. Simon, Optomechanical micro-macro entanglement, Phys. Rev. Lett. 112, 080503 (2014).

[Gro10] C. Gross, T. Zibold, E. Nicklas, J. Estéve, and M.K. Oberthaler, Nonlinear atom interferometer surpasses classical precision limit, Nature 464, 1165 (2010).

[Har13] S. Haroche, Nobel Lecture: Controlling photons in a box and exploring the quantum to classical boundary,  Rev. Mod. Phys. 85, 1083 (2013).

[Leg02] J.D. Leggett, Testing the limit of quantum mechanics: Motivation, state of play, prospects. Phys. Condens. Matter 14, R415 (2002).

[Leh13] K. W. Lehnert, Entangling mechanical motion with microwave fields. Science (New York, N.Y.) 342, 710 (2013).

[Lvo13] A. I. Lvovsky, R. Ghobadi, A. Chandra, A.S. Prasad, and C. Simon, Observation of micromacro entanglement of light. Nature Physics 9, 541 (2013).

[Mar03] W. Marshall, C. Simon, R. Penrose, and D. Bouwmeester, Towards quantum superpositions of a mirror, Phys. Rev. Lett. 91, 130401 (2003).

[Our06] A. Ourjoumtsev, R. Tualle-Brouri, J. Laurat, and P. Grangier, Generating optical Schrödinger kittens for quantum information processing. Science 312, 83 (2006).

[Pal13] T.A. Palomaki et al., Entangling Mechanical Motion with MW Fields, Science, 342, 710 (2013).

[Pen96] R. Penrose, On gravity’s role in quantum state reduction, Gen. Rel. and Grav. 28, 581 (1996).

[Pik12] I. Pikovski, M.R. Vanner, M. Aspelmeyer, M.S. Kim and C. Brukner, Probing Planck-scale physics with quantum optics, Nature Phys. 8, 393 (2012)

[Sek14] P. Sekatski, M. Aspelmeyer, and N. Sangouard, Macroscopic Optomechanics from Displaced Single-Photon Entanglement, Phys. Rev. Lett. 112, 080502 (2014)

[Ved14] V. Vedral, Quantum entanglement, Nature Physics 10, 256 (2014)

[Win13] David J. Wineland, "Nobel Lecture: Superposition, Entanglement, and Raising Schroedinger's Cat", Rev. Mod. Phys. 851103-1114 (2013).

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