**Date:** 16 April 2015, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Charles Adams, Durham University

**Title: Dipole QED: an alternative paradigm for quantum non-linear optics and non-equilbrium dynamics **
**Abstract:**
In cavity QED (cQED), mirrors alter the electromagnetic field in the vicinity of an emitter and thereby modify the light-matter interaction in a significant way. In the strong coupling regime, the effect of the cavity dominates over the coupling to the vacuum. Similarly in dipole QED (dQED), one or more dipoles in the vicinity of the emitter modifies both the vacuum coupling (sub- or super-radiance) and the resonant frequency. The condition for strong coupling is simply that the dipoles must be closer than the reduced wavelength of emission, which is of order tens of nanometers for optical emitters [1] or millimeters for microwave emitters, e.g. transitions between highly-excited Rydberg states in atoms [2]. We will discuss experiments in both these regimes [1,2] and then discuss applications of dQED in single photon non-linear optics [3,4], and non-equilbrium systems with long-range interactions [5].

[1] J. Keaveney et al. Phys. Rev. Lett. 108, 173601 (2012).

[2] D. Barredo et al, Phys. Rev. Lett. to appear, arXiv:1408.1055

[3] J. D. Pritchard et al. Phys. Rev. Lett. 105, 193603 (2010).

[4] D. Maxwell et al, Phys. Rev. Lett. , 110, 103001 (2013).

[5] C. Carr et al. Phys. Rev. Lett. 111, 113901 (2013).

**Date:** 28 May 2015, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Tilman Pfau, University of Stuttgart

**Title: A single charge in a Bose-Einstein condensate: from two to few to many-body physics**
**Abstract:**
Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1].
At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2].
At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, We excite a single Rydberg electron in the BEC, the orbital size of which becomes comparable to the size of the BEC. We study the coupling between the electron and phonons in the BEC [3].
We also observe evidence for ultracold collisions involving a single ion which is shielded by a Rydberg electron. Reactive processes due to few-body Langevin dynamics are mostly l-changing and lead to molecule formation.
As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number, the imaging of the Rydberg electron's wave function by its impact onto the surrounding ultracold cloud as well as the observation of polaron formation seem to be within reach [4].

[1] E. Amaldi and E. Segre, Nature 133, 141 (1934)

[2] C. H. Greene, et al., PRL 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009)

[3] J . B. Balewski, et al., Nature 502, 664 (2013)

[4] T. Karpiuk, et al., arXiv:1402.6875

**Date:** 20 August 2015, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Scott Aaronson , Massachusetts Institute of Technology

**Title: Black Holes, Firewalls, and the Complexity of States and Unitary Transformations**
**Abstract:**
I'll discuss some recent results, motivated by the black-hole firewall paradox and the AdS/CFT correspondence, about the quantum circuit complexity of preparing certain entangled states and implementing certain unitary transformations. One result is a strengthening of an argument by Harlow and Hayden: I'll show that, under plausible assumptions, "decoding" useful information from Hawking radiation, as called for by the AMPS "firewall" thought experiment, requires the computational power to invert arbitrary cryptographic one-way functions, something we think not even quantum computers could do in sub-astronomical time. A second result, joint with Lenny Susskind, considers the circuit complexity of the kinds of states that could arise in AdS/CFT, and shows that, under a reasonable conjecture about complexity classes (PSPACE is not in PP/poly), the complexity indeed becomes superpolynomially large, as predicted by a conjectured relationship between complexity and geometry. I'll also discuss more general problems about the complexities of states and unitary transformations, which I find fascinating even apart from the quantum-gravity motivation.