Cascaded single-photon emission from the Mollow triplet sidebands of a quantum dot

A. Ulhaq, S. Weiler, S. M. Ulrich, R. Roßbach, M. Jetter & P. Michler

Emission from a resonantly excited quantum emitter is a fascinating research topic within the field of quantum optics and is a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line that develops into a triplet above saturation of the quantum emitter. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures. We present a detailed photon statistics analysis of the resonance fluorescence emission triplet from a solid-state-based artificial atom, that is, a semiconductor quantum dot. The photon correlation measurements demonstrate both ‘single’ and ‘cascaded’ photon emission from the Mollow triplet sidebands. The bright and narrow sideband emission (5.9 × 10⁶ photons per second into the first lens) can be conveniently frequency-tuned by laser detuning over 15 times its linewidth (Δv ≈ 1.0 GHz). These unique properties make the Mollow triplet sideband emission a valuable light source for quantum light spectroscopy and quantum information applications, for example.

**Groupmeeting by Amir Feizpour, Apr 11, 2012**

Spontaneous coherence in a cold exciton gas

A. A. High, J. R. Leonard, A. T. Hammack, M. M. Fogler, L. V. Butov, A. V. Kavokin, K. L. Campman & A. C. Gossard

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose–Einstein condensation1, 2. Spontaneous coherence is the key characteristic of condensation in momentum space3. Excitons—bound pairs of electrons and holes—form a model system to explore the quantum physics of cold bosons in solids4, 5. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes6. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state7 and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin.

**Groupmeeting by Xingxing Xing, Apr 4, 2012**

Observation of Quantum Criticality with Ultracold Atoms in Optical Lattices

Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, Cheng Chin

Quantum criticality emerges when a many-body system is in the proximity of a continuous phase transition that is driven by quantum fluctuations. In the quantum critical regime, exotic, yet universal properties are anticipated; ultracold atoms provide a clean system to test these predictions. We report the observation of quantum criticality with two-dimensional Bose gases in optical lattices. On the basis of in situ density measurements, we observe scaling behavior of the equation of state at low temperatures, locate the quantum critical point, and constrain the critical exponents. We observe a finite critical entropy per particle that carries a weak dependence on the atomic interaction strength. Our experiment provides a prototypical method to study quantum criticality with ultracold atoms.

**Groupmeeting by Dylan Jervis, Mar 28, 2012**

Adiabatic Passage with Spin Locking in Tm3+

María Florencia Pascual-Winter, Robert-Christopher Tongning, Romain Lauro, Anne Louchet-Chauvet, Thierry Chanelière, Jean-Louis Le Gouët

In low concentration Tm3+:YAG, we observe efficient adiabatic rapid passage (ARP) of thulium nuclear spin on very long time scales, with flipping time much longer than T2. Even with an optical oscillator strength as small as a few 10−8, optical preparation and detection enable us to work on a sample of about 1010 ions. The impurity-doped crystal strongly differs from monoatomic solids where efficient ARP at slow rate was observed in NMR early days. Explanation in terms of isoentropic reversible thermodynamic transformation does not apply in the present experiment. Instead, this feature can be understood as a spin locking effect.

**Groupmeeting by Chao Zhuang, Mar 21, 2012**

Dynamical Casmir Effect Using SQUIDS

C. M. Wilson, G. Johansson, A. Pourkabirian, M. Simoen, J. R. Johansson, T. Duty, F. Nori & P. Delsing

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. Although initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences—for instance, producing the Lamb shift¹ of atomic spectra and modifying the magnetic moment of the electron². This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed whether it might be possible to more directly observe the virtual particles that compose the quantum vacuum. Forty years ago, it was suggested³that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. The phenomenon, later termed the dynamical Casimir effect^{4, 5}, has not been demonstrated previously. Here we observe the dynamical Casimir effect in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (>10 gigahertz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process.

**Groupmeeting by Dylan Mahler, Mar 14, 2012**

Metastability and Coherence of Repulsive Polarons in a Strongly Interacting Fermi Mixture

Christoph Kohstall, Matteo Zaccanti, Michael Jag, Andreas Trenkwalder, Pietro Massignan, Georg M. Bruun, Florian Schreck, Rudolf Grimm

Ultracold Fermi gases with tuneable interactions represent a unique test bed to explore the many-body physics of strongly interacting quantum systems. In the past decade, experiments have investigated a wealth of intriguing phenomena, and precise measurements of ground-state properties have provided exquisite benchmarks for the development of elaborate theoretical descriptions. Metastable states in Fermi gases with strong repulsive interactions represent an exciting new frontier in the field. The realization of such systems constitutes a major challenge since a strong repulsive interaction in an atomic quantum gas implies the existence of a weakly bound molecular state, which makes the system intrinsically unstable against decay. Here, we exploit radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic 40K impurities resonantly interacting with a Fermi sea of 6Li atoms. In particular, we show that a well-defined quasiparticle exists for strongly repulsive interactions. For this "repulsive polaron" we measure its energy and its lifetime against decay. We also probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that a non-zero range of the order of the interparticle spacing results in a substantial lifetime increase. This major benefit for the stability of the repulsive branch opens up new perspectives for investigating novel phenomena in metastable, repulsively interacting fermion systems.

**Groupmeeting by Alma Bardon, Mar 7, 2012**

Relaxation Dynamics and Pre-thermalization in an Isolated Quantum System

Michael Gring, Maximilian Kuhnert, Tim Langen, Takuya Kitagawa, Bernhard Rauer, Matthias Schreitl, Igor Mazets, David A. Smith, Eugene Demler, Jörg Schmiedmayer

Understanding relaxation processes is an important unsolved problem in many areas of physics. A key challenge in studying such non-equilibrium dynamics is the scarcity of experimental tools for characterizing their complex transient states. We employ measurements of full quantum mechanical probability distributions of matter-wave interference to study the relaxation dynamics of a coherently split one-dimensional Bose gas and obtain unprecedented information about the dynamical states of the system. Following an initial rapid evolution, the full distributions reveal the approach towards a thermal-like steady state characterized by an effective temperature eight times lower than the initial equilibrium temperature of the system before the splitting process. We conjecture that this state can be described through a generalized Gibbs ensemble and associate it with pre-thermalization.

**Groupmeeting by Stefan Trotzky, Feb 29th, 2012**