# Carsten Robens

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· Dr. rer. nat.About

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Research Item (14)

- Jan 2018
- Exploring the World with the Laser

Elitzur and Vaidman have proposed a measurement scheme that, based on the quantum superposition principle, allows one to detect the presence of an object—in a dramatic scenario, a bomb—without interacting with it. It was pointed out by Ghirardi that this interaction-free measurement scheme can be put in direct relation with falsification tests of the macro-realistic worldview. Here we have implemented the “bomb test” with a single atom trapped in a spin-dependent optical lattice to show explicitly a violation of the Leggett–Garg inequality—a quantitative criterion fulfilled by macro-realistic physical theories. To perform interaction-free measurements, we have implemented a novel measurement method that correlates spin and position of the atom. This method, which quantum mechanically entangles spin and position, finds general application for spin measurements, thereby avoiding the shortcomings inherent in the widely used push-out technique. Allowing decoherence to dominate the evolution of our system causes a transition from quantum to classical behavior in fulfillment of the Leggett–Garg inequality.

- Nov 2016

We propose a novel approach to precisely synthesize arbitrary polarization states of light with a high modulation bandwidth. Our approach consists in superposing two laser light fields with the same wavelength, but with opposite circular polarizations, where the phase and amplitude of each light field are individually controlled. To assess the precision of the synthesized polarization states, we characterize static spatial variations of the polarization over the wavefront, as well as the noise spectral density of temporal fluctuations. We find that static polarization distortions limit the extinction ratio to $2\times 10^{-5}$, corresponding to a 0.01% reduction of the degree of polarization (DOP). We also obtain that temporal fluctuations give rise to a $0.2^\circ$ uncertainty in the state of polarization (SOP). We recently demonstrated an application of the polarization synthesizer (Robens et al., arXiv:1608.02410) to create two fully independent, controllable optical lattices, which trap atoms depending on their internal spin state. Probing ultracold atoms in polarization-synthesized optical lattices, we obtain an independent, complementary characterization of the optical performance of the polarization synthesizer.

- Nov 2016

We have designed, built, and characterized a high-resolution objective lens that is compatible with an ultra-high vacuum environment. The lens system exploits the principle of the Weierstrass-sphere solid immersion lens to reach a numerical aperture (NA) of 0.92. Tailored to the requirements of optical lattice experiments, the objective lens features a relatively long working distance of 150 micrometers. Our two-lens design is remarkably insensitive to mechanical tolerances in spite of the large NA. Additionally, we demonstrate the application of a tapered optical fiber tip, as used in scanning near-field optical microscopy, to measure the point spread function of a high NA optical system. From the point spread function, we infer the wavefront aberration for the entire field of view of about 75 micrometers. Pushing the NA of an optical system to its ultimate limit enables novel applications in quantum technologies such as quantum control of atoms in optical microtraps with an unprecedented spatial resolution and photon collection efficiency.

- Sep 2016

Elitzur and Vaidman have proposed a measurement scheme that, based on the quantum superposition principle, allows one to detect the presence of an object --- in a dramatic scenario, a bomb --- without interacting with it. It was pointed out by Ghirardi that this interaction-free measurement scheme can be put in direct relation with falsification tests of the macro-realistic worldview. Here we have implemented the "bomb test" with a single atom trapped in a spin-dependent optical lattice to show explicitly a violation of the Leggett-Garg inequality --- a quantitative criterion fulfilled by macro-realistic physical theories. To perform interaction-free measurements, we have implemented a novel measurement method that correlates spin and position of the atom. This method, which quantum mechanically entangles spin and position, finds general application for spin measurements, thereby avoiding the shortcomings inherent in the widely used push-out technique. Allowing decoherence to dominate the evolution of our system causes a transition from quantum to classical behavior in fulfillment of the Leggett-Garg inequality.

- Aug 2016

We create low-entropy states of neutral atoms by utilizing a conceptually new optical-lattice technique that relies on a high-precision, high-bandwidth synthesis of light polarization. Polarization-synthesized optical lattices provide two fully controllable optical lattice potentials, each of them confining only atoms in either one of the two long-lived hyperfine states. By employing one lattice as the storage register and the other one as the shift register, we provide a proof of concept that selected regions of the periodic potential can be filled with one particle per site. Vibrational entropy is subsequently removed by sideband cooling methods. Our results pave the way for a bottom-up approach to creating ultralow-entropy states of a many-body system.

- Dec 2015

We report on image processing techniques and experimental procedures to
determine the lattice-site positions of single atoms in an optical lattice with
high reliability, even for limited acquisition time or optical resolution.
Determining the positions of atoms beyond the diffraction limit relies on
parametric deconvolution in close analogy to methods employed in
super-resolution microscopy. We develop a deconvolution method that makes
effective use of the prior knowledge of the optical transfer function, noise
properties, and discreteness of the optical lattice. We show that accurate
knowledge of the image formation process enables a dramatic improvement on the
localization reliability. This is especially relevant for closely packed
ensembles of atoms where the separation between particles cannot be directly
optically resolved. Furthermore, we demonstrate experimental methods to
precisely reconstruct the point spread function with sub-pixel resolution from
fluorescence images of single atoms, and we give a mathematical foundation
thereof. We also discuss discretized image sampling in pixel detectors and
provide a quantitative model of noise sources in electron multiplying CCD
cameras.

- Dec 2015

We eport on an ultra-low birefringence dodecagonal glass cell for ultra-high
vacuum applications. The epoxy-bonded trapezoidal windows of the cell are made
of SF57 glass, which exhibits a very low stress-induced birefringence. We
characterize the birefringence $\Delta n$ of each window with the cell under
vacuum conditions, obtaining values around $\num{e-8}$. After baking the cell
at $\SI{150}{\degreeCelsius}$, we reach a pressure below $\SI{e-10}{\milli
\bar}$. In addition, each window is antireflection coated on both sides, which
is highly desirable for quantum optics experiments and precision measurements.

- Nov 2015

We report on the state of the art of quantum walk experiments with neutral
atoms in state-dependent optical lattices. We demonstrate a novel
state-dependent transport technique enabling the control of two spin-selective
sublattices in a fully independent fashion. This transport technique allowed us
to carry out a test of single-particle quantum interference based on the
violation of the Leggett-Garg inequality and, more recently, to probe
two-particle quantum interference effects with neutral atoms cooled into the
motional ground state. These experiments lay the groundwork for the study of
discrete-time quantum walks of strongly interacting, indistinguishable
particles to demonstrate quantum cellular automata of neutral atoms.

- Jan 2015

We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories. We use so-called ideal negative measurements—an essential requisite for any genuine Leggett-Garg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it. The interaction-free measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site. Beyond the fundamental aspect of this test, we demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition. Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.

- Apr 2014

When a physical object is in a quantum superposition, interference among the
possible quantum paths affects the outcomes of a measurement. Gaining knowledge
about the system with an intermediate measurement smears out, if not completely
destroys, the resulting interference pattern. Based on this idea, Leggett and
Garg formulated an inequality relating two-time correlation measurements, which
establishes a quantitative criterion for discerning quantum superposition on a
fundamental ground. Originally proposed to test realism of macroscopic physical
objects, the inequality is here used to prove the non-classicality of the
motion of a cesium atom, which propagates on a space-time discrete lattice.
Making use of ideal negative measurements to test the atom's motion, we obtain
a 6 sigma experimental violation of the Leggett-Garg inequality. From a broader
perspective, our findings provide rigorous validation of the idea that quantum
transport experiments cannot be interpreted in terms of classical trajectories.

- Dec 2013

We present an in situ method to measure the birefringence of a single vacuum window by means of microwave spectroscopy on an ensemble of cold atoms. Stress-induced birefringence can cause an ellipticity in the polarization of an initially linearly polarized laser beam. The amount of ellipticity can be reconstructed by measuring the differential vector light shift of an atomic hyperfine transition. Measuring the ellipticity as a function of the linear polarization angle allows us to infer the amount of birefringence Δn at the level of 10(-8) and identify the orientation of the optical axes. The key benefit of this method is the ability to separately characterize each vacuum window, allowing the birefringence to be precisely compensated in existing vacuum apparatuses.

- Jul 2011

We investigate the implications for supersymmetry from an assumed absence of any signal in the first period of LHC data taking at 7 TeV center-of-mass energy and with 1 to 7fb-1 of integrated luminosity. We consider the zero-lepton plus four jets and missing transverse energy signature, and perform a combined fit of low-energy measurements, the dark matter relic density constraint and potential LHC exclusions within a minimal supergravity model. A nonobservation of supersymmetry in the first period of LHC data taking would still allow for an acceptable description of low-energy data and the dark matter relic density in terms of minimal supergravity models, but would exclude squarks and gluinos with masses below 1 TeV.

- May 2011

Both ATLAS and CMS have published results of SUSY searches putting limits on
SUSY parameters and masses. A non-discovery of SUSY in the next two years would
push these limits further. On the other hand, precision data of low energy
measurements and the dark matter relic density favor a light scale of
supersymmetry. Therefore we investigate if supersymmetry -- more specifically
the highly constraint model mSUGRA -- does at all agree with precision data and
LHC exclusions at the same time, and whether the first two years of LHC will be
capable of excluding models of supersymmetry. We consider the current non
observation of supersymmetry with 35 pb-1 as well as the possible non
observation with 1,2 and 7 fb-1 in a global fit using the framework Fittino.