Latest News | EQOP

APS ViewPoint ''An Accordion Lattice Playing a Soliton Tune''

Sun, 25 Jan 2026 00:00:00 +0000

Tunable optical lattices for the creation of matter-wave lattice solitons

Sat, 10 Jan 2026 00:00:00 +0000

Lattice solitons of matter waves.

Thu, 16 Oct 2025 00:00:00 +0000

New team member

Wed, 01 Oct 2025 00:00:00 +0000

Phonon excitations of Floquet-driven superfluids in a tilted optical lattice

Tue, 02 Jul 2024 00:00:00 +0000

Instabilities of driven matter waves with interactions

Sat, 22 Jul 2023 00:00:00 +0000

New team member

Mon, 01 Aug 2022 00:00:00 +0000

Conference ECAMP14

Mon, 27 Jun 2022 00:00:00 +0000

Finess Meeting 2022

Sat, 05 Feb 2022 00:00:00 +0000

ICONIQ Workshop 2022

Thu, 20 Jan 2022 00:00:00 +0000

Floquet solitons of matter waves

Sun, 01 Aug 2021 00:00:00 +0000

Abstract

We experimentally study the dynamics of a weakly interacting Bose-Einstein condensate of cesium atoms in a 1D optical lattice with a periodic driving force. After a sudden start of the driving we observe the formation of stable wave packets at the center of the fi rst Brillouin zone (BZ) in momentum space, and we interpret these as Floquet solitons in periodically driven systems. The wave packets become unstable when we add a trapping potential along the lattice direction leading to a redistribution of atoms within the BZ. The concept of a negative effective mass and the resulting changes to the interaction strength and effective trapping potential are used to explain the stability and the time evolution of the wave packets. We expect that similar states of matter waves exist for discrete breathers and other types of lattice solitons in periodically driven systems. [Phys. Rev. Lett. 127, 243603 (2021)].

Matter waves with position-dependent interactions

Wed, 28 Oct 2020 00:00:00 +0000

Motivation

Atoms can show attractive or repulsive interactions when they collide. But can atoms be both attractive and repulsive at the same time, e.g. attractive on the left side and repulsive on the right side?

The answer is: yes they can. The wave function of an ultracold atoms can be very large, extending over 100s of micrometers. We are used to the fact that the wave function adapts locally to a varying potential energy over this distance. Surprisingly, the same is true for its scattering properties, which can also vary locally over the extend of the wave function. The concept of an atom with locally changing interactions is only counterintuitive when we think of atoms as point like objects that crash into each other. A description using matter waves with fluid-like properties is well-suited to analyze those so called ‘‘collisionally inhomogeneous’’ systems.

Project summary

Collisionally inhomogeneous fluids exhibit spatially varying interactions between their particles. They frequently occur at interfaces, where interaction properties change due to a variation of an external potential or due to a change of the fluid’s composition. Examples for fluids at interfaces with a collisional inhomogeneity are liquid-vapour surfaces, and material junctions in condensed-matter physics.

We studied the evolution of matter waves for the simplest case, i.e. a spatial gradient of the interaction strength. Starting with a Bose-Einstein condensate with weak repulsive interactions in quasi-one-dimensional geometry, we monitored the evolution of a matter wave that simultaneously extends into spatial regions with attractive and repulsive interactions. We observed the formation and the decay of soliton-like density peaks, counter-propagating self-interfering wave packets, and the creation of cascades of solitons.

The matter-wave dynamics was well reproduced in numerical simulations based on the non-polynomial Schroedinger equation with three-body loss, allowing us to better understand the underlying behavior based on a wavelet transformation [Phys. Rev. Lett. 125, 183602 (2020)].

New Investigator Award

Wed, 01 Jul 2020 00:00:00 +0000

Hurray, we got the New Investigator Award by the EPSRC. The award will provide two years of funding for a project about Quantum bright solitons.

Particles at ultra-low temperatures can show surprising quantum effects, such as tunnelling and entanglement. Typically, those quantum effects are strongest for individual particles but decrease for large many-body systems. Especially many-body systems of bosons, i.e. particles which allow for a simultaneous occupation of the same state, are well described by a collective wave function, with similar properties as a classical fluid. The crossover between this classical, fluid-like regime and systems with individual particles is currently studied, and the particle number, required to observe quantum effects, is actively debated. The goal of this project is to detect and study two states of ultracold atomic gases in this crossover regime - “quantum bright solitons”’ and large “cluster states”. Both states are expected to exist for similar experimental parameters with between five and a few hundred atoms, but they approach the crossover regime from opposite sides.

Bright solitons are dispersionless wave-packets that propagate without changing their shape, and they present a typical property of nonlinear fluids. For reduced particle number, bright solitons are expected to acquire properties which are characteristic for a single quantum object, such as discrete tunnelling, uncertainty relationships and entanglement. Cluster states on the other hand are loosely bound states of few particles, similar to molecules. They are expected to lose quantum properties with increasing particle number. The goal of the project is to experimentally prepare bright solitons and cluster states with a well-defined number of ultracold atoms, and to probe the properties of the system as the atom number is changed.

The research will broaden our understanding of the boundary between few-body and many-body physics, and it has the potential to advance technical applications, e.g. with the development of new quantum technologies based on large and complex quantum states.

You can find more information at the EPSRC webpage.

PhD viva of Dr Andrea Di Carli

Wed, 25 Sep 2019 00:00:00 +0000

Congratulations to Dr Andrea Di Carli for his PhD viva. Well done."

You can find Dr. Andrea Di Carli’s PhD thesis at: PhD thesis, Di Carli

Breathing and higher-order excitations of solitons

Tue, 10 Sep 2019 00:00:00 +0000

Motivation

Bright matter-wave solitons are small dispersionless wave packets that keep its shape while propagating. Typically, this shape-preserving propagation is the result of a delicate balance between attractive interaction energy and dispersive kinetic energy. (Have a look at our project with Floquet solitons for bright matter-wave solitons with repulsive interactions.) The balancing condition relates parameters of the soliton, such as size, atom number, and interaction strength. It is easy to destroy this balance by changing the soliton size. For example, making the soliton larger decreases the kinetic energy while making the soliton smaller increases it.

What happens if we change the size of the soliton, e.g. make it slightly too large? Does the soliton return to its favorite size or does it fall apart?

The answer is: both. The soliton starts to oscillate for small size changes - getting periodically larger and smaller. Its oscillation frequency is a bit unusual, because there is no trap or external potential that could set it. Instead, the frequency is a property just of the soliton, depending on parameters such as atom number, interaction strength, and size.

There are two more options for the soliton to react when its size is far too large: (1) The soliton can shed atoms to get smaller. For example, when it is too large, it can eject atoms until it regains an atom number that fits to its larger size. (2) The soliton starts to oscillate but with a strange breathing-like motion, creating multiple sub-peaks during the oscillation (see cover image at the top of the page). Those oscillations are called higher-order oscillations and appear only for very specific sizes and with discrete frequencies.

Project summary

We experimentally studied the excitation modes of bright matter-wave solitons in a quasi-one-dimensional geometry. The solitons were created by quenching the interactions of a Bose-Einstein condensate of cesium atoms from repulsive to attractive in combination with a rapid reduction of the longitudinal confinement. A deliberate mismatch of quench parameters allowed us to excite breathing modes of the emerging soliton and to determine its breathing frequency as a function of atom number and confinement. In addition, we observed signatures of higher-order solitons and the splitting of the wave packet after the quench [Phys. Rev. Lett. 123, 123602 (2019)].

New team member

Sun, 01 Sep 2019 00:00:00 +0000

PhD viva of Dr Craig Colquhoun

Fri, 21 Jun 2019 00:00:00 +0000

Congratulations to Dr Craig Colquhoun for his PhD viva. Well done."

You can find Dr. Craig Colquhoun’s PhD thesis at: PhD thesis, Colquhoun

Measurement of micro-g acceleration

Wed, 22 May 2019 00:00:00 +0000

Project summary

The sensitivity of atom interferometers is usually limited by the observation time of a free-falling cloud of atoms in Earth`s gravitational field. Considerable efforts are currently made to increase this observation time, e.g. in fountain experiments, drop towers and in space. In this article, we experimentally studied and discussed the use of magnetic levitation for interferometric precision measurements.

We used a Bose-Einstein condensate of cesium atoms with tuneable interaction strength and a Michelson interferometer scheme for the detection of micro-g acceleration. In addition, we demonstrated observation times of 1s, which are comparable to current drop-tower experiments. We also studied the curvature of our force field and observed the effects of a phase-shifting element in the interferometer paths [New J. Phys. 21, 053028 (2019)].

Motivation

Atomic fountain interferometers have demonstrated an staggering measurement precision of order Δg/g~10⁻¹⁰. This project does not aim to compete with atomic fountains, but we were curious to test the limits of out magnetic field control in the lab. Most of the technical work is centered around measuring and controlling current in coils, and determining magnetic field gradients and curvatures. Our take-home message from the project was: magnetic field stabilization with 10^-5 precision for levitation is moderately easy, a 10^-6 precision is do-able, and less than 10⁻⁷ will get really hard and tedious.

Interferometer scheme

We employed a Michelson interferometer scheme that is based on three Kapitza-Dirac pulses with a standing light wave [PRL 56 827 (1986), PRL 75 2633 (1995)]. The pulses change the motional states of the matter waves but leave the internal states of the atoms unchanged. Our pulse sequence and the resulting motion of the matter wave packets are illustrated in the figure below. A first pulse splits the BEC into two wave packets with opposite momenta. The wave packets propagate freely for an evolution time T1 until we apply a second pulse that inverts the direction of the wave packets and changes their momentum again. A third pulse is used after an evolution time T2 to recombine the two wave packets. It is identical to the first pulse and generates three wave packets with momenta p0 = 0, ±p. The acquired phase difference ΔΦ between the wave packets can be determined from the relative population of the recombined wave packets on an absorption image.

Absorption images of 3-pulse Michelson interferometer scheme

Acceleration measurement

The acquired phase difference ΔΦ between the wave packets increases with hold time for external forces. Large forces make ΔΦ oscillate quickly between 0 an 2pi, while small forces result is slow oscillations. The figure below shows this oscillation of ΔΦ (measured by the relative population of the wave packets) for decreasing forces (a,b,c). We experimentally created the forces be changing our levitation current, and thereby changing the levitation gradient, to ΔI/I_lev of (a) 0.003, (b) 0.001, (c) 0.0003. Subplot (d) shows a comparison of the acceleration measurement with the interferometer scheme (red circles) and by a different measurement approach using the center-of-mass motion of a BEC (blue diamonds).

We determine an upper limit for the acceleration of the atoms of 70(10)x10⁻⁶g. To the best of our knowledge, this is the smallest absolute value for an acceleration that is measured directly with ultracold atom interferometry. However, the goal of the project is to evaluate the limits, and the article provides a long discussion of technical reasons that limit the measurement precision.

Oscillation of the phase and population difference for various accelerations

Measuring other potentials

After benchmarking the measurement scheme, we started to play with our setup. The figure below shows a measurement of the phase shift that is created when the BEC propagated through another laser beam. Subplot (a) shows the phase shift for minimized acceleration of the atoms (red circles) and for the addition laser beam in the path of the upper wave packet (blue squares). Subplot (b) illustrates of the path of the wave packets and the additional laser beam during the pulse sequence. Angles and axes are not to scale in the illustration.

Phase shift due to an additional laser beam in the interferometer.

Free vertical expansion of a BEC

Finally, after having minimized residual acceleration with the interferometric measurement scheme, we tried to maximize the evolution time of the BEC with free vertical expansion. Long observation times of an expanding BEC facilitate a sensitive acceleration measurement approach, too. We demonstrate in the article that magnetic levitation allows us to extend the vertical expansion time of a BEC to 1s, and we evaluate advantages and limitations of this scheme for precision measurements.

Typical expansion times for falling BECs are on the order of tens of milliseconds, often limited by the detection area of the imaging system, by the gravitational acceleration and by the expansion velocity of the gas. Usually, the expansion velocity of a quantum gas is not caused by the temperature of the gas but by repulsive interaction during the initial spreading. The current record for long observation times under milli-g acceleration is 1s [Science 329 1540 (2010)]. The experiment was performed in a drop tower, and ballistic expansion was observed over approximately 500 ms, limited by stray magnetic fields. We demonstrate similar observation and expansion times (figure below) for levitated BECs. Sub-panel (a) shows averaged absorption images, (b) measures the broadening of the expanding BEC, and (c,d) show vertical density profiles for expansion times of 400ms and 600ms.

There is a clear kink in the density profile in sub-panel (d). We failed to explain this kink properly at the time of writing the article. It is caused by a position dependent interaction strength due to our levitation gradient. This observation lead to another project and to another article [Phys. Rev. Lett. 125, 183602 (2020)].

Long vertical expansion of a BEC

Experimental setup

Fri, 01 Feb 2019 00:00:00 +0000

The setup is designed to cool a gas of cesium atoms from room temperature to quantum degeneracy. We use a combination of laser cooling techniques, such as 2D and 3D magneto-optical traps (MOTs), degenerate Raman-sideband-cooling, and large dipole traps with laser beam powers up to 200W. The final cooling step is based on evaporative cooling in a crossed-beam dipole trap. We reach Bose-Einstein-condensates (BECs) of 300,000 atoms in 12s.

Vacuum apparatus

We tried to keep the vacuum apparatus as simple as possible. There are two glass cells, one with a moderate pressure of 10^-8 mbar for the 2D MOT and one with 10^-11 mbar for the 3D MOT and the main experiments. All pumps, gauges, vales, and ampules with cesium, are in the middle. The atoms diffuse from the cesium ampule into the glass cess of the 2D MOT. They are cooled with laser beams along two directions, and finally pushed with another laser beam into the main glass cell.

Vaccuum system with two glass cells

Bose-Einstein condensate

The final cooling step is based on evaporative cooling in a crossed-beam dipole trap. Once a critical temperature Tc is reached, the gas shows a phase transition towards a Bose-Einstein-condensates. We create BECs of 300,000 atoms in a total duration of 12s.

The figure below shows absorption images of the momentum profile of a gas of atoms. Left, top to bottoms: The profile changes as the gas is cooled below the critical temperature. Below Tc, a BEC forms and the gas is localized in momentum space. Right: Horizontally integrated density profiles in momentum space. Again, you momentum profiles narrows below the critical temperature.

Latest News | EQOP

APS ViewPoint ''An Accordion Lattice Playing a Soliton Tune''

Tunable optical lattices for the creation of matter-wave lattice solitons

Lattice solitons of matter waves.

New team member

Phonon excitations of Floquet-driven superfluids in a tilted optical lattice

Instabilities of driven matter waves with interactions

New team member

Conference ECAMP14

Finess Meeting 2022

ICONIQ Workshop 2022

Floquet solitons of matter waves

Abstract

Matter waves with position-dependent interactions

Motivation

Project summary

New Investigator Award

PhD viva of Dr Andrea Di Carli

Breathing and higher-order excitations of solitons

Motivation

Project summary

New team member

PhD viva of Dr Craig Colquhoun

Measurement of micro-g acceleration

Project summary

Motivation

Interferometer scheme

Acceleration measurement

Measuring other potentials

Free vertical expansion of a BEC

Experimental setup

Vacuum apparatus

Vaccuum system with two glass cells

Bose-Einstein condensate

Aborption images and profiles of quantum gas of cesium atoms