Atomic Bloch-Zener oscillations and Stückelberg interferometry in optical lattices

Bloch-Zener oscillation with engineered Floquet energy bands in synthetic temporal lattices

Here we experimentally demonstrate the dynamics of Bloch-Zener oscillations (BZOs) in a synthetic temporal lattice formed by the optical pulses in coupled fiber loops. By periodically modulating the phases imposed to the optical pulses in linear driven lattices, a two-band Floquet system with tunable bandgaps is realized, and the related BZOs that occurred in this system are displayed. On this basis, by manipulating the phase difference and coupling angle of the synthetic lattice, the widths of 0-gap and π-gap are tuned feasibly so that a wide variety of the interplays between Bloch oscillations and Landau-Zener tunneling (LZT) are exhibited. As an application, the temporal Mach-Zehnder interferometer utilizing BZOs is realized, where the output patterns could be modulated by the coupling rate of the synthetic lattice. This work lays the foundation for exploring BZO physics with synthetic dimensions, which may find applications in temporal pulse controlling and optical signal processing.

Non-Hermitian Bloch-Zener phase transition

Bloch-Zener oscillations (BZO), i.e., the interplay between Bloch oscillations and Zener tunneling in two-band lattices under an external direct current (DC) force, are ubiquitous in different areas of wave physics, including photonics. While in Hermitian systems such oscillations are rather generally aperiodic and only accidentally periodic, in non-Hermitian (NH) lattices BZO can show a transition from aperiodic to periodic as a NH parameter in the system is varied. Remarkably, the phase transition can be either smooth or sharp, contrary to other types of NH phase transitions which are universally sharp. A discrete-time photonic quantum walk on a synthetic lattice is suggested for an experimental observation of smooth BZO phase transitions.

Landau-Zener-Stückelberg Interference in a Multimode Electromechanical System in the Quantum Regime

The studies of mechanical resonators in the quantum regime not only provide insight into the fundamental nature of quantum mechanics of massive objects, but also introduce promising platforms for novel hybrid quantum technologies. Here we demonstrate a configurable interaction between a superconducting qubit and many acoustic modes in the quantum regime. Specifically, we show how consecutive Landau-Zener-Stückelberg (LZS) tunneling type of transitions, which take place when a system is tuned through an avoided crossing of the coupled energy levels, interfere in a multimode system. The work progresses experimental LZS interference to cover a new class of systems where the coupled levels are those of a quantum two-level system interacting with a multitude of mechanical oscillators. The work opens up applications in controlling multiple acoustic modes via parametric modulation.

Sauter-Schwinger effect with a quantum gas

The creation of particle-antiparticle pairs from vacuum by a large electric field is at the core of quantum electrodynamics. Despite the wide acceptance that this phenomenon occurs naturally when electric field strengths exceed E c ≈ 1018 Vm-1, it has yet to be experimentally observed due to the limitations imposed by producing electric fields at this scale. The high degree of experimental control present in ultracold atomic systems allow experimentalists to create laboratory analogs to high-field phenomena. Here we emulated massive relativistic particles subject to large electric field strengths, thereby quantum-simulated particle-antiparticle pair creation, and experimentally explored particle creation from 'the Dirac vacuum'. Data collected from our analog system spans the full parameter regime from low applied field (negligible pair creation) below the Sauter-Schwinger limit, to high field (maximum rate of pair creation) far in excess of the Sauter-Schwinger limit. In our experiment, we perform direct measurements on an analog atomic system and show that this high-field phenomenon is well-characterized by Landau-Zener tunneling, well known in the atomic physics context, and we find full quantitative agreement with theory with no adjustable parameters.

Precision mapping the topological bands of 2D spin-orbit coupling with microwave spin-injection spectroscopy

To investigate the band structure is one of the key approaches to study the fundamental properties of a novel material. We report here the precision band mapping of a 2-dimensional (2D) spin-orbit (SO) coupling in an optical lattice. By applying the microwave spin-injection spectroscopy, the band structure and spin-polarization distribution are achieved simultaneously. The band topology is also addressed with observing the band gap close and re-open at the Dirac points. Furthermore, the lattice depth and the Raman coupling strength are precisely calibrated with relative errors in the order of 10^-3. Our approach could also be applied for exploring the exotic topological phases with even higher dimensional system.

Observation of Valley Landau-Zener-Bloch Oscillations and Pseudospin Imbalance in Photonic Graphene

We demonstrate intervalley Bloch oscillation (BO) and Landau-Zener tunneling (LZT) in an optically induced honeycomb lattice with a refractive-index gradient. Unlike previously observed BO in a gapped square lattice, we show nonadiabatic beam dynamics that are highly sensitive to the direction of the index gradient and the choice of the Dirac cones. In particular, a symmetry-preserving potential leads to nearly perfect LZT and coherent BO between the inequivalent valleys, whereas a symmetry-breaking potential generates asymmetric scattering, imperfect LZT, and valley-sensitive generation of vortices mediated by a pseudospin imbalance. This clearly indicates that, near the Dirac points, the transverse gradient does not always act as a simple scalar force, as commonly assumed, and the LZT probability is strongly affected by the sublattice symmetry as analyzed from an effective Landau-Zener Hamiltonian. Our results illustrate the anisotropic response of an otherwise isotropic Dirac platform to real-space potentials acting as strong driving fields, which may be useful for manipulation of pseudospin and valley degrees of freedom in graphenelike systems.

Observation and Uses of Position-Space Bloch Oscillations in an Ultracold Gas

We report the observation and characterization of position-space Bloch oscillations using cold atoms in a tilted optical lattice. While momentum-space Bloch oscillations are a common feature of optical lattice experiments, the real-space center-of-mass dynamics are typically unresolvable. In a regime of rapid tunneling and low force, we observe real-space Bloch oscillation amplitudes of hundreds of lattice sites, in both ground and excited bands. We demonstrate two unique capabilities enabled by tracking of Bloch dynamics in position space: measurement of the full position-momentum phase-space evolution during a Bloch cycle, and direct imaging of the lattice band structure. These techniques, along with the ability to exert long-distance coherent control of quantum gases without modulation, may open up new possibilities for quantum control and metrology.

Interaction-Driven Shift and Distortion of a Flat Band in an Optical Lieb Lattice

We report the momentum-resolved measurement of Bloch bands in an optical Lieb lattice for a Bose-Einstein condensate (BEC). A BEC in the lattice is transported to a desired quasimomentum by applying a constant force. The energy dispersion of the lowest band is obtained by integrating measured group velocities. We also measure the gap from the lowest band to the higher bands with the same quasimomentum, which can be extracted from the oscillation of the sublattice populations after preparing a superposition of the band eigenstates. We show that the experimental results agree with a band calculation based on the Bogoliubov approximation. It is revealed that the second band, which should be flat in a single-particle description, is shifted and, in particular, distorted around the Brillouin zone edge as the interaction strength increases.

Landau Levels in Strained Optical Lattices

We propose a hexagonal optical lattice system with spatial variations in the hopping matrix elements. Just like in the valley Hall effect in strained graphene, for atoms near the Dirac points the variations in the hopping matrix elements can be described by a pseudomagnetic field and result in the formation of Landau levels. We show that the pseudomagnetic field leads to measurable experimental signatures in momentum resolved Bragg spectroscopy, Bloch oscillations, cyclotron motion, and quantization of in situ densities. Our proposal can be realized by a slight modification of existing experiments. In contrast to previous methods, pseudomagnetic fields are realized in a completely static system avoiding common heating effects and therefore opening the door to studying interaction effects in Landau levels with cold atoms.

Dynamical Detection of Topological Phase Transitions in Short-Lived Atomic Systems

We demonstrate that dynamical probes provide direct means of detecting the topological phase transition (TPT) between conventional and topological phases, which would otherwise be difficult to access because of loss or heating processes. We propose to avoid such heating by rapidly quenching in and out of the short-lived topological phase across the transition that supports gapless excitations. Following the quench, the distribution of excitations in the final conventional phase carries signatures of the TPT. We apply this strategy to study the TPT into a Majorana-carrying topological phase predicted in one-dimensional spin-orbit-coupled Fermi gases with attractive interactions. The resulting spin-resolved momentum distribution, computed by self-consistently solving the time-dependent Bogoliubov-de Gennes equations, exhibits Kibble-Zurek scaling and Stückelberg oscillations characteristic of the TPT. We discuss parameter regimes where the TPT is experimentally accessible.

Mass and chirality inversion of a Dirac cone pair in Stückelberg interferometry

We show that a Stückelberg interferometer made of two massive Dirac cones can reveal information on band eigenstates such as the chirality and mass sign of the cones. For a given spectrum with two gapped cones, we propose several low-energy Hamiltonians differing by their eigenstates properties. The corresponding interband transition probability is affected by such differences in its interference fringes being shifted by a new phase of geometrical origin. This phase can be a useful bulk probe for topological band structures realized with artificial crystals.

Observation of time-domain Rabi oscillations in the Landau-Zener regime with a single electronic spin

It is theoretically known that the quantum interference of a long sequence of Landau-Zener transitions can result in Rabi oscillations. Because of its stringent requirements, however, this phenomenon has never been experimentally observed in the time domain. Using a nitrogen-vacancy (NV) center spin in isotopically purified diamond, we observed the Rabi oscillations resulting from more than 100 Landau-Zener processes. Our results demonstrate favorable quantum controllability of NV centers, which could find applications in quantum metrology and quantum information processing.

Charge qubit coupled to an intense microwave electromagnetic field in a superconducting Nb device: evidence for photon-assisted quasiparticle tunneling

We study a superconducting charge qubit coupled to an intensive electromagnetic field and probe changes in the resonance frequency of the formed dressed states. At large driving strengths, exceeding the qubit energy-level splitting, this reveals the well known Landau-Zener-Stückelberg interference structure of a longitudinally driven two-level system. For even stronger drives, we observe a significant change in the Landau-Zener-Stückelberg pattern and contrast. We attribute this to photon-assisted quasiparticle tunneling in the qubit. This results in the recovery of the qubit parity, eliminating effects of quasiparticle poisoning, and leads to an enhanced interferometric response. The interference pattern becomes robust to quasiparticle poisoning and has a good potential for accurate charge sensing.

Engineering novel optical lattices

Optical lattices have developed into a widely used and highly recognized tool to study many-body quantum physics with special relevance for solid state type systems. One of the most prominent reasons for this success is the high degree of tunability in the experimental setups. While at the beginning quasi-static, cubic geometries were mainly explored, the focus of the field has now shifted toward new lattice topologies and the dynamical control of lattice structures. In this review we intend to give an overview of the progress recently achieved in this field on the experimental side. In addition, we discuss theoretical proposals exploiting specifically these novel lattice geometries.

Tailoring population inversion in Landau-Zener-Stückelberg interferometry of flux qubits

We distinguish different mechanisms for population inversion in flux qubits driven by dc+ac magnetic fields. We show that for driving amplitudes such that there are Landau-Zener-Stückelberg interferences, it is possible to have population inversion solely mediated by the environmental bath. Furthermore, we find that the degree of population inversion can be controlled by tailoring a resonant frequency Ω(p) in the environmental bath. To observe these effects experiments should be performed for long driving times after full relaxation.

Bloch-Zener oscillations across a merging transition of Dirac points

Bloch oscillations are a powerful tool to investigate spectra with Dirac points. By varying band parameters, Dirac points can be manipulated and merged at a topological transition toward a gapped phase. Under a constant force, a Fermi sea initially in the lower band performs Bloch oscillations and may Zener tunnel to the upper band mostly at the location of the Dirac points. The tunneling probability is computed from the low-energy universal Hamiltonian describing the vicinity of the merging. The agreement with a recent experiment on cold atoms in an optical lattice is very good.

Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice

Dirac points are central to many phenomena in condensed-matter physics, from massless electrons in graphene to the emergence of conducting edge states in topological insulators. At a Dirac point, two energy bands intersect linearly and the electrons behave as relativistic Dirac fermions. In solids, the rigid structure of the material determines the mass and velocity of the electrons, as well as their interactions. A different, highly flexible means of studying condensed-matter phenomena is to create model systems using ultracold atoms trapped in the periodic potential of interfering laser beams. Here we report the creation of Dirac points with adjustable properties in a tunable honeycomb optical lattice. Using momentum-resolved interband transitions, we observe a minimum bandgap inside the Brillouin zone at the positions of the two Dirac points. We exploit the unique tunability of our lattice potential to adjust the effective mass of the Dirac fermions by breaking inversion symmetry. Moreover, changing the lattice anisotropy allows us to change the positions of the Dirac points inside the Brillouin zone. When the anisotropy exceeds a critical limit, the two Dirac points merge and annihilate each other-a situation that has recently attracted considerable theoretical interest but that is extremely challenging to observe in solids. We map out this topological transition in lattice parameter space and find excellent agreement with ab initio calculations. Our results not only pave the way to model materials in which the topology of the band structure is crucial, but also provide an avenue to exploring many-body phases resulting from the interplay of complex lattice geometries with interactions.