Observation of strong correlations and superconductivity in twisted-bilayer graphene has stimulated tremendous interest in fundamental and applied physics. In this system, the superposition of two twisted honeycomb lattices, generating a moiré pattern, is the key to the observed fat electronic bands, slow electron velocity and large density of states. Extension of the twisted-bilayer system to new confgurations is highly desired, which can provide exciting prospects to investigate twistronics beyond bilayer graphene. Here we demonstrate a quantum simulation of superfuid to Mott insulator transition in twisted-bilayer square lattices based on atomic Bose–Einstein condensates loaded into spin-dependent optical lattices. The lattices are made of two sets of laser beams that independently address atoms in diferent spin states, which form the synthetic dimension accommodating the two layers. The interlayer coupling is highly controllable by a microwave feld, which enables the occurrence of a lowest fat band and new correlated phases in the strong coupling limit. We directly observe the spatial moiré pattern and the momentum difraction, which confrm the presence of two forms of superfuid and a modifed superfuid to insulator transition in twisted-bilayer lattices. Our scheme is generic and can be applied to diferent lattice geometries and for both boson and fermion systems. This opens up a new direction for exploring moiré physics in ultracold atoms with highly controllable optical lattices.

The degeneracy of two or more energy bands at a singular point in the band structure, such as a Dirac point, gives rise to intriguing quantum phenomena as well as unusual material properties. Systems at the Dirac points can possess topological charges and their unique properties can be probed by various methods, such as transport measurement, interferometry and momentum spectroscopy. While the topology of Dirac point in the momentum space is well studied theoretically, observation of topological defects in a many-body quantum system at Dirac point remains an elusive goal. Based on atomic Bose–Einstein condensate in a graphene-like optical honeycomb lattice, we directly observe emergence of quantized vortices induced by the non-commutativity between the harmonic trap and the pseudo-spin–orbit coupling at the Dirac point. By adiabatic control of the honeycomb lattice with an additional harmonic trapping potential, the phase diagram of lattice bosons at the Dirac point is revealed. Our work provides a new way of generating vortices in a quantum gas, and the method is generic and can be applied to different types of optical lattices with topological singularities, including topological flat bands near Dirac points in twisted bilayer optical lattices.

This paper presents an experimental study of microwave single-photon transitions that are magnetic-field insensitive in degenerate Fermi gases of 40K. This contrasts with microwave single-photon clock transitions for 0–0 magnetic-field-insensitive states and two-photon clock transitions for non-0–0 magnetic-field-insensitive states in bosonic alkali-metal atoms. We show that there are two sets of special transitions between two dif ferent hyperfine ground states (|F = 9/2, mF = 1/2 ⇔|7/2, −1/2 and |9/2, −1/2 ⇔|7/2, 1/2), whose microwave single-photon transition frequency is insensitive to low magnetic fields, as the first-order Zeeman shift is almost completely canceled. By using the microwave spectrum and Ramsey interference fringes, we demonstrate the long-time stability of the coherent transition under magnetic field fluctuations. These magnetic f ield-insensitive microwave hyperfine transitions in ultracold 40K Fermi gases offer promising applications in quantum information and precision measurements

Dual lattices such as honeycomb and hexagonal lattices typically obey Babinet’s principle in optics, which states that the expected interference patterns of two complementary diffracting objects are identical and in distinguishable, except for their overall intensity. Here, we study Kapitza-Dirac diffraction of Bose-Einstein condensates in optical lattices and find that matter waves in dual lattices obey Babinet’s principle only under the condition of weak-pulse Raman-Nath regime. In contrast, the Kapitza-Dirac matter-wave diffraction in the strong-pulse Raman-Nath regime, which generates subwavelength phase structures via phase wrapping, can break Babinet’s principle and clearly resolve the distinct interference patterns of the dual honeycomb and hexagonal lattices. This method offers exceptional precision in characterizing lattice configurations and advance the study of symmetry-related phenomena, overcoming the limitations of real-space imaging.

We experimentally investigate four-wave mixing (FWM) of matter waves in two geometric configurations in 39K Bose-Einstein condensates with the atomic interaction tuned via Feshbach resonances. For one configuration with the single-spin component, the FWM yield increases with a larger scattering length. For the two-spin component configuration, we specifically investigate FWM in both the droplet and gas parameter regimes. We f ind that the FWM yield reaches its maximum near the critical parameter region between the gas and droplet phases. Our research can help to optimize the FWM yield for matter-wave amplification and entangled atom pair generation, making it conducive to applications in quantum information processing and precision measurement.

Realizing a large-scale fully controllable quantum system is a challenging task in current physical research and has broad applications. In this work, we create a reconfigurable optically levitated nanoparticle array in vacuum. Our optically levitated nanoparticle array allows full control of individual nanoparticles to form an arbitrary pattern and detect their motion. As a concrete example, we choose two nanoparticles without rotation signals from an array to synthesize a nanodumbbell in situ by merging them into one trap. The nanodumbbell synthesized in situ can rotate beyond 1 GHz. Our work provides a platform for studying macroscopic many-body physics and quantum sensing.

Spin-orbit coupling (SOC) in ultracold atoms is engineered by light-atom interaction, such as two-photon Raman transitions between two Zeeman spin states. In this paper, we propose and experimentally realize chiral Raman coupling to generate SOC in ultracold atomic gases, which exhibits high quantization axis direction dependence. Chiral Raman coupling for SOC is created by chiral light-atom interaction, in which a circularly polarized electromagnetic field generated by two Raman lasers interacts with two Zeeman spin states δmF = ±1 (chiral transition). We present a simple scheme of chiral one-dimensional (1D) Raman coupling by employing two Raman lasers at an intersecting angle 90◦ with the proper polarization configuration. In this case, Raman coupling for SOC exists in one direction of the magnetic quantization axis and disappears in the opposite direction. Then we extend this scheme into a chiral two-dimensional (2D) optical square Raman lattice configuration to generate the 1D SOC. There are two orthogonal 1D SOCs, which exist in the positive and negative directions of the magnetic quantization axis respectively. This case is compared with 2D SOC based on the nonchiral 2D optical Raman lattice scheme for studying the topological energy band. This paper broadens the horizon for understanding chiral physics and simulating topological quantum systems.

We report the new observation of several high partial-wave (HPW) magnetic Feshbach resonances (FRs) in 39K atoms of the hyperfine substate |F = 1,mF = -1⟩. These resonances locate at the region between two broad s-wave FRs from 32.6 G to 162.8 G, in which Bose–Einstein condensates can be produced with tunable positive scattering length obtained by magnetic FRs. These HPW FRs are induced by the dipolar spin–spin interaction with s-wave in the open channel and HPW in the closed channel. Therefore, these HPW FRs have distinct characteristics in temperature dependence and loss line shape from that induced by spin–exchange interaction with HPWs in both open and closed channels. Among these resonances, one d-wave and two g-wave FRs are confirmed by the multichannel quantum-defect theory calculation. The HPW FRs have significant applications in many-body physics dominated by HPW pairing.

In recent years, flat electronic bands in twisted bilayer graphene (TBG) have attracted significant attention due to their intriguing topological properties, extremely slow electron velocities, and enhanced density of states. Extending twisted bilayer systems to new configurations is highly desirable, as it offers promising opportunities to explore flat bands beyond TBG. Here, we study both topological and trivial flat bands in a twisted bilayer honeycomb lattice for ultracold atoms and present the evolution of the flat bands with different interlayer coupling strength (ICS). Our results demonstrate that an isolated topological flat band can emerge at the Dirac-point energy for a specific value of weak ICS, referred to as the “critical coupling.” This occurs over a wide range of twist angles, surpassing the limits of the magic angle in TBG systems. When the ICS is slightly increased beyond the critical coupling value, the topological flat band exhibits degenerate band crossings with both the upper and lower adjacent bands at the high-symmetry s point. As the ICS is further increased into the strong coupling regime, trivial flat bands arise around Dirac-point energy. Meanwhile, more trivial flat bands appear, extending from the lowest to higher energy bands, and remain flat as the ICS increases. The topological properties of the flat bands are studied through the winding pattern of the Wilson loop spectrum. Our research provides deeper insights into the formation of flat bands in ultracold atoms with highly controllable twisted bilayer optical lattices, and may contribute to the discovery of new strongly correlated states of matter.

The collective spontaneous emission of many atoms is significantly different from that of a single atom, depending on the geometry and the phase correlation of atomic ensembles. However, experimental observation of arbitrary superradiant and subradiant states of atoms remains challenging due to the difficulties in both preparation and detection of those states. Here we report the time-resolved observation of superradiance from a timed Dicke state in a momentum-space superradiance lattice of Bose-Einstein condensates, which enables an in situ measurement of the coherent lattice dynamics involving both superradiant and subradiant states. The long-lasting oscillation in the superradiant emission is contributed by population transport from the subradiant states in the superradiance lattice. This work paves the way to prepare and observe subradiant states, which has promising applications in quantum information processing...

We report the measurement of the Rydberg excitation spectrum by two-photon process in ultracold ⁴⁰K Fermi gases. Two different methods are employed to measure the Rydberg excitation spectrum, depending on the power of the probe laser. One scheme is to reduce atomic losses by means of electromagnetically induced transparency. The other is to enhance the atomic losses by spontaneous avalanche ionization due to the strong Rydberg-Rydberg interactions. We verify the consistency of both of the methods. The highest Rydberg states detectable in our experiment are limited to n ≤ 62 due to the competition between the long Rydberg blockade effective range and the limited atomic cloud size...

Hall tube with a tunable flux is an important geometry for studying quantum Hall physics, but its experimental realization in real space is still challenging. Here, we propose to realize a synthetic Hall tube with tunable flux in a one-dimensional optical lattice with the synthetic ring dimension defined by atomic hyperfine states. We investigate the effects of the flux on the system topology and study its quench dynamics. Utilizing the tunable flux, we show how to realize topological charge pumping. Finally, we show that the recently observed....