We have experimentally studied the magnetic-field dependence of the decay of a Bose-Einstein condensate of metastable ${}^4$He atoms confined in an optical dipole trap, for atoms in the $m=+1$ and $m=-1$ magnetic substates, and up to 450 G. Our measurements confirm longstanding calculations of the two-body loss rate coefficient that show an increase above 50 G. We demonstrate that for $m=-1$ atoms decay is due to three-body recombination only, with a three-body loss rate coefficient of $6.5{\left(0.4\right)}_{\mathrm{stat}}{\left(0.6\right)}_{\mathrm{sys}}\times {10}^{-27}$ cm${}^6$s${}^{-1}$, which is interesting in the context of universal few-body theory. We have also searched for a recently predicted $d$-wave Feshbach resonance, but did not observe it.

We have measured the electron-transfer process in fast collisions (630–1200 keV/u) of protons with helium, which is dependent on the projectile scattering angle and the final electronic state. The fully differential data accompanied by theoretical second-order perturbation theory allow a detailed insight into the mechanism of electron-transfer processes.

We propose a dynamic pump-control scheme to manipulate the molecular predissociation process of NaI using two color few-cycle laser pulses. We excite the NaI molecule with a pump pulse (648 nm, 35 fs) and employ another control laser pulse (7.53 μm, 50 fs) to modify the molecular potential energy surfaces (MPESs) of NaI in the coupling zone. The intersection of the two diabatic MPESs will move in opposite directions due to the nonresonant dynamical Stark effect (NRDSE) introduced by the control pulse. The final dissociation channels, i.e., ionic channel and covalent channel, can be well controlled by tuning the carrier-envelope phase (CEP) and the time delay of the control pulse. We find that the percentage of the covalent channel can reach ∼91.3$\%$ when the CEP of the control pulse is taken to be ∼0.45π at a delay time slightly before the adiabatic coupling zone. On the contrary, a minimum percentage of 18.3$\%$ can be obtained at CEP ∼1.79π at a delay time slightly after the adiabatic coupling zone.

The dynamics of a vortex dipole in a quasi-two-dimensional two-component Bose-Einstein condensate is investigated. A vortex dipole is shown to penetrate the interface between the two components when the incident velocity is sufficiently large. A vortex dipole can also disappear or disintegrate at the interface, depending on its velocity and the interaction parameters.

In this paper, we investigate systematically the Mott-insulator–superfluid quantum phase transitions for ultracold scalar bosons in triangular, hexagonal, and Kagomé optical lattices. With the help of the field-theoretical effective potential, by treating the hopping term in the Bose-Hubbard model as perturbation, we calculate the phase boundaries analytically for different integer filling factors. Our analytical results are in good agreement with recent numerical results.

In this paper, we explore the nature of three-dimensional Bose gases at large positive scattering lengths via resummation of dominating processes involving a minimum number of virtual atoms. We focus on the energetics of the nearly fermionized Bose gases beyond the usual dilute limit. We also find that an onset instability sets in at a critical scattering length, beyond which the near-resonance Bose gases become strongly coupled to molecules and lose the metastability. Near the point of instability, the chemical potential reaches a maximum, and the effect of the three-body forces can be estimated to be around a few percent.

We study nonequilibrium dynamics of ultracold two-component Fermi gases in low-dimensional geometries after the interactions are quenched from a weakly interacting to a strongly interacting regime. We develop a $T$-matrix formalism that takes into account the interplay between Pauli blocking and tight confinement in low-dimensional geometries. We employ our formalism to study the formation of molecules in quasi-two-dimensional Fermi gases near Feshbach resonance and show that the rate at which molecules form depends strongly on the transverse confinement. Furthermore, Pauli blocking gives rise to a sizable correction to the binding energy of molecules.

A nonlinear interferometer uses nonlinear elements as beam splitters to split and to recombine optical waves for interference. As a result, the interference fringe size has a nonlinear dependence on the intensity of the field for phase sensing and leads to an enhanced phase signal. In this paper, a practical scheme of nonlinear interferometry for precision phase measurement is analyzed with parametric amplifiers as the nonlinear beam splitters. It is found that the signal due to phase shift is enhanced by a factor of the amplification gain as compared to a linear interferometer with the same phase-sensing light intensity while the quantum noise is kept at the vacuum level, thus, effectively increasing the signal-to-noise ratio (SNR) beyond the standard quantum limit. Furthermore, the scheme is not as sensitive to the detection loss as the linear scheme with a squeezed state for noise reduction. However, losses inside the interferometer limit the enhancement factor in SNR. We apply the concept to a Michelson interferometer but with parametric amplifiers involved for gravitational-wave detection. We find that effective power is increased by the gain of the amplifiers without actually increasing the cycling power inside the interferometer. Furthermore, the full benefits with squeezed input and variational output or the combination of a quantum nondemolition interferometer for sensitivity beyond the standard quantum limit apply here with even better results. Such a nonlinear interferometer will find wide applications in precision measurements.

The measurement of photon-number statistics of fields composed of photon pairs, generated in spontaneous parametric down-conversion and detected by an intensified charge-coupled device (iCCD) camera, is described. Final quantum detection efficiencies, electronic noises, finite numbers of detector pixels, transverse intensity spatial profiles of the detected beams, as well as losses of single photons from a pair are taken into account in a developed general theory of photon-number detection. The measured data provided by an iCCD camera with single-photon detection sensitivity are analyzed along the developed theory. Joint signal-idler photon-number distributions are recovered using the reconstruction method based on the principle of maximum likelihood. The range of applicability of the method is discussed. The reconstructed joint signal-idler photon-number distribution is compared with that obtained by a method that uses superposition of signal and noise and minimizes photoelectron entropy. Statistics of the reconstructed fields are identified to be multimode Gaussian. Elements of the measured as well as the reconstructed joint signal-idler photon-number distributions violate classical inequalities. Sub-shot-noise correlations in the difference of the signal and idler photon numbers as well as partial suppression of odd elements in the distribution of the sum of signal and idler photon numbers are observed.

We have studied the decay of a Bose-Einstein condensate (BEC) of metastable helium atoms in an optical dipole trap. In the regime where two- and three-body losses can be neglected we show that the Bose-Einstein condensate and the thermal cloud show fundamentally different decay characteristics. The total number of atoms decays exponentially with time constant $\tau$; however, the thermal cloud decays exponentially with time constant $\frac{4}{3}\tau$ and the condensate decays much faster, and nonexponentially. We show that this behavior, which should be present for all BECs in thermal equilibrium with a considerable thermal fraction, is due to a transfer of atoms from the condensate to the thermal cloud during its decay.

We examine theoretically the visualization of Majorana fermions in a two-dimensional trapped ultracold atomic Fermi gas with spin-orbit coupling. By increasing an external Zeeman field, the trapped gas transits from a nontopological to a topological superfluid, via a mixed phase in which both types of superfluids coexist. We show that the zero-energy Majorana fermion, supported by the topological superfluid and localized at the vortex core, may be visible through (i) the core density and (ii) the local density of states, which are readily measurable in experiment. We present a realistic estimate on experimental parameters for ultracold ${}^{40}$K atoms.

We consider dipolar fermions in a two-dimensional square lattice and a harmonic trapping potential. The anisotropy of the dipolar interaction combined with the lattice leads to transitions between phases with density order of different symmetries. We show that the attractive part of the dipolar interaction results in a superfluid phase that is suppressed by density order. The trapping potential is demonstrated to make the different phases coexist, forming ring and island structures. The phases with density and superfluid order can overlap, forming regions with supersolid order.

We generate narrow-band frequency-tunable entangled photon pairs from spontaneous four-wave mixing in three-level cold atoms and study their two-photon quantum interference after a beam splitter. We find that the path-exchange symmetry plays a more important role in the Hong-Ou-Mandel interference than the temporal or frequency indistinguishability, and observe coalescence interference for both degenerate and nondegenerate photons. We also observe a quantum beat in the same experimental setup using either slow or fast detectors.

A classical Hamiltonian system of a point moving on a sphere of fixed radius is shown to emerge from the constrained evolution of quantum spin. The constrained quantum evolution corresponds to an appropriate coarse graining of the quantum states into equivalence classes, and forces the equivalence classes to evolve as single units representing the classical states. The coarse-grained quantum spin with the constrained evolution in the limit of the large spin becomes indistinguishable from the classical system.

We numerically study the effects of two forms of quenched disorder on the anyons of the toric code. First, a class of codes based on random lattices of stabilizer operators is presented and shown to be superior to the standard square-lattice toric code for certain forms of biased noise. It is further argued that these codes are close to optimal, in that they tightly reach the upper bound of error thresholds beyond which no correctable Calderbank-Shore-Steane codes can exist. Additionally, we study the classical motion of anyons in toric codes with randomly distributed on-site potentials. In the presence of repulsive long-range interaction between the anyons, a surprising increase in the lifetime of encoded states with disorder strength is reported and explained by an entirely incoherent mechanism. Finally, the coherent transport of the anyons in the presence of both forms of disorder is investigated and a significant suppression of the anyon motion is found.

A radical-pair-based chemical reaction might be used by birds for navigation via the geomagnetic direction. The inherent physical mechanism is that the quantum coherent transition from a singlet state to triplet states of the radical pair could respond to a weak magnetic field and be sensitive to the direction of such a field; this then results in different photopigments to be sensed by the avian eyes. Here, we propose a quantum bionic setup, inspired by the avian compass, as an ultrasensitive probe of a weak magnetic field based on the quantum phase transition of the environments of the two electrons in the radical pair. We prove that the yield of the chemical products via recombination from the singlet state is determined by the Loschmidt echo of the environments with interacting nuclear spins. Thus quantum criticality of environments could enhance the sensitivity of detection of weak magnetic fields.

We outline a proof that teleportation with a single particle is, in principle, just as reliable as with two particles. We thereby hope to dispel the skepticism surrounding single-photon entanglement as a valid resource in quantum information. A deterministic Bell-state analyzer is proposed which uses only classical resources, namely, coherent states, a Kerr nonlinearity, and a two-level atom.

Color codes are a class of topological quantum codes with a high error threshold and a large set of transversal encoded gates and are thus suitable for fault-tolerant quantum computation in two-dimensional architectures. Recently, computationally efficient decoders for the color codes were proposed. We describe an alternate efficient iterative decoder for topological color codes and apply it to the color code on the hexagonal lattice embedded on a torus. In numerical simulations, we find an error threshold of 7.8$\%$ for independent dephasing and spin-flip errors.

A scheme based on electromagnetically induced transparency is used to store light in a Bose-Einstein condensate. In this process, a photonic polarization qubit is stored in atomic Zeeman states. The performance of the storage process is characterized and optimized. The average process fidelity is $1.000\pm 0.004$. For long storage times, temporal fluctuations of the magnetic field reduce this value, yielding a lifetime of the fidelity of $(1.1\pm 0.2)$ ms. The write-read efficiency of the pulse energy can reach $0.53\pm 0.05$.

We theoretically investigate the ionization suppression of heteronuclear diatomic molecules and choose NO and NH as examples. These molecules have antibonding molecular orbitals, and hence we expect that the dynamics can be different from those, such as CO, with bonding molecular orbitals we have studied before [Ren and Nakajima, Phys. Rev. A 82, 063410 (2010)]. Calculations are performed within the framework of strong-field approximation. We find that NO generally exhibits a double-core feature at any laser intensity and hence the ionization suppression is significant. However, it is not as significant as that of O${}_2$ because the interference between the two electron wave packets arising from the N and O cores upon photoelectron ejection cannot be as strong as that of O${}_2$ due to the fact that NO is a heteronuclear diatomic molecule. In contrast, NH exhibits a single-core feature at any laser intensity, since the dominant contribution to ionization comes from the low-energy photoelectron ejections which mainly take place from the neighborhood of the N core. These behaviors of NO (always double-core feature) and NH (always single-core feature) are quite different from that of CO whose ionization behavior gradually changes from the single- to double-core features as the laser intensity increases.