Multiple Dark States Research Articles

Quantum Criticality in Open Quantum Spin Chains with Nonreciprocity.

We investigate the impact of nonreciprocity on universality and critical phenomena in open quantum interacting many-body systems. Nonreciprocal open quantum systems often have an exotic spectral sensitivity to boundary conditions, known as the Liouvillian skin effect (LSE). By considering an open quantum XXZ spin chain that exhibits LSE, we demonstrate the existence of a universal scaling regime that is not affected by the presence of the LSE. We resolve the critical exponents, which differ from those of free fermions, via tensor network methods and demonstrate that observables exhibit a universal scaling collapse, irrespective of the reciprocity. We find that the LSE only becomes relevant when a healing length scale ξ_{heal} at the system's edge (which is different from the localization length of the eigenstate of the Liouvillian) exceeds the system size, allowing edge properties to dominate the physics. We expect this result to be a generic feature of nonreciprocal models in the vicinity of a critical point. The driven-dissipative quantum criticality we observe has no classical analog and stems from the existence of multiple dark states.

Photon control and coherent interactions via lattice dark states in atomic arrays

Ordered atomic arrays with subwavelength spacing have emerged as an efficient and versatile light-matter interface, where emitters respond collectively and form subradiant lattice modes with supressed decay rate. Here, we demonstrate that such lattice dark states can be individually addressed and manipulated by applying a spatial modulation of the atomic detuning. More specifically, we show that lattice dark states can be used to store and retrieve single photons with near-unit efficiency, as well as to control the temporal, frequency and spatial degrees of freedom of the emitted electromagnetic field. Additionally, we discuss how to engineer arbitrary coherent interactions between multiple dark states. These results pave the way towards building a quantum platform that can equally act as a quantum memory and a photon shaper capable of producing states of light relevant in quantum information protocols.

Blinking statistics and molecular counting in direct stochastic reconstruction microscopy (dSTORM).

Many recent advancements in single-molecule localization microscopy exploit the stochastic photoswitching of fluorophores to reveal complex cellular structures beyond the classical diffraction limit. However, this same stochasticity makes counting the number of molecules to high precision extremely challenging, preventing key insight into the cellular structures and processes under observation. Modelling the photoswitching behaviour of a fluorophore as an unobserved continuous time Markov process transitioning between a single fluorescent and multiple dark states, and fully mitigating for missed blinks and false positives, we present a method for computing the exact probability distribution for the number of observed localizations from a single photoswitching fluorophore. This is then extended to provide the probability distribution for the number of localizations in a direct stochastic optical reconstruction microscopy experiment involving an arbitrary number of molecules. We demonstrate that when training data are available to estimate photoswitching rates, the unknown number of molecules can be accurately recovered from the posterior mode of the number of molecules given the number of localizations. Finally, we demonstrate the method on experimental data by quantifying the number of adapter protein linker for activation of T cells on the cell surface of the T-cell immunological synapse. Software and data available at https://github.com/lp1611/mol_count_dstorm. Supplementary data are available at Bioinformatics online.

Power quantum control of odd-order multiwave mixing in an electromagnetically induced transparency window

We investigate the power control behavior of odd-order multiwave mixing in an electromagnetically induced transparency window. We successfully obtain the evolution from pure enhancement (bright state), half-enhancement, and half-suppression, to pure suppression (dark state) in a four-wave mixing (FWM) channel. The correlations of two bright and two dark states of FWM are studied under four different kinds of power configurations. Moreover, the power-controlled quantum interference among multiple dark states is also studied in FWM and six-wave mixing channels. Such selective switching among multiple frequency channels could have potential applications in optical switching, optical communication, and quantum information processing.

Spinor Slow-Light and Dirac Particles with Variable Mass

We consider the interaction of two weak probe fields of light with an atomic ensemble coherently driven by two pairs of standing wave laser fields in a tripod-type linkage scheme. The system is shown to exhibit a Dirac-like spectrum for light-matter quasiparticles with multiple dark states, termed spinor slow-light polaritons. They posses an "effective speed of light" given by the group velocity of slow light, and can be made massive by inducing a small two-photon detuning. Control of the two-photon detuning can be used to locally vary the mass including a sign flip. Particularly, this allows the implementation of the random-mass Dirac model for which localized zero-energy (midgap) states exist with unusual long-range correlations.

Control of multitransparency windows via dark-state phase manipulation

We identify a process in which we can control the behavior of an atomic medium to switch between transparent and enhanced-absorption at multiple frequencies. The process relies on the quantum interference between multiple dark states, which can be constructive or destructive at different frequencies and can be altered dramatically by changing the phase difference between the two circularly polarized components of a single coherent field. This phase-controlled switching behavior is obtainable without altering the powers or frequency detunings of the beams. We demonstrate the process experimentally by using two coherent fields and atomic rubidium vapor. Such selective switching between multiple frequency channels could also have applications in optical communication and quantum information processing.

Adiabatic creation of coherent superposition states in atomic beams

We describe a technique for creating superpositions of degenerate quantum states, such as are needed for beam splitters used in matter-wave optics, by manipulating the timing of three orthogonally polarized laser beams through which moving atoms (or molecules) pass; motion across the laser beams produces pulses in the atomic rest frame. As illustrated with representative simulations for transitions in metastable neon, a single pass through three overlapping laser beams can produce superpositions (with preselected phase) of atomic beams differing by transverse momentum corresponding to the momentum of four photons. Like the two-photon momentum transfer of the tripod linkage pattern which it extends, the method relies on controlled adiabatic time evolution in the Hilbert subspace of two degenerate dark states. It is thus a generalization to multiple dark states (and larger transfers of linear momentum to the atomic beam) of the single dark state occurring with the stimulated Raman adiabatic passage (STIRAP) technique, and therefore it is potentially insensitive to decoherence due to spontaneous emission. By extending the tripod-linkage system to more numerous degenerate states, the technique not only increases the atomic beam deflections but, as we demonstrate, allows control over the superposition phase and amplitudes. LIke other techniques based on adiabatic time evolution, the technique is robust with respect to variations of the intensity, timing, and other characteristics of the laser fields. Unlike STIRAP, the same robust partial population transfer occurs for opposite timings of the pulse sequence, as is needed for such procedures as Hadamard gates.

Coherent effects in a driven Vee scheme

We examine coherent effects in a four-level Vee scheme. We show that the systemcan be made to display a number of different phenomena, including the inhibitionof two-photon absorption and the production of multiple dark states. We showthat this latter effect is due to the independent control of the Autler–Townessplitting of the upper and lower probe field by independent driving fields. Wesuggest possible experimental systems in rubidium for the observation of sucheffects.