Session F11: V: General Topics

Single photon routing in position-disordered waveguide quantum electrodynamics ladders

We study the problem of deterministic routing of single photons in waveguide quantum electrodynamics ladders. In the past, we [1] and others have examined the impact of dipole-dipole interactions on the routing scheme when multiple quantum emitters or atoms are coupled with chiral waveguides in the ladder settings. In this work, we analyze how routing efficiency is impacted if a disorder exists in the location of quantum emitters, either due to imperfect atomic trapping or temperature fluctuations in the surroundings of the setup. Average transmission, routing efficiency, and localization lengths have been calculated to explore the interplay between the collective atomic effects and the disorder-induced photon localization, which affects photon routing. Quantum communication and quantum networking are two areas where this work can be applied.

[1] "Collective photon routing improvement in a dissipative quantum emitter chain strongly coupled to a chiral waveguide QED ladder," Bibandhan Poudyal, Imran M Mirza; Physical Review Research, 2 (4), 043048 (2020).   

Interference of two coherent lasers on glass surface assisted by an isotropic energy.

The coherent superposition of cw-TEMoo laser beams on non-conductive surfaces can efficiently inhibit the interaction between a weaker beam and the surface dipoles of a dielectric material. We study the signal reflected by a soft silica glass surface due to the superposition of two 532 nm cw-TEM₀₀ laser beams at various electronic voltages set up across the surface. A regular interference pattern with evenly distributed maxima-minima is observed. The experimental uncertainty is less than 2%, which is much less than the amplitude of the oscillations measured in the interference pattern, of 10 to 12%. We clearly observe a peak of reflectance at Brewster angle of 56.67⁰ for silica glasses irradiated with 532nm, where the weak probe laser vanishes, because the coupling laser dominates the interaction with silica dipoles. This interaction is affected by the voltage set up across the dielectric. Thus, we observe at voltages below 0.4 volts, an interference pattern that has a perfect cos²-variation, as in any double-slit interference pattern, with a mesh of 0.76⁰. The series of maxima-minima agree with our observation for having a minimum at 60.48⁰ and a maximum at 61.24⁰.  At 5.3V, the optical response of silica to 532nm incident radiation changes. The non-linear Cauchy equation leads to a shift of the refractive index for silica from 1.521 to 1.724, which gives a shift in the Brewster angle from 56.67⁰ to 60.648⁰. This makes the maximum reflectance due to the dominance of the coupling laser at 56.67⁰ to shift at 60.648⁰. This finding indicates that the reflection of the weaker laser beam assisted by an isotropic energy can be reversed from a minimum (which shows extinction) at 60.48⁰ for 0volts, to a maximum (which shows a strong reflection) at 60.648⁰ for 5.3volts. A possible application of this configuration is a new optoelectronic switch with ON (maxima) and OFF (minima) positions for the reflected light by a silica surface when an electronic voltage is set up across this surface.   

Non-linear media in weakly curved spacetime: optical solitons and probe pulses for gravimetry

That light propagating in a gravitational field gets frequency-shifted is one of the basic consequences of any metric theory of gravity rooted in the equivalence principle. At the same time, also a time-dependent material's refractive index can frequency-shift light propagating in it. In this talk, we combine these two effects by showing that propagation in non-linear media in the presence of a refractive index perturbation leads to a gravity-dependent blueshift of light. This blueshift surpasses the gravitational redshift even if the medium is considered to be perfectly stiff and, in realistic scenarios, the amplification can be of several orders of magnitudes. We argue that this has the potential to facilitate optical sensing of small gravity gradients.   

Laser harmonic generation with tuneable orbital angular momentum and frequency steps using a structured plasma target

High harmonic generation using a laser pulse with circular polarisation (CP) onto a flat or isotropic target is not generally deemed possible. However, when a target with a structured surface is used, a rich spectrum of harmonic frequencies and/or orbital angular momentum (OAM) levels can be unlocked. Here we show via numerical simulations how a single CP laser pulse, interacting with both reflective and transmissive structured targets, can produce a harmonic progression with tuneable steps in both frequency and OAM level [1]. We introduce a novel way to analyse the frequency, spin and OAM content of the harmonic radiation which provides enhanced insight into this process. Finally, we demonstrate how a controlled frequency comb can be obtained via the preferential selection of a subset of harmonic modes with a specific OAM value.

[1] R. Trines et al., "Laser harmonic generation with independent control of frequency and orbital angular momentum", DOI:10.21203/rs.3.rs-3458883/v1 (2023).   

Entanglement transfer from a many atom Rydberg W state to a multimode photonic cavity state

A robust quantum protocol is realized that achieves high fidelity transfer from a 3-atom Rydberg system initialized in a W state (|rrg>+|rgr>+|grr>)/√3 to an equivalent photonic W state (|110>+|101>+|011>)/√3 in a multimode cavity. The challenge of achieving transfer is addressed by dynamically adjusting the cavity mode frequencies and modulating the coupling rates to restrict the transfer dynamics to a sequence of avoided crossings. Our two-step protocol effectively reduces the complex problem of multimode transfer with 3 atoms to a sequence of highly efficient two-level avoided crossing problems. The fractional STIRAP method, featuring Gaussian modulation of the coupling rates and linearly chirping of the cavity frequencies, is used to execute partial and complete population transfers at individual stages of the entanglement transfer process. Exact expressions for the non-adiabatic transition probability are derived using the Dykhne formula. Moreover, we propose practical approaches for realizing our protocol in the lab.   

Quantum parameter estimation with many-body fermionic systems and application to the quantum Hall effect

Fermionic quantum field theory is the tool of choice for modeling electronic quantum sensors. To this end, we calculate the quantum Fisher information for a generic many-body fermionic system in a pure state depending on a parameter.  The parameter can be imprinted in the basis states, the state coefficients, or in both. We apply our findings to the quantum Hall effect and evaluate the quantum Fisher information associated with the optimal measurement of the magnetic  field for a sensor in the electronic ground state. The occupation of single-particle states with high momentum enforced by the Pauli principle leads to a super-Heisenberg scaling of the sensitivity with a power law that depends on the geometry of the sensor.   

Multiparameter Quantum Metrology: Super-Resolution Imaging in Passive Remote Sensing

We theoretically study super-resolution imaging using an n-mode interferometer in the microwave regime for satellite-based passive remote sensing. We present a comprehensive quantum mechanical analysis concerning the multiparameter estimation of temperatures on the source plane. We construct detection modes from superpositions of the incoming modes with an optimized unitary transformation. This enables the most informative measurement based on photon counting in the detection modes. Our approach saturates the quantum Cramér-Rao bound for a set of temperatures that describes the sources. In our numerical analysis, using a maximum likelihood estimator to reconstruct an image in the source plane, we attain quantum-enhanced super-resolution yielding a pixel size of 3 kilometers, surpassing that of a comparable classical interferometer by more than a factor 10.   

Probing the link between contextuality and quantum coherences using the quantum Cheshire cat paradox

We analyse the quantum Cheshire cat using contextuality theory, to see if this can tell us anything about how best to interpret this paradox. We show that this scenario can be analysed using the relation between three different measurements, which seem to result in a logical contradiction. We discuss how this contextual behaviour links to weak values, and coherences between states prohibited by either the pre-selection or the postselection. Rather than showing a property of the particle is disembodied, the quantum Cheshire cat instead demonstrates the effects of these coherences, which are typically found in pre- and postselected systems. Our results shed a surprising new light on the relation between weak values and contextuality, a key resource for quantum computing. (This talk is based on the published paper, Contextuality, Coherences, and Quantum Cheshire Cats (Jonte R Hance et al 2023 New J. Phys. 25 113028)).   

Long-lived collective Rydberg excitations in atomic gas achieved via ac-Stark lattice modulation

Collective Rydberg excitations are the subject of growing interest in many key domains of physics. In particular, these excitations hold promising potential for applications in fields ranging from quantum information processing, and quantum simulators to ultra-sensitive electrometry. Their vast potential is somewhat limited in real-life scenarios by their short thermal dephasing time, which for typical temperatures in experiments (~100 μK) is not much more than a single microsecond. Some state-of-the-art methods provide means to increase the atomic coherence lifetime, however, they were only ever implemented in ground-state quantum memories. Their application in Rydberg excitations would require a fundamental redesign to work. We propose a novel protocol based on Ac-Stark phase modulation of a quantum memory ensemble, which in principle can freeze the atomic coherence and completely halt the effect of thermal decoherence. Our implementation was realized by interfering two off-resonant laser beams on the atomic medium and demonstrated that a ten-fold increase in lifetime is feasible. We verified our results by simulating the atomic-coherence evolution in phase space, accounting for the finite duration of the modulation pulses, imperfect alignment of the experimental setup, and the movement of atoms in different velocity classes. Our experimental setup achieved more than a 10-fold improvement in lifetime, reaching very close to the natural lifetime of the Rydberg state determined by spontaneous and radiative dephasing. The presented protocol makes measurements of Rydberg interactions possible on longer timescales, which is often crucial in electrometry or for usage of long-lived collective qubits for quantum simulations and computations interfaces with light.