Session M06: Quantum/Coherent Control: Cold Gases and Quantum Information

Measuring the atomic excitation time due to narrowband resonant photons that are transmitted

If a resonant photon traverses a medium and is transmitted on the far side, does it excite any atoms along the way? Previous work (PRX Quantum 3, 010314) provides evidence that it does. Since this work was limited to measurements using only broadband pulses of light, it cannot distinguish between recent theories that make strikingly different predictions in the case of excitation with narrowband pulses and media with low optical depth. In particular, the weak-value formalism suggests that this excitation time could be negative under such conditions. We present experimental progress to investigate this prediction and aim to fully elucidate the history of resonant photons that are ultimately transmitted through a cold cloud of 85Rb atoms.   

Time-Optimal Two- and Three-Qubit Gates for Rydberg Atoms

Entangling gates between two or more qubits stored in the electronic states of neutral atoms can be implemented via the strong and long-range interaction of atoms in highly excited Rydberg states. Two properties of a gate are particularly desirable: Firstly, the gate should be fast, since many types of error can be mitigated by short gate durations. Secondly, it should be implemented by a global laser pulse which does not require single site addressability of the atoms, simplifying experimental implementation of the gate. In this work we use two quantum optimal control techniques, gradient ascent pulse engineering (GRAPE) and Pontryagins’s maximum principle, to determine time-optimal global laser pulses implementing a controlled-Z(CZ) gate and a three qubit C₂Z gate. Our pulses improve upon the traditional non-global pulses for the CZ and the C₂Z gate with just a limited set of variational parameters, demonstrating the potential of quantum optimal control techniques for advancing quantum computing with Rydberg atoms.   

Counterdiabatic Optimised Local Driving

Speeding up adiabatic protocols is key in the future development and use of quantum technologies. We present a new method for solving this problem: counterdiabatic optimised local driving (COLD). It combines two existing complementary methodologies - quantum optimal control and shortcuts to adiabaticity - while taking advantage of the strengths of each. The new method improves upon approximate counterdiabatic driving - an approach in shortcuts to adiabaticity which aims to suppress nonadiabatic transitions - by adding supplementary control fields which increase its effectiveness - as one would in the case of optimal control. We show that COLD can result in a substantial improvement when applied to annealing protocols, state preparation schemes and state transfer on a lattice. Furthermore, we combine counterdiabatic optimised local driving with an existing advanced optimal control method - chopped randomised basis - for a potential further improvement.    

A cryogenic neutral atom optical tweezer array

Scalable ultracold Rydberg atom arrays provide an intriguing platform for programmable quantum computation. We present a new system for a 2D Rydberg qubit array embedded in a low-vibration cryostat. Cryopumping will improve the atom vacuum lifetime to fully leverage the scalability of Rydberg platforms, and a 30 K environment will extend the Rydberg lifetime to several times its value at room temperature. To create a large and controlled array, we will utilize a 2D optical lattice with the site-resolved addressability and interaction control aided by optical tweezers. We will harness a bi-chromatic magic lattice to provide identical confinement for both ground and Rydberg states.   

Hybrid quantum systems formed by cold atoms and levitated nanospheres

Optical potentials created by incident and scattered optical fields around a levitated nanosphere can be used for sympathetic cooling and for creating a bound nanosphere-atom system analogous to a large molecule. We demonstrate that long range potentials can be produced allowing fast sympathetic cooling of a trapped nanosphere to microKelvin temperatures using cold atoms.  We also show how the center-of-mass can be controlled by manipulation of the internal state of the atom to create and detect nonclassical macroscopic motional states of the nanoparticle. Spatial superpositions can be produced that do not require ground state cooling and can be revealed using the Earth's gravitational field.   

Spatial Coherence of Light in Collective Spontaneous Emission

When a quantum system is put into an excited state, it will decay back to the ground state through a process termed spontaneous emission. It is generally assumed that spontaneous emission between different individual emitters would not be coherent with each other; to produce coherent light one would need population inversion and stimulated emission. In this talk, we describe our recent experiments which show how an optically-thin ensemble of 11,000 radiating atoms spontaneously organize to produce spatially coherent light. The reason for this coherence is collective-coupling of the individual emitters via Dicke superradiance and subradiance (as opposed to amplification through stimulated emission).   

Controlling optical response to charged particles in EIT media

There is a good tunability of the optical response of atomic media in electromagnetically induced transparency (EIT) states. In the weak-field approximation, features of the optical response in 3-level atoms include low absorption, strong nonlinearity and slow light for frequencies in a transparency window centered on the transition frequency with KHz bandwidth. Theoretical results show significant variation of pulse properties such as the group velocity and spectral chirp with the atomic system and dressing field parameters. This high sensitivity of the susceptibility to small changes in the parameters implies the pulse spectrum can be controlled with very weak fields probing the transitions. In this work, the detection of perturbative effects in the medium is studied, specifically from a train of high energy charged particles passing through the medium. A control scheme is developed to allow for continuous trapping of spectrally tuned Cherenkov radiation in the medium along a narrow group cone to determine properties of the charge train through change of the optical response.   

Nanoscale addressing and manipulation of neutral atoms using electromagnetically induced transparency

We propose to integrate dark-state-based localization techniques into a neutral atom quantum computing architecture and numerically investigate two specific schemes. The first scheme implements state-selective projective measurement by scattering photons from a specific qubit with very little crosstalk on the other atoms in the ensemble. By exploiting the non-linearity of the Electromagnetically Induced Transparency effect, one can manipulate atoms on the optical lattice with a sub-wavelength resolution. The second scheme performs a single-qubit phase gate with high fidelity. Our numerical simulations in rubidium (Rb) atoms show that for both schemes a spatial resolution at the level of 10 nanometers using visible light can be achieved with experimentally realistic parameters.   

Controlling single photons via lattice dark states in atomic arrays

Ordered atomic arrays with subwavelength spacing have emerged as a versatile quantum optical platform that exhibits strong and directional coupling between light and matter. In this platform, collective interactions give rise to sets of super- and subradiant lattice states. While radiating states can be easily excited, subradiant modes do not couple to the radiative field and cannot be accessed by incoming light fields. In this talk, I will show that spatial modulations of the atomic detuning can be used to individually and selectively address such highly subradiant states, often called dark states. Additionally, I will demonstrate that dark states and spatial detuning patterns can be combined to store and retrieve single photons, as well as to control the temporal, frequency and spatial degrees of freedom of the emitted electromagnetic field.   

Light-Induced Dipole-Dipole Forces in Ultracold Atomic Gases

I will present our work on light induced dipole-dipole (LI-DD) interactions provoked by a spatially homogenous laser beam in a cloud of freely expanding ultracold 87Rb atoms [1]. This light-triggered collective effect results in a self-confining potential with interesting features: it exhibits nonlocal properties, it is attractive for both red and blue-detuned light fields and it induces a strong force that depends on the gradient of the atomic density. For our experimental parameters we reach remarkably deep self-confining potentials corresponding to a trap depth of up to 8 µK. This LI-DD potential minimum is intrinsically tied to the atomic ensemble and can freely evolve in additional external potentials.

[1] Observation of Light-Induced Dipole-Dipole Forces in Ultracold Atomic Gases

M. Maiwöger, M. Sonnleitner, T. Zhang, I. Mazets, M. Mallweger, D. Rätzel, F. Borselli, S. Erne, J. Schmiedmayer, P. Haslinger

arXiv:2202.00562