Symmetry-protected collisions between strongly interacting photons

Abstract

Realizing robust quantum phenomena in strongly interacting systems is one of the central challenges in modern physical science. Approaches ranging from topological protection to quantum error correction are currently being explored across many different experimental platforms, including electrons in condensed-matter systems, trapped atoms and photons. Although photon-photon interactions are typically negligible in conventional optical media, strong interactions between individual photons have recently been engineered in several systems. Here, using coherent coupling between light and Rydberg excitations in an ultracold atomic gas, we demonstrate a controlled and coherent exchange collision between two photons that is accompanied by a π/2 phase shift. The effect is robust in that the value of the phase shift is determined by the interaction symmetry rather than the precise experimental parameters, and in that it occurs under conditions where photon absorption is minimal. The measured phase shift of 0.48(3)π is in excellent agreement with a theoretical model. These observations open a route to realizing robust single-photon switches and all-optical quantum logic gates, and to exploring novel quantum many-body phenomena with strongly interacting photons.

Critical slowdown of non-equilibrium polaron dynamics

“…Therefore, understanding its interplay with coherent driving of internal impurity states is also important for the quality of light matter interfaces involving polaron physics. Examples include the storage [51] and propagation [52] of quantum light in strongly interacting nonlinear media [53] or atomic BECs coupled to an optical cavity [54].…”

A photon–photon quantum gate based on Rydberg interactions

“…One promising candidate is the strong and long-range interaction between Rydberg states of neutral atoms, for which first applications in the fields of quantum computation [1] and quantum simulations [2][3][4] have been reported in recent years. Using electromagnetically induced transparency, these Rydberg interactions can be mapped to light [5][6][7][8][9][10][11][12][13][14][15], thus creating an effective interaction between photons. Here we extend the range of applicability of these interactions to the field of quantum communication and quantum networking [16] by realising a photon-photon quantum gate [17,18].…”

“…However, ambitions to use photons for processing rather than only transmitting qubits are hampered by the fact that photons hardly interact with each other. A solution to this problem is offered by a Rydberg polariton [6][7][8][9][10][11][12][13][14][15]. This intriguing quasiparticle -composed of a photonic component and an atomic Rydberg excitation -is obtained when a photon enters a medium in which electromagnetically induced transparency (EIT) couples the photon to a Rydberg state.The key idea is that the atomic Rydberg components create a strong long-range interaction between two Rydberg polaritons.…”

Two photon conditional phase gate based on Rydberg slow light polaritons

“…intermediate p-level caused by the interaction, • β sc accounts for scattering into additional polariton channels, • β wp includes the impact of the finite length of the wave-packets on (a) the losses from p-state during propagation and (b) the inhomogeneous interactioninduced phase-shift • β at describes the distortion of the phase shift due to spatially varying atomic density, • β Ry denotes losses due to the finite lifetime of the Rydberg state, whereas • β tr depicts finite beam waist effects leading to scattering of photons to other transversal photonic modes. At this point it worth mentioning the family of phase-gate schemes in which at least one photon [58,57,27,68,69,70] is stored as a collective atomic excitation. This type of schemes is superior compared to counter-propagating scheme when it comes to the β sc and β wp in equation Eq.…”