Periodic Reporting for period 4 - Q-PHOTONICS (Quantum fluids of photons in optically-induced structures)
Reporting period: 2020-12-01 to 2021-05-31
We have developed and studied systems that are able to realize "quantum nonlinear optics" – an optical regime in which photons effectively interact and could manifest a quantum fluid. We have focused on reaching the quantum nonlinear regime using atomic gases. In these gases, strong photon-photon interactions are enabled by coupling the photons to interacting atoms. The three primary tools we employ are Rydberg atoms (which are atoms that are excited to high electronic orbitals), light-matter polaritons (which are slowly-propagating objects composed of photons and atomic excitations), and optical structures (either actual structure or effective structures induced by light in space and time). Realizing and engineering quantum nonlinear optics is of high scientific and technological importance. From the fundamental side, we have studied novel many-body physics with photons, namely that of non-equilibrium (driven and dissipative) strongly-correlated systems. This regime gradually becomes very important in the context of the newly arising research field of "waveguide QED." From the practical side, we have developed several approaches towards photon-by-photon control, which are important for quantum light manipulation and metrology, with applications in quantum computation, communication, and sensing.
Our system of choice is a gas of either cold or hot atoms, excited by the propagating photons to high-lying Rydberg levels. The ERC project focuses on optically-driven spatial and spectral structures and on multimode operation. We have built two experimental setups – hot-atom and ultracold-atom systems – for realizing these ideas. We analyzed the enhancement of photonic quantum gates by an optically-induced cavity, that is, by a photonic resonator generated by illuminating the medium with a structured pattern. We showed that the circulation of photons back-and-forth in the resonator enhances the gate fidelity, surpassing the well-known limitations of such gates. Using the cold atoms, we realized a variety of interaction regimes, from strong photon-photon attraction (dominated by one and two bound-states), through single-photon saturable absorption and genuine three-photon interaction, to the formation of ordered states of photons. Using the hot atom, we demonstrated fast, coherent excitation of electronic orbitals, towards the realization of Rydberg-mediated nonlinear optics with hot atoms. Achieving the fast (sub-nanosecond) excitation to the Rydberg level requires strong (kilowatt peak-power) pulses of narrowband infrared light, and we have developed a unique laser system to achieve it. We recently obtained preliminary signals of non-classical light generated by the hot atoms in this configuration. For the same system, we developed and demonstrated methods to counteract the reduction of absorption and reduction of coherence time due to thermal motion by using optical driving, which is crucial for reaching the quantum nonlinear optics regime with hot atoms. We developed unique thin optical fibers, which are optical structures that can guide a light field while still maintaining most of the field outside the structure. This platform is particularly suitable for realizing quantum nonlinear optics with hot atoms. Finally, we developed a novel scheme for a robust two-photon phase gate based on counter-propagating polaritons. We used this idea and others developed in this project to contribute to a recent analysis of the enhancement of photonic quantum computation using quantum nonlinear optics.
As detailed above, we have realized and studied new schemes for Rydberg-mediated quantum nonlinear optics using optically-induced spatial and spectral structures, with cold and hot atoms.
In particular, we identify the following results is particularly significant:
(a) Our study on induced structures concludes a long-standing debate in the field of quantum nonlinear optics, whether finite-range interactions between photons can be enhanced by extending the optical medium beyond the interaction range. We invoke optically-induced structures to provide a positive answer to this important question.
(b) Our study on fast coherent excitation of highly-excited electronic orbital via light-induced absorption has led to developing a new light-storage scheme. This scheme outperforms all others in projected performance for multi-photon synchronization, and it lays the foundation for coherent photon manipulations with Rydberg atoms in room-temperature systems.
(c) We established the concept of continuous protection of a qubit state from inhomogeneous dephasing using an auxiliary (sensor) state, originating from ideas suggested in the original research proposal. We showed that this method can be applied to increase the effective atom-photon cross-section, to decrease the linewidth, and correspondingly to increase the coherence time of ballistic atoms. Moreover, we are aware of several current attempts to use our method on other platforms.
(d) In addition to other new ideas and implementations that were published, there are a few new concepts that are soon to be published. One notable idea is the dipolar collisions between two opposite-parity polaritons, which we believe would be substantially more robust than single-parity polaritons in realizing photonic quantum gates.
In particular, we identify the following results is particularly significant:
(a) Our study on induced structures concludes a long-standing debate in the field of quantum nonlinear optics, whether finite-range interactions between photons can be enhanced by extending the optical medium beyond the interaction range. We invoke optically-induced structures to provide a positive answer to this important question.
(b) Our study on fast coherent excitation of highly-excited electronic orbital via light-induced absorption has led to developing a new light-storage scheme. This scheme outperforms all others in projected performance for multi-photon synchronization, and it lays the foundation for coherent photon manipulations with Rydberg atoms in room-temperature systems.
(c) We established the concept of continuous protection of a qubit state from inhomogeneous dephasing using an auxiliary (sensor) state, originating from ideas suggested in the original research proposal. We showed that this method can be applied to increase the effective atom-photon cross-section, to decrease the linewidth, and correspondingly to increase the coherence time of ballistic atoms. Moreover, we are aware of several current attempts to use our method on other platforms.
(d) In addition to other new ideas and implementations that were published, there are a few new concepts that are soon to be published. One notable idea is the dipolar collisions between two opposite-parity polaritons, which we believe would be substantially more robust than single-parity polaritons in realizing photonic quantum gates.