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Photonics Insights

Optical quantum states based on hot atomic ensembles and their applications

Kai Zhang, Shengshuai Liu, Yingxuan Chen, Xutong Wang, Jietai Jing

Published Jan 1, 2022• 25 citations
Publisher

Abstract

The four-wave mixing process in atomic ensembles has many important applications in quantum information. We review recent progress on the generation of optical quantum states from the four-wave mixing process in hot atomic ensembles, including the production of two-beam, multi-beam, and multiplexed quantum correlated or entangled states. We also review the applications of these optical quantum states in implementing quantum information protocols, constructing SU(1,1) quantum interferometers, and realizing quantum plasmonic sensing. These applications indicate that the four-wave mixing process in hot atomic ensembles is a promising platform for quantum information processing, especially for implementing alloptical quantum information protocols, constructing SU(1,1) interferometers, and realizing quantum sensing.

DOI
10.3788/pi.2022.r06

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The four-wave mixing process in atomic ensembles has many important applications in quantum information. We review recent progress on the generation of optical quantum states from the four-wave mixing process in hot atomic ensembles, including the production of two-beam, multi-beam, and multiplexed quantum correlated or entangled states. We also review the applications of these optical quantum states in implementing quantum information protocols, constructing SU(1,1) quantum interferometers, and realizing quantum plasmonic sensing. These applications indicate that the four-wave mixing process in hot atomic ensembles is a promising platform for quantum information processing, especially for implementing alloptical quantum information protocols, constructing SU(1,1) interferometers, and realizing quantum sensing.
Quantum correlations and entanglement shared among multiple quantum modes are important for both fundamental science and the future development of quantum technologies. This development will also require an efficient quantum interface between multimode quantum light sources and atomic ensembles, which makes it necessary to implement multimode quantum light sources that match the atomic transitions. Here, we report on such a source that provides a method for generating quantum correlated beams that can be extended to a large number of modes by using multiple four-wave mixing (FWM) processes in hot rubidium vapor. Experimentally, we show that two cascaded FWM processes produce strong quantum correlations between three bright beams but not between any two of them. In addition, the intensity-difference squeezing is enhanced with the cascaded system to -7.0±0.1 dB from the -5.5±0.1/-4.5±0.1 dB squeezing obtained with only one FWM process. One of the main advantages of our system is that as the number of quantum modes increases, so does the total degree of quantum correlations. The proposed method is also immune to phase instabilities due to its phase insensitive nature, can easily be extended to multiple modes, and has potential applications in the production of multiple quantum correlated images.
This thesis presents a study of the four-wave mixing process in hot Rubidium vapor and its applicability to generating arbitrary quantum states, both in the optical domain and as atomic collective spin excitations. The four-wave mixing process involves a double Raman scattering event, where conservation of energy and momentum entangle these Raman-scattered photons. Heralding upon detection of one of these photons in a given mode projects the other photon onto a single photon Fock state in a well-defined mode, which may serve as a narrowband quantum light source that is compatible with atom-based quantum communication protocols. We integrate the existing technique of conditional measurements into our detection scheme, which allows us to engineer more complex quantum states. Experimentally we demonstrate the production of arbitrary superpositions of |0> and |1> in the optical Fock basis. We also present a theoretical model of this process which incorporates experimental complications such as losses, higher photon-number states, and a non-trivial temporal mode. This scheme can be extended to generate arbitrary quantum states with increasing photon numbers, at the expense of repetition rate. The atomic nature of this four-wave mixing process has an added benefit in that it allows access to excitations of the collective atomic ensemble. The optical results demonstrate the creation of transient collective spin excitations, and we present a proposal to extend this work to generate longer lived arbitrary collective atomic states.
Multi-photon nonlinear processes in atoms have served as important tools for quantum metrology, quantum communications, and quantum sensing. In this thesis, we experimentally address the interplay of various multi-photon Raman processes in hot Rb vapor, with the four-wave mixing (FWM) process being a central theme. FWM is the nonlinear response of a medium to a strong optical pump field inelastically scattering off atomic resonances and resulting in the generation of additional photons in different modes. FWM is a detrimental, but inherent part of electromagnetically induced transparency (EIT) and Raman based quantum memories. However, we were able to weaken the four-photon resonance by utilizing two-photon absorption to remove the additional photons without interfering with the signal beam. We also demonstrate the ability to tailor FWM to generate new photons in a controlled fashion for mode conversion. With this, we showed the conversion of 795 nm light to 420 nm light. While FWM is a source of noise in quantum memories, it can also be used for the generation squeezed twin-beams. Such beams have relative intensity noise reduced below the classical shot noise limit and share mode dependence based on the phase-matching conditions. Using this, we demonstrated that twin-beams can be generated with largely different spatial structure (optical angular momentum) and still share strong correlations, so long as the phase-matching conditions are satisfied. We then constructed and demonstrated the operation of a polarization-based quantum interferometer using squeezed twin-beams and showed that our beams were entangled under the inseparability condition. Using this interferometer, we were also able to achieve squeezing at low detection frequencies, which is necessary for things like quantum imaging and gravitational wave detection. We also demonstrated that squeezed twin-beams can be utilized to enhance the sensitivity of two-photon absorption spectroscopy. This research has touched on many different subjects related to quantum information science and improved upon some of the tools needed for the implementation of such technologies.
The strong interest in space-division multiplexing of classical communication signals has resulted in development of optical fibers that support propagation of multiple modes. This has paved the way for progress in signal processing using multiple modes and four-wave mixing between these. More specifically, careful design and knowledge of optical properties of individual modes enables phase matching in four-wave mixing that is otherwise not possible with single-mode fibers. This enables signal processing using four-wave mixing between higher-order modes for e.g. frequency conversion over a wide bandwidth. In addition, processing and creation of quantum states has also been demonstrated using four-wave mixing, including generation of photon pairs. In this work we review recent progress in signal processing and generation of quantum states using four-wave mixing.
Being able to generate and process quantum states are essential functionalities in quantum communication. Examples of the processing include conversion of the wavelength of quantum states or modification of their temporal shape. The combined use of four-wave mixing and higher order modes in optical fibers has enabled such functionalities. The use of intermodal four-wave mixing among higher order modes, enable frequency conversion over a wavelength bandwidth, large enough to avoid spontaneous Raman scattering, which is otherwise detrimental for quantum communication. The process of intermodal four-wave mixing furthermore has the advantage that the same process may be used to generate single photon states. We review recent results on the use of intermodal four-wave mixing in optical multi-mode fibers for quantum information science and show a theoretical/numerical model that may be used to predict the performance of quantum state processing devices based on four-wave mixing.