Quantum technologies rely on the generation, manipulation and measurements of quantum superposition states in a highly controlled and coherent fashion using various physical systems, such as atoms, photons, and solid-state platforms. In this respect, an atom-based quantum memory, that can store and retrieve quantum states on demand, is a key component to several quantum technology applications . While preserving the fragile nature of quantum states for very long times, such memory devices also allow for transferring the stored quantum states from atoms to photons in the retrieval process . Based on these unique features, to date, atomic quantum memories have primarily been exploited as hardware for future quantum computers and long-distance quantum communication systems. However, a recent study by M. Mazelanik et al.  shows that their utilization can go beyond these applications; Atomic quantum memories could also serve as enabling tools for quantum imaging and sensing. The authors experimentally demonstrate this possibility by implementing the quantum ghost imaging (QGI) technique with the assistance of a multiplexed atomic quantum memory.
The basic idea of QGI is to form an image of a target object from the detections of photons that never directly interact with that object [4,5]. To achieve such an unconventional way of imaging, QGI employs pairs of photons that feature a type of strong spatial and temporal correlation that is impossible to realize with classical light used in traditional imaging technologies. In the QGI approach, one member of each photon-pair (referred to as the signal photon) is continuously sent to a target object such that it probes “spatial information” contained in the object, as shown in panel (a) of the figure below. After interacting with the object, these photons are simply collected all together without considering their spatial distribution and then used as heralding signals via a “bucket” detector. Hence, signal photons are not directly used for reconstructing the image (transverse spatial profile of the object). Rather, the image is extracted from the detection of the other members of the correlated photons (referred to as the idler photons) by impinging them on a camera sensitive to single photons, which is triggered by the signal-photon detection. Although idler photons never “see” the target object, their spatial correlations with signal photons lead to an image being formed on the camera. In addition to these spatial correlations, the temporal correlations of photon pairs ensure that noise detections, which are random in nature, are filtered based on the precise timing between a signal and idler photon, thus rendering QGI superior to classical techniques under certain conditions . For real-time QGI, this timing needs to be maintained by compensating both electronic delay (e.g., due to the slow triggering time of single-photon sensitive cameras) and optical delay which may arise in certain configurations, where signal photons are sent over long distances for remote sensing or imaging .
To date, almost all of QGI experiments have been realized with photon-pair sources, relying on spontaneous down-conversion (SPDC), as well as optical delay lines when necessary for real-time imaging, as shown in panel (a) of the figure. In the experimental demonstrations of M. Mazelanik et al., an ensemble of laser-cooled atoms combines these two elements (pair generation $+$ delay) in a single and versatile implementation, based on a well-known quantum memory technique, the Duan-Lukin-Cirac-Zoller (DLCZ) protocol , as illustrated in panel (b) of the figure above. In this protocol, an optical pump field (“write”) excites atoms in such a way that a single spin excitation (shared by all atoms across the ensemble) is generated with an accompanying emission of a photon, which corresponds to the signal member of our photon pair. While this spin excitation is correlated with the emitted signal photon through the laws of momentum and energy conversation, such spin-photon correlations can be stored over long timescales from microseconds to a millisecond in cold atoms. Whenever desired, these correlations can be converted into photon-photon correlations in space and time by launching another pump field (“read-out”) that produces an idler photon from the stored spin excitation. In this way, a pair of signal and idler photons can be generated with a dynamically controllable delay between them, which is not possible in the SPDC process with non-linear crystals.
Demonstration of memory-assisted quantum ghost imaging
Although atom-based photon-pair sources have been known and used for two decades, so far, they have not been considered as an alternative to SPDC sources for quantum imaging applications due to certain limitations. One of these limitations has been the orders of magnitude lower pair-generation rate. Another obstacle has been the difficulty to exploit spatial correlations of the produced photon pairs, for example, as in type-2 SPDC sources. The research group of the current study overcomes these problems with their spatial multi-mode memory approach, relying on the fact that write and read-out pump pulses can simultaneously generate signal-idler pairs in arbitrarily oriented wave-vector modes. This feature essentially translates into the generation of multi-mode photon pairs which can be spatially addressed and detected using single-photon cameras, as previously reported by the authors . While enhancing the photon-pair rate substantially, utilizing spatial correlations in this multiplexing configuration makes atomic quantum memories an interesting candidate for quantum imaging.
The authors discover this aspect of their quantum memory by implementing QGI in an elegant fashion, which also allows for real-time observation of entanglement and non-locality. In this implementation, they encode the polarization degree-of-freedom in the quantum state of generated multiplexed photon pairs such that the entangled nature of each spatially correlated pair (occupying a different wave-vector mode) can be independently verified by simultaneous joint projections in this degree of freedom. The phase setting of the polarization projection for each wave-vector mode is given by the path-length difference of the corresponding signal-idler beams, which can be simply controlled by mirrors in the far-field collection optics of these beams. This means that any desired phase pattern of the multiple wave-vector modes can be prepared and then imaged with the outcomes of the projection measurements, as the authors have achieved with the QGI protocol. In their demonstrations, as in an SPDC-based implementation, signal photons were detected using a bucket detector, triggering a camera for the detections of idler photons for image formation. However, unlike an SPDC-based implementation, the emission of the idler photons is delayed by the atomic quantum memory based on the active feedback from the bucket detector, instead of a fixed optical delay line [panel (b) in the figure]. Such a memory-controlled delay was utilized for compensating the electronic delay of the camera between the events of signal trigger and idler detection, which can be on the order of tens of nanoseconds. While this feature allows for observing the image and non-local correlations in real-time, it technically eliminates the complexity of “image preserving delay lines”, requiring free-space optical paths that are tens of meters long .
Perspectives and Future directions
Beyond the above-mentioned fundamental importance and technical advantage, atomic quantum memories may bring new possibilities to quantum imaging technologies. In particular, sensing and imaging of remote targets could benefit from the memory approach in configurations where the object is at an unknown distance or mobile, as shown in panel (c) of the figure above. In this scenario, typically encountered in LiDAR (Light Detection and Ranging) applications [10,11], a signal photon travels to a distant location for probing the target while the correlated idler photon is stored in a memory until detecting the reflected/scattered signal photon from the target for a subsequent correlation or interference measurement. Extending the distance over kilometers (beyond the standard range of the current LiDAR systems) would require tens of microseconds of variable delay time between the signal and idler detections, which is practically not possible with image-preserving optical delay lines. In addition, since the required delay between a signal and idler is not known prior to detections in this kind of applications, it is challenging to identify correlated detections with digital post-processing (e.g., cross-correlogram techniques), which typically suffers from jitter-related issues and is particularly demanding for the purpose of imaging due to high data volumes [12,13]. The use of atomic quantum memories can overcome these limitations with long and fully controllable delays, even up to millisecond time-scales in realizations with cold-atom , warm-atom  and solid-state atomic [16,17] systems. Moreover, the inherently narrow and adjustable bandwidth of photons emitted from an atomic quantum memory may bring an additional advantage for filtering background noise, which can further enhance the signal-to-noise ratio and image contrast. Together with future improvements in photon-pair generation rates, atomic quantum memories hold great promise for advanced sensing applications , including, but not limited to, quantum radar , quantum LiDAR , and quantum ghost imaging.
I would like to thank Dr. Kadir Durak, Dr. Daniel Oblak, Dr. Vahid Salari, Dr. Shabir Barzenjeh, and Dr. Lindsay LeBlanc for fruitful discussions and their feedback.
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