Rydberg superatom
By surpassing conventional technologies in some tasks, quantum technologies which take advantage of quantum mechanical effects have the potential to completely transform a number of industries. Realising quantum networks, which are complex systems made up of several interconnected quantum devices, is a major subject of current research. In order to achieve the ambitious objective of large-scale quantum networks, researchers have been working for years to find and create more promising methods and experimental platforms.
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Because of their intrinsic characteristics that assist preserve their delicate quantum states, photons are the preferred particle for transferring quantum information between devices within these networks due to their exceptional speed and weak interaction with their surroundings. The ability to verify the effective storage of photons without unintentionally damaging the very quantum information they contain has proven to be a recurring difficulty in the development of these networks.
The flash storage is a very promising solution to this problem. A’signal’ or ‘herald’ is emitted when photons are stored with this novel approach, which gives important proof that the storage event was successful.
This heralded storing of photons in a Rydberg superatom was recently demonstrated by researchers at the University of Science and Technology, marking an important milestone. A cloud of atoms that behaves as a single, collective quantum system under particular circumstances is known as a Rydberg superatom. Importantly, new experimental techniques that do not rely on conventional high-finesse optical cavities a common element in many previous works were used to achieve this accomplishment.
Developing a Novel Strategy
The team’s earlier work on a mechanism they called “photon compensation in a Rydberg superatom,” which had been used to establish atom-photon and multiphoton entanglement, served as the inspiration for this new heralded storage method. “In subsequent discussions, explored new potential applications of this technique, leading to the idea of heralded storage,” noted Dr. Xiao-Hui Bao, one of the paper’s co-senior authors.
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The researchers first thought of a more intricate arrangement that would use two Rydberg superatom configurations and encode the input photonic qubit in the spatial degree of freedom. This plan, however, turned out to be resource-intensive and complicated to implement. Bao and his associates came up with a more straightforward and effective method that uses a single superatom to get around these obstacles.
Individual qubits are encoded in the time-bin degree of freedom in this improved approach, which means that the quantum information is saved in the precise moment a photon arrives rather than its polarisation or physical location. “Heralded storage had previously been thought to be a cavity-QED capacity, but concept and experiments show that a Rydberg superatom can do it. this task effectively as well” .
The Experimental Setup and Process
Two Rb 87 atomic ensembles, known as Nodes A and B, were used in the experiment. Laser cooling and optical molasses were used to meticulously prepare these ensembles, which were subsequently trapped at 6 µK in a far red-detuned optical trap.
Using the electromagnetically induced transparency (EIT) technique, an input photon is first stored in the Rydberg superatom at Node B. This entails coupling the Rb 87 atoms’ ground state, intermediate state, and Rydberg state (in this experiment, 91S1/2) using 795 nm and 475 nm lasers. The EIT storage’s control beam is also the 475 nm laser. Node A uses a two-photon Rydberg excitation technique to produce the single photon. A collective excitation results when just one atom within a specific radius is excited to the Rydberg state due to the Rydberg blockage effect. The early and late modes of the time-bin photonic qubit are then produced by reading out this excitation.
Emission of a second photon, the ‘herald’, after the first storage is an important stage in the heralded storage. The production of this herald photon is contingent upon the first photon being successfully stored in the superatom. Using a specifically created “read-and-patch” pulse sequence a method derived from the team’s previous research this is accomplished. The scientists found that the input photon had an overall storage and read-out efficiency of 16.4%.
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Demonstrated Entanglement and Performance
One important use of this method is to entangle two distant Rydberg superatom without the requirement for an intermediary node. Such intermediate stations are frequently needed by conventional interference-based distant entanglement techniques, which raises the complexity and resource overhead.
The “read-and-patch” procedure produces atom-photon entanglement in addition to signalling the storage. This is confirmed by retrieving the atomic qubit and using an interferometer to transform it into a polarization-encoded photon. With an entanglement fidelity of 77.3% ± 0.9%, the scientists showed that photon-photon entanglement could be generated between the herald and recovered photons.
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The researchers created six distinct input states and used state tomography on the recovered photons following heralded storage in order to measure the memory’s performance. For raw data, they reported an average state fidelity of 84.8% ± 0.3%, which improved to 89.9% ± 0.3% when dark counts were taken into consideration. Additionally, the quantum nature of their storage process was confirmed by process tomography, which showed a fidelity of 76.4% ± 0.9% (raw data) and 83.4% ± 0.8% (deducting dark counts), both of which exceeded the classical limit of 69%. The estimated herald efficiency was 2.1%, with motion-induced dephasing during storage being the primary cause of restrictions.
In order to create initial atom-photon entanglement for the two-node entanglement, Node A additionally underwent a “read-and-patch” process to emit a flying qubit while maintaining a superposition of collective Rydberg excitations. Entanglement between the two distant superatoms was established by the announced storing at Node B. By recovering atomic states at both nodes and transforming them into polarisation qubits, this entanglement was confirmed, resulting in an entanglement fidelity of 70.5% ± 0.6%, which is obviously higher than the 50% classical limit.
Future Implications
Through heralded photon storage, this ground-breaking work presents a novel and practical approach to achieving entanglement between two distant superatoms. An important advancement is the generation of entanglement between two quantum memories without the need for an intermediary node.
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Although there is room for improvement in the current herald efficiency and measurement fidelity, the researchers think that these issues can be resolved by using high-performance superconducting nanowire single-photon detectors in conjunction with better cavities and larger optical depths to strengthen the photon-atom interaction. In order to extend storage lifespan, future research will also concentrate on moving Rydberg superatom to ground states and putting optical lattices into practice.
Numerous applications in quantum networks and linear optical quantum computing are made possible by this finding, which was described in an article by Zi-Ye An et al. published in Physical Review Letters. The researchers intend to incorporate this promising method into a quantum network at the city scale soon.
