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UID:69d2561927fe4
DTSTART:20231106T140000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:ICFO Auditorium and Online (Teams)
SUMMARY:ICFO | LUKAS HELLER
CLASS:PUBLIC
DESCRIPTION:Quantum memories are devices that are able to store photonic qu
 antum states and entanglement. Future quantum networks\, which could enhan
 ce cybersecurity through quantum key distribution\, improve the precision 
 in atomic clock networks\, and connect quantum devices over long distances
 \, rely on them. This thesis reports on experiments towards improved quant
 um memories for quantum repeaters used in long-distance quantum communicat
 ion.\nThe quantum memory is based on a cloud of laser-cooled Rubidium-87. 
 Thanks to collective interference effects\, this system enhances the light
 -matter interaction compared to that of a single atom. This is exploited t
 o either create long-lived quantum correlations between light and atomic e
 xcitations through probabilistic light scattering (DLCZ protocol) or to ef
 ficiently absorb an incoming single photon (Raman protocol). In both cases
 \, the excitation is retrieved after a programmable delay as a single phot
 on. An optical cavity around the atoms further enhances the light-matter c
 oupling.\nIn a first experiment\, the DLCZ protocol is combined with a pho
 ton echo protocol\, allowing for the sequential creation of excitations in
  N distinguishable temporal modes. This is known as temporal multiplexing.
  Multiplexing improves the rate at which entanglement is created in a netw
 ork link by a factor N. Here\, the cavity is essential to suppress noise o
 riginating from the probabilistic scattering of light in the DLCZ protocol
 . Ten temporal modes are stored while maintaining strong quantum correlati
 ons between the scattered photon and the atomic excitation.\nIn a second e
 xperiment\, a quasi-deterministic single photon is stored in the cloud fol
 lowing the Raman protocol. The photon originates from an ensemble of laser
 -cooled Rydberg atoms. Strong dipole-dipole interactions prevent the excit
 ation of more than one atom to the Rydberg level\, leading to the creation
  of a single collective Rydberg excitation which is later retrieved as a s
 ingle photon. A deterministic source\, opposed to a probabilistic source\,
  improves the entanglement creation rate as it generates single photons at
  higher rates. The single photon is stored and faithfully retrieved from t
 he memory while maintaining its single-photon nature. The cavity is not be
 ing used in this experiment.\nIn a third experiment\, the retrieval effici
 ency of a stored excitation is increased by cavity-enhancing the read-out 
 process. Highly-efficient memories are important because the entanglement 
 distribution scales strongly with memory efficiency. The intra-cavity effi
 ciency could be improved by a factor 2-3\, depending on the protocol\, eve
 n for a non-optimal cavity setup.\nFinally\, ongoing work towards efficien
 t entanglement between an atomic excitation and a telecom photon is presen
 ted\, involving a cavity-enhanced DLCZ memory\, an atomic dipole trap and 
 quantum frequency conversion (QFC) to the telecom C-band. As the memory op
 erates in the optical domain\, where photonic transmission losses are larg
 e\, QFC will be needed to communicate over large distances. This setup wil
 l be used in the future for a hybrid experiment connecting the cold atomic
  memory to a solid state memory.\nThese investigations target intrinsic li
 mitations of early\, proof-of-principle quantum links. They can therefore 
 help to build practical links in the future.\nWednesday November 6\, 15:00
  h. ICFO Auditorium and Online (Teams)\nThesis Director: Prof Dr. Hugues d
 e Riedmatten
DTSTAMP:20260405T123121Z
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