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UID:69e876c9cf1a7
DTSTART:20221102T140000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:Auditorium and Online (Teams)
SUMMARY:ICFO |  DARÍO LAGO
CLASS:PUBLIC
DESCRIPTION:The future quantum Internet will allow the transfer of quantum 
 states between remote locations. However\, bridging large distances is a c
 hallenging task due to losses in optical fibres. A promising approach to r
 each continental distances is based on quantum repeaters\, where quantum e
 ntanglement is created and distributed between distant quantum memories in
  a heralded fashion. Such a heralding signal should be compatible with the
  already deployed telecommunication infrastructure\, i.e. it should consis
 t of quantum states of light in the telecommunication regime travelling th
 rough optical fibres. On top of this\, a multiplexed operation will largel
 y reduce the entanglement generation time. The specific approach followed 
 in this thesis consists of a hybrid system that combines sources of teleco
 m heralded single photons and multimode solid-state quantum memories based
  on rare-earth doped crystals.\nThe objective of this thesis is the distri
 bution of quantum states along remote distances\, either by storing one qu
 bit in one quantum memory for long enough while the heralding photon trave
 ls through a telecom fibre\, by teleporting an arbitrary qubit onto a dist
 ant matter qubit or by creating entanglement between remote quantum memori
 es. Thanks to the intrinsic temporal multimodality of our system\, the rep
 etition rate of these experiments is decoupled from the distance being bri
 dged. The core of my efforts consisted on building and operating telecom h
 eralded single photon sources based on cavity enhanced spontaneous paramet
 ric down conversion (cSPDC). I was also involved on conceiving and impleme
 nting several enabling technologies to allow for the creation\, distributi
 on and verification of quantum states.\nIn a first project it was possible
  to generate energy-time entangled photon pairs by increasing the coherenc
 e time of the laser used to pump the cSPDC source. We then stored one of t
 he photons for up to 47.7us in a spin wave Praseodymium doped quantum memo
 ry with on-demand read out. The entanglement between the telecom photon an
 d the stored photon was successfully verified in all the explored scenario
 s using the Franson scheme. For this purpose\, I built and stabilized a fi
 bre-based unbalanced Mach-Zehnder interferometer with a length difference 
 between arms of 85m.\nIn a second experiment we performed multiplexed quan
 tum teleportation from the telecom photon to the solid-state memory. We to
 ok advantage of the telecom wavelength featured by one of the energy-time 
 entangled qubits and we sent it through a 1km long optical fibre. After tr
 avelling through that distance it was jointly measured with a second qubit
 . The state of that second qubit was then teleported onto the quantum memo
 ry that was initially storing the second entangled qubit. The storage time
  was enough to allow for further processing of the teleported state\, whic
 h consisted of a unitary transformation based on the result of the remote 
 joint measurement.\nFinally\, in a third experiment we generated telecom h
 eralded entanglement between remote solid-state multimode quantum memories
 . I first worked on the indistinguishability of the photon pairs produced 
 by two cSPDC sources. I then mixed the generated telecom qubits in a beam 
 splitter (BS) such that a detection event after this BS heralded the creat
 ion of entanglement between the two quantum memories in the photon number 
 basis. This way of creating entanglement\, together with its verification\
 , is sensitive to the relative phase of the qubits interfering at the BSs.
  Along these lines\, I also worked on stabilizing the length of the optica
 l fibres involved in this experiment\, which in the end consisted of a ~75
 m long interferometer.\nThese results represent the state of the art in te
 rms of scalability for quantum repeaters using multimode memories and show
  that our system has a huge potential to enable the creation and transfer 
 of quantum states over large distances.\n&nbsp\;\nThesis Director: Prof. D
 r. Hugues de Riedmatten
DTSTAMP:20260422T072041Z
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