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SUMMARY:Polariton Graph Optimisers - Professor Natalia Berloff (DAMTP\, Qu
 antum Fluids Group)
DTSTART:20180124T200000Z
DTEND:20180124T210000Z
UID:TALK99616@talks.cam.ac.uk
CONTACT:Callum Jones
DESCRIPTION:The search for an optimal solution is analogous to looking for
  the lowest point in a mountainous terrain with many valleys\, trenches\, 
 and drops. Such a search may seem daunting in natural terrain\, but imagin
 e its complexity in high-dimensional space! This is exactly the problem to
  tackle when the objective function to minimise represents a real-life pro
 blem with many unknowns\, parameters\, and constraints. Modern supercomput
 ers can only deal with a small subset of such problems when the dimension 
 of the function to be minimised is small or when the underlying structure 
 of the problem allows it to find the optimal solution quickly even for a f
 unction of large dimensionality. Even a hypothetical quantum computer\, if
  realised\, offers at best the quadratic speed-up for the “brute-force
 ” search for the global minimum. What if instead of moving along the mou
 ntainous terrain in search of the lowest point\, one fills the landscape w
 ith a magical dust that only shines at the deepest level\, becoming an eas
 ily detectable marker of the solution? Our "magic dust" is created by shin
 ing a laser at stacked layers of selected atoms such as gallium\, arsenic\
 , indium\, and aluminum. The electrons in these layers absorb and emit lig
 ht of a specific colour. Polaritons are ten thousand times lighter than el
 ectrons and may achieve sufficient densities to form a new state of matter
  known as a Bose-Einstein condensate\, where the quantum phases of polarit
 ons synchronise and create a single macroscopic quantum object that can be
  detected through photoluminescence measurements. To create a potential la
 ndscape that corresponds to the function to be minimised and to force pola
 ritons to condense at its lowest point we focused on a particular type of 
 optimisation problem\, but a type that is general enough so that any other
  hard problem can be related to it\, namely minimisation of the XY model w
 hich is one of the most fundamental models of statistical mechanics. We ha
 ve shown that we can create polaritons at vertices of an arbitrary graph: 
 as polaritons condense\, the quantum phases of polaritons arrange themselv
 es in a configuration that corresponds to the absolute minimum of the obje
 ctive function.
LOCATION:Wolfson Lecture Theatre\,  Department of Chemistry\, Lensfield Ro
 ad
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