Thread by @fgrosshans

Seated for a seminar by Aephraim Steinberg grom @UofT : How to count one photon and get a result of 1000 – and other experiments on quantum measurement

#LTQI

@UofT Aephraim Steinberg’s main question: “Can we ask what a particle was doing before we observed it?”
E.g., a particle can be in one of 3 boxes A, B, C, prepared in superposition A+B. Postselected on a measurement finding it in B+C, what was its probability to be in A?

#LTQI

@UofT Aephraim Steinberg answer below:

0, because of the psot-selection
#LQTI

Aephraïm Steinberg: use cold ⁸⁵Rb to measure cross phase shifts down to pulses with ⟨n⟩=1 (coherent states). He also sees non-linear phase-shift on post-selected single photons.
#LTQI

Aephraim Steinberg: The difference in average photon number post selected on a click and no-click event is always exctly 1 photon, which allows to measure the dephasing by a single quantum, without any calibration.
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Aephraim Steinberg extends that to weak measurements using pointer states. It was introduced in PRL 60 1351 journals.aps.org/prl/abstract/1…
by Aharonov, Albert, Vaidman. IT leads to paradoxes, which people either find interesting to investigate or see as proof it’s senseless #LTQI

Aephraim Steinberg: Essentially, the post selection leads to arenormalization of the average value of the operator A, corresponding to the pointer state. We have ⟨f|A|i⟩/⟨f|i⟩. It can lead to “too high” values #LTQI

Aephraim Steinberg thus observed a 8-photn like NL phaseshift on a pst-selected single photon. Surprizingly, there is a classical interpretation (in his specifc regime), if one thinks in terms on field, and take into account the quantum noise. #LTQI

Aephraim Steinberg now moes to part II, starting with a game. Alice choses |ψ⟩ ant random sends |ψ⟩^⊗n to Bob, Bob should, much letter guess the result on a spin measurement by Alice on |ψ⟩. What is Bob’s best strategy, depending on his quantum memory. #LTQI

Aephraim Steinberg experimentally plays this game with 3 qubits, and only 2 qubits of memory, compressing it using a Schur Transform. He did this optically arxiv:1410.3941 arxiv.org/abs/1410.3941 / PRL 113 160504 doi.org/10.1103/PhysRe…
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Aephraim Steinberg: looks at the way to clock the time for a particle to go through a tunelling barrier. A solution is a Larmor clock (Baz’,Rybachenko, Buttinger 1983), with a B field inside the barrier. He plans to demonstrate it with cold atoms
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Aephraim Steinberg is also working, with Stacy Jeffery and Barry Sanders, on an implementation of quantum homomorphic encryption #LTQI