Double-Fock superposition interferometry for differential diagnosis of decoherence
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| License | CC Attribution 3.0 Unported: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
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Transcript: English(auto-generated)
00:03
In our paper, we use two-mode Fock states and entangled double Fock superpositions to characterize photon sources and diagnose decoherence effects in interferometers. In the laboratory, Mach-Zehnder interferometers are used to estimate relative phase shifts between the interferometric arms.
00:21
Interferometric contrast, however, is typically not optimal, but only achieves a finite visibility. The reasons for such imperfect interference visibility are manifold. The most important decoherence effects are dephasing, that is, loss of phase coherence, distinguishability, that is, mode mismatch between the two arms, and finally, mixing,
00:43
which can be due to residual entanglement to idler photons or some random classical process. But how can we distinguish these three different processes to effectively cure them and achieve optimal visibility? Single photon states, or more generally, noon states, are not suitable for this task, because
01:01
the interference contrast turns out to be simply the product of the three decoherence parameters. Double Fock states feature bosonic bunching, but they are not suitable for decoherence diagnosis in the Mach-Zehnder interferometer either, since they do not carry a relative phase. In our contribution, we showed that superpositions of double Fock states combine the best of
01:20
both worlds. These states feature phase-dependent interference together with bosonic effects. As a consequence, they will allow great insight into the effects that influence interferometers. To see that, we treat many-particle interference using double-sided Feynman diagrams. The probability for a scattering event is then written as the norm of the wave function
01:42
projected onto the desired outcome. We rewrite the probability as the scalar product of the projected vector with itself and use the idempotence of the projector. By expanding the resulting terms, we obtain a sum of double-sided Feynman diagrams. We find classical contributions that are immune to any sort of decoherence and exchange
02:04
terms that suffer from dephasing, distinguishability, and mixing. For noon states, there are only two types of double-sided Feynman diagrams, classical and phase-dependent exchange contributions, hence the inability to resolve different decoherence processes. For double Fock superpositions, we have a much richer picture and a dependence on several
02:24
classes of diagrams with different numbers of exchange terms. Therefore, event probabilities depend on the decoherence parameters in a more intricate way than for noon states. Pictorially speaking, a single-photon interference signal with a certain visibility is compatible
02:41
with a two-dimensional surface in the space of decoherence parameters. While it does not reveal the very location on that surface, but already for the simplest non-trivial double Fock superposition, composed of the Fock states 2,1 and 1,2, the resulting signal reveals the quantitative impact of all three processes and the precise
03:01
location in parameter space. That is to say, the space of parameters is mapped to the three-dimensional space of observables. The signal borne by double Fock superpositions then immediately reveals not only the total degree of decoherence, shown here as a yellow surface, but also to which extent decoherence
03:20
is due to dephasing, distinguishability or mixing, as indicated by the red ball. In our paper, we treat two-mode scattering with double-sided Feynman diagrams and explain in more detail the dependence of transition probabilities on decoherence processes. We show how to differentiate decoherence effects in a more and more detailed way as we increase the number of particles.
03:41
Finally, we discuss the characterization of photon sources via twin Fock states in a way that goes qualitatively beyond the visibility of two-photon hong bundle interferometry.