There is still no complete consensus on what is meant by the term

1. Single molecule experiments challenge the strict particle aspect of the photonThe accumulation time argument is not yet satisfiedKarl Otto Greulich Fritz Lipmann Institute Beutenbergstr. 11 D 07745 Jena There is still no complete consensus on what is meant by the term „particle“ in the context of the photon and the wave particle dualism See next slide The quantum optics community often works with extended /delocalized, not particle like, photons The quantum informarion community needs spatially separated, but not particle like photons The accumulation time argument was coined to distinguish between a highly localized and a more delocalized photon. Until the advent of single molecule techniques it was somewhat uncertain to say whether this argument was satisfied or no, since a very detailed knowledge on detector function is required.

2. Different views on the photongreen: the photon is rather a particlered: the photon may be extended or delocalized Albert Einstein 1955(see for example H.Paul, Photonen) Heute glaubt zwar jeder Lump, er wisse es (was ein Photon ist) aber er täuscht sich Richard Feynman 1985 Princeton University Press QED The strange theory of light and matter p14 and p 16 We know that light is made of particles (NO SPECIFICATION OF THE TERM PARTICLE) because we can take a very sensitive instrument thatmakes click when light shines on it, and if light gets dimmer, the click remain just as loud – there are just fewer … I told you about the photomultiplier in orde to ill ustrate an essential phenomenon that you may not have been familiar with – that light is made of particles Willis E. Lamb jr. 1995Appl. Phys. B Anti Photon (60 th birthday of H.Walther) Only a comedy of errors led to its popularity Thomas Walther / Herbert Walther Was ist Licht C.H. Beck Seite 52 Nach unserer heutigen Vorstellung dürfen wir uns Photonen nicht als lokalisierte Teilchen vorstellen, sie sind dagegen über einen bestimmten Bereich verschmiert Anton Zeilinger 2005Happy centenary, photon Nature 433, 230 – 238 modern experiments – especially those using photon pairs – have beautifully confirmed itscorpuscular character (HOW TO DISTINGIUSH CORPUSCULER CHARACTER FROM PARTICLE CHARAKTER) Roy Glauber 2005Nobel Prize lecture It became clear that the particle picture(NO SPECIFICATION OF THE TERM PARTICLE of light quanta, whatever were the dilemmas that accompanied it, was here to stay” Gerhard Rempe 2005Licht in der Quantenoptik, Sterne und Weltraum Special 1- 2005), 70 - 75 ..welches Experiment muss durchgeführt werden, um die Teilcheneigenschaften des Lichts zweifelsfrei zudokumentieren? Aus heutiger Sicht kann zum Beispiel der Photoeffekt, den Einstein als Argument für die Teilchennatur des Lichts verwendet hat, diese nicht eindeutig bezeugen. Konzeptionell eindeutige Experimente zum einwandfreien Nachweis der Photonenatur des Lichts führten Physiker erst kürzlich durch. (CHANGE IN TERMINOLGY FROM PARTICLE TO PHOTON)

3. What does a point like photon mean in terms of technical physics Short wave radio waves – 50 m wavelength - 10 pW / m2 radio amateurs 1 kW in 1000 km distance, i.e. at a cylinder surface of 4 p 10 km* 1 000 000 km2 ( = approx 1014 m2) E = h * n = h * c / l =(6.625 *10-34 J s ) * (3 * 108 m / s) / 50 m =0,4 * 10-26 J 2.5 * 1026photons / J ------ 2.5 * 1015photons / 10 pJ 10 pW/m2= 2.5 * 1015 radio photons per sec and m2 * 1.5 * 10-7 sec per optical cycle 3.25* 108 radio photons per optical cycle and m2= (1.8 * 104)radio photons p 18 radio photons / mm and optical cycle Situations with 1 Photon / mm are realistic Corresponds transversally to 1 point like photon every 50 mmand in the direction of propagation every 50 m. The rest of the wave is empty These „lone“ photons have to organize themselves into a wave, in spite of the fact that photons should not interact with each other at a distance The notion of a point like photon results in strange technical physics

4. Photons as quanta versus photons as particles photo effect Compton effect antibunching experiments with photon pairs cannot distinguish between extended / delocalized photons and particle („point“) like photons To distinguish this, the accumulation time argument has been coined

5. The accumulation time argument If the photon were extended or delocalized, it would never be possible that a single photon could be absorbed by a single atom or molecule since the flyby time is too short and the local energy density is too small Absorbing atom or molecule: a few Angström l Extended or delocalised photon Particle like photon less than a few Angström The fact that single photons can be measured by suitable detectors is not sufficient to satisfy the accumulation time argument, since details of the primary process of photon conversion are not safely known. The only convincing experiment is absorption of a single, freely travelling photon by a single free (not in a cavity) atom or molecule

6. The accumulation time argumentis not yet satisfiedHowever, microscopy with single photons may satisfy this arguments To excite a single atom or molecule not interacting with a cavity, Kilowatt peak powers are needed 1 kW =1021 Photons/sec = 106 Photons / optical cycle this is pretty close to 1 hn portion per optical cycle on the cross section of a typical dye molecule In any case it is far away from 1 photon

7. Summary The photon as a(point like)particle(in the common sense of the word) would result in essentially empty technical waves The accumulation time argument, thought to prove the strict particle aspect of photons, is not yetsatisfied experimentally If at all, the photon is a delocalized particle – but what is that ?? Without a clear message from the accumulation time argument the photon is rather an electromagnetic field Pulse (a single photon wave packet) than a strict particle, with a similar „corpuscular character“ as a laser pulse

8. Different views on the photon Albert Einstein1955 letter to M.Besso Heute glaubt zwar jeder Lump, er wisse (was ein Photon ist) aber er täuscht sich Richard Feynman Princeton University Press 1985 QED The strange theory of light and matter p14 and p 16 We know that light is made of particles because we can take a very sensitive instrument that makes click when light shines on it, and if light gets dimmer, the click remain just as loud – there are just fewer … I told you about the photomultiplier in orde to ill ustrate an essential phenomenon that you may not have been familiar with – that light is made of particles W.E. Lamb jr. 1995 Appl. Phys. BAnti Photon (60 th birthday of H.Walther) Only a comedy of errors led to its popularity Thomas Walther / Herbert Walther Was ist Licht C.H. Beck Seite 52 Nach unserer heutigen Vorstellung dürfen wir uns Photonen nicht als lokalisierte Teilchen vorstellen, sie sind dagegen über einen bestimmten Bereich verschmiert A Zeilinger Happy centenary, photon 2005 Nature 433, 230 – 238 .. modern experiments – especially those using photon pairs – have beautifully confirmed its corpuscular character Roy Glauber Nobel Prize lecture 2005 It became clear that the particle picture of light quanta, whatever were the dilemmas that accompanied it, was here to stay”