Proof of the Existence of Photons Ryan LaRose April 17, 2017

What is Light? A bit of physics history: 17th Century: Newton says it’s a particle (corpuscles of light) 18th Century: Fresnel, Young say it’s a wave (diffraction, slit experiments) 19th Century: Maxwell gets fancy with PDEs, definitely a wave 20th Century: Planck and Einstein say it’s a particle

What is Light? 21st Century: Alright, enough already. It’s both. Wave Theory: Maxwell’s equations, electricity and magnetism, classical physics Experiment: Interference, diffraction, 2 slit, many slit Particle Theory: Planck, blackbody spectrum, quantum mechanics Experiment? photoelectric effect? not necessarily Question: How can you experimentally prove photons exist?

Argument for the Proof of Photons Light is never detected directly -- we measure currents in a detector → How can we say the granularity of light is due to photons, not electrons? Idea: Show that if a single photon is incident on a beam splitter, it can be transmitted or reflected, but not both In other words... ...If we measure something that cannot be explained by classical physics, we will take this as proof for the existence of photons

The Degree of Second Order Coherence, A measurement of the temporal correlation between transmitted and reflected beam intensities as a function of the time delay (tau) Classically, it can be shown that It follows from the Cauchy-Schwarz inequality that

Experimental Setup Key Components: A: Laser B: Pump half wave plate C: Downconversion crystal D: Detection Assembly

Experimental Procedure Align everything (easier said than done) Obtain maximal photon count rates on coincidence counting unit Measure (done via labpro.vi software) Sample screenshot of measurement shown to the right

Results We’ve got photons, baby! Table below shows results for four different update periods All measurements are significantly (statistically) smaller than one. Hence, photons exist.

Wave Nature Still There The key point of the previous measurement is the presence single photon at the beam splitter → This is ensured by conditioning counts on measuring a photon at the other detector (and made possible by spontaneous parametric downconversion) If this is relaxed, measurements should show photons transmitted AND reflected → ie, light should exhibit wave behavior again This is indeed what we see:

What will light be in the 22nd century? Conclusions Light is particle-like → We showed photons exist by measuring g^(2)(0) < 1, which cannot be accounted for classically Light is wave-like → We showed light reflects and transmits in a single beam splitter when count rates are unconditioned What will light be in the 22nd century?

Questions?