June 2015
Spotlight Summary by Enrique Gálvez
Shot-by-shot imaging of Hong–Ou–Mandel interference with an intensified sCMOS camera
Quantum phenomena baffle us through counter-intuitive predictions that are consistently verified. In a landmark experiment of quantum optics performed by L. Mandel and coworkers C.K. Hong and Z.Y. Ou, also known as the “Hong-Ou-Mandel dip,” two identical photons reach a beam splitter in such a way that they are indistinguishable from each other. What ensues is quantum interference. This is a phenomenon that prescribes the behavior between photons and particles, not through a force field but via the superposition of their quantum amplitudes. It is the same one responsible for the “exchange interaction” between electrons, for example, or even the Pauli exclusion principle, which specifies the orbital order of atoms. In the Hong-Ou-Mandel experiment the photons’ quantum amplitudes interfere destructively resulting in both photons coming out of the same port of the beam splitter (either one) but not through separate ports. With no capability to resolve the number of photons, Mandel and co-workers had detectors at the two ports of the beam splitter, and when scanning a delay that made the quantum amplitudes of the photons overlap (that is, made indistinguishable), they saw a “dip” in the coincidences (the destructive interference). The conclusion then is that the photons went together because they did not part ways at the beam splitter. It is still intriguing to ask: What is of these photon pairs, bi-photons, once they leave the same port?
In a beautiful recent experiment, M. Jachura and R. Chrapkiewicz from the University of Warsaw, report on the life beyond that joint journey that quantum mechanics endows to these photons. The authors use a variation of the classic experiment in which the incident photons are collinear. Their distinguishability is varied by a temporal delay using clever quantum-optics tricks. The photons are distinguishable by polarization, one horizontal and the other vertical, but when faced with a polarizing beam splitter that effectively splits “diagonal” and “antidiagonal” components, amplitudes of which both polarizations have, the pairs face their Hong-Ou-Mandel quantum decision. The polarizing beam splitter merely displaces the paths of the ports, so the photons leave the splitter in one of two spatially distinguishable regions. Once together in flight they face their ultimate demise: detectors; but in this case the authors selected a single-photon imaging device that they developed, which recorded the two output regions of the splitter. Photons carry a “spatial mode.” That is, they have a transverse probability amplitude (note: photons are not “tiny” particles), so they occupy a transverse spatial region as they propagate. Upon reaching the camera, the quantum roll of dice takes place, along with the wholeness of the quantum, and each photon collapses onto a camera pixel according to the probability amplitude of its transverse mode. The authors then recorded the hits on the camera and found the unintuitive but expected result: pairs of hits on each region, with very few mishits, as shown in the video of their detections. The video (and data) also shows that when made temporally distinguishable, quantum interference disappears. With these results the authors retrieved a wealth of quantum data plus a stunning Hong-Ou-Mandel dip with a visibility of 96.3 %. The imaging device became a photon-number detector, increasing their detection fidelity. In addition, the photons were used to trigger one photon’s position off the others detection, and reconstruct the transverse mode amplitude of the individual photons. This experiment underlies a new quantum imaging technique that can be used for inquiring further about correlated spatial modes of photons, a new type of quantum metrology; and for uncovering more (expected) quantum mysteries.
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In a beautiful recent experiment, M. Jachura and R. Chrapkiewicz from the University of Warsaw, report on the life beyond that joint journey that quantum mechanics endows to these photons. The authors use a variation of the classic experiment in which the incident photons are collinear. Their distinguishability is varied by a temporal delay using clever quantum-optics tricks. The photons are distinguishable by polarization, one horizontal and the other vertical, but when faced with a polarizing beam splitter that effectively splits “diagonal” and “antidiagonal” components, amplitudes of which both polarizations have, the pairs face their Hong-Ou-Mandel quantum decision. The polarizing beam splitter merely displaces the paths of the ports, so the photons leave the splitter in one of two spatially distinguishable regions. Once together in flight they face their ultimate demise: detectors; but in this case the authors selected a single-photon imaging device that they developed, which recorded the two output regions of the splitter. Photons carry a “spatial mode.” That is, they have a transverse probability amplitude (note: photons are not “tiny” particles), so they occupy a transverse spatial region as they propagate. Upon reaching the camera, the quantum roll of dice takes place, along with the wholeness of the quantum, and each photon collapses onto a camera pixel according to the probability amplitude of its transverse mode. The authors then recorded the hits on the camera and found the unintuitive but expected result: pairs of hits on each region, with very few mishits, as shown in the video of their detections. The video (and data) also shows that when made temporally distinguishable, quantum interference disappears. With these results the authors retrieved a wealth of quantum data plus a stunning Hong-Ou-Mandel dip with a visibility of 96.3 %. The imaging device became a photon-number detector, increasing their detection fidelity. In addition, the photons were used to trigger one photon’s position off the others detection, and reconstruct the transverse mode amplitude of the individual photons. This experiment underlies a new quantum imaging technique that can be used for inquiring further about correlated spatial modes of photons, a new type of quantum metrology; and for uncovering more (expected) quantum mysteries.
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Article Information
Shot-by-shot imaging of Hong–Ou–Mandel interference with an intensified sCMOS camera
Michał Jachura and Radosław Chrapkiewicz
Opt. Lett. 40(7) 1540-1543 (2015) View: Abstract | HTML | PDF