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## Main Question or Discussion Point

Imagine I have a detector which is "single photon sensitive". What that means, I gather, is that if I shine a pulsed laser at the detector with just the right amount of attenuation, I should get to the regime where there is less than 1 photon arriving each time the laser fires a pulse, so the detector shows a sporadic and random blip. (I have never seen this happen, but my research advisor claims that this is what we would observe if the detector is really that good.)

My question is about whether this "photon" is a monochromatic wave or a wavepacket. It is a basic definition that a photon is a quantization of an electromagnetic field of frequency v, so that each photon has energy hv. If I shine my (say) 50 fs pulse on the detector, and attenuate to just less than hv, I must have a single photon arriving at my detector. And by the textbook definition, the photon has a well defined, discrete energy. But by some sort of Fourier argument, my laser light is no longer in a 50 fs pulse - in fact, for the light to be monochromatic it has to extend infinitely in time. Physically, it doesn't make any sense for the pulse to broaden any more than it would due to normal dispersion through the optic. But then I have the exact same bandwidth that I had before.

So how many photons are reaching my detector? Is it one photon, defined by the fact that the detector only registers an interaction for a small fraction of the incident pulses? In that case the photon could only be viewed as a wavepacket, which seems contrary to how it is always presented in the (admittedly) introductory courses that I have taken.

My question is about whether this "photon" is a monochromatic wave or a wavepacket. It is a basic definition that a photon is a quantization of an electromagnetic field of frequency v, so that each photon has energy hv. If I shine my (say) 50 fs pulse on the detector, and attenuate to just less than hv, I must have a single photon arriving at my detector. And by the textbook definition, the photon has a well defined, discrete energy. But by some sort of Fourier argument, my laser light is no longer in a 50 fs pulse - in fact, for the light to be monochromatic it has to extend infinitely in time. Physically, it doesn't make any sense for the pulse to broaden any more than it would due to normal dispersion through the optic. But then I have the exact same bandwidth that I had before.

So how many photons are reaching my detector? Is it one photon, defined by the fact that the detector only registers an interaction for a small fraction of the incident pulses? In that case the photon could only be viewed as a wavepacket, which seems contrary to how it is always presented in the (admittedly) introductory courses that I have taken.