# is a single photon a wave? Is it a wave packet?

How could it split and interfere? How could a wave packet, with many frequencies, be a photon with one freq.?

Thank you very much.

• Any source for saying photon to be wave packet will be helpful. – Immortal Player Feb 8 '14 at 19:03
• I'm beginner in modern physics. – aayyachi Feb 9 '14 at 0:00
• A one-photon state can be a linear (quantum) superposition of any number of one-photon energy eigenstates, polarisations and directions, so this is how a wavepacket photon looks. See my answer to a very like, if not the same, question here. A wavepacket therefore has an uncertain energy when this quantity is measured (by the Hamiltonian observable). – Selene Routley Feb 9 '14 at 12:24

It's better to know about wave particle duality before going to your question.

Lets know what Broglie says in his noble lecture (December 12, 1929):

This is the extracted passage which makes an attempt to say the importance of both wave and particle nature.

The existence of a granular structure of light and of other radiations was confirmed by the discovery of the photoelectric effect. If a beam of light or of X-rays falls on a piece of matter, the latter will emit rapidly moving electrons. The kinetic energy of these electrons increases linearly with the frequency of the incident radiation and is independent of its intensity. This phenomenon can be explained simply by assuming that the radiation is composed of quanta hv capable of yielding all their energy to an electron of the 246 1929 L.DE BROGLIE irradiated body: one is thus led to the theory of light quanta proposed by Einstein in 1905 and which is, after all, a reversion to Newton’s corpuscular theory, completed by the relation for the proportionality between the energy of the corpuscles and the frequency. A number of arguments were put forward by Einstein in support of his viewpoint and in 1922 the discovery by A. H. Compton of the X-ray scattering phenomenon which bears his name confirmed it. Nevertheless, it was still necessary to adopt the wave theory to account for interference and diffraction phenomena and no way whatsoever of reconciling the wave theory with the existence of light corpuscles could be visualized. As stated, Planck’s investigations cast doubts on the validity of very small scale mechanics. Let us consider a material point which describes a small trajectory which is closed or else turning back on itself. According to classical dynamics there are numberless motions of this type which are possible complying with the initial conditions, and the possible values for the energy of the moving body form a continuous sequence. On the other hand Planck was led to assume that only certain preferred motions, quantized motions, are possible or at least stable, since energy can only assume values forming a discontinuous sequence. This concept seemed rather strange at first but its value had to be recognized because it was this concept which brought Planck to the correct law of black-body radiation and because it then proved its fruitfulness in many other fields. Lastly, it was on the concept of atomic motion quantization that Bohr based his famous theory of the atom; it is SO familiar to scientists that I shall not summarize it here. The necessity of assuming for light two contradictory theories-that of waves and that of corpuscles - and the inability to understand why, among the infinity of motions which an electron ought to be able to have in the atom according to classical concepts, only certain ones were possible: such were the enigmas confronting physicists at the time I resumed my studies of theoretical physics.

In the following passage Broglie predicts the existence of corpuscle accompanied by wave.

When I started to ponder these difficulties two things struck me in the main. Firstly the light-quantum theory cannot be regarded as satisfactory since it defines the energy of a light corpuscle by the relation W = hv which contains a frequency v. Now a purely corpuscular theory does not contain any element permitting the definition of a frequency. This reason alone renders it necessary in the case of light to introduce simultaneously the corpuscle concept and the concept of periodicity. On the other hand the determination of the stable motions of the electrons in the atom involves whole numbers, and so far the only phenomena in which whole numbers were involved in physics were those of interference and of eigenvibrations. That suggested the idea to me that electrons themselves could not be represented as simple corpuscles either, but that a periodicity had also to be assigned to them too. I thus arrived at the following overall concept which guided my studies: for both matter and radiations, light in particular, it is necessary to introduce the corpuscle concept and the wave concept at the same time. In other words the existence of corpuscles accompanied by waves has to be assumed in all cases. However, since corpuscles and waves cannot be independent because, according to Bohr’s expression, they constitute two complementary forces of reality, it must be possible to establish a certain parallelism between the motion of a corpuscle and the propagation of the associated wave. The first objective to achieve had, therefore, to be to establish this correspondence.

The following passage is very much interesting and much related to your question. So, pay little more attention.

We shall content ourselves here by considering the general significance of the results obtained. To sum up the meaning of wave mechanics it can be stated that: "A wave must be associated with each corpuscle and only the study of the wave’s propagation will yield information to us on the successive positions of the corpuscle in space". In conventional large-scale mechanical phenomena the anticipated positions lie along a curve which is the trajectory in the conventional meaning of the word. But what happens if the wave does not propagate according to the laws of optical geometry, if, say, there are interferences and diffraction? Then it is no longer possible to assign to the corpuscle a motion complying with classical dynamics, that much is certain. Is it even still possible to assume that at each moment the corpuscle occupies a well-defined position in the wave and that the wave in its propagation carries the corpuscle along in the same way as a wave would carry along a cork? These are difficult questions and to discuss them would take us too far and even to the confines of philosophy. All that I shall say about them here is that nowadays the tendency in general is to assume that it is not constantly possible to assign to the corpuscle a well-defined position in the wave. I must restrict myself to the assertion that when an observation is carried out enabling the localization of the corpuscle, the observer is invariably induced to assign to the corpuscle a position in the interior of the wave and the probability of it being at a particular point M of the wave is proportional to the square of the amplitude, that is to say the intensity at M.

If we consider a cloud of corpuscles associated with the same wave, the intensity of the wave at each point is proportional to the cloud density at that point (i.e. to the number of corpuscles per unit volume around that point). This hypothesis is necessary to explain how, in the case of light interferences, the light energy is concentrated at the points where the wave intensity is maximum: if in fact it is assumed that the light energy is carried by light corpuscles, photons, then the photon density in the wave must be proportional to the intensity.

By my understanding from the Broglie lecture, photons won't split. Photons are associated with wave, which superimpose constructively and destructively to form interference pattern. And the points of maximum intensity corresponds to the region where the photon of the wave are more likely to be present.

If any correction (or wrong understanding) here, I will be happy to know.

• You can read entire noble lecture of Broglie here – Immortal Player Feb 8 '14 at 8:44
• Should we then consider that a classical electromagnetic wave packet pulse (let's say gaussian) is associated to many kind of photons (different freq.)? Could a short monochromatic pulse have exactly one single photon? Thank you very much! – aayyachi Feb 9 '14 at 0:51

You are confusing terms. Photons have energies, and waves have frequencies.
Generally (and not so accurately), a wave function can be expressed as a superposition of mutually orthogonal wave function, called eigen-functions, each associated with an eigen-value (energy in your case), so that when a measurement (yours, or some interaction with an appropriate macroscopical body), the wave function will collapse to one of the energy eigen-functions, and the corresponding eigen-value is the energy you will measure.

• isn't f = E/h a freq. of the photon associated wave? I'm not talking about measurement, but in general – aayyachi Feb 7 '14 at 21:36
• @aayyachi it is, once there's a definite $E$ for the photon. If there isn't, the nature of It's existence is not really easy for our minds to comprehend. – user76568 Feb 7 '14 at 21:40
• in other words, according to popular concepts, what is the minimal number of photons per electromagnetic wave packet – aayyachi Feb 7 '14 at 23:26