# FeaturedA Phonon detection

1. Jul 20, 2017

### Demystifier

Is it possible to detect a single phonon? If yes, can it be detected at a well-defined position?

2. Jul 20, 2017

I don't know the answer but I'm watching the thread because I think it's an interesting question. I would,however, like to offer some initial thoughts:
1. I think the observation of phonons is known of only indirectly for example by observations of things such as sound propagation and specific heat capacities.
2. Perhaps more observational details can be obtained by observing vibrations of particles that carry the vibrations from place to place. X ray diffraction techniques come to mind.
3. The concept of phonons is probably useful but I'm wondering whether they are actually real things and by that I mean whether it is possible, if only in principle, to actually observe them directly.

3. Jul 20, 2017

### Lord Jestocost

I remember some "listening with a 'quantum ear'": “Local probing of propagating acoustic waves in a gigahertz echo chamber”, Nature Physics 8, 338–343 (2012)

4. Jul 20, 2017

### Demystifier

In principle, a phonon should be possible to detect if there is an interaction of the form
$$H_{\rm int}\propto a_{\rm phonon} b^{\dagger}b + h.c.$$
where $a_{\rm phonon}$ is the destruction operator for the phonon, while $b$ and $b^{\dagger}$ are the destruction and creation operators of some quanta which can absorb and emit phonons. Can someone give an actual example of such an interaction?

Such an interaction can detect a phonon at a well-defined position if the above are local operators, i.e.
$${\cal H}_{\rm int}(x)\propto a_{\rm phonon}(x) b^{\dagger}(x)b(x) + h.c.$$
where $b(x)$ etc are Fourier transforms of the usual operators $b_k$ etc in the momentum space. Examples?

Last edited: Jul 20, 2017
5. Jul 20, 2017

### Demystifier

As far as I can see, this doesn't have much to do with single phonons.

6. Jul 20, 2017

7. Jul 20, 2017

### Demystifier

In the abstract they talk about "coherent phonons in SWNT ensembles". I'm not sure what do they mean by that, but it doesn't sound like a single phonon to me.

EDIT: Indeed, by little googling I found
https://arxiv.org/pdf/1205.6023.pdf
where Eq. (2.1) shows that coherent phonons is a state with an uncertain number of phonons. I want exactly one phonon, not an uncertain number of them.

Last edited: Jul 20, 2017
8. Jul 20, 2017

9. Jul 20, 2017

### TeethWhitener

Phonons are collective lattice motions with well-defined momenta. Thus individual phonons are localized in reciprocal space, not real space.

As for detecting a phonon, why isn't Raman scattering acceptable? It involves either creation (Stokes) or annihilation (anti-Stokes) of a phonon via inelastic photon scattering. Theoretically, you could do the experiment with a single photon, though the cross section of Raman scattering is several orders of magnitude smaller than Rayleigh scattering.

10. Jul 21, 2017

### Demystifier

Phonon (just like photon) does not need to have a well-defined momentum. A superposition of one-phonon states with different momenta is still one phonon.

11. Jul 21, 2017

### Demystifier

12. Jul 21, 2017

### Demystifier

Theoretically, yes. But I would like to know if something like that has been done in practice.

13. Jul 21, 2017

### DrDu

How about Mössbauer effect, specifically the phonon sideband?

14. Jul 23, 2017

### atyy

Maybe

https://www.nature.com/nature/journal/v464/n7289/full/nature08967.html
Qyantum ground state and single-phonon control of a mechanical resonator
A. D. O’Connell, M. Hofheinz, M. Ansmann, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, John M. Martinis & A. N. Cleland

https://arxiv.org/abs/1410.1047
Phonon counting and intensity interferometry of a nanomechanical resonator
Justin D. Cohen, Sean M. Meenehan, Gregory S. MacCabe, Simon Groblacher, Amir H. Safavi-Naeini, Francesco Marsili, Matthew D. Shaw,Oskar Painter

15. Jul 30, 2017

### David Olivier

Plain thermal agitation can be construed as a sea of incoherent phonons, can't it? If so, if you have a molecule that, when excited above a certain threshold, emits a photon, and you maintain a collection of such molecules at a certain temperature such that once in a while the threshold is crossed and a photon is emitted, then it would seem that you have detected a single phonon ‑ namely, the phonon that, added to all others, made the molecule cross that threshold.

16. Jul 31, 2017

### Andy Resnick

17. Jul 31, 2017

### Mech_Engineer

So-called "photon counting" systems do exist, although it does require careful management of your signal to noise ratio. Photon Multiplier Tubes have historically been one method to achieve single photon counting this due to their high gain and high sensitivity.

More reading: Hamamatsu PMT Handbook- Chapter 6: Photon Counting

One modern technology for single photon counting is the "superconducting nanowire single-photon detector," see here:

https://en.wikipedia.org/wiki/Superconducting_nanowire_single-photon_detector
https://www.nist.gov/news-events/news/2013/02/high-efficiency-fastest-single-photon-detector-system

18. Jul 31, 2017

### DrDu

I don't see why not. A photon is a quantum of atomic displacement. It is not required that this quantum is in an eigenstate of energy or momentum.

19. Jul 31, 2017