# Accuracy of various optical instruments

I understand that this may not be the type of question allowed here, but I'm not sure. Feel free to close this if you feel that it shouldn't be here

I'm planning on carrying out a certain set of experiments regarding quantum mechanics and double-slit setups in half a year or so. The experiments will be similar to the quantum eraser. The issue is, I don't know what sort of experiments are feasible. I'd like someone who has experience with such optical instruments to explain the avaialiblity, cost, and feasability of the following instruments. Also if they can be found in a lab of any institution that deals with QM.

Whenever I ask for a value, I only want a ballpark figure or an order of magnitude

1. Single-photon detectors: I've seen these referred to as "click-detectors". How accurate are they? Do they measure every single photon that eneters or only a significant fraction of them?

2. One-photon-at-a-time emitters: How accurate?

3. Fiber optics:

• If I send a single photon through a fiber optic of some length, how much percentage of the time will it be lost? If there are different types of fiber optics, please let me know which one is the best for this.

• If I have a pair of parallel-running fiber optics, how coherent can they keep two coherent sources? For example, if set up a normal YDSE, feed fiber optics into the slits, and feed the source into the fiber optics some $s$ meters away, will I observe a fringe pattern?

4. Screen:What sort of screens do we get nowadays for YDSEs? Can they detect single photons with xyz accuracy? How many photons are required on a screen to see a distinct fringe pattern? In the quantum eraser, they used a detector on a tractor. Any reason for doing this instead of using a screen? Are there any screens which can detect single photons, pinpoint their location with xyz accuracy, and feed the data to a computer?

5. Environment: While doing a single-photon YDSE, what sort of environment is required? After all, we need to block all external photons. Do we need to sheath the YDSE with something? If so, how will we view the results without tampering with them (taking a photographic screen out to view it can easily damage it)?

6. Mirrors/lenses: How good are mirrors for directing single photons? Do they eat up xyz% of the photons? Do they tamper with coherence?

7. Beam splitters: Same situation as mirrors.

8. Beta-barium borate crystal (used in the quantum eraser): Same situation as splitters and mirrors.

An example setup would be where I use a beam splitter to create two possible paths for a single photon. These paths are directed to a YDSE, in such a way that they cannot interfere till they reach the slits. One path corresponds to one slit. I've though of fiber optics so that the paths are kept from interfering with each other. A thin opaque barrier might also do the trick. For this experiment to work, basically, it should be able to build up an interference pattern from one photon emitted at a time, with the interference happening ONLY in the YDSE (the reason for this is that I want to do something else involving the BBO crystal in between).

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Your question might be a bit broad but I like it as it is about the heart of physics: the experiment. Single-photon generation and detection is possible but as far as I know there is not much off-the-shelf equipment. A simple detector with a high efficiency (up to 90%) is an avalanche photo diode. NIST has a nice overview of the technologies you are looking for: qubit.nist.gov/qiset-PDF/Nam.QISET2004.pdf – Alexander Feb 15 '12 at 6:44
Nice pdf! Unfortunately, it seems to be a slideshow background of a presentation, so it doesn't completely answer the question (but it helps). – Manishearth Feb 15 '12 at 8:41
Just so you know, we usually kind of discourage asking a whole bunch of different questions in a single post like this. It's not so bad if all the questions are related, so I wouldn't say you have to split this up, but I think some of the things you're asking could be factored out into their own posts. – David Z Feb 15 '12 at 18:33
Id prefer not to have to write a bunch of teensy posts.. Well, my reasoning was, if someone knows the answer to one of these, then he/she will probably know most of the rest. – Manishearth Feb 15 '12 at 18:37
Updated my answer below. Also check this website:->physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm – Antillar Maximus Feb 21 '12 at 13:42

From my experience:

Single-Photon detectors:

The real problem is not dark counts, but spurious counts. The shot-noise limit is $\frac{1}{\sqrt{N}}$ where N is the number of photons in the field for a coherent source (i.e a laser). Reaching the Heisenberg sensitivity of $\frac{1}{{N}}$ is very difficult to realize experimentally. Detector efficiency depends on the wavelength you wish to employ. Telecom/IR detectors are much noisier (40%) than Blue detectors (70%).

Single-photon source:

I don't think there are any commercially available single photon sources. You may have to setup some kind of an interferometric projection measurement to get it.

Fiber-Optics

Single mode fibers are pretty reliable. I'd suggest a polarization maintaining fiber if your experiments are about the polarization of the light. What do you mean by coherence? Do you mean spatial coherence or temporal coherence?

I'll fill the rest in tomorrow. :)

Are you trying to demonstrate interference between two correlated photons or are you trying to show that a photon interferes with itself? Since you mentioned BBO,a nonlinear crystal used in Parametric downconversion, I suspect you are trying to show the latter. In this case you can try using a modified Hong-Ou-Mandel Interferometer.

Optical Fibers

As for OFCs, fiber beamsplitters are commercially available (check Thorlabs or Newport).

Detection

I am not sure about screens, but photographic film was used in the past. Also note that the human eye is a very sensitive detector in itself, so a scintillation screen may also work (not sure where you will get them). If you are looking for cheap APD (avalanche photodiode) sources, forget it. They are too noisy. Since you are concerned about XYZ accuracy, a CCD imaging system would be your best bet. But these are going to be prohibitively expensive because to keep the noise low, they have to be at liquid He temperatures.

Environment

The environmental conditions are not critical. Just turn off all the lights in the room and cover the photodetector with a cardboard box. Under no circumstances should a single-photon detector be exposed to ambient light when it is active. That is a sure way to burn it.

Linear Optical elements

No special optics are necessary. Of course, they should be coated for the appropriate wavelength but other than that nothing special. It is useful to have a few sets of neutral-density filters (attenuators) at hand to play with the intensity of the input light. As for beam-splitters, you can either use fixed ratio or polarizing beam-splitter cubes.

Nonlinear crystals

Nonlinear crystals such as BBO are more of a need based thing. Just make sure that they are brewster cut.

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I think its temporal. Basically, if I conduct a YDSE with the source far away and pipe a fiber optic to each slit, there should be a constant phase difference; giving an interference pattern. – Manishearth Feb 21 '12 at 2:32
Updated with new comments. – Antillar Maximus Feb 21 '12 at 13:41
Thanks! This helps a lot, and tells me what I can or can't use. I'm not exactly proving single photon interference, but I'm using it to analyse the nature of a QM observation. – Manishearth Feb 21 '12 at 13:53

Single-photon detectors: I've seen these referred to as "click-detectors". How accurate are they? Do they measure every single photon that eneters or only a significant fraction of them?

Roughly 50 %, can be higher nowadays I believe.

If I send a single photon through a fiber optic of some length, how much percentage of the time will it be lost? If there are different types of fiber optics, please let me know which one is the best for this.

Just calculate the transmission T of your fibre, take 1-T and you'll get the probability to loose your photon.

If I have a pair of parallel-running fiber optics, how coherent can they keep two coherent sources? For example, if set up a normal YDSE, feed fiber optics into the slits, and feed the source into the fiber optics some s meters away, will I observe a fringe pattern?

If you use a single-mode fibre, there will be no loss of coherence.

Mirrors/lenses: How good are mirrors for directing single photons? Do they eat up xyz% of the photons? Do they tamper with coherence? Beam splitters: Same situation as mirrors.

I believe the answer is the same as for fibre losses: you always have some losses (reflection at lens surfaces, absorption in mirror coatings, scattering), so some photons will be lost. They do not cause problems with coherence.

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Thanks! That's enough for me to get a general idea! – Manishearth Feb 21 '12 at 1:57