Creating radiowave entangled photons

[Ponderer]

I would like to create entangled photons at radiowave frequencies. To do this I thought it might help to understand as much details as possible how entangled photons are created by parametric down-conversion. Since the down-conversion doesn't happen often, what are the special conditions? Are the two photons created by two electrons, or one? What exactly is happening with the electron(s)?

 

[mfb]

Parametric down-conversion does not use electrons at all - a single photon gets converted into two. I would be surprised to see that effect with radio waves.
If coherence is enough (what do you want to do?) you can use a simple dipole antenna.

 

[Ponderer]

Then how does the photon become down converted if the process is not done by electrons?

 

[Ponderer]

If it's not the electrons, then is it something in the nucleus, perhaps the strong force that causes this down conversion? Virtual particles? It must be something in the crystal that causes this.

No, I'm only interested in studying entanglement at radio wavelengths.

 

[mfb]

The strong force does not act on photons.
Electrons in the crystal are certainly relevant for the process (but not individually, just as the whole material), but the details are not well understood.

No, I'm only interested in studying entanglement at radio wavelengths.

What exactly do you want to study?

 

[Ponderer]

Electrons in the crystal are certainly relevant for the process (but not individually, just as the whole material), but the details are not well understood.

I appreciate the honesty! Sounds like the process is complex, involving multiple electrons like you said. If it could be replicated at radio frequencies then it might take critical timing of numerous small antenna elements to produce entangled photons at radio wavelengths.

What exactly do you want to study?

Everything humanly possible about entanglement from an amateurs perspective. I understand so little about it, but I have a good understanding of electronics, antenna theory, and low noise design. For me it seems easier to work with low frequencies given I can keep the noise down. Last, I have no experience with optics, or the required equipment.

One area of special interest is the quantum wave function. It's my understanding experiments still haven't closed all of the loops in Bell's test. There's a lot of talk about the possibility of instantaneous communication between entangled particles. I'm not doubting QM or relativity, but some scientists believe instantaneous communication could be possible while not breaking the speed of light. I don't know how lol. Perhaps the entanglement link is tunneling through some region of non spacetime we're unaware of?

 

[Cthugha]

I appreciate the honesty! Sounds like the process is complex, involving multiple electrons like you said. If it could be replicated at radio frequencies then it might take critical timing of numerous small antenna elements to produce entangled photons at radio wavelengths.

The process is pretty well understood and: yes, it involves the collective response of electrons in the crystal. See e.g. the thesis of Paul Kwiat and references therein: http://copilot.caltech.edu/classes/APh150_Intro_Quantum_Devices/kwiat-thesis.pdf

In a nutshell, what happens is the following: The electrons in the system will respond collectively to the em field arriving. In a metal they could probably screen the field. In other systems, the response will be somewhat slower. The factor connecting the arriving field and the polarization created in the material is known as optical susceptibility. This may just be a constant value or it may be more complicated. For example for asymmetric crystals it might depend on the direction or for stronger fields, the field itself may change the susceptibility, so that the response becomes nonlinear. A crystal showing such a nontrivial response (a so called chi2 non-linearity) may be useful for down conversion. Just compare it to a classical non-linear oscillator like a non-linear pendulum. If you drive such an oscillator at a fixed frequency, you will also get a response at other frequencies (especially twice or half) than the one you use for driving. That is a pretty trivial result from Fourier analysis. Now the interesting step to "quantum" lies just in the fact, that this kind of response still occurs at the single photon level.

One area of special interest is the quantum wave function. It's my understanding experiments still haven't closed all of the loops in Bell's test. There's a lot of talk about the possibility of instantaneous communication between entangled particles. I'm not doubting QM or relativity, but some scientists believe instantaneous communication could be possible while not breaking the speed of light. I don't know how lol. Perhaps the entanglement link is tunneling through some region of non spacetime we're unaware of?

The loop holes have been closed, just not all of them simultaneously in a single experiment. And no real scientist thinks that instantaneous communication (in the sense of being able to transfer information) could be possible.

 

[vanhees71]

Nevertheless the original question is interesting. I guess it becomes more and more complicated to create Fock states of photons the lower you go in energy. It is not easy to create really single photons. One way is to use parametric down conversion, using one of the photons to detect that such an event occurred and then one has for sure another single photon.

Dimming down light sources does not give you simply single-photon states but coherent states. Dimming a laser down to have on the average only 1 (or even less) photons in a certain interval of time does not mean that you have created a single-photon Fock state but that you have a coherent state with very small average photon number. So it's mostly the vacuum state + a single-photon state, but also all other higher-photon-number states are also present.

So the question is, whether there is some mechanism practically usable to make single photons at radio frequencies or even to make entangled states of two radio-frequency photons somehow. I don't know, how low the frequencies of entangled photon pairs can be made in practice. As far as I know the usual experiments are done in the realm of visible light (roughly 400-800 nm of wavelength).

 

[Cthugha]

Nevertheless the original question is interesting. I guess it becomes more and more complicated to create Fock states of photons the lower you go in energy. It is not easy to create really single photons. One way is to use parametric down conversion, using one of the photons to detect that such an event occurred and then one has for sure another single photon.

Dimming down light sources does not give you simply single-photon states but coherent states. Dimming a laser down to have on the average only 1 (or even less) photons in a certain interval of time does not mean that you have created a single-photon Fock state but that you have a coherent state with very small average photon number. So it's mostly the vacuum state + a single-photon state, but also all other higher-photon-number states are also present.

So the question is, whether there is some mechanism practically usable to make single photons at radio frequencies or even to make entangled states of two radio-frequency photons somehow. I don't know, how low the frequencies of entangled photon pairs can be made in practice. As far as I know the usual experiments are done in the realm of visible light (roughly 400-800 nm of wavelength).

Well, the problem in SPDC is to achieve phase matching conditions, so one would neeed to find a material, where the refractive indices for the pump beam wavelength and the signal/idler wavelength are such that energy and wavevector mismatch are minimal. It is my feeling that it should be pretty complicated to do that for radio waves as you would need very specific crystal dimensions, but I have not done the math.

The lowest single photon frequency I am aware of is in the microwave range ( Nature Physics 7, 154-158 (2011), http://www.nature.com/nphys/journal/v7/n2/full/nphys1845.html). I am not sure about entangled photons, though.

And: yes, of course dimming does not give single photons.

 

[vanhees71]

Great! It's a nice example for, how difficult it is to prepare really one-photon states!

 

[Ponderer]

Cthugha, thanks for the reply. I'm still digesting all of your helpful words.

Nevertheless the original question is interesting. I guess it becomes more and more complicated to create Fock states of photons the lower you go in energy. It is not easy to create really single photons. One way is to use parametric down conversion, using one of the photons to detect that such an event occurred and then one has for sure another single photon.

Dimming down light sources does not give you simply single-photon states but coherent states. Dimming a laser down to have on the average only 1 (or even less) photons in a certain interval of time does not mean that you have created a single-photon Fock state but that you have a coherent state with very small average photon number. So it's mostly the vacuum state + a single-photon state, but also all other higher-photon-number states are also present.

So the question is, whether there is some mechanism practically usable to make single photons at radio frequencies or even to make entangled states of two radio-frequency photons somehow. I don't know, how low the frequencies of entangled photon pairs can be made in practice. As far as I know the usual experiments are done in the realm of visible light (roughly 400-800 nm of wavelength).

I've heard such a packet of photons referred to as a photon wave train. In electronics it's easy enough to control the oscillating electrical current signal, but I'm not aware of a way to force the single radio wave photon to emit when I want. If doesn't emit, then there should be no radiation resistance. When it emits, there should be radiation resistance. So the circuit should know when a single photon was emitted and how much energy was required. What I have in mind is to send a short pulse to the antenna to hopefully guarantee a single photon.

 

[DrChinese]

Here is about all I have seen much outside of the visible range:

http://arxiv.org/abs/1401.7470

"We report the first entanglement generation experiment using an on-chip slow light device. With highly efficient spontaneous four-wave mixing enhanced by the slow light effect in a coupled resonator optical waveguide based on a silicon photonic crystal, we generated 1.5-μm-band high-dimensional time-bin entangled photon pairs. We undertook two-photon interference experiments and observed the coincidence fringes with visibilities >74%. The present result enables us to realize an on-chip entanglement source with a very small footprint, which is an essential function for quantum information processing based on integrated quantum photonics."

 

[Ponderer]

Well, the problem in SPDC is to achieve phase matching conditions, so one would neeed to find a material, where the refractive indices for the pump beam wavelength and the signal/idler wavelength are such that energy and wavevector mismatch are minimal. It is my feeling that it should be pretty complicated to do that for radio waves as you would need very specific crystal dimensions, but I have not done the math.

The lowest single photon frequency I am aware of is in the microwave range ( Nature Physics 7, 154-158 (2011), http://www.nature.com/nphys/journal/v7/n2/full/nphys1845.html). I am not sure about entangled photons, though.

And: yes, of course dimming does not give single photons.

Using crystals at radio band wavelengths is beyond my present means. What I have in mind is setting up an array of antennas.

 

[Ponderer]

Here is about all I have seen much outside of the visible range:

http://arxiv.org/abs/1401.7470

"We report the first entanglement generation experiment using an on-chip slow light device. With highly efficient spontaneous four-wave mixing enhanced by the slow light effect in a coupled resonator optical waveguide based on a silicon photonic crystal, we generated 1.5-μm-band high-dimensional time-bin entangled photon pairs. We undertook two-photon interference experiments and observed the coincidence fringes with visibilities >74%. The present result enables us to realize an on-chip entanglement source with a very small footprint, which is an essential function for quantum information processing based on integrated quantum photonics."

Appreciated! I'm surprised that's the lowest frequency so far. Very interesting!

 

[Ponderer]

I have a lot of experience using antenna design software such as 4NEC. The software could be helpful in designing an antenna to produce a split near field wave signal which hopefully could on occasion produce two entangled photons. There are perhaps endless antenna designs that would produce a split near field pattern. Of course the NEC engine doesn't work with single photons. Rather, the NEC engine shows electric and magnetic fields for both near and far fields. I'm wondering if there's a specific E-H field pattern I could try to achieve to encourage entangled photon generation. That's all the software provides in terms of the radiation. The software calculates the antenna radiation resistance, and gives the total radiated power, which provides energy given a time range.

So let's say we have an antenna, designed such that under normal operating power levels it radiates two beams at say 20 degrees apart, one that is horizontally polarized, the other vertically polarized. We're interested in single photons, so we feed the antenna one pulse at a time where the current is low enough to limit two photons per pulse. My question is, would it help any to design the antenna such that according to NEC the fields of the two beams are semi connected? I know this kind of language must seem odd since radio wavelength entangled photons is uncharted territory, but the E and H fields is all I know of in antenna theory. I mean, the antenna software doesn't work with wave functions. The far fields show how many photons are expected at a specific angle per unit of time. Perhaps this could be seen as the wave function. Is it even remotely possible that entangled photons are created by a link of fields, perhaps near fields? Maybe the near fields are an important role in creating entangled photons.

I'm probably not the best at describing my thoughts. So here's a summary. What I invasion is an antenna that produces two beams, horizontally and vertical polarized, from one source. Furthermore, since the antenna provides radiation resistance, I can calculate the necessary pulse current required to produce two photons. According to antenna theory given the energy calculations, such an antenna would produce two simultaneous photons per pulse at perpendicular polarizations. Let's say the NEC software is correct in that at least sometimes two of such photons are simultaneously detected at the predicted polarizations. Does that guarantee the two photons are entangled?

Thanks for any input and help.

 

[Vanadium 50]

This is not going to work. To be dealing with single photons at radio energies means that the whole system needs to be cryogenic. Probably uK or even nK. Otherwise you are going to drown in thermal emission.

 

[Marceli]

Why are you looking for single photons or electrons and not for entangle group of photons or electrons?

 

[Nugatory]

Why are you looking for single photons or electrons and not for entangle group of photons or electrons?

Groups of entangled particles aren't especially interesting, because they display the same statistical behavior as groups of unentangled particles. To actually observe and study entanglement, you need to match up the members of the individual pairs.

 

[Marceli]

Instead of using photons why you not using electrons to get similar result? Entangle electrons are getting superconductive in high ambient temperatures and can make good low frequency quite sensitive RF antenna. Referring link:
http://www.cifar.ca/live-webcast-cifar-senior-fellow-subir-sachdev-on-quantum-entanglement-and-superconductivity

 

[Ponderer]

This is not going to work. To be dealing with single photons at radio energies means that the whole system needs to be cryogenic. Probably uK or even nK. Otherwise you are going to drown in thermal emission.

What you said surprised me because I always thought these type of experiments would work at room temps. So I've been working hard at this with the aid of simulators such as ltspice and 4nec2. To my surprise the highest temperature I could detect a 10MHz photon was 5uK. By increasing the photon to 10GHz I was able to increase the temp to 5mK. A 100GHz photo would require a 50mK environment. M BTW these simulated experiments were regarding a single photon for the entire experiment, not a continuos stream of photons.

So I thought to incorporate a high Q filter in the circuit and produce a stream of photons, but limited to one photon per wavelength. Actually in my setup it emitted one photon every other wavelength. There's no fixed rate at which the photons must be emitted. For example it could emit one photon every 100 wavelengths, but the filter Q must be higher. So that did the trick, allowing the receiver circuit to detect the photons at room temp. The down side is that I won't be able to see each individual photon pulse on the oscilloscope, but the circuit will know when the stream of photons are being emitted, the polarities of both entangled photons (if they're entangled), the energy per photon, and whatever else might be learned. One could do a lot of experiments such as placing something in the path of one of the entangled photon streams to see if and how it affects the other entangled stream. Although I'm not sure how to tell if the photons are entangled. I'm sure the photon polarities will always be consistent and known because they will be emitted from an antenna where the electrical current axis remains the same.

Any thoughts?

 

[Vanadium 50]

I think you should see what sorts of Qs you need. I also think you should look at the thermal noise in this putative circuit.

 

[Ponderer]

I'm still trying to see what the benefit of seeing a stream photons. Albeit it would be a single photon every few wavelengths. It would be interesting to actually see the signal of a single photon. So if one wanted to see a single photon, rather than a stream, then how expensive is the equipment that would get the temperature down to 5mK?

 

[Vanadium 50]

then how expensive is the equipment that would get the temperature down to 5mK?

Very. This is not my field, but I would guess hundreds of thousands.

 

[Dundeephysics]

I believe dimming an antenna's current requires the current to be ridiculously small (unachievable by todays SMU's). Also, generating single photons requires a two state system that behaves quantum mechanically and these systems need to be in cryogenic temperatures to be isolated from the environment.

 

[Dundeephysics]

Very. This is not my field, but I would guess hundreds of thousands.

Yes depending on the temperature needed to be achieved which might not need to be less than 20k and depending on the volume, cryostates prices change from a minimum of 10K to may be 200K. In our lab we have a cryostat that is probably quarter of a liter and goes down to 15k we bought for 25,000 GBP

 

[Ponderer]

I believe dimming an antenna's current requires the current to be ridiculously small (unachievable by todays SMU's). Also, generating single photons requires a two state system that behaves quantum mechanically and these systems need to be in cryogenic temperatures to be isolated from the environment.

But why can't a resistor be used to lower the current?

 

[vanhees71]

The difficulty is not to produce a photon but to produce one and only one photon. I don't know the specific buildup of your source in your simulator, but isn't it rather a coherent wave at very low intensity what's simulated? This then is not a one-photon Fock state but mostly vacuum in superposition with all other Fock states with arbitrary photon number.

 

[Dundeephysics]

But why can't a resistor be used to lower the current?

Sorry, I didnt mean simply lowering the current would generate single photons. But regarding that I think it's difficult to achieve single current path using resistors that are under cryogenic temperatures and if that would be achievable I think it would still not work as single electrons oscillating in an antenna are not the ones that create single radiowaves in an antenna.
The method that I am aware of that scientists use to achieve single photons today is using quantum mechanical two states systems like quantum dots that can confine single electrons to produce single photons when they relax.

 

[mfb]

15 K is achievable with conventional compression/expansion cycles and helium. Going below ~2K is much more tricky, especially if the cooled volume is large (for an antenna).
It is possible, sure - the CUORE detector has 1m³ cooled to 6 mK (news) - but it will certainly require expensive equipment and probably some custom development.

 

[Vanadium 50]

CUORE's cryogenics cost something around $2M - perhaps more, since these are capital costs and don't include scientific labor.

This whole thing was triggered by "I know a lot about radio, wouldn't that be easier?" and it's pretty clear that it is not easier. It's not been stated how you create a single pair of photons in the radio spectrum, much less an entangled pair. Also, the background is huge: let's say we're working at 1 GHz, 1 square meter (probably way too small) and 6 mK. Black body radiation is 100 million photons per second. One of which is the one you want.