Imagine you order a delivery of several glass vases in different colors. Each vase is sent as a separate parcel. What would you think of the courier if the parcels arrive apparently undamaged, yet when you open them, it turns out that all the red vases are intact and all the green ones are smashed to pieces? Physicists from the University of Warsaw and the Gdansk University of Technology have demonstrated that when quantum information is transmitted, nature can be as whimsical as this crazy delivery man.

Experiments on individual photons, conducted by physicists from the Faculty of Physics at the University of Warsaw (FUW) and the Faculty of Applied Physics and Mathematics at the Gdansk University of Technology (PG), have revealed yet another counterintuitive feature of the quantum world. When a quantum object is transmitted, its quantum property – whether it behaves as a wave or as a particle – appears to depend on other properties that at first glance have nothing to do with the transmission. These surprising results were published in the research journal *Nature Communications*.

Wave-interference experiments are some of the simplest and most elegant, and can be conducted by almost anyone. When a laser beam is directed at a plate with two slits, we observe a sequence of light and dark fringes. It has long been known that the fringes are visible even when just individual particles – single electrons or photons – pass through the slits. Physicists assume that every individual particle exhibits wave properties, passing through both slits at once and interfering with itself.

The situation is very different when it is possible to detect the path taken by a given photon or electron and determine which slit the particle has passed through, at least in principle. When information about the particle path leaks from the system to the observer, the interference disappears and instead of interference fringes no pattern is observed.

In order for photons to exhibit interference, their wavelengths must be the same, while electrons must have the same energy. However, quantum particles have a number of other properties. For example, they can be polarized (their electrical field vibrates in a certain plane) or have different spin orientations (a quantum property describing the dynamics of an object at rest).

"So far, it has been generally assumed that additional properties such as spin and polarization do not have a non-trivial impact on interference. We decided to study the topic in more depth, and we were surprised by the results we obtained," says Prof. Konrad Banaszek (FUW).

The experiments by physicists from the University of Warsaw and the Gdańsk University of Technology started by generating heralded photons. "The name sounds complicated, but the idea is simple in itself," according to Prof. Czeslaw Radzewicz (FUW). "We generate photons using a process in which they must be created in pairs. When we register one photon, we can be certain that the second was also born, and we know its properties such as direction or wavelength without destroying it. In other words, we use one photon to herald the generation of the second photon."

Each heralded photon was directed individually towards an interferometer, comprising two calcite crystals. In the first crystal, the photon was split and then sent through both arms of the interferometer at the same time. In each arm, researchers altered the polarization of the photon (the plane of vibration of its electrical field) by introducing noise. In the second calcite crystal, the paths were recombined to create a distinctive set of interference fringes, provided that the system did not leak any information as to which arm the given photon travelled along. The final stage of the experiment involved measuring the interference fringes using silicon avalanche photodiodes.

"It turned out that we were able to use measurements of interference fringes to determine how much information had leaked during transmission of the photon through the interferometer. In other words, we could be certain whether any eavesdropping had taken place during transmission," says Dr Michal Karpinski (University of Warsaw, currently University of Oxford), responsible for building the experimental system and conducting the measurements.

The results have revealed a new, surprising property of reality: the polarization of photons, or other internal degrees of freedom, play a highly non-trivial role in interference between the two paths.

"It is almost as though the quality of a courier delivery – for example, whether a glass vase delivered inside a securely packed parcel is still in one piece – depends on whether the vase is green or red. In our world the color has no bearing on whether the vase arrives intact or not. However, the condition of the parcels our 'quantum courier' delivers does indeed depend on internal properties that seem to have nothing to do with interference," according to Prof. Pawel Horodecki (PG).

The results allow physicists to examine the fundamental properties of reality in new, more comprehensive ways, as well as having practical applications in quantum cryptography. The Warsaw and Gdansk physicists have successfully derived a general inequality making it possible to precisely estimate the volume of information leaking from the measurement system.

**Explore further:**
Quantum physics first: Researchers observe single photons in two-slit interferometer experiment

## LarryD

Are we going back to 'hidden variables?

## Zera

## vlaaing peerd

But yes, the more we know, the more we know we don't know. It has always been like that.

## antialias_physorg

This is a very cool result and the ramifications for data transmission security are potentially pretty big. One could gather a running statistic of the information 'leakage'. A jump in the leakage would indicate an eavesdrop attempt (e.g. via weak coupling) that isn't detectable otherwise using quantum encrypted channels. At the very least one could set a 'minimum acceptable level' of leakage for a given channel.

## Doug_Huffman

## LarryD

What I'm suggesting is that as far as the experiment is concerned, which I find most interesting, wouldn't we have to know beforehand the '...internal degrees of freedom..' = 'hidden varaibles' so that we could be sure the interference 'jumps' weren't caused by other factors. Would that mean EPR was correct or that a modification of Bell's theorem is required?

Maybe I'm get getting my slits and polarization thoughts mixed up here.

## antialias_physorg

Where do you get that? Fractals are an ideal construct. Nature is not unlimirted fractal in any way. It does impose limits. E.g. if you talk about landscapes being fractal then you cannot apply this below the scape of atoms or above the scale of the entire planet.

Especially in the quantum realm the notion of fractals becomes meaningless.

The entire gist of the analogy is that a seemingly unrelated propery (color) can have an effect on another seemingly unrelated property (broken/not broken). Don't take the analogy further than that.

## Moebius

## Noumenon

It's because we supply the conceptual framework in which we formulate a means of ordering what is observed. For example, "Waves/particles" are macroscopically derived concepts, while the underlying reality is neither. QM tells us about our experience, not about independent reality,... i.e. depending on experimental arrangement and concepts used one could observe an electron as a wave or as a particle, like wise for all subatomic constituents.

The spin, angular momentum, and magnetic quantum numbers (degrees of quantum freedom), I believe were basically derived from formulating the Laplacian of the Schrodinger equation in spherical form.

## Noumenon

Local hidden variables, of any sort, that is, in general, were disproved by Bell, so knowing beforehand couldn't possibly help anyway. Btw, "non-local hidden variables" were evidently not disproved, but the "conceptual damage" is already done.

## no fate

Put a little simpler than Nounenon's description, in order to obtain information about SA particles they either have to be absorbed, deflected, or impacted by another SA particle. Any interaction alters at least one aspect of their physical properties, so that what it is after measurment is never the same as what you measured.

Good call on our grasp of reality.

## Noumenon

If you observe the same particle with the same measurement (quick enough), subsequent measurements WILL actually be the same, as the wave-function containing all the information of the particle, collapses to a well defined state.

You mean to say that the complementary variable to the one measured if non-commuting, will be unknown in proportion as the one measured is known.

It is merely an analogy, albeit an acceptable one, that an interaction disturbs the object being measured in a way that results in uncertainty. This was already the case in classical physics and is known as the 'observer effect'. Unfortunately Heisenberg didn't want anyone to think he was nuts so he put it forward as a description of the uncertainty relation.

Prior to measurement there are NO "observable values" per say, ...i.e. there is no counterfactual definiteness. Observable values are created at observation.

## LarryD

'Local hidden variables, of any sort, that is, in general, were disproved by Bell, so knowing beforehand couldn't possibly help anyway. Btw, "non-local hidden variables" were evidently not disproved, but the "conceptual damage" is already done.'

Yes the point about colour I understand but perhaps I should rephrase: to have many bits with the same colour as the whole would mean that one didn't have what one thought.

'hidden variables' EPR/Bells theorem I am aware of hence my question but "non-local hidden variables" is something I don't know about. Where may read about this? Thanks in advance

## Noumenon

## LarryD

## antialias_physorg

I'd not be so sure. If you measure a property you have to interact with the property - i.e. you're taking information away from the measured object regarding this property which should change its value. The uncertainty in your own measuring device means that you are - to a certain degree - uncertain as to how much information you took out. This means that you don't know how much you have changed the property and hence makes any pronouncement of the post-measurement state uncertain.

(If this weren't true you could artificially up the information contained in the universe. And I don't think that's possible)

## vlaaing peerd

## Noumenon

But you are referencing the http://en.wikiped...rinciple again, and is nothing new in physics.

I believe Heisenberg used it as a qualitative description of the uncertainty principal, as a way of 'justifying' qm to people who were otherwise "rational" at the time.

The 'uncertainty' in qm does not refer to repeated measurements on the Same entity**, but rather on a series of entities individually, all prepared in the same state, ...a state that say has a spread in its wavefunction indicating a spread in possible values upon measurements.

Notice above I said the "same particle with the same measurement".

All the information about the qm entity is contained in the wave-function, and once a measurement is perform, the wave-function collapses into a well defined sharp spike. It will begin to spread again according to Schrodinger's prescription, but if measured quick enough, you will get the ~same measured value.

**it does on the complimentary variable

## antialias_physorg

You are aware that the wave function (squared) is a PROBABILITY density?

## Noumenon

Yes, once normalized for that purpose. Information is observer dependent by definition. The wave-function is not itself a physical entity, but represents what is observable given the particular representation.

## Nancy G

## Nancy G

## Nancy G

## LarryD

antialias_physorg, I think you are right! i In any case isn't normalizing just a method we use so that we can gain some understanding without involving the infinite? So that @Noumenon hasn't got it quite right; whether we normalize or not the wave function still reflects a probability.

## Nancy G

## Nancy G

## mohammadshafiq_khan_1

Nov 08, 2013## beleg

Does the Zeno Quantum Effect qualify? How can you delay decay without 'upping the information contained' locally?

On the other hand I see your point from the principle of (information) conservation.

## Mimath224