Naked Science Forum

On the Lighter Side => New Theories => Topic started by: McQueen on 19/03/2017 15:39:25

Title: Quantum Encryption:
Post by: McQueen on 19/03/2017 15:39:25
In spite of all the hype about quantum encryption and the promise that quantum encryption is just around the corner, IMHO  there appears to be some  justification for scepticism; nothing new has emerged over the past twenty or so years.

Is this important ? Yes vitally so, much of the physical claims made about quantum mechanics are intimately tied in to what  quantum encryption hopes to achieve. One of the cornerstones of quantum mechanics, namely superposition of  states of particles wherein sub-atomic particles can physically occupy two discrete and distinct positions in space at the same time is vital to the theory of quantum qubits.  The same theory also proposes that super position plays a part in the binding together of atoms to make molecules by occupying simultaneously orbits in both atoms.

Another extremely challenging principle of quantum mechanics and one that was strongly opposed by Einstein, that will be physically proved or disproved  through the implementation of quantum encryption is the principle of quantum entanglement.  This is a physical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This leads to correlations between observable physical properties of the systems. It is thought that such correlations exist even when the particles are separated  whole galaxies apart . 

Measurements of physical properties such as position, momentum, spin, and polarization, performed on entangled particles are found to be appropriately correlated. For example, if a pair of particles are generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, the spin of the other particle, measured on the same axis, will be found to be counterclockwise, as to be expected due to their entanglement. However, this behavior gives rise to paradoxical effects: any measurement of a property of a particle can be seen as acting on that particle (e.g., by collapsing a number of superposed states) and will change the original quantum property by some unknown amount; and in the case of entangled particles, such a measurement will be on the entangled system as a whole. It thus appears that one particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances.

As stated previously a lot depends on how research into quantum computing and quantum encryption goes.  To date the advances that have been made have not been directly related to quantum qubits or  with other quantum properties in any significant way. The advances that have been made  in quantum encryption so far relate to  the formulation and transmission of an encryption key that would be impossible to intercept.

This has been achieved with photons. Photons possess the property of polarization. It is possible to polarise photons in four different orientations, vertical, horizontal and diagonal left and diagonal right.  This is fairly straight forward and the rules for polarisation are as follows:
The Standard Experiment :

( (  Figure 1.

In Figure 1, an unpolarized, parallel light source is fired through a polarizing filter, and the light strongly registers in a light meter at the other end.

( ( Figure 2

In Figure 2, a second filter is introduced, oriented at 90 to the first one. Now, no light gets through.

( ( Figure 3. [br\
In Figure 3, a third filter is placed in between the first two, at 45 to each of them. Suddenly, the light meter registers a significant amount of light, although not as much as in Figure 1. Spooky ! Say quantum physicists. Nothing spooky about it say the cynic.

I am inclined to go with the cynic Why ? Because classical physics tells us that when light passes through a polariser it acquires the polarisation of the filter through which it passes.  It has already been established that when a diagonal polariser is placed in front of a vertical polariser,  light passes through  in proportion to the relationship Icos2 α.  When individual photons pass through in sufficient numbers they also obey the same relation, so there is nothing spooky about it.  A certain number of photons get through and a certain number are blocked. That's all there is to it.

This property of photons to be polarised is used in quantum encryption to send a supposedly  unbreakable key from sender to receiver.  It works something like this:

( (
A sends a sequence of signals to B randomly polarised. B uses a random sequence of filters to read the message. Then B sends his sequence to A and A sends his sequence to B, Then the key can be worked out.  But surely this is a bit dotty, wouldn't it be necessary to encrypt both the sending sequence and the receiving sequence, and lastly apart from the fact that it is possible to know when someone attempts to tamper with the signal what is the benefit? Further where is the quantum process involved in this ? Then again such polarised photons are very susceptible to the slightest change any imperfection in the conductor or other conditions would result in loss of the polarisation. Therefore such encryption is possible only over distances of 100 km or so, which  is not much good to anyone.

Lastly, where does this leave theories like quantum entanglement and super position ?
Title: Re: Quantum Encryption:
Post by: Bored chemist on 20/03/2017 21:30:14
The quantum physicists wouldn't describe the fact that light gets through 3 filters as "spooky".
Title: Re: Quantum Encryption:
Post by: McQueen on 21/03/2017 00:04:12
Bored chemist : The quantum physicists wouldn't describe the fact that light gets through 3 filters as "spooky".

Quite possibly true.  Even more interesting though, is the fact that all the equipment to prove or disprove some of these 'spooky' theories is in place.  For instance it is possible to isolate an individual photon, something that was quite impossible to contemplate even a few decades ago.   If a photon with a frequency of  10 14 THz  is used for example, the capability exists to measure intervals of 10-15 sec and so it would be seemingly well within the capability of present technology to switch a laser of the required frequency off and on in such  a time interval that only a single photon is released. This photon can be passed through a beam splitter and one of the resultant photons sent in one direction and the other in another direction. If now one of the photons is sent through a polarising filter, then the other photon must have the opposite polarity.

Several significant points arise from this short passage. Firstly, although a photon is not a particle in the classical sense, the theory of super position of states can be proved  (or disproved) within the very first stage of the experiment (i.e., is it possible to split a single photon into two separate photons possessing the same properties? ), Secondly, in the second half of the experiment, which again is well within the range of present technological capability, namely the measurement of time intervals needed for light to travel a nanometer. It will be possible , even if the distances are small to verify the concept of quantum entanglement.   

These technological capabilities have existed for at least the past fifteen years. Why, during that time has more concrete proof not been forthcoming?  This is a question deserving rigorous introspection.