The Spooky Factor And Teleportation

A few weeks ago we just got one big step closer to having quantum computers and teleportation. This is because atoms have an almost ‘psychic´ ability that Einstein once termed spooky. It seems that distant atoms are almost ‘telepathically´ linked to each other in what scientists call entanglement. Plainly put, it means that a group of atoms ‘over here´ knows what a group of atoms ‘over there´ are doing. This is one of the properties of atoms described by of a branch of science called Quantum Theory.

The word ‘quantum´ means a discrete or separate unit of energy, and Quantum Theory tries to explain the properties of these basic units of nature. When people think of atoms, they usually envision solid, separate balls of matter like a group of billiard balls. However, according to Quantum Theory, atoms are much less tangible, with properties that can only be described (right now) as spooky.

One of the first weird things we learned about quantum particles was that we could know where a particle was, or when a particle was, but we could never know both at the same time. Because it reminds me of the socks I always lose in the wash, I call it "Beth´s Principle of Lost Socks and Quantum Particles." (I would know when I had a full pair of socks, or where that full pair had been, but do the laundry and the full pair would be gone.) In fancy terms, we´d say we knew a particle´s location or it´s velocity, but not both. This discovery is called the Heisenberg Uncertainty Principle, and understanding it is critical to understanding the possibilities inherent in Quantum Theory.

Quantum particles such as atoms or photons can exist in distinct states, like the head or tail of a coin. But these same particles can also exist in both states at once (known formally as superposition). This is comparable to a coin spinning in the air before it lands.

Now let´s suppose we toss two coins at once. Whichever way one coin lands has no bearing on how the other coin lands. Because of this, we say their outcomes are independent -- if one coin lands heads up, it will not effect the way the other coin lands. However, two entangled quantum particles are not like coins. The fate of one effects the fate of the other. For instance, if one entangled quantum particle is in a 'heads' state, the other must be in a 'tails' state. We say that they are interdependent. And this interdependence is the whole key to teleportation.

For any practical applications, entanglement has to embrace thousands, or even millions, of particles, and maintaining total entanglement is very difficult. However, the scientists have found a way around this problem.

They do without complete entanglement, where the state of each particle depends on the state of every other particle. Instead, they generate two loosely entangled clouds of cesium gas, one with slightly more atoms in a 'heads' state and the other with slightly more in a 'tails' state. (These two states are actually defined by the directions of the atoms' magnetic fields.) By doing it this way, many more atoms can be entangled, and stay that way for a longer period of time.

So how does this translate into teleportation? Well, it does and it doesn´t. But the final effect is the same as teleportation. One set of quantum particles can be instantaneously reproduced somewhere else. So unlike Star Trek, objects are not broken down and their particles ‘beamed´ somewhere. Instead, they are reproduced somewhere. In this way a message encoded in photons of light could be transmitted from one place to another without sending the photons across the intervening space, effectively bypassing the speed-of-light barrier.

Up until now, the maximum amount of particles that scientists could entangle were a measly four atoms. This most recent experiment entangled about a million atoms, bringing us much closer to the realization of teleportation, quantum computers, and a new form of instant communication over vast distances.

With such a short distance involved, that much wouldn't matter a great deal. From what I have been taught, it is like a tube filled with golf balls. You push one in and at the same exact time, one comes out.

But the golf ball at the other end isn't pushed out at exactly the same time. That signal propagates at the speed of sound through the golf balls, which is actually quite slow.

But here's the correct answer, chapter and and verse being in section 7.11 of Classical Electrodynamics by J.D. Jackson (2nd edition, p. 319):

The qualitative features of the propagation of a signal are now clear. Some minute part of the wave propagates with the velocity of light in vacuum. This initial signal, called the first or Sommerfeld precursor, is very small and oscillates rapidly. At a later time t1 there is a sudden change when omega=0 becomes a point of stationary phase. The second or Brillouin precursor, of greater amplitude and longer period, arrives. At later times, depending on the details of n(omega) and the incident wave, the signal settles down to the expected steady-state behavior. It is evident that the exact build up of the signal is a complicated matter, that causality and relativity are obeyed regardless of the detailed dispersive properties of the medium, and that the arrival of the signal cannot be given an unambiguous definition. The general usage is to take the group velocity of the dominant frequency component as the signal velocity and velocity of energy transport. This suffices in most circumstances, but with sensitive enough detectors the signal velocity can evidently be pushed close to the velocity of light in vacuum, independent of medium.

Noting like the words of a master. The only thing I can add to this wonderful passage is that recently, techniques have been developed for "decapitating" the precursor and discarding the rest of the signal, resulting in effective phase velocities that are greater than c. Despite the lurid headlines this past summer, this does not mean that information can be transmitted faster than c.

Imagine, if you will a long train leaving Baltimore for DC at 30 mph, where the train is as long as the distance between the cities. The long train officially leaves Baltimore when the center of the train leaves Baltimore. Unfortunately, just as the front of the train is about to arrive in DC, the first few cars (the Sommerfeld precursor) break away and the rest of the train gets stuck. The short train officially gets to DC when its center arrives in DC. The calculated speed of the train is 60 mph, even though no car in the train ever went faster than 30 mph.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.