Shining a Light on Interplanetary Communication

An anonymous reader writes "Researchers at the MIT have developed a new device that they claim could one day boost interplanetary communication to broadband speeds. From the article: 'The new light detector improves detection efficiency to 57 percent at a wavelength of 1,550 nanometers--the same wavelength used by optical fibers on Earth to carry broadband signals to homes and offices. Currently, light detectors only absorb about 20 percent of the light they receive. "It can take hours with the existing wireless radio frequency technology to get useful scientific information back from Mars to Earth," said study team member Karl Berggren from the Massachusetts Institute of Technology. "But an optical link can do that thousands of times faster."'"

Comments

Latency

[(1 -sqrt(5))*(2**-1)]

Currently, the maximum data rate between Earth and Mars is about 128,000 bits per second.

They keep harping on data rate, but what about latency? Given that Mars are Earth are anywhere from 40 to 160 million miles apart, perhaps it suffice merely to:

100 * 10^6 mi / 186 * 10^3 mi/sec = 538 sec

to estimate its order of magnitude.

Re:Latency

[maxwell demon]

No, it will not help. Entanglement cannot be used to communicate faster than light. In quantum teleportation, until you get the classical bits (through the classical, speed of light bound channel), you have a random choice of one of several quantum states, ensuring that any measurement you do will get completely random classical results (or more exactly, just as random as if the other side had done nothing). It's only after you got the classical bits that you can "decode" the quantum state.

Ridiculous, if you do the math....

[Ancient_Hacker]

A ridiculous article. Even if they boosted the efficiency by a factor of three, without hurting any other specs, you could accomplish the same thing, or better, just by increasing the diameter of the light-collecting lens. To get three times the light in you just need to increase the diameter by the square root of three-- 1.63 something IIRC. Not a big deal. And lenses and mirrors can be increased in size almost indefinitely, while you can only increase the efficiency of the detector another 30-some percent, and it gets harder and harder to get every extra percentage.

Re:Ridiculous, if you do the math....

[Xaositecte]

A Ridiculous Comment. Over the past few decades, we've boosted the efficiency of recepters an incredible amount, while simultaniously reducing the dish size on Satellite and space communications terminals. Take military Satellite communications Terminals (My Job) - we've gone from a 20' Antenna to a 6' with comparable data rates and greater reliability off the 6' dish in just the time I've been in (almost two years now). This is possible largely due to increased effiency in Dish coatings and designs.

Meanwhile, the size of a dish is often a limiting factor in space vehicle design, making every advance in getting better reception out of a same-sized or smaller dish that much more important.

Besides, it's a LOT easier to transport and set up the 6' dish compared to the 20'.

Re:Ridiculous, if you do the math....

[farnsaw]

A couple of things here.

1) Cost - creating a mirror that much bigger becomes very very expensive over a certain size. Even if this technology to improve the sensitivity makes the sensor twice as expensive you are still saving money. Remember, these are not bathroom mirrors, we are talking about optically perfect mirrors of great size.

2) Size and Weight - If we are using the satelites to capture this information rather than ground based devices then size and weight are a critical factor. This technology would weigh nothing more (or minimally more) while a 1/3 bigger mirrow would weigh 1/3 more.

3) Currently, I believe, we are using radio waves and so therefore we would not be using mirrors. If we were to go to light transmittion, we would probably need to have detectors in space, and I would bet that at least one of them would be in orbit around mars. That said, a bigger mirror again means more size and weight that would have to be transported all the way to Mars.

Also, your math is wrong, 1.63 is not the square root of 3, 1.732050808 is the square root of 3.

For the latency crowd out there, use UDP packets rather than TCP packets and then re-request the sending of any missing packets over time. This emulates TCP over UDP but at a higher level that allows transmittion to continue while waiting for acknowledgement of packets received..

Re:SETI?

[maxwell demon]

Shorter wavelengths are more energetic, so I would expect it takes more energy to transmit light than radio.

You are mixing up the energy of a single photon with the total energy of your transmission. Yes, a single light photon has much more energy than a single microwave photon. But a 100W light source and a 100W radio source both emit exactly the same energy, 100 Joule per second.

Re:SETI?

[AKAImBatman]

Masers (microwave amplification by stimulated emission of radiation) predate the invention of the laser. When the laser was invented, it was mostly ignored as nothing more than a special case of the maser.

Re:S/N?

[kirinyaga]

Redundancy code is something perfectly mastered today. In fact, interplanetary communication drove the technology to where it is now (and is one of the factor leading to the last increases of hard disk bit density). The modern algorithms are able to automatically update the Signal+RedundancyCode output depending of the signal/noise ratio to obtain an optimal transmission rate. Thus "increasing the bandwith" means "increasing the S/N ratio", since this is today the only limitation. So the whole point of this new technology is about increasing the S/N.

Earth and Mars shadow.

[Nowhere.Men]

Considering that we are talking about communication between 2 satellites orbiting 2 differents planets.
The line of sight will be blocked much more often than that by Earth and Mars. there are no stable orbits that would allow permanent line of sight between 2 spacecrafts.

A system to have almost permanent conection would need 2 or 3 geostationary s/c around each body and 1 at the Earth or Mars lagrange point 4 or 5.

For those who praised the superiority of optical communications : The signal would then need to be radioed to the ground unless you live in the Space Station.

More details

[Steve525]

The device in the article is a photon counting device. That means that each individual photon causes an event that can be measured. (Actually, the 57% efficiency means that 57% of the photons cause as event, the other 43% are missed for one reason or another). There are other types of photon counting devices such as avalanche photodiodes, proportional counters (for xrays), and maybe photomultiplier tubes under the right conditions. The problem with avalanche photodiodes is that they take some time to reset after an event, which limits their speed for communications. This is being improved, but this new type of device may offer an intrinsically faster reset time, as well as high efficiency.

How this new device works is that a thin (9 nm?) superconductor wire (100 nm wide) is patterned into a serpentine path. A current, just below the critical current is driven though the superconduncting wire. (The critical current is the current at which the superconductor is no longer superconducting). Any photon that is aborbed by the wire causes local heating, and the wire can no longer be a superconductor with the amount of current going through it. This causes a sudden increase in resistance which can be measured.

Understanding Quantum Entanglement

[TexVex]

Here is how so-called photon "teleportation" works. This should explain why nothing happens faster than light, and why otherwise this isn't causality-destroying voodoo.

You have a source that spits out pairs of polarization-entangled photons. Each particular photon is random -- it'll either be "horizontal" or "vertical" with 50% probability each. But, entanglement means that when one member of each pair is vertical, the opposite member of that pair is horizontal, and vice-versa.

Because of the way QM works, we can't know the polarization of any particular photon pair in advance -- we only know when we pass the photon through a filter and then try to detect it. Both the filter and the detector change the photon, though, so any photon that we measure becomes completely worthless to us thereafter.

So, we know that our photon pairs always have opposite polarization, but we don't know the exact state for each pair in advance. Now, let's cheat a bit and peek behind the veil, pretending we know the state of each photon in a sample stream. I'll use a 0 to encode one polarization state and 1 to encode its opposite:

Stream 1: 0010110101
Stream 2: 1101001010

Now, right off the bat, suppose we read Stream 1 here and send Stream 2 to Mars. By looking at the values we read locally, and flipping each bit, we know what data Mars will receive. But, there's no way we can inform Mars of the contents of the bitstream ahead of time, because nothing travels faster than light.

So, what's all this quantum teleportation stuff about? Well, it's like this. Our Stream 1 and Stream 2 above are random, so they're useless to us for transmitting anything but white noise. But, we can do a cool trick and transmit information in that white noise. We can't exceed lightspeed with it, but we can guarantee that the information can't be undetectably intercepted.

Let's add in Stream 3, which contains data we want to transmit. I pick an arbitrary message -- suppose I want to send alternating bits, like so:

Stream 3: 101010101010

Now, I want to send Stream 3 to Mars, but I want it encrypted in the randomness of Streams 1 and 2. To do this, I read in Stream 1 and perform an operation on each result based on the contents of the corresponding bit in Stream 3: whenever a bit in Stream 3 is a 1, then I flip the result that I read in from Stream 1. Otherwise, I keep the Stream 1 bit unmodified:

Stream 1: 0010110101
Stream 3: 1010101010
Stream 4: 1000011111

So, Stream 4 now contains the data I want to send, mixed with the randomness in one of the two entangled streams. By itself, Stream 4 is meaningless. Also, Stream 1 has been destroyed by reading it. So, I can only decrypt Stream 4 using the data I have from reading Stream 1 -- or by using Stream 2.

Now, I send Stream 2 to Mars unmodified. Anyone reading that stream destroys it and gets random data out of it. Using a separate beam, I send Stream 4 to Mars. Anyone can intercept this and get the data out of it, but it's useless without Stream 2. At the receiving station, they can combine Stream 2 and Stream 4 using a variation on the rule used to encrypt the data, to learn the contents of Stream 3, and they can be guaranteed that the data wasn't intercepted without them knowing about it:

If someone intercepts Stream 2, reads it, and substitutes in another random photon stream, then the decryption on Mars will fail, and so the interception will be detected. If someone intercepts Stream 2, reads it, and manages to make a passable copy to beam to Mars, the time delay will be detected. (Not only that, but QM "no cloning" says you can't make a good enough copy anyway.).

In all of this, nothing at all is happening faster than light. The veil of QM simply says that we can't know the contents of Stream 1 and Stream 2 until we measure them. When we do our encryption operation, we are putting useful data behind that veil, and when we "teleport" the data to the destination, we are getting it back out from behind that veil. But we still have to send everything at light speed.