PhD student explains Quantum Communications

Рет қаралды 5,137

Department for Science, Innovation and Technology

Күн бұрын

Пікірлер: 13

@jbkrobin Ай бұрын

I don’t understand why distance would be a problem? Is it because of noise?

@venkatabhignan8410 Ай бұрын

When photons travels through a medium it causes losses. Here the fiber optic cable which connects Alice and Bob causes losses.

@jbkrobin Ай бұрын

@@venkatabhignan8410 So, this losses due to classical communication?

@lepidoptera9337 Күн бұрын

@@jbkrobin It's just classical absorption. A photon is a small amount of energy. Any impurities in the fiber will scatter or absorb that energy and then the signal that we are looking for is lost.

@grindupBaker 41 минут бұрын

For the old copper wiring back in the 1960s,70s when I was a computer programmer and did some hardware prototyping it was "parasitic capacitance" that llimited the speed of signals. I was finding the highest speed I could send data at down our cable and it was just 300KHz reliably as I recall. The scope showed voltage building from 0V to 5V over some nanoseconds as a rounded type of voltage rising like a soft shoulder and something like 0.0V to 1.0V is a reliable "1", something like 4.0V to 5.0V a reliable "0". As I increased the rate of the pulse train it was able to make it (by filling the parasitic capacitance) to 5.0, 4.9, ... 4.1, 4.0V before the rising pulse ended and the next drop back to 0.0V occurred. At 300KHz it was only reaching 4.0V and I found that was 100% reliable for being taken as a 5.0V logic "0" so I made 300KHz be the rate that my computer program sent bits down the wiring to communicate with my 100 of 4021 CMOS 8-bit shift registers in the 10 big box contraptions at the other end. It was a parallel-to-serial, 120VAC input parallel-loading instantaneously a gang of 100 of the 4021 CMOS 8-bit shift registers in the 10 big box contraptions (we designed them so you could walk on them up to 300 pounds bloke because they had to be all over the floor in a large floor area in a mechanical room). My Zilog MCZ 1/05 Zilog 80 Assembler program sends a freeze-lock-signal pulse instantaneously to 100 of 4021 CMOS 8-bit shift registers in the 10 big box contraptions, freezing the 120VAC "Hot" or 0V "Cold" state of each 120VAC-powered circuit, simultaneousness of all 800 devices being crucial. Then it clocks out the 800 frozen bits serially to the Zilog MCZ 1/05 computer at 300KHz the maximum speed I found reliably achievable with that low-voltage cable of type & length we used, to guarantee a minimum 4.0V at the Zilog for 5.0V at the remote CMOS 4021 chip. So it sampled 800 of 120VAC circuits simultaneously for "on" or "off" at 300KHz/800 = 375 samples per second rate. The reason why going 4.0-5.0V to 0V wasn't an issue was that it was tied to 0V through 100k Ohm resistor at the Zilog MCZ 1/05 computer end so the remote-device CMOS 4021 chips only ever had to pull it up to at least 4.0V through the cable wiring parasitic capacitance coupled with the 100k Ohm resistor leakage, never needed to force it back to 0V because that was done locally, instantaneous. It did the job. "parasitic capacitance" limited the speed. I did something else similar with RS232C USARTS (+/- 12 volts from 5V / 0V) but geezer memory has faded, it was 2 separate hardware prototyping & computer programming (my expertise) projects in the 1970s.

@lepidoptera9337 35 минут бұрын

@@grindupBaker Parasitic capacitance only limits data rates if you are driving that cable with the wrong impedance. You simply didn't know enough electronics to properly terminate and equalize long cables. Today we can send Gbit/s rates over the same cables.

@ronaldluning4010 2 ай бұрын

Sound travels faster and stronger in water than air. Finding a way to communicate through dark matter? Is that what your saying. Advanced extraterrestrial worlds probably have achieved this, which makes me think that planet earth is much louder than the light\ radio waves its been sending out since the '30s.