0 00:00:00,000 --> 00:00:30,000 Dear viewer, these subtitles were generated by a machine via the service Trint and therefore are (very) buggy. If you are capable, please help us to create good quality subtitles: https://c3subtitles.de/talk/52 Thanks! 1 00:00:09,210 --> 00:00:10,949 Please give us a warm round of applause 2 00:00:10,950 --> 00:00:11,950 for Christophe. 3 00:00:21,960 --> 00:00:23,219 Oh, excellent. 4 00:00:25,500 --> 00:00:28,169 OK, I'm pissed off that I'm a physicist, 5 00:00:28,170 --> 00:00:30,599 and today I would like to talk about 6 00:00:30,600 --> 00:00:33,059 long distance quantum cryptography, 7 00:00:33,060 --> 00:00:34,079 communication. 8 00:00:34,080 --> 00:00:35,080 It's the same. 9 00:00:36,270 --> 00:00:38,819 So at first I would like to give a small 10 00:00:38,820 --> 00:00:39,820 introduction. 11 00:00:40,460 --> 00:00:41,999 Then I would like to talk about quantum 12 00:00:42,000 --> 00:00:44,129 communication, then about 13 00:00:44,130 --> 00:00:46,019 extending the reach and finally about the 14 00:00:46,020 --> 00:00:47,020 quantum repeater. 15 00:00:48,150 --> 00:00:50,349 So the introduction, 16 00:00:50,350 --> 00:00:52,439 um, let's assume we would 17 00:00:52,440 --> 00:00:54,869 like to talk would like to communicate 18 00:00:54,870 --> 00:00:56,279 in secrecy. 19 00:00:56,280 --> 00:00:58,349 And it would be preferable 20 00:00:58,350 --> 00:01:00,869 if you can use a provable 21 00:01:00,870 --> 00:01:03,029 security and for approvable 22 00:01:03,030 --> 00:01:05,189 security, we need one at the one 23 00:01:05,190 --> 00:01:07,379 time. Pett the one 24 00:01:07,380 --> 00:01:09,569 time. It is a very simple 25 00:01:09,570 --> 00:01:11,879 symmetric encryption algorithm 26 00:01:11,880 --> 00:01:15,029 and it was invented in 1882 27 00:01:15,030 --> 00:01:18,119 by Frank Miller and later I think in 1917 28 00:01:18,120 --> 00:01:21,239 and 1980, reinvented, 29 00:01:21,240 --> 00:01:23,129 um, as you can see here, 30 00:01:25,050 --> 00:01:27,159 as you can see, 31 00:01:27,160 --> 00:01:29,579 as you can see, as you can see here, 32 00:01:29,580 --> 00:01:32,069 there is a plaintext, there is a key 33 00:01:32,070 --> 00:01:34,519 module of the alphabet and 34 00:01:34,520 --> 00:01:36,779 the alphabet here is two 35 00:01:36,780 --> 00:01:39,479 because we are using zeros and ones. 36 00:01:39,480 --> 00:01:41,999 And if we just apply this machine, 37 00:01:42,000 --> 00:01:43,380 we're getting a cipher text, 38 00:01:45,360 --> 00:01:47,509 which is not decrypted 39 00:01:47,510 --> 00:01:49,099 for a secure application. 40 00:01:49,100 --> 00:01:50,100 We have got some 41 00:01:51,450 --> 00:01:52,619 requirements. 42 00:01:52,620 --> 00:01:54,749 We need a random 43 00:01:54,750 --> 00:01:56,999 secret key. We need a secret as 44 00:01:57,000 --> 00:01:59,489 long as the data and we need 45 00:01:59,490 --> 00:02:01,009 a one time secret key. 46 00:02:01,010 --> 00:02:03,149 So in the end, we Single-Use 47 00:02:03,150 --> 00:02:04,890 long, random secret keys. 48 00:02:07,330 --> 00:02:09,910 The problem is the key distribution, 49 00:02:11,500 --> 00:02:13,720 the key must be printed, crafted 50 00:02:14,770 --> 00:02:17,289 or saved on a physical object 51 00:02:17,290 --> 00:02:19,959 and then the key could be copied 52 00:02:19,960 --> 00:02:21,879 during mechanical transport. 53 00:02:21,880 --> 00:02:24,159 And transport is a logistical 54 00:02:24,160 --> 00:02:26,649 expensive and 55 00:02:26,650 --> 00:02:28,989 without a quantum regime in the clerical 56 00:02:28,990 --> 00:02:31,219 regime, the solution 57 00:02:31,220 --> 00:02:33,099 relies on trust. 58 00:02:33,100 --> 00:02:35,169 But you would like go to the 59 00:02:35,170 --> 00:02:36,369 quantum regime. 60 00:02:36,370 --> 00:02:38,169 Over here, you can see some photons. 61 00:02:42,960 --> 00:02:45,089 And I would 62 00:02:45,090 --> 00:02:47,609 like to introduce, introduce, to 63 00:02:47,610 --> 00:02:49,919 introduce to the 64 00:02:49,920 --> 00:02:51,419 quantum effect of entanglement 65 00:02:53,220 --> 00:02:55,559 first in the quantum regime, 66 00:02:55,560 --> 00:02:56,669 when you measure things, 67 00:02:58,140 --> 00:03:00,359 you don't measure deterministic 68 00:03:00,360 --> 00:03:01,979 but probabilistic. 69 00:03:01,980 --> 00:03:03,899 So when you measure the photon, one in 70 00:03:03,900 --> 00:03:05,459 the state, you measure it with the 71 00:03:05,460 --> 00:03:06,839 probability of alpha. 72 00:03:07,980 --> 00:03:10,059 The same goes with the full time 73 00:03:10,060 --> 00:03:12,569 to which is higher probability 74 00:03:12,570 --> 00:03:14,999 of better and also 75 00:03:15,000 --> 00:03:16,919 of photon three. 76 00:03:16,920 --> 00:03:19,019 But if the photon three 77 00:03:19,020 --> 00:03:21,299 is is part of an entangled photon 78 00:03:21,300 --> 00:03:23,429 pair and the 79 00:03:23,430 --> 00:03:26,009 measurement of the photon four 80 00:03:26,010 --> 00:03:27,989 depends on the previous measurement of 81 00:03:27,990 --> 00:03:30,449 photons. Three Einstein 82 00:03:30,450 --> 00:03:33,089 called this book after found vehicle 83 00:03:33,090 --> 00:03:34,709 spooky action at a distance. 84 00:03:36,780 --> 00:03:38,909 OK, so now we have 85 00:03:38,910 --> 00:03:40,289 got some some 86 00:03:41,670 --> 00:03:43,709 introduction. We come to the quantum 87 00:03:43,710 --> 00:03:46,409 communication and we can 88 00:03:46,410 --> 00:03:48,299 think of a simplified quantum 89 00:03:48,300 --> 00:03:49,949 communication system. 90 00:03:49,950 --> 00:03:51,179 It goes like this. 91 00:03:51,180 --> 00:03:53,869 There is a quantum entanglement 92 00:03:53,870 --> 00:03:54,870 chomper, 93 00:03:55,950 --> 00:03:58,079 um, Eliz purpose of 94 00:03:58,080 --> 00:04:00,089 prepress, not the state of the Fortran 95 00:04:00,090 --> 00:04:02,219 one, but then 96 00:04:02,220 --> 00:04:04,469 copies to state a fortune to 97 00:04:04,470 --> 00:04:07,229 two to four, two and three, 98 00:04:07,230 --> 00:04:09,419 then measures and then measures 99 00:04:09,420 --> 00:04:10,409 for two and three. 100 00:04:10,410 --> 00:04:13,229 And as it is probabilistic, 101 00:04:13,230 --> 00:04:15,569 Bob, repeat steps two and three until 102 00:04:15,570 --> 00:04:17,509 the issue about his measurement. 103 00:04:17,510 --> 00:04:19,789 So, yeah, he 104 00:04:19,790 --> 00:04:22,169 he's sure about his measurement. 105 00:04:22,170 --> 00:04:24,599 The only problem is this 106 00:04:24,600 --> 00:04:27,209 scheme is oversimplified 107 00:04:27,210 --> 00:04:29,819 and it is possible 108 00:04:29,820 --> 00:04:31,919 with this if if this 109 00:04:31,920 --> 00:04:34,199 system would be possible 110 00:04:34,200 --> 00:04:36,269 for information distribution would 111 00:04:36,270 --> 00:04:37,619 be faster than light. 112 00:04:37,620 --> 00:04:39,959 And that contradicts the theory of 113 00:04:39,960 --> 00:04:42,239 relativity, which is 114 00:04:42,240 --> 00:04:43,240 bad. 115 00:04:46,190 --> 00:04:48,419 OK, and you can 116 00:04:48,420 --> 00:04:50,759 see the original author, although 117 00:04:50,760 --> 00:04:53,099 I don't think he 118 00:04:53,100 --> 00:04:54,509 talked about entangled photocopiers. 119 00:04:56,010 --> 00:04:58,799 So, um, it's 120 00:04:58,800 --> 00:04:59,819 not only contradicts 121 00:05:01,030 --> 00:05:03,149 the theory of relativity, 122 00:05:03,150 --> 00:05:05,309 but there is also the no colonial 123 00:05:05,310 --> 00:05:07,949 theory since no clue, 124 00:05:07,950 --> 00:05:10,069 there's no cloning theory was just 125 00:05:10,070 --> 00:05:13,019 rented to counter the possibility 126 00:05:13,020 --> 00:05:15,449 of faster than light information 127 00:05:15,450 --> 00:05:17,279 communication, faster than light 128 00:05:17,280 --> 00:05:18,599 communication. 129 00:05:18,600 --> 00:05:21,119 And the proof depends on the linearity 130 00:05:21,120 --> 00:05:22,979 of quantum mechanics. 131 00:05:22,980 --> 00:05:24,179 Therefore, it's not very 132 00:05:25,530 --> 00:05:26,489 sophisticated. 133 00:05:26,490 --> 00:05:27,899 It is sophisticated. 134 00:05:27,900 --> 00:05:30,479 Anyway, um, 135 00:05:30,480 --> 00:05:32,159 what we have to keep in mind is that a 136 00:05:32,160 --> 00:05:34,679 single photon cannot be cloned. 137 00:05:34,680 --> 00:05:37,649 While this destroys or oversimplified 138 00:05:37,650 --> 00:05:39,809 quantum communications team, it 139 00:05:39,810 --> 00:05:42,419 also gives some kind of copy protections 140 00:05:42,420 --> 00:05:44,789 for single photons, single Quantum's. 141 00:05:44,790 --> 00:05:46,829 In our case, we would like to talk about 142 00:05:46,830 --> 00:05:49,619 photons and 143 00:05:49,620 --> 00:05:51,019 the important the. 144 00:05:51,020 --> 00:05:53,789 Yeah, it's important to remember 145 00:05:53,790 --> 00:05:55,859 that this copy protection is 146 00:05:55,860 --> 00:05:56,999 only for single photons. 147 00:05:57,000 --> 00:05:58,919 So when we have when we have got a 148 00:05:58,920 --> 00:06:01,139 thousand photons and measure 100 149 00:06:01,140 --> 00:06:03,269 of them, then we have got still 150 00:06:03,270 --> 00:06:04,859 nine hundred photons that are not 151 00:06:04,860 --> 00:06:07,469 disturbed by the measurement and 152 00:06:07,470 --> 00:06:09,569 nobody will miss the hundred. 153 00:06:09,570 --> 00:06:11,759 So we need a single photon 154 00:06:11,760 --> 00:06:12,760 sources. 155 00:06:13,530 --> 00:06:15,179 What are single photon sources? 156 00:06:16,410 --> 00:06:17,759 Single photon sources 157 00:06:18,930 --> 00:06:19,930 can be 158 00:06:21,300 --> 00:06:23,369 can be characterized by 159 00:06:23,370 --> 00:06:25,329 an experiment you see down below. 160 00:06:27,000 --> 00:06:29,399 So you 161 00:06:29,400 --> 00:06:31,409 take the photons of your light source, 162 00:06:31,410 --> 00:06:33,649 put it here in the beam splitter, a 163 00:06:33,650 --> 00:06:35,129 beam symmetry. If it's perfect 164 00:06:37,500 --> 00:06:39,749 that 50 percent of the photons 165 00:06:39,750 --> 00:06:42,509 to this Avalon's fortitude 166 00:06:42,510 --> 00:06:44,819 and the other 50 percent to this 167 00:06:44,820 --> 00:06:46,889 forward to the old avalanched, 168 00:06:46,890 --> 00:06:49,439 40 odd, the coincidence 169 00:06:49,440 --> 00:06:51,779 contours and measure 170 00:06:51,780 --> 00:06:54,179 the time difference between the events 171 00:06:54,180 --> 00:06:56,969 on this day or 172 00:06:56,970 --> 00:06:59,399 between the events on this, the old 173 00:06:59,400 --> 00:07:00,400 and the old. 174 00:07:01,470 --> 00:07:02,470 So 175 00:07:04,020 --> 00:07:06,089 as you can see here, there is 176 00:07:06,090 --> 00:07:08,249 a graph to see 177 00:07:08,250 --> 00:07:09,929 different sources. 178 00:07:09,930 --> 00:07:12,359 The one the first one is the light bulb 179 00:07:12,360 --> 00:07:14,489 of on top here, 180 00:07:14,490 --> 00:07:17,309 which is a chaotic, chaotic source. 181 00:07:17,310 --> 00:07:20,009 And in in a certain 182 00:07:20,010 --> 00:07:22,229 time interval, it's very 183 00:07:22,230 --> 00:07:24,060 probable that when we have 184 00:07:25,110 --> 00:07:27,509 measured one event that we measured 185 00:07:27,510 --> 00:07:28,859 a second the wind. 186 00:07:28,860 --> 00:07:30,419 You as you can see, the photons are 187 00:07:30,420 --> 00:07:31,889 bunching here. 188 00:07:31,890 --> 00:07:34,199 OK, then we have got a laser. 189 00:07:34,200 --> 00:07:36,329 The laser is a coherent 190 00:07:36,330 --> 00:07:38,729 photon source and 191 00:07:40,580 --> 00:07:41,599 the. 192 00:07:41,600 --> 00:07:44,639 Event of an event 193 00:07:44,640 --> 00:07:46,789 that the event is 194 00:07:46,790 --> 00:07:48,889 an event is unrelated of 195 00:07:48,890 --> 00:07:51,529 any, if any event has been 196 00:07:51,530 --> 00:07:54,469 previous fiercely measured. 197 00:07:54,470 --> 00:07:56,869 So if, 198 00:07:56,870 --> 00:07:59,149 as you can see, this line 199 00:07:59,150 --> 00:08:01,729 is quite straight, this photo 200 00:08:01,730 --> 00:08:02,839 needs a bunch, not a.. 201 00:08:02,840 --> 00:08:04,939 Bunch a 202 00:08:04,940 --> 00:08:06,439 single photo. 203 00:08:06,440 --> 00:08:08,629 Well, the truth single 204 00:08:08,630 --> 00:08:10,969 photon source, which is perfect 205 00:08:10,970 --> 00:08:13,099 in theory, 206 00:08:13,100 --> 00:08:14,689 does a.. Bunching. 207 00:08:14,690 --> 00:08:16,579 So you as you can see here, he has a 208 00:08:16,580 --> 00:08:17,580 certain 209 00:08:19,320 --> 00:08:21,439 is a certain time interval. 210 00:08:21,440 --> 00:08:23,809 And it is 211 00:08:23,810 --> 00:08:26,539 the probability that a second event is 212 00:08:26,540 --> 00:08:28,969 registered in this time interval 213 00:08:28,970 --> 00:08:30,270 is nearly zero. 214 00:08:31,410 --> 00:08:33,558 So but for 215 00:08:33,559 --> 00:08:34,969 real significance also, 216 00:08:36,049 --> 00:08:38,779 we say that the second autocorrelation 217 00:08:38,780 --> 00:08:40,999 below zero point five is 218 00:08:41,000 --> 00:08:43,460 a single photon source and 219 00:08:45,920 --> 00:08:46,920 therefore 220 00:08:48,230 --> 00:08:50,809 it is in its eye. 221 00:08:50,810 --> 00:08:53,509 It is in discussion if if attenuated 222 00:08:53,510 --> 00:08:55,879 lasers can also be single 223 00:08:55,880 --> 00:08:58,069 photon sources, because 224 00:08:58,070 --> 00:09:00,229 as you can see here, when you drop 225 00:09:00,230 --> 00:09:02,359 every second photon off of this 226 00:09:02,360 --> 00:09:04,429 measurement, you might 227 00:09:04,430 --> 00:09:06,769 get something similar to this 228 00:09:06,770 --> 00:09:08,359 to the screen graph. 229 00:09:10,400 --> 00:09:12,799 OK, so 230 00:09:12,800 --> 00:09:15,769 what are the possible sources 231 00:09:15,770 --> 00:09:18,499 of the possible photon sources 232 00:09:18,500 --> 00:09:20,539 are quantum notes, which are atomic 233 00:09:20,540 --> 00:09:21,540 structures 234 00:09:23,240 --> 00:09:25,639 on a normal scale, 235 00:09:25,640 --> 00:09:26,640 and 236 00:09:27,710 --> 00:09:29,369 they are somehow 237 00:09:32,540 --> 00:09:34,579 engineered out of semiconductors, but 238 00:09:34,580 --> 00:09:36,199 they are also not in semiconductor 239 00:09:36,200 --> 00:09:37,200 quantum dots. 240 00:09:37,940 --> 00:09:40,009 And then there are optical parametric 241 00:09:40,010 --> 00:09:41,010 oscillators. 242 00:09:42,170 --> 00:09:44,719 This is a very interesting source because 243 00:09:44,720 --> 00:09:47,239 as this opio 244 00:09:47,240 --> 00:09:49,849 generates untenured photon pairs 245 00:09:49,850 --> 00:09:52,339 and you can then measure one photon 246 00:09:53,780 --> 00:09:56,179 and know that there is a second photon 247 00:09:56,180 --> 00:09:58,249 flying around and then in a different 248 00:09:58,250 --> 00:10:00,799 state, then called a 249 00:10:00,800 --> 00:10:02,959 heralded photon 250 00:10:02,960 --> 00:10:03,960 source. 251 00:10:04,520 --> 00:10:06,649 You can use single photons and you can 252 00:10:06,650 --> 00:10:09,070 use Nigerien vacancy centers, diamonds, 253 00:10:10,160 --> 00:10:12,130 um, just arm. 254 00:10:13,640 --> 00:10:15,439 It's a five year old that is a diamond 255 00:10:15,440 --> 00:10:17,479 made out of carbon and one of the carbon 256 00:10:17,480 --> 00:10:20,089 of one of the carbon opto. 257 00:10:20,090 --> 00:10:22,159 One of the carbon atoms in 258 00:10:22,160 --> 00:10:24,259 this crystal is not is 259 00:10:24,260 --> 00:10:26,479 not carbon, but it's nitrogen. 260 00:10:26,480 --> 00:10:28,699 And these are very stable and very 261 00:10:28,700 --> 00:10:30,499 popular right now. 262 00:10:30,500 --> 00:10:33,289 Yeah. So, um, 263 00:10:33,290 --> 00:10:35,699 it's completely anti bunching behavior. 264 00:10:35,700 --> 00:10:38,179 Correct. Desire's characterizes 265 00:10:38,180 --> 00:10:41,479 our single photon source, and 266 00:10:41,480 --> 00:10:43,939 the non single photon sources 267 00:10:43,940 --> 00:10:45,829 would allow non disturbing measurement of 268 00:10:45,830 --> 00:10:48,499 the state, which would 269 00:10:48,500 --> 00:10:50,689 in the end harm our security 270 00:10:50,690 --> 00:10:51,690 of our device. 271 00:10:53,840 --> 00:10:55,999 OK, we, 272 00:10:57,470 --> 00:10:59,629 um, here is a 273 00:10:59,630 --> 00:11:01,699 quantum key concept, quantum 274 00:11:01,700 --> 00:11:03,439 key distribution concept. 275 00:11:03,440 --> 00:11:06,079 And you have over here, 276 00:11:06,080 --> 00:11:08,359 over here Ellas a 277 00:11:08,360 --> 00:11:11,449 photon and a certain polarization. 278 00:11:11,450 --> 00:11:13,939 Then send this photon to the electoral 279 00:11:13,940 --> 00:11:16,429 optical modulator which 280 00:11:16,430 --> 00:11:19,109 shifts the polarization at random. 281 00:11:19,110 --> 00:11:20,839 Uh, the photons then send over the 282 00:11:20,840 --> 00:11:23,419 quantum channel to the BOP 283 00:11:23,420 --> 00:11:24,420 where BOP 284 00:11:26,480 --> 00:11:28,639 modulates the incoming phone to at 285 00:11:28,640 --> 00:11:30,739 random and then measure it with a 286 00:11:30,740 --> 00:11:32,869 Paula popularizing beam 287 00:11:32,870 --> 00:11:33,870 splitter 288 00:11:35,180 --> 00:11:37,129 and detect it was the property of the 289 00:11:37,130 --> 00:11:39,109 polarizing beams, which is different from 290 00:11:39,110 --> 00:11:41,209 the beam spectrum as 291 00:11:41,210 --> 00:11:43,669 it transmits 292 00:11:43,670 --> 00:11:45,829 or reflects photon. 293 00:11:45,830 --> 00:11:47,159 This is here. 294 00:11:47,160 --> 00:11:49,219 It reflects 295 00:11:49,220 --> 00:11:51,349 a photon based on 296 00:11:51,350 --> 00:11:52,669 on its polarization. 297 00:11:54,110 --> 00:11:55,470 So, um, 298 00:11:56,720 --> 00:11:58,789 now Ellerson bobbleheads have 299 00:11:58,790 --> 00:12:01,279 done some measurement and 300 00:12:01,280 --> 00:12:03,439 then Alice and Bob 301 00:12:03,440 --> 00:12:04,369 compare. 302 00:12:04,370 --> 00:12:06,799 They are random modulation 303 00:12:06,800 --> 00:12:09,019 of the um, of 304 00:12:09,020 --> 00:12:11,269 a verified, verifiable 305 00:12:11,270 --> 00:12:12,349 channel. 306 00:12:12,350 --> 00:12:13,520 Um, this channel 307 00:12:14,870 --> 00:12:17,449 can be public but must not. 308 00:12:17,450 --> 00:12:19,249 But it is must be verifiable. 309 00:12:28,170 --> 00:12:30,299 Um, just 310 00:12:30,300 --> 00:12:32,819 in Beebee, 1884, 311 00:12:32,820 --> 00:12:35,279 there are two to two states for encoding 312 00:12:35,280 --> 00:12:36,870 in two states for randomizing, 313 00:12:37,890 --> 00:12:40,079 so you can use two states 314 00:12:40,080 --> 00:12:42,569 and here you are using also two states. 315 00:12:42,570 --> 00:12:45,119 And as an important they're also 316 00:12:45,120 --> 00:12:46,559 two states you can use. 317 00:12:48,750 --> 00:12:51,359 OK, there are various protocols 318 00:12:51,360 --> 00:12:53,879 you can use. But I just 319 00:12:53,880 --> 00:12:55,379 mentioned it was the first point of 320 00:12:55,380 --> 00:12:57,449 protocol and is intended 321 00:12:57,450 --> 00:12:59,919 for for states to enact 322 00:12:59,920 --> 00:13:02,039 and ninety one is used 323 00:13:02,040 --> 00:13:04,769 for users and on transmission 324 00:13:04,770 --> 00:13:07,289 and zerk protocol 325 00:13:07,290 --> 00:13:09,689 users is used in some commercial 326 00:13:09,690 --> 00:13:11,669 and available quantum key distribution 327 00:13:11,670 --> 00:13:12,670 devices. 328 00:13:13,620 --> 00:13:16,049 Of course, the security 329 00:13:16,050 --> 00:13:19,019 depends on on the implementation. 330 00:13:19,020 --> 00:13:21,299 And you might have seen the nice 331 00:13:21,300 --> 00:13:23,759 talk in 2007. 332 00:13:23,760 --> 00:13:26,249 It's a college communication device 333 00:13:26,250 --> 00:13:28,499 where they show some practical 334 00:13:28,500 --> 00:13:30,839 attacks on 335 00:13:30,840 --> 00:13:31,840 quantum cryptography. 336 00:13:34,720 --> 00:13:36,010 So we have got to 337 00:13:37,720 --> 00:13:40,749 we have got some quantum communication 338 00:13:40,750 --> 00:13:42,369 going on in the laboratory, which is 339 00:13:42,370 --> 00:13:44,559 awesome, but we would like to go 340 00:13:44,560 --> 00:13:46,629 beyond and therefore we need 341 00:13:46,630 --> 00:13:48,189 to extend the range. 342 00:13:48,190 --> 00:13:50,259 One thing we could do is we can use 343 00:13:50,260 --> 00:13:53,109 the free space like the 344 00:13:53,110 --> 00:13:55,809 the air, which is 345 00:13:55,810 --> 00:13:58,239 has an advantage that is very flexible 346 00:13:58,240 --> 00:14:00,070 because it's nearly everywhere. 347 00:14:01,630 --> 00:14:02,630 And 348 00:14:03,820 --> 00:14:06,189 the disadvantage is that 349 00:14:06,190 --> 00:14:08,319 there's background light like the sun or 350 00:14:08,320 --> 00:14:11,349 streetlamps or street lamps. 351 00:14:11,350 --> 00:14:14,589 And when you have got 352 00:14:14,590 --> 00:14:17,649 a photon count in your APD, 353 00:14:17,650 --> 00:14:19,839 the SPDR burns 354 00:14:19,840 --> 00:14:22,059 out or just is separated and doesn't 355 00:14:22,060 --> 00:14:23,139 detect any events. 356 00:14:24,940 --> 00:14:26,319 You can handle others. 357 00:14:26,320 --> 00:14:28,809 But it is a, doesn't it? 358 00:14:28,810 --> 00:14:30,669 It is a disadvantage. 359 00:14:30,670 --> 00:14:32,649 And other disadvantage is that the 360 00:14:32,650 --> 00:14:34,719 distribution medium, the air is not 361 00:14:34,720 --> 00:14:35,720 very stable. 362 00:14:37,600 --> 00:14:39,639 That's, for example, the fog or heavy 363 00:14:39,640 --> 00:14:41,439 rain or something like this. 364 00:14:41,440 --> 00:14:43,899 And this would degrade your 365 00:14:43,900 --> 00:14:44,900 your 366 00:14:46,000 --> 00:14:47,289 key rate. 367 00:14:47,290 --> 00:14:49,419 And yeah, it's 368 00:14:49,420 --> 00:14:50,769 not what we would like to have. 369 00:14:51,850 --> 00:14:54,009 And there has been 370 00:14:54,010 --> 00:14:55,839 some experiments and they reached, I 371 00:14:55,840 --> 00:14:58,179 think, about between 50 and 100 372 00:14:58,180 --> 00:15:00,279 kilometers a 373 00:15:00,280 --> 00:15:01,880 free space, quantum communication. 374 00:15:02,950 --> 00:15:05,239 The main problem of them for 375 00:15:05,240 --> 00:15:07,639 for for reaches beyond 376 00:15:07,640 --> 00:15:09,069 100 kilometers. 377 00:15:09,070 --> 00:15:10,539 It's a problem that the earth is not 378 00:15:10,540 --> 00:15:11,709 flat. 379 00:15:11,710 --> 00:15:13,629 You can't reach Tokyo from Hamburg 380 00:15:13,630 --> 00:15:16,479 without a very high tower or a satellite. 381 00:15:16,480 --> 00:15:18,999 So there was quantum condition 382 00:15:19,000 --> 00:15:21,219 proposed by satellite and 383 00:15:21,220 --> 00:15:24,039 by satellite. You sending, for example, 384 00:15:24,040 --> 00:15:26,199 a single photon and tangled 385 00:15:26,200 --> 00:15:28,719 photon power source up was the satellite. 386 00:15:28,720 --> 00:15:30,939 And the satellite sends one photon to 387 00:15:30,940 --> 00:15:33,399 the receiver, one and one for to receiver 388 00:15:33,400 --> 00:15:36,009 to each receiver measures, 389 00:15:36,010 --> 00:15:37,010 then 390 00:15:38,080 --> 00:15:40,179 measure, then the 391 00:15:40,180 --> 00:15:41,229 photon. 392 00:15:41,230 --> 00:15:43,359 And through entanglement 393 00:15:43,360 --> 00:15:45,399 of the photons, the measurements are 394 00:15:45,400 --> 00:15:46,619 correlated. 395 00:15:46,620 --> 00:15:48,969 Uh, you can see a paper 396 00:15:48,970 --> 00:15:49,929 that has been published. 397 00:15:49,930 --> 00:15:52,599 So that shows some 398 00:15:52,600 --> 00:15:54,849 more, uh, satellite 399 00:15:54,850 --> 00:15:56,470 seems to communicate. 400 00:15:58,330 --> 00:16:00,489 OK, then the next 401 00:16:00,490 --> 00:16:02,769 thing we could do is we couldn't do 402 00:16:02,770 --> 00:16:05,199 quantum communication by fiber. 403 00:16:05,200 --> 00:16:07,569 And the advantages here that the signal 404 00:16:07,570 --> 00:16:09,939 in the fiber is a relatively independent 405 00:16:09,940 --> 00:16:12,729 of the environment. I say relatively 406 00:16:12,730 --> 00:16:14,979 because this has 407 00:16:14,980 --> 00:16:17,469 shaken his fiber during an experiment, 408 00:16:17,470 --> 00:16:19,599 knows how hard it can get to 409 00:16:19,600 --> 00:16:20,860 get any signal out of it. 410 00:16:22,150 --> 00:16:24,089 The fiber is, of course, flexible, but it 411 00:16:24,090 --> 00:16:27,039 is definitely not mobile. 412 00:16:27,040 --> 00:16:29,169 And the good thing is also that you can 413 00:16:29,170 --> 00:16:30,400 use commercially available 414 00:16:31,540 --> 00:16:33,729 fibers and 415 00:16:35,290 --> 00:16:37,119 these fibers are also already in the 416 00:16:37,120 --> 00:16:40,419 ground and can be used as Advil. 417 00:16:40,420 --> 00:16:42,579 So the disadvantage is that 418 00:16:42,580 --> 00:16:45,009 that you can't use every wavelength 419 00:16:45,010 --> 00:16:47,559 because the most wavelengths are, 420 00:16:47,560 --> 00:16:49,839 uh, have a high loss in the fiber. 421 00:16:49,840 --> 00:16:52,389 The wavelengths that are used today 422 00:16:52,390 --> 00:16:54,789 for network traffic around 423 00:16:54,790 --> 00:16:57,099 Etemad are around one thousand 424 00:16:57,100 --> 00:16:59,439 three ten and around 425 00:16:59,440 --> 00:17:01,629 one thousand five hundred fifty. 426 00:17:01,630 --> 00:17:03,909 But even the the last one has 427 00:17:05,170 --> 00:17:08,229 zero zero point two DB 428 00:17:08,230 --> 00:17:10,659 damping per kilometer and 429 00:17:10,660 --> 00:17:12,789 the reach is limited to about 100 430 00:17:12,790 --> 00:17:13,790 kilometers. 431 00:17:14,710 --> 00:17:16,868 So so what we should 432 00:17:16,869 --> 00:17:19,059 do now is find 433 00:17:19,060 --> 00:17:21,309 a scheme to enhance this reach. 434 00:17:23,589 --> 00:17:25,749 One possibility is that 435 00:17:25,750 --> 00:17:28,088 node concepts here you 436 00:17:28,089 --> 00:17:30,369 combine quantum transmission like 437 00:17:30,370 --> 00:17:33,189 here one, two and three with classic 438 00:17:33,190 --> 00:17:35,469 transmissions like and B, 439 00:17:35,470 --> 00:17:37,689 the problem is obvious that you 440 00:17:37,690 --> 00:17:40,359 need to trust the persons or the machines 441 00:17:41,770 --> 00:17:43,869 or whatever works at this 442 00:17:43,870 --> 00:17:44,889 place. 443 00:17:44,890 --> 00:17:46,119 Here at the 444 00:17:47,200 --> 00:17:48,999 transmission takes place. 445 00:17:49,000 --> 00:17:51,099 It relies on trust that 446 00:17:51,100 --> 00:17:53,259 we also already had before. 447 00:17:53,260 --> 00:17:55,089 I was always might be interesting. 448 00:17:55,090 --> 00:17:57,339 Interesting test case for some things. 449 00:17:59,050 --> 00:18:01,239 Instead, you would like to reach 450 00:18:01,240 --> 00:18:02,289 the quantum repeater. 451 00:18:05,620 --> 00:18:07,479 The quantum computer looks like this. 452 00:18:07,480 --> 00:18:09,699 It's, uh, consists out of a quantum 453 00:18:09,700 --> 00:18:12,189 memorium. And like here at one 454 00:18:12,190 --> 00:18:14,259 like here it has 455 00:18:14,260 --> 00:18:16,480 got some entangled photon pairs 456 00:18:17,800 --> 00:18:20,349 of Inspecteur and two photoed yours. 457 00:18:20,350 --> 00:18:22,539 It is an idea published in this 458 00:18:22,540 --> 00:18:23,540 paper Downbelow. 459 00:18:24,520 --> 00:18:26,649 So let's take a look at 460 00:18:26,650 --> 00:18:27,579 first. It's a beam. 461 00:18:27,580 --> 00:18:29,799 Spitzer It's a 462 00:18:29,800 --> 00:18:32,089 common swapping and 463 00:18:32,090 --> 00:18:33,429 Tengblad swapping means. 464 00:18:33,430 --> 00:18:35,589 That you got here two on 465 00:18:35,590 --> 00:18:37,779 one and here 42, and 466 00:18:37,780 --> 00:18:40,209 you sent those photons, which are part of 467 00:18:40,210 --> 00:18:42,539 a entangled photon pass to the speed 468 00:18:42,540 --> 00:18:44,769 superior when this 469 00:18:44,770 --> 00:18:46,959 photons are indistinguishable, 470 00:18:46,960 --> 00:18:49,239 just photons bunch and 471 00:18:49,240 --> 00:18:51,369 end up. Is that this totally accurate 472 00:18:51,370 --> 00:18:53,230 and distorted fortitude 473 00:18:55,360 --> 00:18:57,879 when this photon bunch and 474 00:18:57,880 --> 00:18:59,949 entanglement between these two 475 00:18:59,950 --> 00:19:02,079 and the entanglement 476 00:19:02,080 --> 00:19:04,509 between two and three is swapped 477 00:19:04,510 --> 00:19:06,519 and afterwards you have got entanglement 478 00:19:06,520 --> 00:19:08,619 between three and 479 00:19:08,620 --> 00:19:10,719 four and 480 00:19:10,720 --> 00:19:12,969 thereby you have swapped in internment 481 00:19:12,970 --> 00:19:16,359 if it have have reached a greater 482 00:19:16,360 --> 00:19:17,360 range. 483 00:19:18,760 --> 00:19:21,729 So now we have got entanglement 484 00:19:21,730 --> 00:19:24,429 and entanglement swapping 485 00:19:24,430 --> 00:19:26,709 and we would like to have also 486 00:19:26,710 --> 00:19:29,139 memory quantum memory element because 487 00:19:30,940 --> 00:19:33,459 it enlightens 488 00:19:33,460 --> 00:19:35,859 networks and synchronization 489 00:19:35,860 --> 00:19:38,319 and it might be possible 490 00:19:38,320 --> 00:19:40,539 to connect this quantum memory event 491 00:19:40,540 --> 00:19:42,759 to other devices you would like 492 00:19:42,760 --> 00:19:43,760 to use. 493 00:19:47,960 --> 00:19:50,229 OK, the quantum memory element 494 00:19:50,230 --> 00:19:52,329 there has been defined some pretty for 495 00:19:52,330 --> 00:19:53,469 memory elements. 496 00:19:53,470 --> 00:19:55,389 As I said before, the wavelength is very 497 00:19:55,390 --> 00:19:56,709 important. 498 00:19:56,710 --> 00:19:58,839 The bandwidth is important, which means 499 00:19:58,840 --> 00:20:01,539 that it's a frequency 500 00:20:01,540 --> 00:20:04,269 interval in which photons 501 00:20:04,270 --> 00:20:06,429 would be still stored in 502 00:20:06,430 --> 00:20:09,129 the quantum memory the field entity 503 00:20:09,130 --> 00:20:11,199 describes as a quality 504 00:20:11,200 --> 00:20:13,389 of the photon. So if I send in a four 505 00:20:13,390 --> 00:20:14,499 photon instead, are 506 00:20:15,910 --> 00:20:18,309 the fortunate that the fortunate 507 00:20:18,310 --> 00:20:20,499 comes out of the quantum memory element 508 00:20:20,500 --> 00:20:22,749 should also be a state and 509 00:20:22,750 --> 00:20:24,909 not something else, because then 510 00:20:24,910 --> 00:20:26,979 it would be a random generator, but not a 511 00:20:26,980 --> 00:20:27,980 memory element. 512 00:20:29,080 --> 00:20:31,599 And then the efficiency, that means 513 00:20:31,600 --> 00:20:34,179 how often I have to send in a photon 514 00:20:34,180 --> 00:20:35,709 to get a photon out. 515 00:20:35,710 --> 00:20:38,469 And of course, the storage time and the 516 00:20:38,470 --> 00:20:40,700 capacity is also important. 517 00:20:42,010 --> 00:20:44,799 Again, we can use 518 00:20:44,800 --> 00:20:46,779 quantum dots in recent years in 519 00:20:48,010 --> 00:20:50,409 atomic gases and atoms 520 00:20:50,410 --> 00:20:52,510 for for quantum memory. 521 00:20:54,830 --> 00:20:57,069 OK, quantum dots, you 522 00:20:57,070 --> 00:20:58,849 can see them here on the right hand side. 523 00:20:58,850 --> 00:21:01,629 I think it's an atomic force, microscopy 524 00:21:01,630 --> 00:21:03,219 and other sizes. 525 00:21:03,220 --> 00:21:06,369 And this is out of semiconductors. 526 00:21:06,370 --> 00:21:08,200 So you've got a semiconductor 527 00:21:09,550 --> 00:21:11,889 of a certain lattice and 528 00:21:11,890 --> 00:21:14,409 other semiconductors with similar 529 00:21:14,410 --> 00:21:16,719 but not the same letters on top 530 00:21:16,720 --> 00:21:18,999 and a bunch because they thereby 531 00:21:19,000 --> 00:21:20,680 reduce the energy 532 00:21:22,120 --> 00:21:23,230 in the surface energy. 533 00:21:24,580 --> 00:21:26,500 OK, yeah, we have got 534 00:21:27,830 --> 00:21:30,759 the the conducting band 535 00:21:30,760 --> 00:21:33,579 as the band structure of the 536 00:21:33,580 --> 00:21:36,099 quantum dot and the upper left. 537 00:21:36,100 --> 00:21:38,409 On the other side is the conducting 538 00:21:38,410 --> 00:21:40,749 band, which is an 539 00:21:40,750 --> 00:21:43,599 electron ensemble of electrons 540 00:21:43,600 --> 00:21:47,109 far away from the forms, the atomic 541 00:21:47,110 --> 00:21:49,419 core and Downbelow. 542 00:21:49,420 --> 00:21:52,329 You've got to balance electrons which are 543 00:21:52,330 --> 00:21:54,760 electrons near the atomic core. 544 00:21:55,810 --> 00:21:57,939 And then there are 545 00:21:57,940 --> 00:21:58,940 some some 546 00:22:00,070 --> 00:22:02,519 some states in this this 547 00:22:02,520 --> 00:22:04,839 this band, when 548 00:22:04,840 --> 00:22:07,809 a photon comes into 549 00:22:07,810 --> 00:22:11,349 into the quantum dot, it is absorbed 550 00:22:11,350 --> 00:22:12,350 and 551 00:22:13,480 --> 00:22:15,879 it generates an exit on an electron 552 00:22:15,880 --> 00:22:18,189 is an electron holper. 553 00:22:19,330 --> 00:22:21,639 So because quantum 554 00:22:21,640 --> 00:22:23,949 dot is biased, 555 00:22:25,000 --> 00:22:27,099 um, the, the 556 00:22:27,100 --> 00:22:29,739 whole as you can see in the middle, 557 00:22:29,740 --> 00:22:31,929 is moved out of the quantum dot 558 00:22:31,930 --> 00:22:34,119 and the electron can't 559 00:22:34,120 --> 00:22:36,639 recombine, at 560 00:22:36,640 --> 00:22:37,609 least for a certain time. 561 00:22:37,610 --> 00:22:39,759 That's not stable for hundreds of years. 562 00:22:41,350 --> 00:22:42,350 So then you have got 563 00:22:44,590 --> 00:22:45,640 up there and 564 00:22:47,100 --> 00:22:49,989 the incoming incoming photon 565 00:22:49,990 --> 00:22:52,389 and to the information 566 00:22:52,390 --> 00:22:53,949 on the incoming photon and the 567 00:22:53,950 --> 00:22:56,649 information on on the electron 568 00:22:56,650 --> 00:22:59,229 are correlated 569 00:22:59,230 --> 00:23:01,389 over a polarization to 570 00:23:01,390 --> 00:23:03,410 spin correlation. 571 00:23:04,430 --> 00:23:06,070 So then for a readout, 572 00:23:07,210 --> 00:23:10,389 you would you would 573 00:23:10,390 --> 00:23:12,309 bias the quantum lot again. 574 00:23:12,310 --> 00:23:14,379 But this time to move in some 575 00:23:14,380 --> 00:23:17,019 holes, as you can see down below, 576 00:23:17,020 --> 00:23:19,329 then to enter the quantum 577 00:23:19,330 --> 00:23:21,579 dot, then the electron 578 00:23:21,580 --> 00:23:24,069 and one how we can combine and 579 00:23:24,070 --> 00:23:26,169 and photon will be sent 580 00:23:26,170 --> 00:23:28,609 out if the fidelity is perfect, 581 00:23:28,610 --> 00:23:30,699 is the same 582 00:23:30,700 --> 00:23:32,460 photon we have sent in. 583 00:23:34,560 --> 00:23:35,560 So 584 00:23:37,020 --> 00:23:38,699 in this paper, they mentioned that their 585 00:23:38,700 --> 00:23:40,829 fidelity was around 80 percent plus 586 00:23:40,830 --> 00:23:43,139 minus 10 and they 587 00:23:43,140 --> 00:23:45,389 had only efficiency of 588 00:23:45,390 --> 00:23:47,579 20 percent, but 589 00:23:47,580 --> 00:23:50,129 others reach out to 97 590 00:23:50,130 --> 00:23:51,029 percent. 591 00:23:51,030 --> 00:23:53,519 So which is a very high 592 00:23:53,520 --> 00:23:55,709 value. Of course, the start 593 00:23:55,710 --> 00:23:58,409 time is about 100 microseconds. 594 00:23:58,410 --> 00:24:00,659 And just recently I read 595 00:24:00,660 --> 00:24:02,789 the paper or 596 00:24:02,790 --> 00:24:05,699 at least the abstract where they thought 597 00:24:05,700 --> 00:24:07,799 a photon for about 39 598 00:24:07,800 --> 00:24:10,829 minutes. So it's a 599 00:24:10,830 --> 00:24:12,209 sure enough probable 600 00:24:13,290 --> 00:24:15,779 long, long enough storage time. 601 00:24:15,780 --> 00:24:17,849 The bandwidth is one 602 00:24:17,850 --> 00:24:20,039 one gigahertz with 603 00:24:20,040 --> 00:24:22,889 which I think is also wide enough 604 00:24:22,890 --> 00:24:25,319 as as 605 00:24:25,320 --> 00:24:27,509 you would like to 606 00:24:27,510 --> 00:24:30,509 entangles and swap this photons 607 00:24:30,510 --> 00:24:32,759 and four and taking swopping as a mind 608 00:24:32,760 --> 00:24:34,049 to remind you, you need to 609 00:24:34,050 --> 00:24:35,759 indistinguishable photons. 610 00:24:35,760 --> 00:24:38,069 And I'm 611 00:24:38,070 --> 00:24:39,799 not not so sure where the 612 00:24:41,320 --> 00:24:43,679 where you can't and where you about 613 00:24:43,680 --> 00:24:45,149 photons are not indistinguishable 614 00:24:45,150 --> 00:24:46,890 anymore. But I think 615 00:24:47,910 --> 00:24:50,099 one gigahertz is good enough 616 00:24:50,100 --> 00:24:52,409 for using it as a quantum theory. 617 00:24:52,410 --> 00:24:55,529 So as we have got quantum dots, 618 00:24:55,530 --> 00:24:57,719 um, the wavelength of the quantum 619 00:24:57,720 --> 00:25:00,109 memory can be tuned as 620 00:25:00,110 --> 00:25:02,309 as you wish and for the 621 00:25:02,310 --> 00:25:04,559 capacity you can do, 622 00:25:04,560 --> 00:25:07,529 of course, if can frequency multiplexing. 623 00:25:07,530 --> 00:25:09,899 And on the other hand, 624 00:25:09,900 --> 00:25:12,029 multi photon storage is not likely, 625 00:25:12,030 --> 00:25:13,920 as they say in their paper. 626 00:25:15,690 --> 00:25:17,849 OK, now 627 00:25:17,850 --> 00:25:19,650 we have got the quantum repeater. 628 00:25:20,790 --> 00:25:22,859 We have got here all quantum memory, 629 00:25:22,860 --> 00:25:25,079 which is toss photons 630 00:25:25,080 --> 00:25:27,509 for us for a certain time. 631 00:25:27,510 --> 00:25:29,880 And over here, the entangled photon paths 632 00:25:31,110 --> 00:25:33,479 which are useful 633 00:25:33,480 --> 00:25:35,399 for the entanglement swapping. 634 00:25:35,400 --> 00:25:37,529 If we combine if we combine 635 00:25:37,530 --> 00:25:40,049 a long chain of this quantum repeaters, 636 00:25:40,050 --> 00:25:42,299 we can reach any distance 637 00:25:42,300 --> 00:25:44,299 we would like to have. 638 00:25:44,300 --> 00:25:45,300 OK, 639 00:25:47,070 --> 00:25:49,589 so coming to my conclusion is 640 00:25:49,590 --> 00:25:51,959 talk to you something about the freespace 641 00:25:51,960 --> 00:25:54,149 quantum communication, the quantum 642 00:25:54,150 --> 00:25:56,849 communication by fiber and 643 00:25:56,850 --> 00:25:59,039 quantum computer. Was it was it 644 00:25:59,040 --> 00:26:01,229 was it was 645 00:26:01,230 --> 00:26:03,449 there the main parts of 646 00:26:03,450 --> 00:26:05,489 the bedside measurement and the quantum 647 00:26:05,490 --> 00:26:08,339 memory, if you like, 648 00:26:08,340 --> 00:26:10,379 if you like the quantum communication, 649 00:26:10,380 --> 00:26:11,969 you also could take a look at quantum 650 00:26:11,970 --> 00:26:13,889 dots, which are a very interesting 651 00:26:13,890 --> 00:26:16,439 nanoscale structure and frequency 652 00:26:16,440 --> 00:26:18,569 down, down and up conversion. 653 00:26:19,800 --> 00:26:21,989 And yeah, I would like to thank you 654 00:26:21,990 --> 00:26:22,990 for your attention. 655 00:26:31,540 --> 00:26:33,249 Now, if you have questions, there are 656 00:26:33,250 --> 00:26:35,379 mikes there, there, and 657 00:26:35,380 --> 00:26:37,749 I think this one also works also, 658 00:26:37,750 --> 00:26:39,849 if you're on the stream, you can use 659 00:26:39,850 --> 00:26:40,850 your Twitter. 660 00:26:41,500 --> 00:26:43,299 Any questions in the room? 661 00:26:43,300 --> 00:26:44,949 OK, you go ahead. 662 00:26:44,950 --> 00:26:47,529 Yeah, you 663 00:26:47,530 --> 00:26:49,299 showed us with a repeater. 664 00:26:49,300 --> 00:26:51,069 You have this beam splitter. 665 00:26:51,070 --> 00:26:53,679 Yeah. And you just set the photons 666 00:26:53,680 --> 00:26:55,179 can bunch. 667 00:26:55,180 --> 00:26:56,410 How do they do that? 668 00:26:57,550 --> 00:27:00,069 Um, I don't know how, 669 00:27:00,070 --> 00:27:02,289 but the important point, um, 670 00:27:02,290 --> 00:27:04,359 because it's quantum mechanics. 671 00:27:04,360 --> 00:27:07,749 The important point is this is photons 672 00:27:07,750 --> 00:27:09,749 come to the beam splitter. 673 00:27:09,750 --> 00:27:12,939 OK, and if they're just indistinguishable 674 00:27:12,940 --> 00:27:14,019 then they are. 675 00:27:14,020 --> 00:27:15,519 This is the concept. 676 00:27:15,520 --> 00:27:17,049 Why is that bunch. It's not part of 677 00:27:17,050 --> 00:27:19,209 physics, but, 678 00:27:19,210 --> 00:27:20,949 uh, it's it's. 679 00:27:20,950 --> 00:27:22,900 Yeah, it's in the quantum mechanics. 680 00:27:24,640 --> 00:27:25,640 OK, thank you. 681 00:27:27,280 --> 00:27:28,510 Internet, please. 682 00:27:31,720 --> 00:27:32,679 Audio, please. 683 00:27:32,680 --> 00:27:34,869 Thanks. Um, so I only 684 00:27:34,870 --> 00:27:36,700 have one question. 685 00:27:38,920 --> 00:27:41,469 How do you fix the static magnetic 686 00:27:41,470 --> 00:27:43,199 symmetry problem for the cube? 687 00:27:43,200 --> 00:27:45,489 It does that problem 688 00:27:45,490 --> 00:27:46,490 even exist? 689 00:27:48,400 --> 00:27:50,469 It's called static magnetic. 690 00:27:50,470 --> 00:27:52,629 The symmetry problem for 691 00:27:52,630 --> 00:27:53,630 the cube. It 692 00:27:55,210 --> 00:27:56,799 yes. Does that exist? 693 00:27:57,970 --> 00:27:59,109 I'm not so sure. 694 00:27:59,110 --> 00:28:01,209 OK, I think the 695 00:28:01,210 --> 00:28:02,989 one who posed the question wasn't so sure 696 00:28:02,990 --> 00:28:04,689 either. So never mind. 697 00:28:04,690 --> 00:28:05,690 Then 698 00:28:07,300 --> 00:28:08,529 you go ahead. 699 00:28:08,530 --> 00:28:09,799 I don't know if I could follow. 700 00:28:11,280 --> 00:28:13,409 Probably just wouldn't be possible 701 00:28:13,410 --> 00:28:15,489 to have men in the middle attacks, 702 00:28:15,490 --> 00:28:17,729 so if there is one untrusted 703 00:28:17,730 --> 00:28:20,489 step in this whole chain, 704 00:28:20,490 --> 00:28:22,679 would you detect it or is 705 00:28:22,680 --> 00:28:24,779 it undetectable this 706 00:28:24,780 --> 00:28:26,309 way, though? 707 00:28:26,310 --> 00:28:28,589 You mean with a quantum key distribution? 708 00:28:28,590 --> 00:28:30,689 Not if you have a chain, but not 709 00:28:30,690 --> 00:28:33,099 normally in a bar you have to 710 00:28:33,100 --> 00:28:35,459 do to amplify the photons. 711 00:28:35,460 --> 00:28:37,559 This is done with some notation 712 00:28:37,560 --> 00:28:39,749 now and classical way 713 00:28:39,750 --> 00:28:42,209 with. Yes, some some 714 00:28:42,210 --> 00:28:44,459 rough elements and something 715 00:28:44,460 --> 00:28:45,869 like this. 716 00:28:45,870 --> 00:28:47,669 But this is all classical and that use 717 00:28:47,670 --> 00:28:50,639 stimulated emission to simply multiply 718 00:28:50,640 --> 00:28:51,449 the photons. 719 00:28:51,450 --> 00:28:54,059 So we would have one photon, which is 720 00:28:54,060 --> 00:28:56,459 exactly the same quantum numbers but 721 00:28:56,460 --> 00:28:57,809 hasn't been measured. 722 00:28:57,810 --> 00:28:59,609 I never understood why this is not a 723 00:28:59,610 --> 00:29:01,679 problem for for quantum 724 00:29:01,680 --> 00:29:02,969 cryptography. 725 00:29:02,970 --> 00:29:05,129 I think you're talking about the no 726 00:29:05,130 --> 00:29:06,149 cloning theory. 727 00:29:06,150 --> 00:29:08,219 You can't clone a single 728 00:29:08,220 --> 00:29:10,289 photon state perfectly as 729 00:29:10,290 --> 00:29:11,699 far as I could read. 730 00:29:11,700 --> 00:29:14,019 It only applies for a 731 00:29:14,020 --> 00:29:16,649 system. Professor Hamiltonian and 732 00:29:16,650 --> 00:29:18,899 stimulated emission has a 733 00:29:18,900 --> 00:29:21,269 completely different Hamiltonian, 734 00:29:21,270 --> 00:29:23,489 which which is not. 735 00:29:23,490 --> 00:29:25,799 This is not a play as simple as I think. 736 00:29:26,850 --> 00:29:27,839 I don't think so. 737 00:29:27,840 --> 00:29:30,119 As the no cloning theory 738 00:29:30,120 --> 00:29:32,249 says that, you know, you can't 739 00:29:32,250 --> 00:29:34,619 copy a perfect 740 00:29:34,620 --> 00:29:37,199 perfectly copy quantum and 741 00:29:37,200 --> 00:29:38,849 arbitrary state. 742 00:29:38,850 --> 00:29:42,089 So you maybe generate 743 00:29:42,090 --> 00:29:44,189 a photon in a 744 00:29:44,190 --> 00:29:45,190 certain state 745 00:29:46,860 --> 00:29:48,479 because they have been produced before. 746 00:29:48,480 --> 00:29:50,699 So if I use the same process, I might get 747 00:29:50,700 --> 00:29:52,349 the same photon again. 748 00:29:52,350 --> 00:29:54,660 But you can't measure it perfectly 749 00:29:56,550 --> 00:29:58,649 with a laser. But in my opinion, does 750 00:29:58,650 --> 00:30:01,859 exactly this simply copy? 751 00:30:01,860 --> 00:30:04,290 No, they they are not in the same state 752 00:30:05,520 --> 00:30:07,679 and then they don't have 753 00:30:07,680 --> 00:30:08,880 the same polarization 754 00:30:10,380 --> 00:30:11,639 in the laser. 755 00:30:11,640 --> 00:30:12,839 That's the answer. 756 00:30:12,840 --> 00:30:15,419 OK, OK. 757 00:30:15,420 --> 00:30:16,420 So can you 758 00:30:17,610 --> 00:30:19,649 can you say anything about the loss rate 759 00:30:19,650 --> 00:30:21,899 of the repeater or is it perfect. 760 00:30:21,900 --> 00:30:23,999 It's perfect, of course. 761 00:30:24,000 --> 00:30:27,119 Uh, uh, 762 00:30:27,120 --> 00:30:28,979 well right now there is no quantum 763 00:30:28,980 --> 00:30:31,349 repeater that it is an active 764 00:30:31,350 --> 00:30:32,729 research topic. 765 00:30:32,730 --> 00:30:34,829 And right 766 00:30:34,830 --> 00:30:36,569 now there is a loss rates. 767 00:30:36,570 --> 00:30:37,570 I mean, 768 00:30:38,970 --> 00:30:40,529 the problem right now is that the crunch 769 00:30:40,530 --> 00:30:41,909 memory is not not there. 770 00:30:41,910 --> 00:30:43,649 So we can talk about a quantum repeater, 771 00:30:43,650 --> 00:30:45,299 but the rates are right now are very 772 00:30:45,300 --> 00:30:46,259 high. 773 00:30:46,260 --> 00:30:48,449 So there's something that we have 774 00:30:48,450 --> 00:30:50,939 which is actively research. 775 00:30:50,940 --> 00:30:51,940 OK, thanks. 776 00:30:52,860 --> 00:30:54,419 Please go ahead. 777 00:30:54,420 --> 00:30:56,519 I am so usually when 778 00:30:56,520 --> 00:30:59,849 people talk about entanglement, they 779 00:30:59,850 --> 00:31:01,859 consider properties of particles, for 780 00:31:01,860 --> 00:31:03,449 example, photons in two different 781 00:31:03,450 --> 00:31:04,559 locations. 782 00:31:04,560 --> 00:31:05,609 So the two particles have 783 00:31:07,920 --> 00:31:09,899 essentially pretty applications. 784 00:31:09,900 --> 00:31:12,329 And the Heisenberg's uncertainty 785 00:31:12,330 --> 00:31:14,429 principle, of course, and the momentum 786 00:31:14,430 --> 00:31:16,409 component of the particles, properties 787 00:31:16,410 --> 00:31:17,559 moving kind of broad. 788 00:31:17,560 --> 00:31:19,979 So the wavelength is not 789 00:31:19,980 --> 00:31:21,449 very sharply defined for the photons, 790 00:31:21,450 --> 00:31:23,219 which I think is fine in this case. 791 00:31:23,220 --> 00:31:25,289 But I'm saying in 792 00:31:25,290 --> 00:31:26,399 the way people normally talk about 793 00:31:26,400 --> 00:31:28,439 entanglement, the space component is very 794 00:31:28,440 --> 00:31:30,599 sharp and the momentum component isn't. 795 00:31:30,600 --> 00:31:32,549 So my question is, in principle, I think 796 00:31:32,550 --> 00:31:35,429 you could also have a sharp, um, 797 00:31:35,430 --> 00:31:38,039 momentum component and a very undefined 798 00:31:38,040 --> 00:31:39,479 space component. For example, if you look 799 00:31:39,480 --> 00:31:41,129 at electrons and carbon nanotubes, they 800 00:31:41,130 --> 00:31:43,619 might be delocalized 801 00:31:43,620 --> 00:31:44,969 essentially, and one might be going this 802 00:31:44,970 --> 00:31:46,680 way around the nanotube 803 00:31:47,860 --> 00:31:49,619 government going the other way around. 804 00:31:49,620 --> 00:31:51,359 So my question is, have there been any 805 00:31:51,360 --> 00:31:53,459 experiments in using entanglement, not 806 00:31:53,460 --> 00:31:56,249 in space, but in momentum space? 807 00:31:56,250 --> 00:31:58,289 And if so, what could be practical 808 00:31:58,290 --> 00:32:00,469 applications for encryption and 809 00:32:00,470 --> 00:32:01,470 stuff? 810 00:32:01,960 --> 00:32:04,259 Uh, because it would be possible 811 00:32:04,260 --> 00:32:05,260 that Senator. 812 00:32:07,440 --> 00:32:09,890 I can't remember people. 813 00:32:11,430 --> 00:32:13,709 This was described so 814 00:32:13,710 --> 00:32:15,779 I don't know every paper, but 815 00:32:15,780 --> 00:32:16,780 it might be possible, 816 00:32:18,030 --> 00:32:19,789 so. OK, thanks. 817 00:32:21,120 --> 00:32:23,699 OK, do we have any more questions? 818 00:32:23,700 --> 00:32:24,700 Yes, but 819 00:32:26,370 --> 00:32:28,499 the entanglement of the polarizations of 820 00:32:28,500 --> 00:32:30,959 two photons in two different locations. 821 00:32:30,960 --> 00:32:32,609 Yeah. And you could also, for example, 822 00:32:32,610 --> 00:32:34,799 two electrons in the same position 823 00:32:34,800 --> 00:32:36,899 in the same nanotube and one is going 824 00:32:36,900 --> 00:32:37,919 counterclockwise. 825 00:32:37,920 --> 00:32:39,839 In the other one's going clockwise. 826 00:32:39,840 --> 00:32:41,159 And that is two different locations in 827 00:32:41,160 --> 00:32:43,499 momentum space where they overlap 828 00:32:43,500 --> 00:32:45,869 in the direct conventional 829 00:32:45,870 --> 00:32:46,829 space. 830 00:32:46,830 --> 00:32:47,830 That's what I meant. 831 00:32:49,210 --> 00:32:50,210 Hmm. OK. 832 00:32:52,640 --> 00:32:54,349 Any more questions? 833 00:32:54,350 --> 00:32:55,350 OK, go ahead. 834 00:32:57,620 --> 00:32:59,839 I have only a very small 835 00:32:59,840 --> 00:33:02,059 question, so I was wondering 836 00:33:02,060 --> 00:33:04,219 what is the problem with 837 00:33:04,220 --> 00:33:07,189 conserving the state 838 00:33:07,190 --> 00:33:09,379 when you, for example, when 839 00:33:09,380 --> 00:33:10,380 you use 840 00:33:13,520 --> 00:33:16,129 when you try to to save 841 00:33:16,130 --> 00:33:18,499 phantom states, 842 00:33:18,500 --> 00:33:20,749 you just have that you can save 843 00:33:20,750 --> 00:33:23,989 it up to one hundred 844 00:33:23,990 --> 00:33:25,429 microseconds. 845 00:33:25,430 --> 00:33:27,589 Also, what the problem 846 00:33:27,590 --> 00:33:29,719 why can't I only save the state of 847 00:33:29,720 --> 00:33:30,770 such a short time. 848 00:33:32,300 --> 00:33:33,300 The. 849 00:33:35,890 --> 00:33:37,959 And this is only one 850 00:33:37,960 --> 00:33:40,089 from the memory, so there might be others 851 00:33:40,090 --> 00:33:42,309 I just mentioned, there has 852 00:33:42,310 --> 00:33:44,499 been published a paper in 853 00:33:44,500 --> 00:33:46,569 November would say that 854 00:33:46,570 --> 00:33:50,019 they can save the state for about 39 855 00:33:50,020 --> 00:33:52,269 minutes at room temperature 856 00:33:52,270 --> 00:33:54,369 and a sink for about three 857 00:33:54,370 --> 00:33:55,509 hours, an 858 00:33:56,680 --> 00:33:58,420 encouraging environment. 859 00:33:59,770 --> 00:34:01,839 The problem here is that the occurrence 860 00:34:01,840 --> 00:34:04,389 of the electron, you see 861 00:34:04,390 --> 00:34:06,669 this electron is not stable for 862 00:34:07,840 --> 00:34:09,399 arbitrary time. 863 00:34:09,400 --> 00:34:11,859 And after 864 00:34:11,860 --> 00:34:14,309 and after more time, they 865 00:34:14,310 --> 00:34:16,569 I think they had problems 866 00:34:16,570 --> 00:34:18,209 through the doubt. 867 00:34:18,210 --> 00:34:19,210 Thank you very much. 868 00:34:21,320 --> 00:34:24,468 I have a question concerning detector's 869 00:34:24,469 --> 00:34:26,718 how do you actually detect a thought 870 00:34:26,719 --> 00:34:28,819 on state and get the 871 00:34:28,820 --> 00:34:30,968 on state out of the photon? 872 00:34:30,969 --> 00:34:32,030 So it's, 873 00:34:33,350 --> 00:34:35,809 uh, I can't really imagine 874 00:34:35,810 --> 00:34:37,968 how to build and detect that can 875 00:34:37,969 --> 00:34:39,529 catch these different states. 876 00:34:39,530 --> 00:34:41,929 Do you apply to this with you, with 877 00:34:44,480 --> 00:34:45,709 the beam splitter to do. 878 00:34:45,710 --> 00:34:46,710 Yeah. 879 00:34:47,300 --> 00:34:49,759 When when I talk about states, 880 00:34:49,760 --> 00:34:52,189 I mean, just one certain property, 881 00:34:52,190 --> 00:34:53,658 like for example, for example, the 882 00:34:53,659 --> 00:34:56,119 polarization, of course you can use other 883 00:34:57,200 --> 00:34:59,569 properties, but the polarization 884 00:34:59,570 --> 00:35:01,869 you can catch with this 885 00:35:01,870 --> 00:35:04,039 polarizing beam splitter, which 886 00:35:04,040 --> 00:35:06,289 can differ between 887 00:35:06,290 --> 00:35:08,359 two polarization and 888 00:35:08,360 --> 00:35:10,759 when it's of one paralyzation, it goes 889 00:35:10,760 --> 00:35:12,889 to the front 890 00:35:12,890 --> 00:35:14,779 of your Downbelow and when it's off, 891 00:35:14,780 --> 00:35:17,019 another polarization, it goes to the 40 892 00:35:17,020 --> 00:35:18,249 year old on the other side. 893 00:35:18,250 --> 00:35:20,509 But if you if you if you supported 894 00:35:20,510 --> 00:35:22,879 old, you never have actually 895 00:35:22,880 --> 00:35:25,069 one single photon detection 896 00:35:25,070 --> 00:35:26,639 with an avalanche. What do you do to your 897 00:35:26,640 --> 00:35:28,820 diet? You only can detect 898 00:35:30,320 --> 00:35:32,329 a bunch of photons coming to the light. 899 00:35:32,330 --> 00:35:33,979 You're not able to detect one thing 900 00:35:33,980 --> 00:35:36,049 before because this will not cause 901 00:35:36,050 --> 00:35:38,549 the avalanche effect. 902 00:35:38,550 --> 00:35:40,709 How do you come across this? 903 00:35:40,710 --> 00:35:43,049 No, I mean, you use 904 00:35:43,050 --> 00:35:45,169 an avalanche dude for detecting this 905 00:35:45,170 --> 00:35:47,479 photon, but you can't detect one single 906 00:35:47,480 --> 00:35:48,520 photon with an avalanche. 907 00:35:49,570 --> 00:35:50,599 You do know. 908 00:35:50,600 --> 00:35:52,459 I mean, it did. 909 00:35:52,460 --> 00:35:55,519 Yeah. But you don't have one photon done 910 00:35:55,520 --> 00:35:57,469 because it does not have a single 911 00:35:57,470 --> 00:35:58,699 thoughtful resolution. 912 00:35:58,700 --> 00:36:00,289 This type of detector. 913 00:36:00,290 --> 00:36:03,229 It depends on what kind of avalanche 914 00:36:03,230 --> 00:36:04,129 you use. 915 00:36:04,130 --> 00:36:06,349 But there are there 916 00:36:06,350 --> 00:36:09,349 are avalanche Forteo that, uh, 917 00:36:09,350 --> 00:36:11,209 that are used for a single photon 918 00:36:11,210 --> 00:36:12,210 detection. 919 00:36:16,630 --> 00:36:17,829 I'm very sure, 920 00:36:19,330 --> 00:36:20,330 very sure. 921 00:36:21,250 --> 00:36:22,250 OK. 922 00:36:24,790 --> 00:36:27,310 Any more questions, anyone? 923 00:36:28,500 --> 00:36:30,549 No. OK, then. Thank you. 924 00:36:30,550 --> 00:36:31,670 And another round of applause.