1
00:00:00,499 --> 00:00:01,890
PROFESSOR: So far so good.

2
00:00:01,890 --> 00:00:06,420
So here is the kind of
very entertaining thing

3
00:00:06,420 --> 00:00:10,740
that happens when you try to
do some physics with this.

4
00:00:10,740 --> 00:00:19,230
And this was done by two
physicists, Elitzur and Vaidman

5
00:00:19,230 --> 00:00:23,820
in Tel Aviv, they
invented or fantasized

6
00:00:23,820 --> 00:00:27,270
about some sort of bombs--

7
00:00:27,270 --> 00:00:28,570
things that explode.

8
00:00:28,570 --> 00:00:37,740
So they're called
Elitzur-Vaidman bombs.

9
00:00:42,830 --> 00:00:46,040
And you could invent
different things,

10
00:00:46,040 --> 00:00:48,770
but here is what an
Elitzur-Vaidman bomb is--

11
00:00:48,770 --> 00:00:54,490
some sort of bomb,
and the way it works

12
00:00:54,490 --> 00:00:56,110
is with a photon detector.

13
00:00:58,620 --> 00:01:02,770
So there's a little
tube in the bomb,

14
00:01:02,770 --> 00:01:05,990
and there's a photon detector.

15
00:01:05,990 --> 00:01:10,220
And you have your
bomb, and you want

16
00:01:10,220 --> 00:01:15,180
to detonate it-- you
send the photon in,

17
00:01:15,180 --> 00:01:18,260
you send the photon
in through the tube.

18
00:01:18,260 --> 00:01:23,850
And the photon, it's
detected by the detector.

19
00:01:23,850 --> 00:01:27,120
And the bomb explodes.

20
00:01:27,120 --> 00:01:31,870
On the other hand, if
the bomb is defective,

21
00:01:31,870 --> 00:01:35,500
the photon goes in, and
the detector doesn't work.

22
00:01:35,500 --> 00:01:37,510
The photon goes out.

23
00:01:37,510 --> 00:01:41,470
Just goes through.

24
00:01:41,470 --> 00:01:45,100
So that's an
Elitzur-Vaidman bomb.

25
00:01:45,100 --> 00:01:48,550
And here is the puzzle for you--

26
00:01:48,550 --> 00:01:55,370
suppose you have those bombs,
and unfortunately, those bombs,

27
00:01:55,370 --> 00:01:57,490
after time, they decay.

28
00:01:57,490 --> 00:02:00,730
And sometimes the
detectors go wrong,

29
00:02:00,730 --> 00:02:03,630
and they don't work anymore.

30
00:02:03,630 --> 00:02:08,060
So you have 10
bombs, and you know,

31
00:02:08,060 --> 00:02:09,755
maybe five have gone wrong.

32
00:02:12,500 --> 00:02:17,300
And now, you have maybe
a very important mission

33
00:02:17,300 --> 00:02:20,890
and you need the bomb
that really works.

34
00:02:20,890 --> 00:02:23,290
So what do you do?

35
00:02:23,290 --> 00:02:26,440
Let's assume you cannot
break apart the detector--

36
00:02:26,440 --> 00:02:28,220
it's just too complicated.

37
00:02:28,220 --> 00:02:32,830
So you have the bombs,
and you want to test them.

38
00:02:32,830 --> 00:02:35,700
If you send in a photon
and nothing happens,

39
00:02:35,700 --> 00:02:39,000
the bomb is not working.

40
00:02:39,000 --> 00:02:43,180
But if you send in a photon,
and the bomb is working--

41
00:02:43,180 --> 00:02:45,060
explodes.

42
00:02:45,060 --> 00:02:48,140
So you cannot use it anymore.

43
00:02:48,140 --> 00:02:51,620
So the question that
Elitzur and Vaidman pose,

44
00:02:51,620 --> 00:02:56,690
is there a way to
certify that the bomb is

45
00:02:56,690 --> 00:03:01,210
working without exploding it?

46
00:03:01,210 --> 00:03:02,140
Can you do that?

47
00:03:04,830 --> 00:03:07,830
The answer looks
absolutely impossible.

48
00:03:07,830 --> 00:03:10,260
And certainly, in
classical physics,

49
00:03:10,260 --> 00:03:12,570
it's completely impossible.

50
00:03:12,570 --> 00:03:16,260
You either do the measurement
to see if the detector works,

51
00:03:16,260 --> 00:03:19,410
and if it works,
your lab goes off.

52
00:03:19,410 --> 00:03:22,040
It's totally destroyed.

53
00:03:22,040 --> 00:03:24,700
And if it doesn't
work, well, OK,

54
00:03:24,700 --> 00:03:26,720
it's not a good bum anyways.

55
00:03:26,720 --> 00:03:30,620
So there's no way out.

56
00:03:30,620 --> 00:03:34,370
But there is a
way out, and it is

57
00:03:34,370 --> 00:03:40,010
to insert this bomb in the mass
[? center ?] interferometer.

58
00:03:40,010 --> 00:03:42,401
So here we go.

59
00:03:42,401 --> 00:03:51,780
We put the mass
[? center ?] interferometer,

60
00:03:51,780 --> 00:04:00,140
and we insert the bomb here with
the detector along this place.

61
00:04:07,500 --> 00:04:10,750
D 0 and D 1 are still here.

62
00:04:18,019 --> 00:04:28,390
And now, you put this,
and you send in a photon.

63
00:04:28,390 --> 00:04:33,730
So let's see what happens
if you send in a photon.

64
00:04:33,730 --> 00:04:36,490
Suppose the bomb is defective--

65
00:04:39,640 --> 00:04:41,800
bomb is defective.

66
00:04:47,560 --> 00:04:50,140
So what are the
possible outcomes?

67
00:04:50,140 --> 00:04:53,100
Outcome and probability.

68
00:04:59,460 --> 00:05:02,580
Photon goes to D 0--

69
00:05:07,050 --> 00:05:08,150
0.

70
00:05:08,150 --> 00:05:14,890
Photon to D 1, bomb explodes.

71
00:05:20,350 --> 00:05:23,000
Well, we said the
bomb is defective.

72
00:05:23,000 --> 00:05:25,660
So if the bomb is
defective, we said

73
00:05:25,660 --> 00:05:28,510
it's like a detector that
doesn't work, and lets

74
00:05:28,510 --> 00:05:30,790
the photons go through.

75
00:05:30,790 --> 00:05:38,630
So if the bomb is defective,
it's as if there no bomb here,

76
00:05:38,630 --> 00:05:43,650
and you have the situation
where all is open.

77
00:05:43,650 --> 00:05:51,760
So there will be a probability
of 1 to get the photon to D 0--

78
00:05:51,760 --> 00:05:56,930
a probability of 0 to
get the photon to D 1.

79
00:05:56,930 --> 00:06:01,120
And the bomb, of course,
doesn't explode--

80
00:06:01,120 --> 00:06:04,240
probability of 0.

81
00:06:04,240 --> 00:06:08,790
On the other hand,
suppose the bomb is good--

82
00:06:15,490 --> 00:06:16,712
bomb is good.

83
00:06:22,400 --> 00:06:24,130
And then, what are the outcomes?

84
00:06:27,230 --> 00:06:29,140
And what are the probabilities?

85
00:06:38,140 --> 00:06:42,820
Well, you know, more or less,
what's happening already.

86
00:06:42,820 --> 00:06:47,890
The bomb is good means there
is a detector that never

87
00:06:47,890 --> 00:06:50,360
fails to detect the photon.

88
00:06:50,360 --> 00:06:54,340
And if a photon comes
in, it will capture it--

89
00:06:54,340 --> 00:06:55,990
it will block it.

90
00:06:55,990 --> 00:06:59,380
The bomb will explode.

91
00:06:59,380 --> 00:07:02,290
So you have your mass
[? center ?] interferometer,

92
00:07:02,290 --> 00:07:06,510
and you've really done
the equivalent of this--

93
00:07:06,510 --> 00:07:10,280
if the bomb is really working.

94
00:07:10,280 --> 00:07:12,410
You've put a block
of concrete-- it's

95
00:07:12,410 --> 00:07:15,960
going to absorb the photon.

96
00:07:15,960 --> 00:07:22,340
So if the bomb is
really working,

97
00:07:22,340 --> 00:07:25,470
the outcome are the following--

98
00:07:29,720 --> 00:07:38,170
well, I'm sorry to say, your lab
will explode half of the times,

99
00:07:38,170 --> 00:07:41,850
because the photon
on the block happens,

100
00:07:41,850 --> 00:07:50,145
and bomb explodes
with probability 1/2.

101
00:07:53,160 --> 00:07:57,650
On the other hand,
in this situation,

102
00:07:57,650 --> 00:07:59,755
it is possible that the photon--

103
00:08:03,140 --> 00:08:11,102
photon-- at D 0, and
bomb doesn't explode--

104
00:08:11,102 --> 00:08:13,860
not explode.

105
00:08:13,860 --> 00:08:15,810
And there is a probability 1/4.

106
00:08:18,540 --> 00:08:25,020
And there is a probability,
1/4, that the photon is at D 1,

107
00:08:25,020 --> 00:08:28,065
and the bomb does not explode.

108
00:08:32,960 --> 00:08:35,960
But here is the catch now--

109
00:08:35,960 --> 00:08:41,010
yes, half of the bombs exploded,
we're sorry about that.

110
00:08:41,010 --> 00:08:45,140
But if the bomb
doesn't work, there

111
00:08:45,140 --> 00:08:50,690
is no way a photon can reach
D 1, because if a bomb doesn't

112
00:08:50,690 --> 00:08:55,280
work, all photons go to D 0.

113
00:08:55,280 --> 00:08:59,990
So the fact that some
photons, a quarter percent

114
00:08:59,990 --> 00:09:01,940
of the time, 1/4--

115
00:09:01,940 --> 00:09:04,010
25% of the time--

116
00:09:04,010 --> 00:09:09,470
reach D 1 implies
that photon is at D 1,

117
00:09:09,470 --> 00:09:13,070
and bomb did not explode.

118
00:09:13,070 --> 00:09:16,250
But the bomb is good.

119
00:09:16,250 --> 00:09:20,760
So look what has happened--
it's really strange.

120
00:09:20,760 --> 00:09:27,360
The photon went-- the bomb was
here, it was ready to explode.

121
00:09:27,360 --> 00:09:32,610
The photons kept the
bomb, and ended at D 1,

122
00:09:32,610 --> 00:09:36,030
and you still know that
the bomb works now--

123
00:09:36,030 --> 00:09:39,810
even though the photon never
went through the detector.

124
00:09:39,810 --> 00:09:44,400
It never touched here, it never
went inside and get detected.

125
00:09:44,400 --> 00:09:46,830
Somehow it went
through the other way,

126
00:09:46,830 --> 00:09:51,660
but you know that the bomb is
working, with a quarter percent

127
00:09:51,660 --> 00:09:53,160
efficiency.

128
00:09:53,160 --> 00:09:56,250
We will do exercises, and
it's possible to raise

129
00:09:56,250 --> 00:09:58,890
the efficiency to 50%.

130
00:09:58,890 --> 00:10:02,980
And if you put the bomb inside
a cavity, a resonant cavity

131
00:10:02,980 --> 00:10:07,540
with photons going around,
you can reach 99% efficiency.

132
00:10:07,540 --> 00:10:12,558
So the probability of blowing
up MIT goes down to 1%.

133
00:10:12,558 --> 00:10:13,980
[LAUGHTER]

134
00:10:13,980 --> 00:10:15,750
I don't know if we
can live with that--

135
00:10:15,750 --> 00:10:17,560
I don't think so.

136
00:10:17,560 --> 00:10:20,100
But anyway, this
is a true fact--

137
00:10:20,100 --> 00:10:23,430
experiments without
bombs have been done,

138
00:10:23,430 --> 00:10:26,170
and it shows that in
quantum mechanics,

139
00:10:26,170 --> 00:10:29,480
you can do very
surprising measurements.