to unscramble it.
Of course an eavesdropper might be snooping. Let’s call that eavesdropper
Eve. Eve could intercept each particle heading toward Bob, measure its spin, and so learn each digit of the key. Furthermore, she could disguise her presence by producing a new particle of her own, a particle with the same spin as the one she has just measured, and sending it off to Bob. Alice and Bob would be unaware that their supposedly secret key had been intercepted.
But in a brilliant invention, Ekert figured out how to get around this danger.
The method is to have Alice and Bob randomly twist the axes of their spin
detectors about, sometimes using one orientation and sometimes another.
Through a complicated series of steps, both sender and recipient would be
able to learn if Eve was present. Furthermore, Eve would be prevented from knowing whether the information she received was real or bogus.
A similar scheme, not relying on entangled particles, had actually been implemented in 1984 by Charles Bennett and Gilles Brassard. Their device (it happened to use particles of light) transferred information along a 30 centimeter box, cheerfully dubbed “Aunt Martha’s Coffin.” That was in 1984. Over
the next few decades the field matured. By 2005 four companies had been
formed to develop commercial products: prices ran into the hundreds of
thousands of dollars. Shortly thereafter the state of Geneva announced its intention to use quantum cryptography to ensure the security of its network linking ballot data entry during the Swiss national elections.
Anton Zeilinger (we first met him in chapter 12’s freedom of choice
experiment) was the leader of the 2004 demonstration involving the bank
transfer of funds. His bank transfer demonstration involved a collaboration with a corporation interested in creating a marketable quantum cryptography product, a second collaboration with a corporation that laid the optical fiber along which the particles of light were sent, yet another with the receiving bank and finally one with Vienna’s city hall. Within the offices of the bank pairs of photons— particles of light— were produced. One was
sent down the optical fiber to city hall, while the other remained within the
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bank. When detectors measured the properties of the photons the key was
produced and used to decode a secret message.
A milestone was reached in 2017, when a group led by the Chinese physicist Jian Wei Pan managed to transmit a cryptographic key from interplanetary space down to the ground. The year before an entire satellite devoted to the study of quantum entanglement had been launched from China.
Named after the Chinese philosopher/scientist Micius (roughly contemporary with Socrates) it flew at an altitude of some 300 miles, in an orbit designed to carry it over Beijing every night shortly after midnight.
On board that satellite was equipment designed to implement the
Bennett– Brassard technique of quantum key distribution— and a small telescope, pointed downward. Below it, in a suburb of Beijing, stood a larger telescope. Executing a carefully choreographed swivel, it tracked the satellite as it zoomed overhead at 17,000 miles per hour. Within five minutes the satellite had passed over the horizon … but in that five minutes a cryptographic key had been passed down from space.
Zeilinger and Pan are also pioneers in the field of quantum teleportation.
Figure 15.1
Jian Wei Pan at an experiment. Photo courtesy of the Micius Group.
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Figure 15.2
Launch of the Micius Satellite. Photo courtesy of the Micius Group.
Figure 15.3
The Micius Satellite.
Jian Wei Pan is the leader of a group that launched a “quantum machine” into orbit about the earth. This machine, the Micius satellite, has been used to securely transmit a cryptographic key from one place to another, and to teleport a quantum state from one place to another. Photo courtesy of the Micius Group.
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Every science fiction addict is enamored of teleportation. I certainly was as a kid. I would imagine closing my eyes, trying very hard, and opening
them to find myself instantly transported to Mars. I would then enthrall
myself for hours imagining various adventures as I explored the Red Planet.
As for how I had gotten there, and what that “trying very hard” entailed …
well, this was a technical matter best left unexplored.
Now that I am older I have a clearer idea how this might be accomplished. We normally suppose that to teleport something you need it to arrive at its destination without having traversed the intervening spaces. But perhaps you do not need to actually send the object being teleported. Perhaps it might be sufficient to send nothing but information— information describing in excruciating detail exactly how your object was constructed, information sufficient to enable a factory at the destination to construct a perfect replica of your object. That might be considered teleportation.
Quantum nonlocality provides yet another a way to realize this dream—
and this is a way that actually works. Zeilinger has done it. His system began with a quantum particle in some state, and it transferred that state over a distance of six city blocks. It did so beneath— beneath— the river Danube.
There is an island in the Danube as it runs through the city of Vienna.
On that island is an underground pumping station for the city’s sewage
system. Once on the island you take an elevator down to that station. From it, tunnels lead beneath the river over to the mainland. Through those tunnels run the sewage pipes, together with a maze of electrical cables. One of those cables was laid by Zeilinger’s group: it was a state of the art optical fiber carrying quantum particles— photons.
The process producing these photons began with a laser. Housed within
the underground pumping station, it was big and expensive— it cost as
much as a house. The laser shone on a translucent wafer of crystal. In contrast to the laser, the crystal was fragile and tiny, a mere two millimeters thick. When illuminated by the laser, it produced a pair of photons entangled to form a nonlocal state.
One of these quantum particles stayed where it was produced. It ran
through a length of optical fiber within the pumping station. The other
went into a second optical fiber that led through the underwater tunnel
and then up to a receiving station on the opposite bank of the river. There was another communication link involved as well: this one was entirely
nonquantum, and involved nothing more than a radio signal. The signal
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went from a transmitter on the island to an antenna on the roof of the
receiving station.
The experimenters needed their quantum particles of light to arrive at
the receiving station after the radio signal. This they achieved by delaying the photons— by increasing the distance they had to travel. The experimenters did this by sending them through extra lengths of optical fiber lying coiled around on the floor.
Once the radio and quantum signals had arrived at the receiving station,
delicate and complex electronics together with computer algorithms combined to yield a photon at the receiving end arranged to be in precisely the same state as the original photon that had been produced underground six
blocks away. In the process, the original particle had been destroyed. But no matter: its state had been teleported. The information contained in the particle had been teleported.
Since then, the field has developed. In 2015, Zeilinger’s group achieved
teleportation between two islands in the Atlantic: two years later Pan’s
group teleported a photon from Tibet up into their Mic
ius satellite.
Eventually teleportation might be used to connect quantum computers.
Gordon Moore, cofounder of Intel Corporation, once observed that, as
they came out of the factories, the individual components of which integrated circuits were formed were growing smaller and smaller. As a consequence, the number of such components that could be crammed into an integrated circuit was increasing. Working out the numbers, he found that it was doubling every two years. So computers were doubling in power every two years.
That simple observation has come to be known as Moore’s Law, and it
has stayed valid. If things continue this way much longer, the components of which computers are built will have shrunk to quantum dimensions within a mere few decades. The rapid advance of technology will have brought us right into the quantum realm.
Nowadays, your computer works on “0’s” and “1’s: binary digits, or “bits”
for short. But in this new realm the bits will be “qubits”— quantum bits.
Just as electron spin can be along or against the direction of a detector, so quantum mechanics also allows it to be in a more ambiguous state— a state
that cannot be described in plain language, one with no correspondence in
ordinary experience, but that in some loose sense might be thought of as
an electron in both configurations at once. That electron could embody a
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qubit that was both a “0” and a “1” at the same time. And a computer that
worked with qubits would be a quantum computer.
Indeed, a qubit has not just two, but an entire range of values simultaneously, with “0” and “1” merely indicating the extreme values. Present day computers, working only at these two extremes, do one thing at a time, or
at best a few things. Their immense power stems from the fact that they do these things very rapidly. But if a qubit can be a whole range of things at once— why then, a computer working with qubits can do a whole range of
things at once. That would make them faster— much faster. And this enormous increase in computing power will bring with it an enormous increase in what computers might do.
What is your favorite unsolved problem? Predicting the weather? Developing new pharmaceuticals? Mastering the vagaries of the stock market?
Transferring the latest viral video to kids worldwide? You might find such hyper powerful computers just what you need.
Governments and corporations need them too. One of the most famous
methods of encoding a message involves a secret key generated by what is
known as RSA encryption. That method of generating a secret key can be
broken by a computer— but a current day machine, chugging away at the
problem, would take a century to crack the code. If quantum computers
live up to expectations they will be able to do it in a matter of days.
So a horse race is underway. Quantum computers are in a primitive state
right now, but many people are in the race, and most think their efforts
will bear fruit in a matter of decades at most. Research teams at many of the world’s leading universities are working on the problem.
And so are corporations. Large sums of money are involved. I recently
read an article about the potential uses of quantum computers on a website devoted not to science, but to business news. The site quoted the musings of the director of engineering at Google— as well as revelations from Edward Snowden’s leaked documents.
Indeed, government has always been interested in these matters. Astonishingly, as far back as 1972 the Defense Intelligence Agency developed a long and highly classified discussion of how the Soviet Union was spending great sums of money on ESP phenomena, and how the United States might want to do so as well— and, as part of this effort, to fund research on quantum nonlocality. I’m only guessing of course, but I wonder if people in
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Washington were wondering whether a nuclear weapon could be rendered
harmless by nonlocal influences. Or detonated.
But there is no need to wonder about one thing. Not too many years ago
the United States government brought before a grand jury a researcher who
had developed a powerful new method of encryption and given it away for
free. They were investigating him for having disseminated a new kind of
weapon.
The amazing thing about these quantum machines is how they work. For
that is the point of this book, and the point of Bell’s Theorem: normally
explanations involve properties, processes, causes, and effects— and we
now know that these concepts simply do not apply to the quantum world.
But so what? Quantum machines do work. They work just fine. And we know how to build them. More and more in coming years we will be using
machines that live in the world of the bizarre.
16 A New Universe
It was many years ago that I first encountered the Great Predictor.
I was thrilled to meet him. I’d been looking forward to the encounter for
years. The Predictor was famous— world famous. He was legendary for the
number of his predictions, and for their amazing accuracy. Many people
had relied on those predictions, and always with profit.
What intrigued me the most, however, was how bizarre were some of his
predictions. “Tomorrow you will be in two places at once” was one. “On
Wednesday an event will occur for which there is no cause” was another.
How could such things be? I was captivated by the strangeness of these
prognostications. Could such weird things really come to pass? That’s why
I had been so anxious to meet the Predictor. For years I had looked forward to finally getting to know him.
At long last I was getting my wish. I was twenty years old, and I was thrilled.
I was sitting in a classroom, in college, on the first day of a course called Introduction to Quantum Mechanics.
That was many long years ago. And throughout my career I have maintained my early fascination with quantum mechanics. Somehow, though, I never felt that I really understood the theory. It always sat lodged in the back of my mind— enigmatic, mysterious, enticing. Over and over again, I
found myself thinking that someday I really ought to go back and figure it all out, and finally put all those early juvenile confusions to rest.
Part of that project was an effort to understand Bell’s Theorem. To be honest I found myself dreading getting to work on that particular topic. While I had never felt comfortable with quantum mechanics in general, Bell’s
Theorem was a topic that I felt positively unnerved by. Over and over again I had tried to master it, and over and over again I had failed.
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Eventually I did come to some sort of understanding of Bell’s work.
I recall feeling pretty pleased with things … until the fateful day when I looked at my reflection in the mirror— and this is literally true— and I spoke aloud. “Greenstein,” I said to my reflection “you were just kidding yourself, weren’t you? You never really understood Bell’s Theorem at all, did you?”
“Time to get going,” I told my reflection.
And I did.
What did I do? I read some books. I read some scientific articles. I have
already mentioned one: there were others. But, truth to say, not that many books, and not that many articles. I talked to colleagues— but not that often.
I took long walks and stewed things over. Mostly that’s what it was: thinking. I thought and I thought and I thought. It went on for several years.
What was I thinking about? At the beginning I could not even say. I would
go back and
look at the proof of Bell’s Theorem. I would work through the
mathematics for myself— remarkably, the actual calculations involved are not so very hard, now that Bell has shown the way. But even after I had done this I still felt mystified. The math wasn’t the point— the point was what it all meant. I knew damn well that Bell’s discovery was important— everybody else was going around saying that it was important, and it certainly felt important to me. But why? What was his theorem telling us? And why would my mind
go blank whenever I tried to think about it?
That last question was a signal. By now I know myself well enough to
realize that if I find it hard to even think about something, it is a signal that there is some enormous gap in my understanding. Somewhere, something
was missing from my thoughts. But what?
For months it would feel that nothing was happening— but then one day I
would cast my mind backward and realize that my thinking about Bell’s Theorem was different. In the intervening months my thinking had changed without my even being aware of the change. As a matter of fact, that was pretty much what it was like all along: I was hardly ever aware of what was going on.
It felt like I was walking backward— I wasn’t able to see where I was going until I got there. I never knew what was happening until it had finished happening.
Often I was not even aware that I was thinking about the issue. I would
be doing something else— washing the dishes, driving to the store— and
without the slightest warning a thought concerning quantum mechanics
would pop into my head. A pain in the neck? A delight? Yes— yes to both.
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Writing was helpful. It always clarifies my thoughts to get something
down in plain English. That’s why I wrote this book: to clarify my thoughts.
Of course I couldn’t write the book until I had cleared up my thinking—
and I couldn’t clear up my thinking without writing the book. So it was a
back and forth process.
Looking back on it all, I now feel that what I was doing was facing for
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