`15 Quantum Machines`

The birth of a baby next door … or the secure transfer of funds from Vienna’s city hall to the Bank Austria Creditanstalt—a transfer initiated by the city’s mayor, executed by the bank’s director, and announced at a press conference. That was in 2004.

What began as a philosophical difference among a small group of physicists nearly a century ago has blossomed into what promises to be a worldwide industry—an industry based on quantum mechanics. Billions of dollars are involved. Google is interested in quantum nonlocality. So are Facebook and MIT. Venture capital firms are sitting down at the table, as well as the CIA. Above the boardrooms of mighty governments and corporations float the ghosts of Einstein, Bohr, and Bell. Mild, philosophical, perhaps a bit otherworldly, they have been shoved aside by the new breed: those can-do types who roll up their sleeves, brush aside the niceties, and get down to cases. The old stigma has passed—passed with a vengeance.

But why? What has happened to the old antipathy to philosophically tinged questions, the antipathy that stifled the field for years? Part of the answer is a simple matter of time. A new generation of researchers has come of age—researchers no longer in thrall to Von Neumann’s erroneous proof or to the necessities of the Cold War. These people never experienced the old days, were hardly touched by that subtle, all-pervasive amusement that once greeted those who asked such questions—an amusement I must have unconsciously sensed when I was a student, and that steered me away from these matters for so many years.

But I think there is another factor at play. As I wrote in chapter 12, I think it is also a matter of the advance of technology. In the intervening years technology has progressed so rapidly that it is now possible to actually do the experiments of which previous generations could only dream. Thought experiments have been replaced by real ones. It is no longer a matter of arguing about what such-and-such an experiment might reveal: now you can actually find out. There it is again: the thing I love about science—the wonderful sense of freedom and openness and possibility to the business of research. If you can do something you do do it, and all those psychological and historical issues be damned. Science is a can-do enterprise. So philosophy is invading industry.

And strange to say, all this has been made possible by a breed pretty much uninterested in philosophy. The new age of experimental metaphysics has been made possible by people in love, not with philosophy, but with gadgets. These are people who delight in inventing new devices, new procedures, and new ways of doing experiments. If their wonderful new gadgets can be pressed into service to answer a primarily philosophical question, all well and good. But it was never their primary concern. So industry is invading philosophy.

And so experimental metaphysics.

If you are transferring funds you are transferring information—information about your bank account number, let us say. This might be done in person, or by email. But if your name happens to be Alice, and the teller at the bank is named Bob, a whole new dimension of the situation just might occur to you. Can information be conveyed by quantum particles?

When we measure the spin of a particle, we learn whether that spin is along or against the direction of our detector. Suppose we agree that a spin along represents a “0” and a spin against a “1.” Then our measurement has told us a number.

That number is in binary—the number system of base two. We are used to writing numbers in base ten. But the translation is straightforward:

Base 10		Base 2
0		0
1		1
2		10
3		11
4		100
. . .		. . .

Suppose Alice sends Bob three electrons, the first with spin against his detector’s axis–that’s a “1”—and the next two along the axis—these are “0s.” Then this represents the sequence “100”—which, if we think of it as the binary representation of a number, we would interpret as a “4.” Using those electrons, Alice has told Bob a number.

We can also transfer letters of the alphabet. It can be so simple a matter as agreeing that “1” represents the letter “a,” “2” represents a “b,” and so forth. There are more sophisticated codings, but the principle is the same. In every case we can find a means of translating the information we wish to convey into binary numbers, and then we can use electron spins to encode those numbers. Alternatively, we can use particles of light—photons.

It’s not enough to transfer information. The transfer needs to be secure. We need to make our information available to the intended recipient, but not available to anybody else. The world is full of eavesdroppers—hackers trying to steal our credit card numbers, wartime enemies trying to steal our battle plans. If you are a certain soft drink company, you might wish to keep the formula for Coca-Cola secret from competitors, while revealing it to your factories worldwide. How to guard against snoops?

Secrecy in the transfer of information has a long and fascinating history. In one ancient method, the head of a courier was shaved, the message was written on his scalp, and his hair was allowed to grow back. The courier could then travel to the intended recipient, where his head was shaved, thus revealing the message. A more recent technique was to lock the secret message into a briefcase, handcuff the briefcase to a courier, and send the courier off to the recipient—a recipient who had the only key to the briefcase. I recall sitting next to such a courier once in an airplane.

Both of these methods entailed trusting the couriers—and a long trip too, one that might very well be expensive and time-consuming. Far cheaper, and far more rapid, would be something like a telephone call or a transfer over the Web. How to guard against eavesdroppers in these situations?

The method is to encrypt the message—to scramble it in some way. Suppose we send the message “MN.” An eavesdropper could easily intercept it, but she would not have the slightest idea what it means. If, however, you and your intended recipient had agreed beforehand that you would send the letter of the alphabet lying before each letter of your message, the recipient would know that in fact you had sent the word “NO.”

Such a method of encryption is known as the “key”—it unlocks the message. This one, of course, is very simple, and it is one that any smart eavesdropper could foil with ease. It is far better to use a more complicated key. Suppose, for example, that we use some random string of numbers, such as

726 …

and suppose we simply add each digit to the corresponding digit in the message we wish to send. If, for instance, Alice’s credit card number begins with the digits

547 …

then she would add the key to her number

7 + 5, 2 + 4, 6 + 7 …

and send the message

12,6,13 …

If Bob knew the key, he could decode the message and so learn the first three digits of Alice’s credit card number. And if nobody else knew the key, eavesdroppers would be foiled.

Of course, Alice has to tell Bob about the key she used—and this “telling” itself must be secret. There is a whole branch of mathematics devoted to the study of encrypting and decrypting messages. It is known as cryptography. No cryptographic method is foolproof. Some brilliant new technique is invented … and then, far sooner than anybody expected, some brilliant hacker finds a means of circumventing it. For obvious reasons, large corporations are interested in cryptography, as are governments. Billions of dollars are involved.

People’s lives are involved as well. During the Second World War, Germany encrypted its messages using a devilishly complicated device known as the “enigma” machine. In absolute secrecy, code breakers stationed at Bletchley Park in England succeeded in deciphering the code, revealing vitally important military plans. Their triumph shortened the war, probably by several years, saving untold numbers of lives. Had they failed, would an atomic bomb have been dropped on Berlin?

In 1991 the physicist Artur Ekert invented a way to encrypt a key relying on quantum mechanics. The method is to use our old familiar pair of entangled particles, for which measurements of spins always yield opposite results. Suppose we send out a series of such entangled pairs. For each one, Alice and Bob can translate the results of their spin measurements into binary numbers—and then, if Bob simply reverses his digits, his number is guaranteed to be the same as Alice’s. After measuring all the particles, Alice and Bob will have the same series of numbers—the same key. If Alice uses her key to scramble the message she wishes to send, Bob can use his 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 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.

`Figure 15.2`

Launch of the Micius Satellite. Photo courtesy of the Micius Group.

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.

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 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 Micius 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 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 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.