The Information (64 page)

For that matter, Christopher Fuchs argues that it is no use talking about quantum states at all. The quantum state is a construct of the observer—from which many troubles spring. Exit states; enter information. “Terminology can say it all: A practitioner in this field, whether she has ever thought an ounce about quantum foundations, is just as likely to say ‘quantum information’ as ‘quantum state’…‘What does the quantum teleportation protocol do?’ A now completely standard answer would be: ‘It transfers quantum information from Alice’s site to Bob’s.’ What we have here is a change of mind-set.”
^♦

The puzzle of spooky action at a distance has not been altogether resolved.
Nonlocality
has been demonstrated in a variety of clever experiments all descended from the EPR thought experiment. Entanglement turns out to be not only real but ubiquitous. The atom pair in every hydrogen molecule, H
₂
, is quantumly entangled (
“verschränkt,”
as Schrödinger said). Bennett put entanglement to work in quantum teleportation, presented publicly for the first time in 1993.
^♦
Teleportation uses an entangled pair to project quantum information from a third particle across an arbitrary distance. Alice cannot measure this third particle directly; rather, she measures something about its relation to one of the
entangled particles. Even though Alice herself remains ignorant about the original, because of the uncertainty principle, Bob is able to receive an exact replica. Alice’s object is disembodied in the process. Communication is not faster than light, because Alice must also send Bob a classical (nonquantum) message on the side. “The net result of teleportation is completely prosaic: the removal of [the quantum object] from Alice’s hands and its appearance in Bob’s hands a suitable time later,” wrote Bennett and his colleagues. “The only remarkable feature is that in the interim, the information has been cleanly separated into classical and nonclassical parts.”

Researchers quickly imagined many applications, such as transfer of volatile information into secure storage, or memory. With or without goulash, teleportation created excitement, because it opened up new possibilities for the very real but still elusive dream of quantum computing.

The idea of a quantum computer is strange. Richard Feynman chose the strangeness as his starting point in 1981, speaking at MIT, when he first explored the possibility of using a quantum system to compute hard quantum problems. He began with a supposedly naughty digression—“Secret! Secret! Close the doors …”
^♦

He knew very well what the problem was for computation—for simulating quantum physics with a computer. The problem was probability.
Every quantum variable involved probabilities, and that made the difficulty of computation grow exponentially. “The number of information bits is the same as the number of points in space, and therefore you’d have to have something like
N
^N
configurations to be described to get the probability out, and that’s too big for our computer to hold.… It is therefore impossible, according to the rules stated, to simulate by calculating the probability.”

So he proposed fighting fire with fire. “The other way to simulate a probabilistic Nature, which I’ll call
N
for the moment, might still be to simulate the probabilistic Nature by a computer
C
which itself is probabilistic.” A quantum computer would not be a Turing machine, he said. It would be something altogether new.

“Feynman’s insight,” says Bennett, “was that a quantum system is, in a sense, computing its own future all the time. You may say it’s an analog computer of its own dynamics.”
^♦
Researchers quickly realized that if a quantum computer had special powers in cutting through problems in simulating physics, it might be able to solve other types of intractable problems as well.

The power comes from that shimmering, untouchable object the qubit. The probabilities are built in. Embodying a superposition of states gives the qubit more power than the classical bit, always in only one state or the other, zero or one, “a pretty miserable specimen of a two-dimensional vector,”
^♦
as David Mermin says. “When we learned to count on our sticky little classical fingers, we were misled,” Rolf Landauer said dryly. “We thought that an integer had to have a particular and unique value.” But no—not in the real world, which is to say the quantum world.

In quantum computing, multiple qubits are entangled. Putting qubits at work together does not merely multiply their power; the power increases exponentially. In classical computing, where a bit is either-or,
n
bits can encode any one of 2
ⁿ
values. Qubits can encode these Boolean values along with all their possible superpositions. This gives a quantum computer a potential for parallel processing that has no classical equivalent.
So quantum computers—in theory—can solve certain classes of problems that had otherwise been considered computationally infeasible.

An example is finding the prime factors of very large numbers. This happens to be the key to cracking the most widespread cryptographic algorithms in use today, particularly RSA encryption.
^♦
The world’s Internet commerce depends on it. In effect, the very large number is a public key used to encrypt a message; if eavesdroppers can figure out its prime factors (also large), they can decipher the message. But whereas multiplying a pair of large prime numbers is easy, the inverse is exceedingly difficult. The procedure is an informational one-way street. So factoring RSA numbers has been an ongoing challenge for classical computing. In December 2009 a team distributed in Lausanne, Amsterdam, Tokyo, Paris, Bonn, and Redmond, Washington, used many hundreds of machines working almost two years to discover that
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