Quantum computers

The most ambitious proposal is to use quantum computers, which actually compute on individual atoms themselves. Some claim that quantum computers are the ultimate computer, since the atom is the smallest unit that one can calculate on.

An atom is like a spinning top. Normally, you can store digital information on spinning tops by assigning the number “0” if the top is spinning upward, or “1” if the top is spinning down. If you flip over a spinning top, then you have converted a 0 into a 1 and have done a calculation.

But in the bizarre world of the quantum, an atom is in some sense spinning up and down simultaneously. (In the quantum world, being several places at the same time is commonplace.) An atom can therefore contain much more information than a 0 or a 1. It can describe a mixture of 0 and 1. So quantum computers use “qubits” rather than bits. For example, it can be 25 percent spinning up and 75 percent spinning down. In this way, a spinning atom can store vastly more information than a single bit.

Quantum computers are so powerful that the CIA has looked into their code-breaking potentials. When the CIA tries to break the code of another nation, it searches for the key. Nations have devised ingenious ways of constructing the key that encodes their messages. For example, the key may be based on factorizing a large number. It’s easy to factorize the number 21 as the product of 3 and 7. Now let’s say that you have an integer of 100 digits, and you ask a digital computer to rewrite it as the product of two other integers. It might take a digital computer a century to be able to factorize this number. A quantum computer, however, is so powerful that in principle it can

effortlessly crack any such code. A quantum computer quickly outperforms a standard computer on these huge tasks.

Quantum computers are not science fiction but actually exist today. In fact, I had a chance to see a quantum computer for myself when I visited the MIT laboratory of Seth Lloyd, one of the pioneers in the field. His laboratory is full of computers, vacuum pumps, and sensors, but the heart of his experiment is a machine that resembles a standard MRI machine, except much smaller. Like the MRI machine, his device has two large coils of wire that create a uniform magnetic field in the space between them. In this uniform magnetic field, he places his sample material. The atoms inside the sample align, like spinning tops. If the atom points up, it corresponds to a 0. If it points down, it corresponds to a 1. Then he sends an electromagnetic pulse into the sample, which changes the alignment of the atoms. Some of the atoms flip over, so a 1 becomes a 0. In this way, the machine has performed a calculation.

So why don’t we have quantum computers sitting on our desks, solving the mysteries of the universe? Lloyd admitted to me the real problem that has stymied research in quantum computers is the disturbances from the outside world that destroy the delicate properties of these atoms.

When atoms are “coherent” and vibrating in phase with one another, the tiniest disturbances from the outside world can ruin this delicate balance and make the atoms “decohere,” so they no longer vibrate in unison. Even the passing of a cosmic ray or the rumble of a truck outside the lab can destroy the delicate spinning alignment of these atoms and destroy the computation.

The decoherence problem is the single most difficult barrier to creating quantum computers. Anyone who can solve the problem of decoherence will not only win a Nobel Prize but also become the richest man on earth.

As you can imagine, creating quantum computers out of individual coherent atoms is an arduous process, because these atoms quickly decohere and fall out of phase. So far, the world’s most complex calculation done on a quantum computer is 3 × 5 = 15. Although this might not seem much, remember that this calculation was done on individual atoms.

In addition, there is another bizarre complication coming from the quantum theory, again based on the uncertainty principle. All calculations done on a quantum computer are uncertain, so you have to repeat the experiment many times. So 2 + 2 = 4, at least sometimes. If you repeat the calculation of 2 + 2 a number of times, the final answer averages out to 4. So even arithmetic becomes fuzzy on a quantum computer.

No one knows when one might solve this problem of decoherence. Vint Cerf, one of the original creators of the Internet, predicts, “ By 2050, we will surely have found ways to achieve room-temperature quantum computation.”

We should also point out that the stakes are so high that a variety of computer designs have been explored by scientists. Some of these competing designs include:

• optical computers: These computers calculate on light beams rather than electrons. Since light beams can pass through each other, optical computers have the advantage that they can be cubical, without wires. Also, lasers can be fabricated using the same lithographic techniques as ordinary transistors, so you can in theory pack millions of lasers onto a chip.

• quantum dot computers: Semiconductors used in chips can be etched into tiny dots so small they consist of a collection of perhaps 100 atoms. At that point, these atoms can begin to vibrate in unison. In 2009, the world’s smallest quantum dot was built out of a single electron. These quantum dots have already proven their worth with light-emitting diodes and computer displays. In the future, if these quantum dots are arranged properly, they might even create a quantum computer.

• DNA computers: In 1994, the first computer made of DNA molecules was created at the University of Southern California. Since a strand of DNA encodes information on amino acids represented by the letters A,T,C,G instead of 0s and 1s, DNA can be viewed as ordinary computer tape, except it can store more information. In the same way that a large digital number can be manipulated and rearranged by a computer, one can also perform analogous manipulations by mixing tubes of fluids containing DNA, which can be cut and spliced in various ways. Although the process is slow, there are so many trillions of DNA molecules acting simultaneously that a DNA computer can solve certain calculations more conveniently than a digital computer. Although a digital computer is quite convenient and can be placed inside your cell phone, DNA computers are more awkward, involving mixing tubes of liquid containing DNA.

Дата добавления: 2015-11-14