Scott H. Clearwater

Swatting Quantum Bugs

Up to now we have blithely cavorted through idealized applications of quantum computation with little heed for what might go wrong. But things can go wrong. They go wrong in classical computers, and they go wrong in quantum computers in similar as well as novel ways. The delicate quantum superpositions and entanglements must be maintained in a bath of constantly jostling molecules, radioactive decays from the computing constituents themselves, and decays from outside sources such as comic rays. In a quantum computer the size of a piece of dust, these once happily ignored effects can throw everything completely off. These kinds of problems have led some researchers to question the feasibility of quantum computers (Landauer, 1995; Unruh, 1995). If we can’t maintain the needed quantum effects at least as long as it takes to complete the computation, we’re sunk. Has all the elegant theory of quantum computers been for naught?

Computing at the Edge of Nature

The spread of computer technology into every aspect of modern civilization ranks as one of the greatest achievements in human history. The ability to access information at the touch of a button and to mix voice, image, and data in one torrent of information has revolutionized the way we communicate, solve problems, plan, shop, and even play.

Breaking “Unbreakable” Codes

The first computational tasks for which quantum computers were found to outperform classical computers were artificial problems that had been specially contrived to demonstrate a quantum advantage. A good example is the “Deutsch-Jozsa” problem, which was described in Chapter 2. As you may recall, this problem involves deciding whether a mystery n-bit function is constant (i.e., gives the same output on all possible inputs) or balanced (i.e., gives an output of 0 on half of the possible inputs and an output of 1 on the other half). This problem does not arise in practical computing applications very often! This lack of applicability in part explains why the field of quantum computing languished for almost a decade. For quantum computing to become a hot research area, someone needed to find at least one important application for which a quantum computer outperformed any classical computer.

The Keys to Quantum Secrets

Modern schemes for exchanging secret messages, such as the one-time pad and public key procedures that we saw in Chapter 4, rely on the sender and receiver to possess certain “keys.” Such keys are simply large numbers that have been carefully constructed so as to have special mathematical properties. If the appropriate keys are known, then any encrypted messages are easily unscrambled. But without the keys it is computationally intractable, at least with any classical computer, to crack a coded message. Consequently, the integrity of these cryptosystems relies on the keys being kept secret.

Generation-Q Computing: Where Do You Want to Go Tomorrow?

It should be clear by now that quantum computers are theoretically possible. The questions are whether they can be built and, if so, how to build them. Throughout this book we have been discussing the one-atom-per-bit limit that, judging on the basis of current extrapolations, will be reached by the year 2020. If that goal is two decades away, why should we be worrying today about how to build quantum computers? The reason is that quantum computers need not wait for the one-atom-per-bit limit to be reached. Just as transistor technology achieved astounding miniaturizations from the first coin-sized transistors, we can expect the same sort of evolution from quantum computers. As we will see, true quantum switches and logic gates have already been built. These tabletop-sized devices are obviously not feasible for any practical quantum computer, but they represent the necessary first steps in the process.

The Crapshoot Universe

In this chapter we explore the concept of randomness and how we can and must harness it in the service of computation. At one extreme, randomness is as mundane as the outcome of a coin toss or the roll of dice. At the other extreme, randomness reaches to the very core of the metaphysical profundities of quantum mechanics. Quantum physics, as it is usually taught and practiced, relies heavily on the concept of randomness. In particular, when a physical system that is in a superposition of states is observed, it is as though that superposed state collapses, as a result of this measurement, into one of the eigenstates. We cannot predict which of those states will appear as a result of our measurement. All we can do is give the probability of obtaining the various possible outcomes. The inability to predict the state into which a system will collapse upon being observed adds the element of randomness to quantum theory.

Teleportation: The Ultimate Ticket to Ride

In science fiction stories, teleportation is usually depicted as a routine means of relocating an object by a process of dissociation, information transmission, and reconstitution. When all goes well, the original object is scanned and disassembled at one place only to shimmer reassuringly back into existence at another. Occasional blunders corrupt the object en route or leave it suspended in some nebulous state. Hapless bit-part actors are especially prone to difficulties.

What Can Computers Do

In Chapter 1, we discussed the trend toward miniaturization that is luring the computer industry into the unpredictable realm of quantum mechanics. In Chapter 2, we described how a computer that operates quantum-mechanically can harness exotic phenomena, such as entanglement and non-clonability, that have no parallels in the everyday world around us. The question is whether such phenomena confer an advantage. Do they make the capabilities of a quantum computer surpass those of a classical computer? This is an important question because it will require a massive financial investment to create quantum computers. We have to be able to determine whether the effort and expense will be justified.

It Is Now Safe to Turn Off Your Quantum Computer

By now you will realize that many of the assumptions we make about classical computers cease to be correct at the quantum scale. We have seen, for example, that a quantum bit is not necessarily a 0 or a 1, but can be a superposition of both 0 and 1 simultaneously. We have seen that a quantum bit does not necessarily have a definite bit-value until the moment after it has been read. We have seen that reading one qubit can have an effect on the value of another, unread qubit, if the two qubits are initially entangled with each other.