Computing with Quantum Cats (21 page)

Ion trap array incorporating a junction and linear ion trap sections, used by Winfried Hensinger and colleagues to demonstrate ion transport through a junction for the first time. Such a device is an important building block for a large-scale quantum computer.

The great appeal of the Many Worlds Interpretation of quantum mechanics (MWI) is that it avoids the problem of the collapse of the wave function, for the simple reason that the wave function never collapses. The problem with other interpretations—often known as the measurement problem—is deciding at what point between the quantum world and the everyday world the collapse occurs. This is what the Schrödinger's cat puzzle is all about. Physicists have few qualms about accepting the possibility of a radioactive atom being in a superposition of states, but we all have qualms about the cat being in such a superposition. Does the collapse happen when the detector measures the radioactive material to see if it has decayed? Or is the cat's consciousness necessary to make the collapse happen? Could an ant be “aware” enough to cause the collapse? Or a bacterium? These are not facetious questions, because larger and larger molecules have been sent through the experiment with two holes and behaved
in line with quantum mechanics; there is even talk of doing it with molecules of DNA, if not yet with bacteria or cats. John Bell highlighted the ludicrousness of trying to apply the Copenhagen Interpretation to the Universe as a whole:

Was the world wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some more highly qualified measurer—with a PhD? If the theory is to apply to anything but idealized laboratory operations, are we not obliged to admit that more or less “measurement-like” processes are going on more or less all the time more or less everywhere?
¹

EVERETT SETS THE SCENE

The Many Worlds Interpretation avoids these difficulties by saying, for example, that in the case of the cat in the box there are two universes, one with a dead cat and one with a live cat; and similarly in other situations, every quantum possibility is realized. In the mid-1950s the American researcher Hugh Everett III put the MWI on a proper mathematical footing, and showed that for all practical purposes it is exactly equivalent to the Copenhagen Interpretation. Since this meant it made no difference to their calculations, most practicing quantum mechanics ignored it. Unfortunately, there was a flaw in Everett's presentation of the idea which meant that even the few theorists who did think about the implications also found it hard to take seriously.

Everett described the many worlds of his model in terms of splitting. In the case of Schrödinger's cat, this would mean that in the course of the “experiment” we start out with a single cat in a single universe (or world) and that the world then
splits into two, one with a live cat and one with a dead cat. Everett used a different analogy, at least in the first draft of his idea, which he showed to his supervisor at Princeton, John Wheeler, in the autumn of 1955. He used the word “splitting,” and made an analogy with the splitting of an amoeba. You start with one amoeba, and then have two, each of which, if it had a memory, would remember the same experiences, or history, up to the point of the split. At that point, the two individuals go their separate ways, eventually to split again, and their offspring split in their turn, and so on. Wheeler “persuaded” Everett to leave the amoeba analogy out of his PhD thesis and the published version of his work, which appeared in the journal
Reviews of Modern Physics
in 1957. But Everett did say in print that “no observer will ever be aware of any ‘splitting' process,” and clearly had the idea of one “history” branching repeatedly as time progressed.
²
This was spelled out by Bryce DeWitt, an enthusiastic supporter of Everett's idea, who wrote: “Every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself.” Or, as John Bell put it: “Quite generally, whenever there is doubt about what can happen, because of quantum uncertainty, the world multiplies so that all possibilities are actually realized. Persons of course multiply with the world, and those in any particular branch would experience only what happens in that branch.”
³

Again, the language is that of branching and multiplication of worlds by splitting. Bell is not enthusiastic, but, almost in spite of himself, does not dismiss the idea out of hand:

The “many worlds interpretation” seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet…It may have something distinctive to say in connection with the “Einstein Podolsky Rosen puzzle,” and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world…which seems to be in some ways a highly improbable one.
⁴

Although Wheeler was initially enthusiastic about Everett's idea, as the years passed he developed qualms. Two decades later, he said: “I confess that I have reluctantly had to give up my support of that point of view in the end—much as I advocated it in the beginning—because I am afraid it carries too great a load of metaphysical baggage.”
⁵
It seems to me to carry a lesser load of metaphysical baggage than the idea of the collapse of the wave function, even in this imperfect form. But there is a more serious objection to Everett's version of MWI than metaphysics.

For the Everett MWI seems to contain the same flaw, the measurement problem, as the Copenhagen Interpretation itself. It's just that in one case the measurement puzzle refers to the moment of collapse, and in the other it refers to the moment of splitting. This might look like the death knell for the idea, at least as an alternative to the Copenhagen Interpretation; but Everett (and Wheeler, DeWitt and other supporters of the idea) missed a trick.

I confess that I missed the same trick, long ago, although I had fewer qualms than Bell about espousing the MWI, which I enthusiastically endorsed in my book
In Search of
Schrödinger's Cat
. But someone who did not miss this trick was Schrödinger himself, as I learned to my surprise when writing his biography.

SOLVING THE MEASUREMENT PROBLEM

In 1952, Schrödinger published a scientific paper entitled “Are There Quantum Jumps?” Arguing that there is no reason for a quantum superposition to collapse just because we look at it, or because it is measured, he said that “it is patently absurd to let the wave function be controlled in two entirely different ways, at times by the wave equation, but occasionally by direct interference of the observer, not controlled by the wave equation.” His solution was that the wave function does not collapse, and no choice is ever made between a superposition of states. Although Schrödinger himself—perhaps surprisingly—never pointed out the implications in terms of his famous cat puzzle, this neatly demonstrates the point he is making: in effect, he is saying that in the cat experiment, the wave functions leading to the “dead cat” and the “live cat” are equally real, and remain so both before and after the box is opened. In everyday language, there are two parallel worlds, one with a live cat and one with a dead cat;
and
—this is the crucial point—there
always were
two worlds, each starting out with a live cat but becoming different when one of the cats, but not the other, dies. There is no splitting, and no measurement problem. For once, the popular term “parallel worlds” is the most apt, and removes the image of branching realities from our minds. The two worlds (or universes) have identical histories up until the point where the experiment is carried out, but in one universe the cat lives and in the other the cat dies. They are like parallel lines, running alongside
each other. And running on either side of those two parallel worlds are more parallel worlds, each slightly different from its immediate neighbors, with close neighbors having very similar histories and widely separated universes differing more significantly from one another. This is not, strictly speaking, an “interpretation” at all; it is, as Schrödinger pointed out, what the equations tell us. It is the simplest way to understand those equations. If we ever did an experiment like the one Schrödinger envisaged, we would not be forcing the universe to split into multiple copies of itself, but merely finding out which reality we inhabit.

The response of the few people who noticed this idea at the time was summed up rhetorically by Schrödinger himself in a talk he gave in Dublin in 1952. I have quoted it before, but it is surely worth quoting again:

Nearly every result [the quantum theorist] pronounces is about the probability of this or that or that…happening—with usually a great many alternatives. The idea that they may not be alternatives but all really happen simultaneously seems lunatic to him, just impossible. He thinks that if the laws of nature took this form for, let me say, a quarter of an hour, we should find our surroundings rapidly turning into a quagmire, or sort of a featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jelly fish. It is strange that he should believe this. For I understand he grants that unobserved nature does behave this way—namely according to the wave equation. The aforesaid alternatives come into play only when we make an observation—which need, of course, not be a scientific observation. Still it would seem that, according to the quantum theorist, nature is prevented from rapid jellification only by our perceiving or observing it…it is a strange decision.

All of this has led me to change my view on the nature of quantum reality. As Bell's theorem and the experiments described in
Chapter 4
make clear, the world is either real but non-local, or local, but not real. In
In Search of Schrödinger's Cat
, I came down in favor of locality, and concluded (echoing John Lennon) that “nothing is real,” at least until it is measured. Now, I am inclined to accept non-locality, with the corollary that the world is real—or rather, that the many worlds are
all
real. Not “nothing is real,” but “everything is real,” since wave functions never collapse.
⁶

If Everett had been aware of Schrödinger's views when he came up with his own version of MWI a few years later, he could have produced an even more satisfactory package of ideas than he did. But it is still unlikely that many people would have taken it very seriously until the experimental proof of Bell's theorem had arrived. Schrödinger's insight languished in obscurity for more than thirty years, and when David Deutsch elaborated the modern version of the Many Worlds Interpretation in the 1980s he did so without drawing directly on this aspect of Schrödinger's work—indeed, without being aware of Schrödinger's contribution to MWI.

THE WORLDS OF DEUTSCH

David Deutsch is an unusual physicist with unconventional habits. Although he doesn't like too much attention being given to his eccentricities, since he feels this may divert attention from the underlying importance of his work, and might be taken as implying (incorrectly) that you have to be weird to be creative, the stories are as irresistible (if as irrelevant) as those about Einstein not wearing socks or Turing's bicycle chain. Deutsch lives in an ordinary, rather
unkempt-looking house in a suburb of Oxford, and as I discovered the visitor has to be prepared to negotiate their way from the front door past piles of boxes and papers to the darkened room, dominated by computer screens, in which he works. The curtains are almost invariably drawn shut, and Deutsch tends to work at night and sleep (a little) in the day—lunchtime is typically around 8 pm, followed by a solid twelve hours' work. Although he is affiliated with the Center for Quantum Computation at Oxford's Clarendon Laboratory, and is a non-stipendiary visiting professor of physics, Deutsch has no paid academic post, living off lecturing and writing (plus the proceeds of various prizes he has been awarded
⁷
); colleagues are more likely to encounter him at an international meeting in some far-off land than among the dreaming spires of Oxford.