Quanta at large: 101 things to do with Schrodinger's cat.
Is a zebra a white animal with black stripes or a black animal with white stripes? Either-or? Neither-nor? Both-and? Sometimes-one-sometimes-the-other? This question might be an illustration (analogy, metaphor?) of what a macroscopic superposition of states could be.
Superposition of states used to be confined to the microscopic world of quantum mechanics. In the microcosm, objects have a characteristic duality of nature, a mysterious union of opposites for which Niels Bohr adopted the word "complementarity." As long as it was safely quarantined where we couldn't really see it, we weren't really afraid of it. Now it is breaking loose into the macroscopic world of our ordinary perceptions, and physicists are conerned for the future of classical physics and for our ordinary ideas of what's what.
"Do we really think there's a possibility of seeing quantum mechanical effects on a macroscopic scale?" asked Anthony J. Leggett of the University of Illionois at Urbana-Champaign during the recent conference on New Techniques and Ideas in Quantum Measurement Theory, held in New York City under the sponsorship of the New York Academy of Sciences. "Can you see a pure quantum state in a macroscopic system?" asked Sean Washburn of the IBM Thomas J. Watson REsearch Center in Yorktown Heights, N.Y.
The questions are rhetorical. Leggett and Washburn told how to do it. Claudia Tesche, also of the IBM Watson Research Center, described a "system to undergo marcroscopic quantum oscillations." Helmut Rauch of the Atomic Institute of the Austrian Universities in Vienna declared that experiments involving wave-like interference effects of neutrons are testing quantum mechanical properties on a macroscopic scale. Leggett's opening remark could sum up the discussion: "The quantum measurement paradox [in which these questions of superposition and duality play a prominent role] is no longer a matter of 'theology. It has become an experimental subject."
With recent improvements in experimental techniques, physicists are beginning to do experiments that for 50 or 60 years they could only dream about (SN: 1/11/86, p. 28; 2/1/86, p. 70). These thought experiments or "gedankenexperiments," as physicists tend to call them, were originally devised to illustrate principles of make a point in an argument, not with the expectation that anyone, could actually carry them out. In the words of Anton Zeilinger of the Atomic Institute of the Australian Universities, physicists are now learning "how to ungedanken gedankenexperiments."
This could prove something of a shock to physics and philosophy. For decades there has been a great gulf fixed between the microcosm, where the ambiguities and uncertainties of quantum mechanics prevail, and the macrocosm, where common from our bodily senses dominate. A school of physicists usually named for Albert Einstein and Louis de Broglie has contended that the mysteries dualities and superpositions of quantum mechanics are appearances derived from too little knowledge. If we can gain deeper knowledge of the situation, we will see how to resolve the difficulties, and the unambiguous certainties characteristic of classical physics (or some suitable modification of them) will be seen to apply to the microcosm. Macroscopic experimentation with quantum effects seems likely to test some of these expectations.
The other main current of opinion, historically led by Niels Bohr, tends to believe that the dualities. uncertainties and superpositions of quantum physics are fundamental to nature. If that is true and if they should be shown to rule the macroscopic world, some interesting and perhaps spooky changes in our way of perceiving the universe would result.
A zebra is not really a superposition of states, but it can be a kind of both-and, sometimes-this-sometimes-that perception. Macroscopic objects are usually one-thing-or-the-other, either-or kinds of beings. A hand is either a left or a right hand. A superposition of states that makes it both at once is something we can hardly imagine let alone describe in words, but we could have to face such situations.
In recent years nature has provided physicists with several phenomena in which macroscopic quantities seem to be quantized or in which macroscopic phenomena are driven by quantized effects. These include certain aspects of superconductivity and superfluidity and the more recently discovered quantized Hall effect. To prove that quantum effects are really operating on a macroscopic scale, experimenters have to suppress or get around the things that make classical physics classical: heat (dissipation) and noise, and environmental connections generally.
Quantum effects have a numerical precision. Changes come in integral multiples of a basic amount (a quantum) characteristic of the system or the situation. Unless they see such quantum multiples and a hierarchy of energy states separated by these quantum jumps, experimenters cannot say they have really seen quantized phenomena. In the macrospcopic world, classical nature tends to intervene with environmental effects, heat and noise, that destroy the sharpness (in technical terms, the coherence) of these quantized states and changes.
Experimenters are now learning to suppress these environmetal effects. Washburn cites experiments he and Richard A. Webb of the IBM Watson Research Center have done with SQUID circuitry. A SQUID (Superconducting Quantum Interference Device) is a ring of superconducting material interrupted by a thin piece of insulating material. The insulating material makes a tunneling junction, or Josephson junction, and it imposes a hierarchy of quantized states on the relation between current and voltage in these circuits.
The electrons that form the current in the circuit exist in a series of quantized levels that form a kind of cascade from the higher amounts of current the circuit can sustain to the lower ones. To get out of any particular level and cascade to the next level below, the electrons must surmount or pass through an energy barrier. That is, each of these energy states is metastable: The electrons need to get a little energy -- from the effects of heating perhaps -- to get out one state and enable themselves to slide down to the next level below.
Quantum mechanics gives the electrons another out. Instead of getting heated until they have enough energy to surmount the barrier, they can tunnel through it. That is, they never have enough energy to get over the barrier in a classical way, but they get through it nevertheless. From the classical physics point of view this looks like magic, but it is well known to happen on the microscopic level. It arises from the wavelike nature of electrons and so is a direct expression of the duality of quantum physics.
On the macroscopic level the enivronment intervenes with all its connections to the system and all the possibilities of change they offer to average the quantum states and smear everything out. (On the microscopic level, inside an atom, for instance, there are far fewer environmental connections to do this sort of smearing out.) Washburn says he and Webb have been able to get around these difficulties, and in a series of experiments that manipulated the voltage, current and capacitances of series of SQUIDs and played up-and-down games with the energy barries, they have shown that macroscopic quantum tunneling occurs.
Tesche intends to play another kind to game with these energy barriers. Physicists imagine that in each of the quantum states the electrons exist in the bottom of a kind of energy well, known as a potential well. Tesche plans to slosh these wells back and forth. By linking SQUID circuits together she intends to set up an oscillation that will alternately raise and lower the barriers that prevent the electrons from getting out of the wells. Such an oscillation would be an oscillation between quantum states in a macroscopic system, a macroscopic demonstration of a superpositions of states: She hopes to do it within a year.
This realizes in very different form a famous thought experiment of Erwin Schriodinger, in which a cat is alternately killed and resurrected by a quantum oscillation.
The wave-particle dualities of quantum physics also drive macroscopic effects in the neutron interference experiments. Starting more than 10 years ago, these experiments have demonstrated more and more wavelike behaviors of neutrons. In neutron interference a beam of neutron is split in two, sent over different paths and then recombined. In the recombination the experiments show all the effects of adding and subtracting waves of different (or the same) phases.
Now these experiments have reached the point where they can operate with only one neutron in the apparatus." Only one neutron at a time," says Rauch. "the next neutron is not yet born. It is still in a uranium nucleus in the reactor fuel." As Zeilinger of Vienna describes it, with one neutron in an apparatus 10 meters long they will be able to do analogs to all the experiments that in the 19th century proved the wave nature of light -- single slit, double slit, sharp edge, etc. -- and hope to see what the single neutron does.
The single neutron intereferes with itself. It behaves as if it were a wave splitting in two and following separate paths. Yet it is only one neutron. Which path did the single neutron follow? The followers of Bohr (generally known as the Copenhagen school) would say that duality and complementarity make it impossible to talk about a neutron moving in space and time as a billiard ball or a planet does. All we can say is that something we call a neutron entered one end of the apparatus, and at the other end a characteristic interference pattern appeared. The Einsteinian school would say that even though the neutron behaves like a wave it is nevertheless a particle, and it has to take one route or the other. Eventually physics should be able to find out which one.
A way experimenters are planning to find out is to put magnetic coils called spin flippers on the paths. A spin flipper will reverse the spin of a passing neutron by taking a small quantum of energy from it. Neutron spins are quantized, so this is itself a quantum effect on a macroscopic object. It also could identify the path taken by a neutron. Furthermore, manipulating the spins of neutrons in flight should permit the experimenters to study the effects of such changes on quantum coherence and the production of superpositions of neutron spin states in the macroscopic apparatus.
As this kind of experimentation proceeds, physcists hope to learn more about the mysteries of quantum physics. They may also find that those mysteries apply to our familiar macroscopic world. We could then find ourselves in the situation where the left hand can know what the right is doing in ways that it couldn't under classical physics, but at the cost of not knowing whether it is really a right or a left hand.
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|Title Annotation:||quantum mechanics on macroscopic scale|
|Author:||Thomsen, Dietrick E.|
|Date:||Feb 8, 1986|
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