Schrödinger’s SQUID

In superconducting loops, electric
current flows both ways at once

Schrödinger’s cat turned 65 this year, but instead of thinking about retirement, the quantum feline is making increasingly bolder appearances. Recently two independent groups have demonstrated the largest examples of Schrödinger’s cat states by using superconducting loops. In the original thought experiment, quantum effects and a Rube Goldberg like poison apparatus rendered the cat simultaneously alive and dead inside its sealed torture chamber. In the new experiments, an electric current stood in for the cat and flowed both ways around a loop at the same time. Tony Leggett of the University of Illinois at Urbana-Champaign, one who suggested in the 1980s that such large quantum mechanical systems could be demonstrated, calls the research “a milestone in experimental quantum physics.”

 The key phenomenon at work is superposition of waves--similar to the way different individual sound waves from people chatting at a party overlap and add up to a total sound wave that goes into our ears. In quantum mechanics, matter itself behaves like a wave: electrons and other particles can exist in superpositions of different states.

 The problem, as Erwin Schrödinger pointed out in 1935, is to understand why “quite ridiculous” superpositions like that of his cat are never seen in reality, despite there being no prohibition of them in unadulterated quantum mechanics. Today theorists have a much better understanding of how tiny disturbances from the environment tend to upset quantum superpositions and turn them into the unambiguous reality that we see around us every day--a process known as decoherence. Conversely, in the past decade experimenters have created and scrutinized coherent quantum states with a degree of control only dreamed of in idealized textbook descriptions. Experiments have superposed small numbers of particles and put individual atoms in two places at once.

 The two new experiments take things to a substantially more macroscopic level. They were conducted by Jonathan Friedman, James Lukens and their co-workers at the State University of New York at Stony Brook and by Caspar van der Wal, Johan E. Mooij and their co-workers at the Delft University of Technology in the Netherlands. Both groups used SQUIDs--superconducting quantum interference devices. Quantum effects permit only certain discrete amounts of magnetic flux to thread through such a superconducting loop. If a field is applied that lies between the allowed values, an electric current flows around the loop, generating just the right additional field to round off the total flux to an allowed value.

 Things get interesting when the applied flux is midway between two allowed values. That makes the SQUID equally inclined to produce a clockwise or a counterclockwise current--to round up or round down the incommensurate flux--and conditions are most favorable for producing a superposition of these two alternatives. For the Stony Brook SQUID, these currents amounted to flows of billions of electrons, totaling microamps, traveling around a 140-micron-square loop, large enough to encircle a human hair--gargantuan by quantum standards and truly macroscopic. The Delft design was smaller, 1/30 the size.

 The superposition state does not correspond to a billion electrons flowing one way and a billion others flowing the other way. Superconducting electrons move en masse. All the superconducting electrons in the SQUID flow both ways around the loop at once when they are in the Schrödinger’s cat state.

 Important differences remain, however, between these devices and the canonical thought experiment. In the imaginary scenario, the superposition of alive and dead cat inside the box is static, from the time it is created until the lid is opened and the experimenter sees one outcome or the other. Two idealizations are at work here. One is that the interior of the box is so well isolated that the superposition remains undisturbed until the lid is opened. In the present SQUID experiments nearby devices spoil this isolation, and the superpositions decohere rapidly, possibly within a few nanoseconds.

 The second idealization relates to there being two closely related superpositions: one can have the alive state plus the dead state, or the alive state minus the dead state. So far as the cat is concerned, this mathematical distinction is immaterial. Either case amounts to a 50 percent mortality rate. In the SQUID experiments, in contrast, quantum mechanics predicts that the two alternatives will have slightly different energies. Detecting this energy difference is how the two experiments inferred that the cat state was achieved.

 The existence of two different superposition states with unequal energy also implies that an oscillating state should be observable: in this case, the probability of detecting the clockwise current, say, would oscillate between zero and 100 percent, depending on the time delay from the preparation of the state to the measurement. Although it may sound less bizarre than a static superposition, consider what it would mean for the cat: while the box remained sealed, the cat would be oscillating from 100 percent alive to 100 percent dead and back!

 Leggett considers such oscillation experiments (on SQUIDs, not cats) to be the crucial goal in testing the quantum mechanical predictions. Then physicists will be examining just what happens inside Schrödinger’s sealed cat box. Both groups are working on developing just that capability.