Schrödinger's Carousel

Every club has its mascot and among my tribe of physicists no mascot is more fitting than Schrödinger's Cat.

Erwin Schrödinger was a sophisticated Austrian, a reluctant participant in the Quantum Revolution sweeping through Europe in the late 1920's. Schrödinger expressed his displeasure at the new quantum world he was helping to construct by focusing attention on its strangenesses. He was the first to notice that quantum entanglement--an odd kind of instant voodoo action--was an inescapable consequence of his own quantum equations when applied to two or more particles of matter.

And he was particularly troubled by the curious notion of quantum superposition. In theory at least, an atom could not only be in state A or in state B but could exist also in a superposition state 1/2 A and 1/2 B with some "quantum phase" between them. Today this superposition principle forms the basis for quantum computers which instead of manipulating binary bits 1 and 0, compute using "qubits", quantum superpositions of binary bits.

Schrödinger tried to demonstrate the absurdity of the New Physics by taking the notion of quantum superposition to extremes. If one could superpose any number of atoms, as the equations allowed, then why could not one superpose a cat in two different states, say, alive and dead? Schrödinger showed, in a famous thought experiment, exactly how this might be accomplished and left us with an image of not just an atom, but a macroscopic object (covered with fur) existing at the same time in two extremely different states of being.

The quantum rules describe the world as waves (of possibility) when not observed and particles (of actuality) when observed. So Schrödinger's Cat maintains her twofold existence only as long as she is wavelike, that is only until she is observed, whereupon she departs her superposition state (and in a manner complete mysterious to physicists) becomes definitively either dead or alive but not both.

Schrödinger's Cat has been the subject of much theoretical discussion and a kind of informal contest among experimentalists to see how big a system they can coax into a quantum superposition. Superpositions of "big" systems have come to be called "cat states" as in the case of a recent experiment at Oxford University in which researchers placed 13 nuclear spins in phased-linked superposition--Magnetic Field Sensors Using Large Cat States.

Thirteen spins may seem a long way from a 10-pound pussy cat, but the recent production of Bose-Einstein Condensates has now opened the way to manipulate not just a few spins but a few million rubidium atoms into a single state and then coax that state into a quantum superposition. The mass of a few million rubidium atoms is comparable to the mass of a virus so we are still a very long way from being able to quantum-superpose cats.

A Bose-Einstein Condensate (BEC) is a dilute gas of atoms confined by electromagnetic fields and cooled to a super-low temperature (a few nanodegrees above Absolute Zero) until the atoms collapse spontaneously into the same quantum state.

A new way to turn a BEC into a cat state was recently proposed by three theorists (Thanvanthri, Kapale and Dowling) at Louisiana State University. Instead of confining the BEC inside a simple potential well, TKD propose to use a "sombrero potential" for confinement. In a sombrero potential the particles are trapped in the brim of an electromagnetic Mexican hat to form a kind of "moat of quantum waviness" around the central peak of the hat.

Now that you have an unmoving "ring of bright water" made of quantum stuff, the next thing to do is to get that ring to rotate by shining light that contains angular momentum on the ring. When the BEC absorbs the spinning light, it too begins to spin, forming a Quantized Vortex State, a million atoms all rotating CW, say, around the central sombrero peak. You can also get the Vortex to rotate CCW by illuminating it with light spinning in the opposite direction.

Now comes the Schrödinger Cat part. Compared to getting matter to superpose, light is easy. So now you shine a superposition of CW and CCW light on the BEC and a million Rubidium atoms begin to swirl in two directions at once--an odd state of excitation one might call "Schrödinger's Carousel".

Make no mistake, Schrödinger's Carousel is not a simple classical situation where half of the atoms are rotating in one direction and half in the other, but a truly quantum state in which ALL OF THE ATOMS are rotating halfly CW and halfly CCW around the sombrero peak. Cool quantum cat, man.

But wait, there's more. Because this experiment is carried out on the Earth which is rotating once a day, one of the cat halves finds itself spinning slightly faster than the other which gives rise to a phase shift between CW and CCW vortex matter waves that can be easily measured. The Schrödinger Carousel then behaves as an ultrasensitive gyrocompass that can accurately pinpoint the direction of True North. Nice Kitty.

The title of TKD's LSU article says it all: Ultra-Stable Matter-Wave Gyroscopy with Counter-Rotating Vortex Superpositions in Bose-Einstein Condensates.

Sombrero Potential