The quantum physics of Libertango

Physicist-ballerina Merritt Moore danced her PhD to Astor Piazzolla’s Libertango – a brilliant cascade of metaphors that kicks off with the choice of tango, Moore’s razor-sharp pointework tracing the contours of the Bloch sphere of a two-level quantum mechanical system.

The Bloch sphere provides a geometric representation of a qubit. In classical information theory, a bit can exist in one of two possible states ‘0’ or ‘1’, represented by the poles of the sphere. The state of a qubit can be any one of an infinite number of points along the surface of the Bloch sphere. The poles of the sphere represent the computational states Embedded Image and Embedded Image of the qubit and all other points (such as that indicated by the blue arrow) describe a coherent superposition of these states Embedded Image, subject to the normalization condition Embedded Image. The counterintuitive property of quantum coherence leads to many uniquely quantum phenomena, such as the superior power of quantum information processing protocols or the formation of Bose–Einstein condensation in gasses of ultra-cold atoms.

The Bloch sphere provides a geometric representation of a qubit. In classical information theory, a bit can exist in one of two possible states ‘0’ or ‘1’, represented by the poles of the sphere. In quantum information theory, the state of a qubit can be any one of an infinite number of points along the surface of the Bloch sphere. The counterintuitive property of quantum coherence leads to many uniquely quantum phenomena, such as the superior power of quantum information processing protocols.

 

Moore and her partner Nuno Fernandez represent an entangled pair of photons that emerge from a non-linear crystal (the Clarendon Laboratory at Oxford University) after having been zapped with a laser (the pianist playing the music). The red and black elements of their costumes serve as visual signifiers of the nature of quantum entanglement.

The pair are separated by a beam splitter (the split staircase) and he runs off with her red scarf – the essential string of information coded in one of the photons. Her bright red dress identifies her as the “signaling” photon that alerts us to his presence.

The electricity of the score summons up the pioneering use of quantum information theory for important stuff like secure encryption and superfast computing. Libertango’s groundbreaking fusion of traditional tango, classical music and jazz, with its harsh, driving ostinato, is a perfect backdrop for this union of physics and ballet.

 

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