Emulation of a Quantum Spin with a Superconducting Phase Qudit

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Science  07 Aug 2009:
Vol. 325, Issue 5941, pp. 722-725
DOI: 10.1126/science.1173440

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Higher-Level Quantum Emulation

At the heart of a quantum computer is the device on which information is to be encoded. This is typically done with a qubit, a two-level quantum system analogous to the two-level bit that encodes 0 and 1 in classical computers. However, there need not be just two quantum energy levels. There could be three (a qutrit), or more generally, d-levels (a qudit) in the device. Neeley et al. (p. 722; see the Perspective by Nori) demonstrate a five-level quantum device and show that their qudit can be used to emulate the processes involved in manipulating quantum spin. The use of multilevel qudits may also have potential in quantum information processing by simplifying certain computational tasks and simplifying the circuitry required to realize the quantum computer itself.


In quantum information processing, qudits (d-level systems) are an extension of qubits that could speed up certain computing tasks. We demonstrate the operation of a superconducting phase qudit with a number of levels d up to d = 5 and show how to manipulate and measure the qudit state, including simultaneous control of multiple transitions. We used the qudit to emulate the dynamics of single spins with principal quantum number s = 1/2, 1, and 3/2, allowing a measurement of Berry’s phase and the even parity of integer spins (and odd parity of half-integer spins) under 2π-rotation. This extension of the two-level qubit to a multilevel qudit holds promise for more-complex quantum computational architectures and for richer simulations of quantum mechanical systems.

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