Entanglement is one of the key resources required for quantum computation1, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers2. In superconducting devices3, two-qubit entangled states have been demonstrated and used to show violations of Bell’s inequality4 and to implement simple quantum algorithms5. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways6. These are typified by the states |GHZ〉 = (|000〉 + |111〉)/ and |W〉 = (|001〉 + |010〉 + |100〉)/. Here we demonstrate the operation of three coupled superconducting phase qubits7 and use them to create and measure |GHZ〉 and |W〉 states. The states are fully characterized using quantum state tomography8 and are shown to satisfy entanglement witnesses9, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.
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Devices were made at the UCSB Nanofabrication Facility, a part of the NSF-funded National Nanotechnology Infrastructure Network. This work was supported by IARPA under grant W911NF-04-1-0204. M.M. acknowledges support from an Elings Fellowship.
The authors declare no competing financial interests.
This file contains Supplementary Methods and Discussion describing experimental methods and analysis procedures, Supplementary Tables 1-2, Supplementary Figure 1 with legend and additional references. (PDF 276 kb)
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Neeley, M., Bialczak, R., Lenander, M. et al. Generation of three-qubit entangled states using superconducting phase qubits. Nature 467, 570–573 (2010). https://doi.org/10.1038/nature09418
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