A “Spins-inside” Quantum Processor
Lieven Vandersypen
QuTech and Kavli Institute of Nanoscience Delft, TU Delft
Quantum computation has captivated the minds of many for almost two decades. For much of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer can actually be built. Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays, have emerged as a highly promising direction [1]. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision.
First, we created local registers of spin qubits with sufficient control that we can program arbitrary sequences of operations. We show the creation of each of the Bell states with fidelities of 85-90% and the implementation of the four instances of the Deutsch-Jozsa and the Grover algorithms on two qubits [2].
Second, we have explored coherent coupling of spin qubits at a distance via two routes. In the first approach, the electron spins remain in place and our coupled via an intermediary degree of freedom. After showing this principle with an ancillary quantum dot as a coupler [3], we recently observed strong coupling of a single spin to a single microwave photon in a superconducting resonator [4]. In the second approach, spins are shuttled along a quantum dot array, preserving both the spin projection [5] and spin phase [6].
Third, we have developed new concepts and techniques that make quantum dot arrays a credible platform for quantum simulation of the Mott-Hubbard model. As a first demonstration, we map out the transition from Coulomb blockade to collective Coulomb blockade, the finite-size analogue of the Mott insulator transition [7].
When combined, the progress along these various fronts can lead the way to scalable networks of high-fidelity spin qubit registers for computation and simulation.
[1] L.M.K. Vandersypen, H. Bluhm, J.S. Clarke, A.S. Dsurak, R. Ishihara, A. Morello, D.J. Reilly, L.R. Schreiber, M. Veldhorst, Interfacing spin qubits in quantum dots and donors – hot, dense and coherent, npj Quantum Information 3, 34 (2017).
[2] T. F. Watson, S. G. J. Philips, E. Kawakami, D. R. Ward, P. Scarlino, M. Veldhorst, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, L. M. K. Van- dersypen, A programmable two-qubit quantum processor in silicon, Nature 555, 633 (2018)
[3] T.A. Baart, T. Fujita, C. Reichl, W. Wegscheider, L.M.K. Vandersypen, Coherent spin-exchange via a quantum mediator, Nature Nanotechnology, 12, 26 (2017)
[4] N. Samkharadze, G. Zheng, N. Kalhor, D. Brousse, A. Sammak, U. C. Mendes, A. Blais, G. Scappucci, L. M. K. Vandersypen, Strong spin-photon coupling in silicon, Science 359, 1123 (2018)
[5] T. A. Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Single-Spin CCD, Nature Nanotechnology 11, 330 (2016)
[6] T. Fujita, T. A. Baart, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Coherent shuttle of electron-spin states, npj Quantum Information 3, 22 (2017)
[7] T. Hensgens, T. Fujita, L. Janssen, Xiao Li, C. J. Van Diepen, C. Reichl, W. Wegschei- der, S. Das Sarma, L. M. K. Vandersypen, Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array, Nature, 548, 70 (2017)