Resonantly driven CNOT gate for electron spins

Abstract

Abstract:Single qubit rotations and two-qubit CNOT operations are crucial ingredients for universal quantum computing. While high fidelity single qubit operations have been achieved using the electron spin degree of freedom, realizing a robust CNOT gate has been a major challenge due to rapid nuclear spin dephasing and charge noise. We demonstrate an efficient resonantly-driven CNOT gate for electron spins in silicon. Our platform achieves single-qubit rotations with fidelities >99%, as verified by randomized benchmarking. Gate control of the exchange coupling allows a quantum CNOT gate to be implemented with resonant driving in ~200 ns. We use the CNOT gate to generate a Bell state with 75% fidelity, limited by quantum state readout. Our quantum dot device architecture opens the door to multi-qubit algorithms in silicon.Main Text: Gate defined semiconductor quantum dots are a powerful platform for isolating and coherently controlling single electron spins (1, 2). Silicon quantum dots can leverage state-ofthe-art industrial nanofabrication capabilities for scalability, and support some of the longest quantum coherence times measured in the solid-state (3-5). By engineering local magnetic field gradients, electron spins can be electrically controlled (6, 7) with single qubit gate fidelities exceeding 99% (8). Despite this progress, demonstrations of two-qubit gates with quantum dot spins are scarce due to technological and materials challenges (9, 10). While exchange control of spins was demonstrated as early as 2005, high fidelity exchange gates have been difficult to achieve due to nuclear spin dephasing and charge noise (10, 11). A demonstration of an efficient CNOT gate for spins in silicon will open a path for multi-qubit algorithms in a scalable semiconductor system.Here we demonstrate a ~200 ns CNOT gate in a silicon semiconductor double quantum dot (DQD), nearly an order of magnitude faster than the previously demonstrated composite CNOT gate (9). The gate is implemented by turning on an exchange interaction, which results in a state-selective electron spin resonance (ESR) transition that is used to implement a CNOT gate with a single microwave (MW) pulse. Local magnetic field gradients allow for all-electrical control of the spin states with single qubit gate fidelities exceeding 99%, enabled by the largely nuclear-spin-free environment of the silicon host lattice. In contrast with previous DQD implementations of the exchange gate, our CNOT gate is implemented at a symmetric operating point, where the exchange coupling J is first-order insensitive to charge noise (12,13). By combining the CNOT with single qubit gates we create a Bell state with a fidelity F = 75%, limited primarily by the qubit readout visibility (14). Our demonstration of a universal set of fast quantum gates for spins in silicon paves the way for the first multi-qubit algorithms with semiconductor spin qubits (15).The spin of a single electron is used to encode a qubit (16). A gate-defined DQD (Fig. 1A) is used to isolate two electron...

Universal quantum logic in hot silicon qubits

“…From the decay of the Ramsey fringes ( Fig. 2d, e) we extract dephasing times T * 2(Q1) = 2.1 µs and T * 2(Q2) = 2.7 µs, comparable to experiments at similar high temperature [8] and still longer than dephasing times for natural silicon at mK temperatures [16,17].…”

Silicon gains ground in quantum-computing race

“…Petta and his collaborators achieved that feat 3 , as did a separate team 4 led by Lieven Vandersypen at Delft.…”

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“…Experiments with multiple coupled qubits have demonstrated proof-of-principle computations and preliminary steps towards a logical qubit. The field of quantum-processing technology includes photons [17,22], trapped ions [14,15,23], superconducting qubits [18,19,25,[28][29][30], and spins in diamond [24,26], gallium arsenide [20,[32][33][34][35][36][37][38], and silicon [27,[39][40][41][42][43][44][45][46][47]. However, there are unique advantages to a silicon quantum processor, and the potential for high-fidelity control of longlived spin qubits motivates this proposal.…”