Browsing by Author "Monroe, Christopher"
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Item Open Access Fault-tolerant control of an error-corrected qubit.(Nature, 2021-10) Egan, Laird; Debroy, Dripto M; Noel, Crystal; Risinger, Andrew; Zhu, Daiwei; Biswas, Debopriyo; Newman, Michael; Li, Muyuan; Brown, Kenneth R; Cetina, Marko; Monroe, ChristopherQuantum error correction protects fragile quantum information by encoding it into a larger quantum system1,2. These extra degrees of freedom enable the detection and correction of errors, but also increase the control complexity of the encoded logical qubit. Fault-tolerant circuits contain the spread of errors while controlling the logical qubit, and are essential for realizing error suppression in practice3-6. Although fault-tolerant design works in principle, it has not previously been demonstrated in an error-corrected physical system with native noise characteristics. Here we experimentally demonstrate fault-tolerant circuits for the preparation, measurement, rotation and stabilizer measurement of a Bacon-Shor logical qubit using 13 trapped ion qubits. When we compare these fault-tolerant protocols to non-fault-tolerant protocols, we see significant reductions in the error rates of the logical primitives in the presence of noise. The result of fault-tolerant design is an average state preparation and measurement error of 0.6 per cent and a Clifford gate error of 0.3 per cent after offline error correction. In addition, we prepare magic states with fidelities that exceed the distillation threshold7, demonstrating all of the key single-qubit ingredients required for universal fault-tolerant control. These results demonstrate that fault-tolerant circuits enable highly accurate logical primitives in current quantum systems. With improved two-qubit gates and the use of intermediate measurements, a stabilized logical qubit can be achieved.Item Embargo New Techniques for Fast and High-Fidelity Trapped Ion Photonic Interconnects(2024) O'Reilly, JamesonTrapped atomic ions are a leading candidate platform for quantum simulation and computing but system sizes are limited by motional mode crowding and transport overhead. Multiple reasonably-sized, well-controlled modules can be connected into one universal system using photonic interconnects, in which photons entangled with ions in each trap are collected into and detected in a Bell-state analyzer. The speed of these interconnects has heretofore been limited by the use of 0.6 NA objectives and the need to periodically pause entanglement attempts for recooling. In this work, we characterize a system with two in-vacuo 0.8 NA lenses on either side of an ion trap and use it to demonstrate the most efficient free-space ion trap photonic interconnect to date. In addition, we introduce an ytterbium ion as a sympathetic coolant during the entangling attempts cycle to remove the need for recooling, enabling a record photon-mediated entanglement rate of 250 cps between two trapped ions. We collect 493 nm photons from barium ions, which allows us to make use of an in-fiber beamsplitter, thus eliminating the spatial mismatch error that has contributed significant infidelity to all previous ion trap photonic interconnect demonstrations. The major remaining error source is imperfections in the photon polarization encoding, so we also develop a new protocol for remotely entangling two ions using time-bin encoded photons and present preliminary results of an experimental implementation. Finally, we prepare the first remote entangled state involving two barium ions in separate vacuum chambers.
Item Open Access Robust 2-Qubit Gates in a Linear Ion Crystal Using a Frequency-Modulated Driving Force.(Physical review letters, 2018-01) Leung, Pak Hong; Landsman, Kevin A; Figgatt, Caroline; Linke, Norbert M; Monroe, Christopher; Brown, Kenneth RIn an ion trap quantum computer, collective motional modes are used to entangle two or more qubits in order to execute multiqubit logical gates. Any residual entanglement between the internal and motional states of the ions results in loss of fidelity, especially when there are many spectator ions in the crystal. We propose using a frequency-modulated driving force to minimize such errors. In simulation, we obtained an optimized frequency-modulated 2-qubit gate that can suppress errors to less than 0.01% and is robust against frequency drifts over ±1 kHz. Experimentally, we have obtained a 2-qubit gate fidelity of 98.3(4)%, a state-of-the-art result for 2-qubit gates with five ions.