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Demonstration of a small programmable quantum computer with atomic qubits
S. Debnath, N. M. Linke, C. Figgatt, K. A. Landsman, K. Wright, C. Monroe
TL;DR
The paper examines programmable quantum operations in a trapped-ion system and implements native single- and two-qubit gates. It characterizes CNOT gates across all ion pairs and identifies laser-intensity fluctuations as a key source of XX-gate error.
Problem
The paper addresses how to implement and characterize programmable single- and two-qubit operations across ion pairs in a trapped-ion system.
Method
The experiment uses native equatorial single-qubit rotations and XX-gates mediated by collective transverse motional modes.
Results
CNOT gates are performed between all ion pairs and characterized using the four classical two-qubit input states.
Takeaways & Limitations
The trapped-ion platform provides native entangling operations whose geometric phase can be varied through Raman-beam intensity.
Takeaways & Limitations
XX-gates are most sensitive to laser-intensity fluctuations, which produce maximum errors.
Abstract
from arXiv · showhide
Quantum computers can solve certain problems more efficiently than any possible conventional computer. Small quantum algorithms have been demonstrated on multiple quantum computing platforms, many specifically tailored in hardware to implement a particular algorithm or execute a limited number of computational paths. Here, we demonstrate a five-qubit trapped-ion quantum computer that can be programmed in software to implement arbitrary quantum algorithms by executing any sequence of universal quantum logic gates. We compile algorithms into a fully-connected set of gate operations that are native to the hardware and have a mean fidelity of 98 %. Reconfiguring these gate sequences provides the flexibility to implement a variety of algorithms without altering the hardware. As examples, we implement the Deutsch-Jozsa (DJ) and Bernstein-Vazirani (BV) algorithms with average success rates of 95 % and 90 %, respectively. We also perform a coherent quantum Fourier transform (QFT) on five trappedion qubits for phase estimation and period finding with average fidelities of 62 % and 84 %, respectively. This small quantum computer can be scaled to larger numbers of qubits within a single register, and can be further expanded by connecting several such modules through ion shuttling or photonic quantum channels.
METHODS
The experiment uses a five-ion linear Paul trap with state-dependent fluorescence detection and software-configurable native single- and two-qubit gates. Gate implementations are calibrated and characterized across ion pairs, with procedures specified for Deutsch-Jozsa and QFT state preparation.
- Experimental techniques: A segmented four-blade rf Paul trap operates at 23.83 MHz, with actively stabilized transverse secular frequency and fluorescence imaging at 0.55 µm resolution.State-dependent fluorescence is collected through a 0.38 numerical aperture objective.
- Experimental techniques: 99.74(3)% and 99.09(5)% are the single-shot detection fidelities for states |0⟩ and |1⟩, respectively, for a single qubit.Detection fidelity degrades when measuring n = 5 qubits.
- Native gate implementation: 235 µs is the duration of two-qubit XX-gates for any ion pair in the n = 5 setup.The gates use a 9-segment piecewise-constant Rabi-frequency modulation, with pulse shapes optimized separately for each ion pair.
- Algorithm implementation: The Deutsch-Jozsa sequence prepares an input superposition, applies the function and ancilla phase kick-back, then rotates and measures the first four qubits.The ancilla is ignored after measurement because it is not entangled with the other qubits.
- Native gate implementation: Native single-qubit operations rotate the Bloch vector about equatorial axes, while native two-qubit XX-gates implement a σxσx-Ising interaction mediated by collective transverse motional modes.The geometric phase χij is varied through Raman-beam intensity.
- QFT state preparation: QFT period-finding states are prepared by applying individual single-qubit rotations that create amplitude or phase modulation in the input-state coefficients Ck.Table 3 lists input states for various measured periodicities.