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Nature Communications

Preserving a qubit during state-destroying operations on an adjacent qubit at a few micrometers distance

Sainath Motlakunta, Nikhil Kotibhaskar, Chung-You Shih, Anthony Vogliano, Darian McLaren, Lewis Hahn, Jingwen Zhu, Roland Hablützel, Rajibul Islam

Published Aug 3, 2024• 8 citations
Publisher

Abstract

Protecting qubits from accidental measurements is essential for controlled quantum operations, especially during state-destroying measurements or resets on adjacent qubits, in protocols like quantum error correction. Current methods to preserve atomic qubits against such disturbances waste coherence time, extra qubits, and introduce additional errors. We demonstrate the feasibility of in-situ state-reset and state-measurement of trapped ions, achieving >99.9% fidelity in preserving an 'asset' ion-qubit while a neighboring 'process' qubit is reset, and >99.6% preservation fidelity while applying a detection beam for 11 μs on the same neighbor at a distance of 6 μm. This is achieved through precise wavefront control of addressing optical beams and using a single ion as both a quantum sensor for optical aberrations and an intensity probe with >50 dB dynamic range. Our demonstrations advance quantum processors, enhancing speed and capabilities for tasks like quantum simulations of dissipation and measurement-driven phases, and implementing error correction.

DOI
10.1038/s41467-024-50864-2

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•Jan 1, 2015•0 citations
Protecting qubits from accidental measurements is essential for controlled quantum operations, especially during state-destroying measurements or resets on adjacent qubits, in protocols like quantum error correction. Current methods to preserve atomic qubits against such disturbances waste coherence time, extra qubits, and introduce additional errors. We demonstrate the feasibility of in-situ state-reset and state-measurement of trapped ions, achieving >99.9% fidelity in preserving an 'asset' ion-qubit while a neighboring 'process' qubit is reset, and >99.6% preservation fidelity while applying a detection beam for 11 μs on the same neighbor at a distance of 6 μm. This is achieved through precise wavefront control of addressing optical beams and using a single ion as both a quantum sensor for optical aberrations and an intensity probe with >50 dB dynamic range. Our demonstrations advance quantum processors, enhancing speed and capabilities for tasks like quantum simulations of dissipation and measurement-driven phases, and implementing error correction.
Protecting a quantum object against irreversible accidental measurements from its surroundings is necessary for controlled quantum operations. This becomes especially challenging or unfeasible if one must simultaneously measure or reset a nearby object's quantum state, such as in quantum error correction. In atomic systems - among the most established quantum information processing platforms - current attempts to preserve qubits against resonant laser-driven adjacent measurements waste valuable experimental resources such as coherence time or extra qubits and introduce additional errors. Here, we demonstrate high-fidelity preservation of an `asset' ion qubit while a neighboring `process' qubit is reset or measured at a few microns distance. We achieve <1×10−3< 1\times 10^{-3}<1×10−3 probability of accidental measurement of the asset qubit while the process qubit is reset, and <4×10−3< 4\times 10^{-3}<4×10−3 probability while applying a detection beam on the same neighbor for experimentally demonstrated fast detection times, at a distance of 6 μm6\ \rm{μm}6 μm or four times the addressing Gaussian beam waist. These low probabilities correspond to the preservation of the quantum state of the asset qubit with fidelities above 99.9%99.9\%99.9% (state reset) and 99.6%99.6\%99.6% (state measurement). Our results are enabled by precise wavefront control of the addressing optical beams while utilizing a single ion as a quantum sensor of optical aberrations. Our work demonstrates the feasibility of in-situ state reset and measurement operations, building towards enhancements in the speed and capabilities of quantum processors, such as in simulating measurement-driven quantum phases and realizing quantum error correction.
Quantum-enhanced measurements hold the promise to improve high-precision sensing ranging from the definition of time standards to the determination of fundamental constants of nature. However, quantum sensors lose their sensitivity in the presence of noise. To protect them, the use of quantum error-correcting codes has been proposed. Trapped ions are an excellent technological platform for both quantum sensing and quantum error correction. Here we present a quantum error correction scheme that harnesses dissipation to stabilize a trapped-ion qubit. In our approach, always-on couplings to an engineered environment protect the qubit against spin-flips or phase-flips. Our dissipative error correction scheme operates in a continuous manner without the need to perform measurements or feedback operations. We show that the resulting enhanced coherence time translates into a significantly enhanced precision for quantum measurements. Our work constitutes a stepping stone towards the paradigm of self-correcting quantum information processing.
Abstract Quantum-enhanced measurements hold the promise to improve high-precision sensing ranging from the definition of time standards to the determination of fundamental constants of nature. However, quantum sensors lose their sensitivity in the presence of noise. To protect them, the use of quantum error-correcting codes has been proposed. Trapped ions are an excellent technological platform for both quantum sensing and quantum error correction. Here we present a quantum error correction scheme that harnesses dissipation to stabilize a trapped-ion qubit. In our approach, always-on couplings to an engineered environment protect the qubit against spin-flips or phase-flips. Our dissipative error correction scheme operates in a continuous manner without the need to perform measurements or feedback operations. We show that the resulting enhanced coherence time translates into a significantly enhanced precision for quantum measurements. Our work constitutes a stepping stone towards the paradigm of self-correcting quantum information processing.
We implement midcircuit measurement and reset (MCMR) operations on a full-scale trapped-ion quantum computing system by using metastable qubit states in Yb + 171 ions. We compare two methods for isolating data qubits from measured qubits: one shelves the data qubits into the metastable state and the other drives the measured qubit to the metastable state without disturbing the other qubits. We experimentally demonstrate both methods on a crystal of two Yb + 171 ions using both the S 1 / 2 ground-state hyperfine clock qubit and the S 1 / 2 − D 3 / 2 optical qubit. These MCMR methods result in errors on the data qubit of about 2 % without degrading the measurement fidelity. With straightforward reductions in laser noise, these errors can be suppressed to less than 0.1 % . The demonstrated methods allow MCMR to be performed in a single-species ion chain without shuttling or additional qubit-addressing optics, greatly simplifying the system architecture and allowing straightforward integration with existing trapped-ion quantum computers.
Mid-circuit measurement and reset of subsets of qubits is a crucial ingredient of quantum error correction and many quantum information applications. Measurement of atomic qubits is accomplished through resonant fluorescence, which typically disturbs neighboring atoms due to photon scattering. We propose and prototype a new scheme for measurement that provides both spatial and spectral isolation by using tightly-focused individual laser beams and narrow atomic transitions. The unique advantage of this scheme is that all operations are applied exclusively to the read-out qubit, with negligible disturbance to the other qubits of the same species and little overhead. In this letter, we pave the way for non-invasive and high fidelity mid-circuit measurement and demonstrate all key building blocks on a single trapped barium ion.
The trapped atomic ion qubits feature desirable properties for use in a quantum computer such as long coherence times, high qubit readout fidelity, and universal logic gates. While these essential properties have been demonstrated, the ability to scale a trapped ion quantum system has not yet been shown. The challenge of scaling the system calls for methods to realize high-fidelity logic gates in scalable trap structures. Surface electrode ion traps, that are microfabricated from a silicon substrate, provide a scalable platform for trapping ion qubits only if high-fidelity operations are achievable in these structures. Here, we present a system for trapping and manipulating ions in a scalable surface trap. Trapping times exceeding 20 minutes without laser cooling, and heating rates as low as 0.8 quanta/ms indicate stable trapping conditions in these microtraps. Coherence times of more than one second verify adequate qubit and control field stability. We demonstrate low-error single-qubit gates performed using stimulated Raman transitions driven by lasers that are tightly focused on the ion qubit. Digital feedback loops are implemented to control the driving field's amplitude and frequency. Gate errors are measured using a randomized benchmarking protocol for single qubit gates, where residual amplitude error in the control beam is compensated using various pulse sequence techniques. Using pulse compensation, we demonstrate single qubit gates with an average error per randomized Clifford group gate of 3.6(3)times10−43.6(3)\\times10^{-4}3.6(3)times10−4, which is below the fault-tolerant threshold for some error-correction schemes.