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Physical Review Applied

Improved laser-filtering scheme for quantum manipulations with the 171 Yb + optical qubit

Nikita V. Semenin, Ilia V. Zalivako, Vasilii N. Smirnov, Ilya A. Semerikov, Alexander Borisenko, Korolkov Andrey Evgenyevich, Pavel Sidorov, Kristina P. Galstyan, Ksenia Yu. Khabarova, N. Kolachevsky

Published Mar 26, 2025• 1 citation
Publisher

Abstract

We report on a laser-frequency-stabilization and phase-noise-filtering scheme at 871 nm for high-quality quantum operations with an optical ${}^{2}{\mathrm{S}}_{1/2}$--${}^{2}{\mathrm{D}}_{3/2}$ qubit in the ${}^{171}\mathrm{Yb}{}^{+}$ ion. The phase noise of the laser field in a frequency range close to the trapped ion's secular frequencies (0.5--2 MHz) causes undesirable excitations and thus reduces the fidelities of both single- and two-qubit gates. The most common way to overcome this problem is to filter this noise by passing the laser light through a cavity that suppresses high-frequency phase noise and provides a pure laser spectrum. Usually, the same cavity also serves as a reference for long-term frequency stabilization. We propose and characterize an improved laser-stabilization scheme consisting of (i) a reference laser locked to a high-finesse optical cavity providing a narrow linewidth and long-term frequency stability and (ii) a main laser locked to a tunable medium-finesse cavity for phase-noise filtering. The phase locking of the medium-finesse cavity resonance to the reference laser transfers its long-term stability to the main laser. This approach allows independent optimization of both the phase-noise filtering and long-term frequency-stabilization processes, as well as ensuring the robustness of the setup. The resulting laser spectrum is characterized by direct spectroscopy of an ${}^{171}\mathrm{Yb}{}^{+}$ ion in a linear Paul trap. We estimate the expected two-qubit gate infidelity with the filtered laser field to be well below 1%.

DOI
10.1103/physrevapplied.23.034080

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We report on a laser-frequency-stabilization and phase-noise-filtering scheme at 871 nm for high-quality quantum operations with an optical 2S1/2{}^{2}{\mathrm{S}}_{1/2}2S1/2​--2D3/2{}^{2}{\mathrm{D}}_{3/2}2D3/2​ qubit in the 171Yb+{}^{171}\mathrm{Yb}{}^{+}171Yb+ ion. The phase noise of the laser field in a frequency range close to the trapped ion's secular frequencies (0.5--2 MHz) causes undesirable excitations and thus reduces the fidelities of both single- and two-qubit gates. The most common way to overcome this problem is to filter this noise by passing the laser light through a cavity that suppresses high-frequency phase noise and provides a pure laser spectrum. Usually, the same cavity also serves as a reference for long-term frequency stabilization. We propose and characterize an improved laser-stabilization scheme consisting of (i) a reference laser locked to a high-finesse optical cavity providing a narrow linewidth and long-term frequency stability and (ii) a main laser locked to a tunable medium-finesse cavity for phase-noise filtering. The phase locking of the medium-finesse cavity resonance to the reference laser transfers its long-term stability to the main laser. This approach allows independent optimization of both the phase-noise filtering and long-term frequency-stabilization processes, as well as ensuring the robustness of the setup. The resulting laser spectrum is characterized by direct spectroscopy of an 171Yb+{}^{171}\mathrm{Yb}{}^{+}171Yb+ ion in a linear Paul trap. We estimate the expected two-qubit gate infidelity with the filtered laser field to be well below 1%.
Lasers with high spectral purity are indispensable for optical clocks and coherent manipulation of atomic and molecular qubits for applications such as quantum computing and quantum simulation. Stabilisation of the laser to a reference can provide a narrow linewidth and high spectral purity. However, widely-used diode lasers exhibit fast phase noise that prevents high fidelity qubit manipulation. Here we demonstrate a self-injection locked diode laser system utilizing a medium finesse cavity. The cavity not only provides a stable resonance frequency, but at the same time acts as a low-pass filter for phase noise beyond the cavity linewidth of around 100 kHz, resulting in low phase noise from dc to the injection lock limit. We model the expected laser performance and benchmark it using a single trapped 40^{40}40Ca+^{+}+-ion as a spectrum analyser. We show that the fast phase noise of the laser at relevant Fourier frequencies of 100 kHz to >2 MHz is suppressed to a noise floor of between -110 dBc/Hz and -120 dBc/Hz, an improvement of 20 to 30 dB over state-of-the-art Pound-Drever-Hall-stabilized extended-cavity diode lasers. This strong suppression avoids incoherent (spurious) spin flips during manipulation of optical qubits and improves laser-driven gates in using diode lasers with applications in quantum logic spectroscopy, quantum simulation and quantum computation.
Lasers with high spectral purity are indispensable for optical clocks and for the coherent manipulation of atomic and molecular qubits in applications such as quantum computing and quantum simulation. While the stabilization of such lasers to a reference can provide a narrow linewidth, the widely used diode lasers exhibit fast phase noise that prevents high-fidelity qubit manipulation. In this paper, we demonstrate a self-injection locked diode laser system that utilizes a high-finesse cavity. This cavity not only provides a stable resonance frequency, it also acts as a low-pass filter for phase noise beyond the cavity linewidth of around 100 kHz, resulting in low phase noise from dc to the injection lock limit. We model the expected laser performance and benchmark it using a single trapped 40 Ca + -ion as a spectrum analyzer. We show that the fast phase noise of the laser at relevant Fourier frequencies of 100 kHz to >2 MHz is suppressed to a noise floor of between −110 dBc/Hz and −120 dBc/Hz, an improvement of 20 to 30 dB over state-of-the-art Pound-Drever-Hall-stabilized extended-cavity diode lasers. This strong suppression avoids incoherent (spurious) spin flips during manipulation of optical qubits and improves laser-driven gates when using diode lasers in applications involving quantum logic spectroscopy, quantum simulation, and quantum computation.
We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity (HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound–Drever–Hall (PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 kHz, meanwhile the obtained lasers’ phase noise around 750 kHz are suppressed about 30 dB. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of (67 ± 37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ∼3.3 kHz and the maximum long-term laser frequency drift of a single laser is ∼46 kHz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.
For optical qubit encoding, laser phase noise limits the coherence time for qubit manipulations.In this thesis, we address this challenge by replicating a low-phase-noise laser system for qubit manipulation of trapped ions in a cryogenic environment. Using injection locking technology, we inject a laser diode with another laser, ensuring the replicated light retains the same spectral properties. We describe the implementation of this technique, analyse its impact on the laser spectrum, and validate it by comparing its coherence time on a trapped-ion qubit with the previously used laser.Our results demonstrate that the new laser system preserves the coherence time achieved with the seed laser, with a qubit coherence time of $T_{coh}= \SI{2.52\pm0.15}{\milli\second} $, paving the way for scalable and robust quantum computing and simulation in the cryogenic setup. In the future, the laser phase noise may be further reduced by spectral filtering techniques with a narrowband optical cavity. This work contributes to overtake one of the key obstacles in trapped-ion quantum computation: having long qubit coherence time.
This project sets out to frequency stabilize external cavity diode lasers of wavelengths λ = 935 nm and 739 nm, with an eye to Ytterbium-171 ion trapping.The desired laser linewidths were below 1 MHz at 1 second observation time and the desired long term frequency drifts were below a megahertz over the course of a few hours.To stabilize the lasers, two Pound-Drever-Hall laser frequency stabilisation systems were designed, constructed and characterized.The optical frequency references used were sub-megahertz linewidth optical cavities with Invar spacers.These cavities were designed and constructed inhouse.The achieved long term stability of the stabilized lasers was 1 MHz/h over a 5 hour measurement period.The laser frequency long term stabilities are limited by the thermal stabilities of the cavity spacers.The post stabilisation laser frequency noise was lowered when compared to the free running lasers.This was observed by determining the laser frequency noise power spectral densities.From the frequency noise power spectral densities lower bounds on the locked laser linewidths were established.This lower bound is 11.92 kHz for the 739 nm wavelength laser at 1 second observation time and 5.4 kHz for the 935 nm wavelength laser.An additional system was designed and constructed ii
We demonstrate a method to simultaneously suppress the linewidths and stabilize the long-term frequency drifts of both 780-nm and 960-nm lasers to the 10-kHz level. We first use an iodine frequency stabilization laser as a frequency reference to stabilize the length of a tunable high-finesse transfer cavity (THFTC). Then two slave lasers of 780 nm and 960 nm are locked to this THFTC. In this way, the linewidths of slave lasers are suppressed to the order of 10 kHz, which are measured by delayed self-heterodyne measurements. Meanwhile, the long-term frequency drifts are stabilized to be less than 22 kHz for 10 h. With the locked lasers, we realize the coherent Rydberg excitation of a single Rb87 atom and keep the exciting stable and repeatable for 10 h, which is essential for Rydberg-mediated single-atom quantum information processing. This dual-laser frequency stabilization and transfer method can also be used in other high-resolution experiments in atomic physics, precision metrology, and molecular spectroscopy.