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Optics Express

A versatile multi-tone laser system for manipulating atomic qubits based on a fiber Mach–Zehnder modulator and second harmonic generation

Ruirui Li, Ran He, Jin‐Ming Cui, Yan Chen, Weiran Ye, Yilong Chen, Yun‐Feng Huang, Chuan‐Feng Li, Guang‐Can Guo

Published Jul 22, 2022• 4 citations
Publisher

Abstract

Stimulated Raman transition is a fundamental method to coherently manipulate quantum states in different physical systems. Phase-coherent dichromatic radiation fields matching the energy level splitting are the key to realizing stimulated Raman transition. Here we demonstrate a flexible-tuning, spectrum-clean and fiber-compatible method to generate a highly phase-coherent and high-power multi-tone laser. This method features the utilization of a broadband fiber Mach–Zehnder modulator working at carrier suppression condition and second harmonic generation. We generate a multi-tone continuous-wave 532 nm laser with a power of 1.5 Watts and utilize it to manipulate the spin and motional states of a trapped 171Yb+ ion via stimulated Raman transition. For spin state manipulation, we acquire an effective Rabi frequency of 2π × 662.3 kHz. Due to the broad bandwidth of the fiber modulator and nonlinear crystal, the frequency gap between tones can be flexibly tuned. Benefiting from the features above, this method can manipulate 171Yb+ and 137Ba+ simultaneously in the multi-species ion trap and has potential to be widely applied in atomic, molecular and optical physics.

DOI
10.1364/oe.462737

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Stimulated Raman transition is a fundamental method to coherently manipulate quantum states in different physical systems. Phase-coherent dichromatic radiation fields matching the energy level splitting are the key to realizing stimulated Raman transition. Here we demonstrate a flexible-tuning, spectrum-clean and fiber-compatible method to generate a highly phase-coherent and high-power multi-tone laser. This method features the utilization of a broadband fiber Mach–Zehnder modulator working at carrier suppression condition and second harmonic generation. We generate a multi-tone continuous-wave 532 nm laser with a power of 1.5 Watts and utilize it to manipulate the spin and motional states of a trapped 171Yb+ ion via stimulated Raman transition. For spin state manipulation, we acquire an effective Rabi frequency of 2π × 662.3 kHz. Due to the broad bandwidth of the fiber modulator and nonlinear crystal, the frequency gap between tones can be flexibly tuned. Benefiting from the features above, this method can manipulate 171Yb+ and 137Ba+ simultaneously in the multi-species ion trap and has potential to be widely applied in atomic, molecular and optical physics.
Phase-coherent multi-tone lasers play a critical role in atomic, molecular, and optical physics.Among them, the Raman opeartion laser for manipulating atomic hyperfine qubits requires gigahertz bandwidth and low phase noise to retain long-term coherence.Raman operation lasers generated by directly modulated and frequency-multipled infrared lasers are compact and stable but lack feedback control to actively suppress the phase noise, which limits their performance in practical applications.In this work, we employ a fiber electro-optical modulator driven by a voltage-controlled oscillator (VCO) to modulate a monochromatic laser and employ a second-harmonic generation process to convert it to the visible domain, where the beat note of the Raman operation laser is stabilized by controlling the output frequency of VCO with a digital phase-locked loop (PLL).The low-frequency phase noise is effectively suppressed compared to the scheme without active feedback and it reaches -80 dBc=Hz@5 kHz with a 20 kHz loop bandwidth.Furthermore, this compact and robust scheme effectively reduces the system's complexity and cost, which is promising for extensive application in atomic, molecular, and optical physics.
Quantum computers are typically composed of an array of two-level systems, or qubits, encoded in some information carrier, such as an electron, photon, or quantized circuit. The size of this array is restricted by finite access to resources like laser power, cooling capacity, and control lines for trapping and manipulation. Under these constraints, the system's processing power can be increased by using more energy levels per information carrier, but common techniques for qubit control provide only limited connectivity between these additional states. We experimentally demonstrate transitions between electronic angular momentum states with a difference in magnetic quantum numbers $Δ\mathrm{m_J} = $ 3, 4, and 5 via resonant four- and six-photon stimulated Raman transitions in a single trapped atom. Derivation of the corresponding Rabi frequencies, which are verified experimentally, follows the standard treatment of two-photon transitions including the adiabatic elimination of intermediate states. Finally, we discuss pathways to increase the observed multi-photon transition fidelities to >99.99%>99.99\%>99.99%, providing a tool for efficient, high-fidelity control of qudits and single-atom logical qubits.
In order to control the state of a two-level quantum system (e.g. the spin state of an ion qubit), optical frequency combs perform a two-photon stimulated Raman process through stimulated absorption from one comb tooth and stimulated emission into another comb tooth. If the two-level energy gap is an integer multiple of the repetition rate of the laser, resonant Rabi oscillations are excited. When these latter have a frequency close to the qubit’s transition one, a strongly anharmonic phase-locked cycle may exist on the Bloch sphere, which generates a sub-harmonic series of very narrow, equally spaced, spectral lines. If the repetition rate of an optical frequency comb is appropriately tuned to these latter (up to the average carrier envelope frequency), a highly resonant dynamical regime of the two-level system should be reached where the Raman stimulated absorption and emission processes would occur for any pair of adjacent comb teeth.
We produced two Raman-laser beams with a frequency offset of 6.834GHz by injection-locking of a master diode-laser to a slave diode-laser. The master laser was phase-modulated at 6.834 GHz with an Electro-Optic Modulator and then injected into the slave laser that was oscillating around one of the side-bands. The relative linewidth of the two lasers was less than 1 Hz. Utilizing these laser beams, we realized the coherent manipulation of atomic wave packets in a Mach-Zehnder type atom interferometry.
Two-photon Raman process (TPRP) is an important technique in controlling the atomic internal states. It plays an important role in quantum manipulation and quantum information process. A reliable Raman laser for specific atom is the first step to demonstrate TPRP and quantum manipulation of an atom. In this paper, we theoretically analyze the two-photon Raman process regarding to Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0, and we obtain the dependences of the corresponding Rabi frequency on one-and two-photon detunings and one-photon Rabi frequencies in a realistic multi-level Cesium atom system. We find that to obtain an atom state flopping efficiency of 0.99 the Raman laser power fluctuation should be controlled to be smaller than 3.2%. We also report our simple experimental Raman laser system for TPRP of Cesium atom based on a fiber waveguide phase modulator. The phase modulator is driven by a 4.6 GHz microwave source and the two first-order sidebands with a frequency difference of 9.19 GHz are filtered out by a Fabry-Pérot cavity with a finesse of 48. After an amplitude-modulator-based intensity stabilization system, a total power of 73 μupW with a fluctuation of 2.2% within 90 min is obtained. By applying this Raman laser to a single Cesium atom trapped in a micrometer size far-off resonant trap (FORT), we obtain Raman spectra between Cesium “clock states” |6S1/2, F=4, mF=0 and |6S1/2, F=3, mF=0. The discrepancy between the two-photon resonance frequency and the defined clock frequency 9.192631770 GHz is due to the differential Stark shifts by FORT beam and Raman beams as well as the inaccuracy of the microwave source. By varying the Raman pulse length we also show the corresponding Rabi flopping with a rate of 153 kHz, which is consistent with the theoretical calculation. The obtained state transfer efficiency of 0.75 is much smaller than theoretical expectation 0.99, which is mainly limited by the state initialization efficiency. The Raman laser system reported in this paper is simple and reliable to realize and it provides a reliable method to manipulate the Cesium internal state. Moreover it could also be easily extended to other system for quantum manipulation of other species of atom.
Hardware-Economic Manipulation of Dual-Type 171{}^{171}171Yb+^++ Qubits
The dual-type qubit scheme is an emerging method to suppress crosstalk errors in scalable trapped-ion quantum computation and quantum network. Here we report a hardware-economic way to control dual-type 171Yb+^{171}\mathrm{Yb}^+171Yb+ qubits using a single 355 355\,355nm mode-locked pulsed laser. Utilizing its broad frequency comb structure, we drive the Raman transitions of both qubit types encoded in the S1/2S_{1/2}S1/2​ and the F7/2F_{7/2}F7/2​ hyperfine levels, and probe their carrier transitions and the motional sidebands. We further demonstrate a direct entangling gate between the two qubit types. Our work can simplify the manipulation of the 171Yb+^{171}\mathrm{Yb}^+171Yb+ qubits both at the hardware and the software level.