Laser system for reducing relative intensity noise

Abstract

The disclosure relates to a laser system comprising a first pump source configured to provide a first pump signal at a first pump power; a second pump source configured to provide a second pump signal at a second pump power; and a frequency conversion module configured to generate a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals; wherein the laser system is configured to operate the frequency conversion module in a saturation regime with respect to the first pump signal. The disclosure further relates to a method of reducing the relative intensity noise (RIN) in a frequency conversion process. The disclosure further relates to a quantum computing system and an atomic clock.

Description

[0001] LASER SYSTEM FOR REDUCING RELATIVE INTENSITY NOISE

[0002] Technical field

[0003] The present disclosure relates to a laser system. In particular, it relates to a system and method for reducing the relative intensity noise of an optical signal, such as a frequency-converted optical signal. The disclosure further relates to systems and methods for frequency-conversion of optical signals. The disclosure further relates to a quantum computing system and an atomic clock.

[0004] Background

[0005] Relative intensity noise (RIN) is often considered a critical parameter in laser systems, such as in systems for nonlinear optical frequency conversion. Examples of nonlinear frequency conversion processes include sum-frequency generation and second-harmonic generation. Excess RIN can limit the applicability of such systems in precision applications, including quantum computing, spectroscopy, and atomic systems.

[0006] Traditional approaches often optimize the system in terms of conversion efficiency or output power, but sometimes neglect the impact on the RIN, which may remain significant. The RIN of frequency-converted signals is often determined directly by the RIN of the pump sources used for the frequency conversion process. However, there are physical limitations to how low RIN can be achieved on the pump sources.

[0007] There is a need for a method and system that can address RIN in laser systems, such as RIN in optical frequency conversion processes, ideally without resorting to addressing the RIN of the pump sources of such laser systems.

[0008] Summary

[0009] The above-mentioned challenges are solved by providing a laser system, such as a laser system for quantum applications, said laser system comprising a first pump source configured to provide a first pump signal at a first pump power; a second pump source configured to provide a second pump signal at a second pump power; and a frequency conversion module configured to generate a frequency-converted optical signal through a frequency conversion process, such as sum-frequency generation (SFG), of the first and second pump signals, wherein the frequency conversion module is operated in saturation, or near saturation, with respect to the first pump signal and / or with respect to the second pump signal.In accordance with some embodiments, the laser system comprises a first pump source configured to provide a first pump signal at a first pump power; a second pump source configured to provide a second pump signal at a second pump power; and a frequency conversion module configured to generate a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals, optionally wherein the first pump power is sufficiently high that the frequency conversion module is operated in saturation, or near saturation, with respect to the first pump signal. Advantageously, the first pump power is selected to operate the frequency conversion module in a saturation regime with respect to the first pump signal. More generally, the laser system is preferably configured to operate the frequency conversion module in a saturation regime with respect to one of the pump signals, such as the first pump signal.

[0010] In some embodiments, the laser system comprises a first pump source configured to provide a first pump signal at a first pump power; a second pump source configured to provide a second pump signal at a second pump power; and a frequency conversion module configured to receive the first and second pump signals and generate a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals; wherein the system is configured to establish a power asymmetry between the first and second pump signals, the first pump signal having a higher power than the second pump signal, such that the frequency conversion module is operated in a depletion regime with respect to the first pump signal. A technical effect hereof is that the frequency-converted optical signal is rendered substantially insensitive to the relative intensity noise of the first pump signal. A further advantage hereof is that the laser system is suitable for reducing relative intensity noise in a frequency-converted optical signal and / or suitable for generating a low-noise frequency-converted optical signal.

[0011] The present inventors have realized that the relative intensity noise (RIN) of frequency-converted optical signals, such as generated from sum-frequency generation, can be addressed in the frequency conversion module alone, i.e., without necessarily resorting to addressing the RIN of the pump sources used in the conversion process. The benefits may also be combined with efforts of reducing RIN of the utilized pump sources, which may even further reduce the RIN of the frequency-converted optical signal. In particular, the present inventors have realized that by operating the frequency conversion module in saturation, or near saturation, with respect to at least one of the pump signals, such as the first pump signal or the second pump signal, the noise contributions from said pump signal are significantly reduced, such that the frequency-converted optical signal has a lower RIN than typically expected.Generally, saturation may be achieved by ensuring that the number of photons in one pump signal, such as the first pump signal, is higher than the number of photons in the other pump signal, such as the second pump signal. The number of photons can be increased in one of the pump signals by increasing the pump power of one of the pump sources. Thus, at least one way to achieve saturation with respect to one of the pump signals, such as the first pump signal, is to ensure a higher optical power in one of the pump signals. Therefore, in preferred embodiments, the first pump power is higher than the second pump power, or vice versa. By having more power in one of the pump signals than the other, the frequencyconversion operates in saturation with respect to the pump signal of the highest power. This may further cause the power of the output signal from the frequency conversion module to be substantially insensitive to fluctuations in the input pump signal, such as insensitive to fluctuations in the pump signal having the higher power. A further technical effect hereof is that the relative intensity noise in the frequency-converted optical signal is reduced.

[0012] The present inventors have quantified the noise reductions in experiments, the results of which are shown in figures 3-4. As evident, a significant RIN reduction can be achieved from the presently disclosed system and method, such as a RIN reduction exceeding 10 dB, or even exceeding 15 dB, at least in a range of noise frequencies. While the experiments are shown for an SFG process, similar results can be expected for other nonlinear frequencyconversion processes, such as second-harmonic generation (SHG), and differencefrequency generation (DFG).

[0013] The present disclosure further relates to a method of reducing the relative intensity noise (RIN) in a frequency conversion process, the method comprising the steps of: providing a first pump signal at a first pump power; providing a second pump signal at a second pump power; and generating a frequency-converted optical signal through a frequency-conversion process, such as sum-frequency generation (SFG), of the first and second pump signals, optionally wherein the first pump power is sufficiently high that the frequency conversion process operates in saturation with respect to the first pump signal, whereby the RIN of the frequency-converted optical signal is reduced compared to a nonsaturated situation. Preferably, the first pump power is selected to operate the frequency conversion module in a saturation regime with respect to the first pump signal. More generally, the method may comprise the step of operating the frequency conversion module in a saturation regime with respect to one of the pump signals, such as the first pump signal. In some cases, the power of the frequency-converted optical signal is substantially insensitive to fluctuations in the power of the first pump signal and / or the second pump signal. The disclosed method may be carried out, either fully or partially, by any of the embodied laser systems according to the present disclosure.A further aspect of the present disclosure relates to a quantum computing system. The quantum computing system may comprise a plurality of qubits and the laser system as described herein. In such a system, the laser system is optically coupled to the qubits, and its frequency-converted optical signal is preferably configured to control and / or read out a quantum state of the qubits. Control operations may include laser cooling and / or trapping of the quantum particles, initializing them into a specific state, and precisely manipulating their quantum states to perform logic gates.

[0014] A typical requirement for high-fidelity quantum computation is minimizing decoherence. A frequency-converted optical signal having reduced intensity fluctuations (low RIN) is advantageous as it enables more stable Rabi frequencies, leading to fewer systematic gate errors. Furthermore, lower RIN suppresses noise-induced variations of AC Stark shifts, thereby decreasing phase noise on qubit transitions during single- and two-qubit gates. Thus, the low RIN frequency-converted optical signal from the disclosed laser system ensures that quantum states are manipulated with high precision and are not inadvertently disturbed, leading to higher fidelity gate operations and more reliable computational results.

[0015] The qubits in the quantum computing system may be realized in various ways. In some embodiments, the qubits comprise, or constitute, trapped ions, where the laser system provides specific wavelengths for cooling and / or state manipulation. In other embodiments, the qubits are formed by neutral atoms held in an optical lattice. For example, in systems based on neutral Strontium (Sr) atoms, the laser system may be configured to generate a low-RIN optical signal at approximately 689 nm. This wavelength may be used to address the narrow1S0— >3Pi intercombination line, which is ideal for performing high-precision single-qubit or two-qubit logic gates. The stability of the 689 nm light is considered important, as intensity fluctuations typically directly translate into gate errors. By providing a highly stable signal, the present invention enables the execution of complex quantum algorithms with a lower error rate, a crucial step towards building a fault-tolerant quantum computer.

[0016] A further aspect of the present disclosure relates to an optical atomic clock. The optical atomic clock may comprise an atomic frequency reference, comprising a plurality of atoms defining a stable clock transition. The disclosed laser system is preferably integrated as the source for a probe laser, which is configured to generate the frequency-converted optical signal to interrogate the clock transition. The atomic clock may further comprise a feedback control system configured to lock the frequency of the probe laser to the atomic transition. An advantage hereof is a highly stabilized frequency output. A fundamental limitation to the stability and accuracy of atomic clocks is the AC Stark shift, an effect where intensity fluctuations in the probe laser perturb the energy levels of the atoms. By providing a probelaser with exceptionally low relative intensity noise (RIN), the present invention directly minimizes these fluctuations. This reduction in the AC Stark shift leads to a more precise and stable frequency lock, significantly improving the overall performance of the atomic clock.

[0017] The disclosed system and method provide an advantage over existing laser systems and methods employing nonlinear frequency conversion, such as sum-frequency generation (SFG), since a reduction in the relative intensity noise (RIN) is provided and demonstrated. In particular, the presently disclosed system and method are advantageous in applications such as precision atomic physics, quantum technologies, such as quantum computers, and high-end sensing and metrology, where intensity noise directly limits performance.

[0018] Brief description of the drawings

[0019] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0020] Fig. 1 shows a schematic drawing of an embodiment of a laser system in accordance with the present disclosure.

[0021] Fig. 2 shows a schematic drawing of another embodiment of a laser system in accordance with the present disclosure.

[0022] Fig. 3 shows experimental data of measured relative intensity noise (RIN) in a sumfrequency conversion process.

[0023] Fig. 4 shows experimental data of measured relative intensity noise (RIN) in a sumfrequency conversion process.

[0024] Fig. 5 shows a schematic drawing of an embodiment of a quantum computing system comprising a plurality of qubits, and the laser system disclosed herein.

[0025] Detailed description

[0026] Sum-frequency conversion

[0027] Sum-frequency Generation (SFG) is a nonlinear optical process in which two input photons with different frequencies interact in a nonlinear medium to produce a photon with a frequency equal to the sum of the two input frequencies. SFG is commonly used for wavelength conversion, optical signal processing, and spectroscopy applications, enabling the generation of light at new frequencies not directly available from laser sources.

[0028] Relative intensity noiseRelative Intensity Noise (RIN) may be understood as a measure of the fluctuations in the optical power of a laser or light source relative to its average power. RIN is typically expressed as the ratio of the power spectral density of intensity noise to the square of the average intensity, often in units of dB / Hz. It may also be expressed in units of dBc / Hz, where dBc means dB relative to the carrier, such that dBc / Hz means dBc in a 1-Hz bandwidth. RIN typically quantifies noise across different frequencies and is often considered critical in evaluating the stability and performance of optical systems, such as laser systems.

[0029] RIN can be measured using conventional techniques, such as by sampling the output current of a photodetector, such as a photodiode, over time and transforming this data into frequencies using a Fourier transform, or by analyzing the spectrum of the photo-detected signal using an electrical spectrum analyzer. The spectrum analyzer may be used to measure the power spectral density of intensity fluctuations across a range of frequencies. As an example, the RIN may be calculated as the ratio of the noise power spectral density to the square of the average optical power. The RIN may be expressed in dB / Hz or dBc / Hz. The RIN profile is typically a frequency-dependent profile such as the ones shown in figures 3 and 4.

[0030] Pump sources

[0031] The laser system may comprise one or more pump sources, such as two or more pump sources. The pump sources may be selected from the group of pulsed lasers or continuous-wave lasers. In some embodiments, the pump sources are fiber lasers, such as narrow-linewidth single-frequency fiber lasers. As an alternative, solid-state lasers can be utilized. In some cases, only a single pump source is necessary. This may be the case for second-harmonic generation conversion processes. In other cases, such as for sumfrequency generation, at least two pump sources are required.

[0032] The laser system may comprise a first pump source configured to provide a first pump signal, and a second pump source configured to provide a second pump signal. The first and second pump signals may be provided at first and second pump powers, respectively. Furthermore, the first and second pump signals may be provided at first and second frequencies or wavelengths, respectively. Advantageously, the presently disclosed system and method utilize uses two dedicated pump sources to provide energy for the frequency conversion process. In preferred embodiments, the system is configured to provide a power imbalance between the two pump sources, i.e., between the first and second pump sources.

[0033] In some embodiments, the first pump signal has a first wavelength, and the second pump signal has a second wavelength, wherein the first and second wavelengths are different. In some cases, the first wavelength is higher than the second wavelength. As anexample, the first wavelength may be selected in a range from about 1750 nm to about 2150 nm, such as from about 1850 nm to about 2050 nm, such as from about 1900 nm to about 2000 nm. As an example, the second wavelength may be selected in a range from about 850 nm to about 1250 nm, such as from about 950 nm to about 1150 nm, such as from about 1000 nm to about 1100 nm.

[0034] In some embodiments, the first or the second wavelength may be selected in a range from about 0.9 pm to about 1.1 pm, such as about 1 pm. Additionally, or alternatively, the first or the second wavelength may be selected in a range from about 1.4 pm to about 1.6 pm, such as about 1.5 pm. Additionally, or alternatively, the first or the second wavelength may be selected in a range from about 1.9 pm to about 2.1 pm, such as about 2 pm. As a more specific example, the wavelength of the first pump signal may be about 1 pm and the wavelength of the second pump signal may be about 1.5 pm, or about 2 pm. As another example, the wavelength of the first pump signal may be about 1.5 pm and the wavelength of the second pump signal may be about 2 pm. Other wavelengths can be envisaged without departing from the scope of the disclosure. The wavelength of the frequency-converted optical signal may be a third wavelength corresponding to a sum-frequency of the first and second wavelengths. As an example, the third wavelength may be about 689 nm.

[0035] Amplifier(s)

[0036] The laser system may comprise one or more optical amplifiers for amplifying one or more optical signals, such as for amplifying the pump signals. In particular, the optical amplifier(s) may be configured to amplify the pump signal(s) to generate amplified pump signal(s). In some embodiments, the laser system comprises one or more optical amplifiers, such as fiber amplifiers, arranged downstream of the pump sources and upstream of the frequency conversion module. Figure 2 provides an example of where such amplifiers can be located in the disclosed laser system.

[0037] Frequency conversion module

[0038] The laser system may comprise a frequency conversion module configured to generate a frequency-converted optical signal through a nonlinear frequency conversion process, such as second-harmonic generation (SHG) or sum-frequency generation (SFG). In preferred embodiments, the frequency conversion module is configured to perform sumfrequency generation (SFG) of the first and second pump signals. This may generate a frequency-converted optical signal with a frequency that is the sum of the frequencies of the first and second pump signals. Thus, the frequency-converted optical signal may have a third wavelength, such as a wavelength resulting from sum-frequency of the first and second wavelengths. In some embodiments, the third wavelength is selected in a range from about450 nm to about 850 nm, such as from about 550 nm to about 750 nm, such as from about 600 nm to about 700 nm. As an example, the frequency-converted optical signal may have a wavelength of about 689 nm. Other values can be envisaged without departing from the scope of the disclosure.

[0039] The frequency conversion module may be selected from the group of nonlinear crystals and nonlinear waveguides. In some embodiments, the frequency conversion module comprises a material selected from the group of lithium niobate and lithium tantalate. Alternatively, the frequency conversion module may comprise potassium titanyl phosphate (PPKTP) as a nonlinear material. In some cases, the frequency conversion module is periodically poled, such as a periodically poled lithium niobate waveguide. The frequency conversion module may be a fiber-coupled device. The fiber-coupled device may comprise at least two input ports, each configured for receiving and attaching an optical fiber. More generally, the frequency conversion may be a multi-input frequency conversion module, i.e. , comprising two or more input ports for receiving two or more optical signals.

[0040] The frequency conversion module may be configured to operate outside a quadratic regime, such that the power of the frequency-converted optical signal does not scale quadratically with the first pump power. Additionally, or alternatively, the frequency conversion module may be configured to operate outside a linear regime, such that the power of the frequency-converted optical signal does not scale linearly with the first pump power. In some embodiments, the frequency conversion module is operated in a depletion regime, such that a variation in the pump power, such as the first pump power, causes only a slight variation in the power of the frequency-converted optical signal.

[0041] Pump depletion may refer to a reduction in the intensity of the pump signal(s) in a nonlinear optical process due to the transfer of its energy to generate a converted signal at a different frequency, such as in sum-frequency generation (SFG) or other frequency-mixing processes. In other words, pump depletion may be understood as the phenomenon that the pump power, such as the first and / or the second pump power, for some nonlinear process occurring in a frequency conversion module can be depleted or reduced.

[0042] In preferred embodiments, the first, or the second, pump power is sufficiently high that the frequency conversion module is operated in saturation with respect to the first, or the second, pump signal. In other words, the first pump power may be selected to operate the frequency conversion module in a saturation regime with respect to the first pump signal. The present inventors have realized that by operating the frequency conversion module in saturation, or near saturation, with respect to at least one of the pump signals, the relative intensity noise (RIN) of the frequency-converted optical signal can be reduced. Saturationmay be achieved by ensuring that the number of photons in one pump signal, such as the first pump signal, is higher than the number of photons in the other pump signal, such as the second pump signal. The number of photons can be increased in one of the pump signals by increasing the pump power of one of the pump sources.

[0043] Thus, the presently disclosed system is preferably configured to induce saturation with respect to the higher-power pump signal for the specific purpose of suppressing its noise contribution, thereby reducing the RIN of the final frequency-converted optical signal.

[0044] In some embodiments, the reduction in RIN is at least 10 dB, such as at least 15 dB, such as at least 20 dB, at least in a range of noise frequencies. As an example, the range of noise frequencies in which a RIN reduction is achieved may cover at least a 10 Hz bandwidth, such as at least a 100 Hz bandwidth, in some cases at least a 500 Hz bandwidth.

[0045] In some embodiments, the number of photons is higher in the first pump signal than in the second pump signal at a given pump power. In some embodiments, the first pump power is equal to, or higher, than the second pump power. As an example, the first pump power may be at least 25 % higher than the second pump power, such as at least 33 % higher, such as at least 65 % higher. As another example, the first pump power may be at least 1.5 times higher, such as at least 2 times higher, such as at least 3 times higher than the second pump power. In some embodiments, the first pump power and / or the second pump power is between 1 mW and 1000 W, such as between 10 mW and 500 W, such as between 100 mW and 50 W In other embodiments, the pump power of the first and / or second pump signal is even lower, such as in a range between 100 mW and 15 W

[0046] Temperature controller

[0047] In some embodiments, the laser system comprises a temperature controller thermally coupled to the frequency conversion module. The temperature controller may be configured to regulate and maintain the temperature of the module to optimize the sum-frequency generation (SFG) process. The efficiency of the SFG process is sensitive to temperature, as it may impact the phase-matching condition within the nonlinear material of the module. The temperature controller is preferably configured to maintain the module at an optimal and stable phase-matching temperature. Operating the frequency conversion module at the phase-matching temperature generally corresponds to a maximized conversion efficiency. Alternatively, the temperature controller may be configured to maintain the module at a temperature different from the phase-matching temperature, which would cause the conversion efficiency to deviate from its maximum.A stable temperature, and thereby stable conversion efficiency, is beneficial for driving and maintaining the system in the desired saturation regime. A further beneficial effect hereof is a consistent and significant reduction in the relative intensity noise (RIN) of the frequency-converted optical signal, even in the presence of ambient temperature fluctuations. The temperature controller may, for example, comprise a thermoelectric cooler (TEC) coupled with a temperature sensor, such as a thermistor, to provide closed-loop feedback for stable temperature regulation.

[0048] Summarizing, the disclosed laser system exhibiting low RIN in the output is especially well suited for precision atomic physics, quantum technologies, such as quantum computers, and high-end sensing and metrology, where intensity noise directly limits performance.

[0049] Detailed description of the drawings

[0050] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, figures, and graphs.

[0051] Fig. 1 shows a schematic drawing of an embodiment of a laser system (100), such as a laser system for quantum applications, in accordance with the present disclosure. In this embodiment, the laser system (100) comprises a first pump source (102) configured to provide a first pump signal (104) at a first pump power; and a second pump source (106) configured to provide a second pump signal (108) at a second pump power. The laser system (100) further comprises a frequency conversion module (110) configured to generate a frequency-converted optical signal (112) through sum-frequency generation (SFG) of the first and second pump signals (104, 108). The laser system (100) may further comprise one or more optical amplifiers. The frequency conversion module (110) may comprise two or more input ports. In particular, the conversion module may be configured to receive the first and second pump signals (104, 108).

[0052] Advantageously, the pump power of the first pump signal (104) is sufficiently high that the frequency conversion module (110) is operated in saturation with respect to the first pump signal (104). More generally, the laser system is configured to operate the frequency conversion module (110) in a saturation regime with respect to the first pump signal (104). This causes a reduction in the relative intensity noise (RIN) of the frequency-converted optical signal (112). The reduction in RIN may constitute a reduction compared to a situation in which the frequency conversion module (110) is operated in a non-saturated regime, such as in a quadratic regime or a linear regime, with respect to the first pump signal (104). A nonsaturated regime may occur if the pump power of the first pump signal (104) is not sufficiently high compared to the pump power of the second pump signal (108). Morespecifically, this may be the case if the number of photons forming part of the first pump signal (104) is not sufficiently high compared to the number of photons forming part of the second pump signal (108). In some embodiments, the number of photons is significantly higher, such as two times higher, or three times higher, or even 10 times higher, in one of the pump signals (104, 108) compared to the other pump signal (108, 104). This may result in a significant RIN reduction in the frequency-converted optical signal (112). In some cases, the reduction in RIN is at least 10 dB, such as at least 15 dB, such as at least 20 dB, at least in a range of noise frequencies.

[0053] Fig. 2 shows a schematic drawing of an embodiment of a laser system (200), such as a laser system for quantum applications, in accordance with the present disclosure. The laser system (200) comprises the same components as the embodiment (100) described in relation to figure 1, however, with the addition of one or more optical amplifiers (214, 218) for amplifying the pump signals (204, 208). Thus, the laser system (200) may comprise a first optical amplifier (214) configured to amplify the first pump signal (204) to generate an amplified first pump signal (216). The laser system (200) may additionally, or alternatively, comprise a second optical amplifier (218) configured to amplify the second pump signal (208) to generate an amplified second pump signal (220). The first and / or the second optical amplifiers (214, 218) may also form part of any other embodiments of laser systems described herein. The frequency conversion module (210) may comprise two or more input ports. In particular, the conversion module may be configured to receive the amplified first and second pump signals (216, 220). Similar to the previous embodiment, the pump power of the amplified first pump signal (216) is preferably sufficiently high such that the frequency conversion module (210) is operated in saturation with respect to the amplified first pump signal (216). The same effects and advantages apply as described elsewhere herein.

[0054] Fig. 3 shows experimental data of measured relative intensity noise (RIN) in a nonlinear frequency conversion process. This specific example shows the RIN of a sumfrequency generation process, however, the same principle could apply to other frequency conversion processes, such as second-harmonic generation (SHG). The figure shows a plot of the RIN of a frequency-converted optical signal for five scenarios, specifically five different first pump powers, each scenario represented by a graph. The RIN is measured in dBc / Hz. The RIN is shown for a range of noise frequencies in Hz, which is common for RIN measurements. In this experiment, first and second pump signals were provided to a frequency conversion module at first and second pump powers, respectively. The frequency conversion module generated a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals. The RIN of the frequency-converted optical signal was measured for different values of the first pump power. In this specificexample, the first pump power was varied from about 1 Wto about 10 W. The second pump power was held constant, in this case at about 3 W. Other pump powers can be envisaged without departing from the scope of the disclosure.

[0055] From Fig. 3, it is observed that the RIN of the frequency-converted optical signal is reduced with increasing values of the first pump power, at least in a range of noise frequencies. In particular, this is the case when the first pump power is sufficiently high that the frequency conversion module is operated in saturation with respect to the first pump signal. Thus, the first pump power is preferably selected to operate the frequency conversion module in a saturation regime with respect to the first pump signal. Additionally, or alternatively, the frequency conversion module may be operated in a depletion regime, which has the technical effect that variations in the first pump power cause only slight variations, or even no variations, in the power of the frequency-converted optical signal. This has the further technical effect that the output power, i.e., the power of the frequency-converted optical signal, is less sensitive to fluctuations in the input power, thereby effectively reducing the noise contribution from the first pump signal. Consequently, the RIN of the frequency-converted optical signal can be significantly suppressed or reduced.

[0056] Fig. 4 shows experimental data of measured relative intensity noise (RIN) in a nonlinear frequency conversion process. This experiment has the same setup as described in relation to figure 3. In fact, the two experiments are identical with the exception of the value of the pump powers used for the first and second pump signals. In this case, the first pump power was similarly varied between about 1 W to about 10 W, however, the second pump power was held constant at about 8 W. This causes a lower degree of saturation of the first pump signal in the frequency conversion process, and consequently a lower degree of RIN reduction in the frequency-converted optical signal. In other words, the effect of RIN reduction is more pronounced at higher degrees of saturation of the first pump signal. One way to achieve a high degree of saturation is to ensure that the number of photons available for the frequency-conversion process is much higher in one of the pump signals, such as in the first pump signal, than in the other pump signal, such as the second pump signal. Thus, it is preferred that the first pump power is equal to, or higher, than the second pump power. It is noted that even at an equal pump power, the number of photons may be different in the two pump signals in case two different pump frequencies are used, since the photon energy generally depends on the frequency.

[0057] Fig. 5 shows a schematic drawing of an embodiment of a quantum computing system (500) comprising a plurality of qubits (522), and the laser system disclosed herein. In this embodiment, the laser system is configured to deliver the frequency-converted optical signal(512) to one or more of the plurality of qubits (522). The frequency-converted optical signal (512) may be used for controlling and / or reading out a state of the plurality of qubits. As further examples, it may be configured for laser cooling, trapping, qubit initialization, state manipulation of quantum logic gates, and / or combinations thereof. As an example, the plurality of qubits may comprise trapped ions, such as ions selected from the group of: Ytterbium (Yb+), Barium (Ba+), or Rubidium (Rb+). Alternatively, the plurality of qubits comprises neutral atoms, such as atoms selected from the group of: Rubidium (Rb) or Strontium (Sr). In such case, the frequency-converted optical signal may be used for atom cooling or magneto optical trapping of the neutral atoms.

[0058] In this embodiment, the laser system comprises a first pump source (502) configured to provide a first pump signal (504) at a first pump power; a second pump source (506) configured to provide a second pump signal (508) at a second pump power; and a frequency conversion module (510) configured to receive the first and second pump signals. The optical amplifiers (514, 518) are optional, and if included, configured to amplify the first and second pump signals (504, 508). Thus, the frequency conversion module (510) may be configured to receive the first and second pump signals (504, 508) or alternatively configured to receive amplified first and second pump signals (516, 520). The frequency conversion module (510) is further configured to generate a frequency-converted optical signal (512) through sum-frequency generation (SFG) of the first and second pump signals. The system is further configured to establish a power asymmetry between the first and second pump signals, the first pump signal having a higher power than the second pump signal, such that the frequency conversion module (510) is operated in a depletion regime with respect to the first pump signal. An advantage hereof is the provision of a frequency-converted optical signal (512) which has a reduced relative intensity noise (RIN) level, e.g., compared to a non-saturated regime or compared to a situation with lower degree of power asymmetry.

[0059] A low RIN level is beneficial in quantum computing applications, since a low RIN reduces unwanted spontaneous-emission events that cause decoherence. For optical traps and lattices, a low RIN stabilizes trap depths. Furthermore, a stable intensity, i.e., a low intensity noise, improves repeatability of state preparation, cooling, and readout of pulses. Summarizing, a low RIN optical signal provides several important advantages for quantum computing systems that use atoms or ions as qubits.

[0060] Further details of the disclosure

[0061] 1. A laser system, comprising:a first pump source configured to provide a first pump signal at a first pump power;

[0062] a second pump source configured to provide a second pump signal at a second pump power; and

[0063] a frequency conversion module configured to generate a frequency-converted optical signal through a nonlinear frequency conversion process, such as a sumfrequency generation (SFG) process, of the first and second pump signals.

[0064] The laser system according to item 1 , wherein the nonlinear frequency conversion process is a sum-frequency generation (SFG) process

[0065] The laser system according to any of the preceding items, wherein the frequency conversion module is configured to generate a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals.

[0066] The laser system according to any of the preceding items, wherein the nonlinear frequency conversion process is a difference-frequency generation (DFG) process.

[0067] The laser system according to any of the preceding items, wherein the nonlinear frequency conversion process is a second-harmonic generation (SHG) process.

[0068] The laser system according to any of the preceding items, wherein the first pump power is selected to operate the frequency conversion module in a saturation regime with respect to the first pump signal.

[0069] The laser system according to any of the preceding items, wherein the first pump power is sufficiently high that the frequency conversion module is operated in saturation with respect to the first pump signal.

[0070] The laser system according to any of the preceding items, wherein the second pump power is sufficiently high that the frequency conversion module is operated in saturation with respect to the second pump signal.

[0071] The laser system according to any of the preceding items, wherein the frequency conversion module is configured to operate outside a quadratic regime, such that thepower of the frequency-converted optical signal does not scale quadratically with the first pump power.

[0072] 10. The laser system according to any of the preceding items, wherein the frequency conversion module is configured to operate outside a linear regime, such that the power of the frequency-converted optical signal does not scale linearly with the first pump power.

[0073] 11. The laser system according to any of the preceding items, wherein the frequency conversion module is operated in a depletion regime.

[0074] 12. The laser system according to any of the preceding items, wherein the frequency conversion module is operated in a depletion regime, such that a variation in the first pump power causes only a slight variation in the power of the frequency-converted optical signal.

[0075] 13. The laser system according to any of the preceding items, wherein the saturation with respect to the first pump signal causes a reduction in the relative intensity noise (RIN) of the frequency-converted optical signal.

[0076] 14. The laser system according to any of the preceding items, wherein the output power from the frequency conversion module is substantially insensitive to fluctuations in the first and / or the second pump power.

[0077] 15. The laser system according to any of the preceding items, wherein the first pump signal has a higher relative intensity noise (RIN) than the second pump signal, or vice versa.

[0078] 16. The laser system according to any of the preceding items, wherein the reduction in RIN is at least 10 dB, such as at least 15 dB, such as at least 20 dB, at least in a range of noise frequencies.

[0079] 17. The laser system according to item 16, wherein the range of noise frequencies covers at least a 10 Hz bandwidth, such as at least a 100 Hz bandwidth.18. The laser system according to any of the preceding items, wherein the number of photons is higher in the first pump signal than in the second pump signal at a given pump power, or vice versa.

[0080] 19. The laser system according to any of the preceding items, wherein the first pump power is equal to, or higher, than the second pump power, or vice versa.

[0081] 20. The laser system according to any of the preceding items, wherein the first pump power is substantially higher than the second pump power, or vice versa.

[0082] 21. The laser system according to any of the preceding items, wherein the first pump power is at least 25 % higher than the second pump power, such as at least 33 % higher, such as at least 65 % higher, or vice versa.

[0083] 22. The laser system according to any of the preceding items, wherein the first pump power is at least 1.5 times higher than the second pump power, such as at least 2 times higher such as at least 3 times higher, or vice versa.

[0084] 23. The laser system according to any of the preceding items, wherein the first pump power and / or the second pump power is between 1 mW and 1000 W, such as between 10 mW and 500 W, such as between 100 mW and 100 W

[0085] 24. The laser system according to any of the preceding items, wherein the first pump power and / or the second pump power is between 1 mW and 100 W, such as between 100 mW and 50 W, such as between 1 W and 15 W

[0086] 25. The laser system according to any of the preceding items, wherein the frequency conversion module is selected from the group of nonlinear crystals and nonlinear waveguides.

[0087] 26. The laser system according to any of the preceding items, wherein the frequency conversion module comprises a material selected from the group of lithium niobate and lithium tantalate.

[0088] 27. The laser system according to any of the preceding items, wherein the frequency conversion module is periodically poled.28. The laser system according to any of the preceding items, wherein the frequency conversion module is a periodically poled nonlinear waveguide.

[0089] 29. The laser system according to any of the preceding items, wherein the frequency conversion module comprises a nonlinear waveguide, and wherein the waveguide has a chirped or a periodically-poled grating structure.

[0090] 30. The laser system according to any of the preceding items, wherein the frequency conversion module comprises a nonlinear crystal having an interaction length of at least 20 mm, such as at least 30 mm, such as at least 50 mm.

[0091] 31. The laser system according to any of the preceding items, wherein the frequency conversion module comprises a nonlinear crystal, or nonlinear waveguide, made of periodically poled lithium niobate (PPLN).

[0092] 32. The laser system according to any of the preceding items, wherein the frequency conversion module comprises a nonlinear crystal, or nonlinear waveguide, made of periodically poled potassium titanyl phosphate (PPKTP).

[0093] 33. The laser system according to any of the preceding items, wherein the first pump signal has a first wavelength, and the second pump signal has a second wavelength, wherein the first and second wavelengths are different.

[0094] 34. The laser system according to any of the preceding items, wherein the frequency conversion module is operated in saturation with respect to the pump signal having the highest wavelength of the two pump signals.

[0095] 35. The laser system according to any of the preceding items, wherein the first wavelength is higher than the second wavelength.

[0096] 36. The laser system according to any of the preceding items, wherein the second wavelength is higher than the first wavelength.

[0097] 37. The laser system according to any of the preceding items, wherein the first wavelength is in a range from about 1750 nm to about 2150 nm, such as from about 1850 nm to about 2050 nm, such as from about 1900 nm to about 2000 nm.38. The laser system according to any of the preceding items, wherein the second wavelength is in a range from about 850 nm to about 1250 nm, such as from about 950 nm to about 1150 nm, such as from about 1000 nm to about 1100 nm.

[0098] 39. The laser system according to any of the preceding items, wherein the first or the second wavelength is in a range from about 0.9 pm to about 1.1 pm, such as about 1 pm.

[0099] 40. The laser system according to any of the preceding items, wherein the first or the second wavelength is in a range from about 1.4 pm to about 1.6 pm, such as about 1.5 pm.

[0100] 41. The laser system according to any of the preceding items, wherein the first or the second wavelength is in a range from about 1.9 pm to about 2.1 pm, such as about 2 pm.

[0101] 42. The laser system according to any of the preceding items, wherein the first pump signal is provided at a first wavelength selected in a range from about 1.4 pm to about 1.6 pm, and the second pump signal is provided at a second wavelength selected in a range from about 0.9 pm to about 1.1 pm.

[0102] 43. The laser system according to any of the preceding items, wherein the first pump signal is provided at a first wavelength selected in a range from about 1.9 pm to about 2.1 pm, and the second pump signal is provided at a second wavelength selected in a range from about 1.4 pm to about 1.6 pm.

[0103] 44. The laser system according to any of the preceding items, wherein the first pump signal is provided at a first wavelength at about 2 pm, and the second pump signal is provided at a second wavelength at about 1 pm.

[0104] 45. The laser system according to any of the preceding items, wherein the frequency- converted optical signal has a third wavelength.

[0105] 46. The laser system according to any of the preceding items, wherein the third wavelength is in a range from about 450 nm to about 850 nm, such as from about 550 nm to about 750 nm, such as from about 600 nm to about 700 nm.47. The laser system according to any of the preceding items, wherein the third wavelength is approximately 689 nm.

[0106] 48. The laser system according to any of the preceding items, wherein the first and / or second pump sources are pump lasers, such as fiber lasers.

[0107] 49. The laser system according to any of the preceding items, wherein the first and / or second pump sources are single-frequency pump lasers.

[0108] 50. The laser system according to any of the preceding items, wherein the first and / or second pump sources are narrow-linewidth pump lasers.

[0109] 51. The laser system according to any of the preceding items, wherein the first and / or second pump sources are continuous-wave lasers.

[0110] 52. The laser system according to any of the preceding items, wherein the frequency conversion module is a fiber-coupled device.

[0111] 53. The laser system according to item 52, wherein the fiber-coupled device comprises at least two input ports, each configured for receiving and attaching an optical fiber.

[0112] 54. The laser system according to any of the preceding items, wherein the frequency conversion module comprises multiple input ports, such as two or more input ports, for receiving two or more optical fibers.

[0113] 55. The laser system according to any of the preceding items, wherein the laser system is fiber-based such that at least the pump lasers are fiber lasers, and the frequency conversion module is a fiber-coupled device.

[0114] 56. The laser system according to any of the preceding items, wherein the laser system further comprises one or more optical fibers for optically connected one or more optical components of the system, such as for connecting the pump lasers and the frequency conversion module.

[0115] 57. The laser system according to any of the preceding items, wherein the system is configured to establish a power asymmetry between the first and second pump signals, the first pump signal having a higher power than the second pump signal.58. The laser system according to any of the preceding items, wherein the frequency conversion module is operated in a depletion regime with respect to the first pump signal.

[0116] 59. The laser system according to any of the preceding items, further comprising a temperature controller thermally coupled to the frequency conversion module.

[0117] 60. The laser system according to any of the preceding items, wherein the temperature controller is configured to substantially maintain the frequency conversion module at a phase-matching temperature.

[0118] 61. The laser system according to any of the preceding items, wherein the laser system is suitable for quantum applications, such as quantum computing systems, atomic clocks, quantum sensing, cold-atom interferometry, or quantum gravimeters.

[0119] 62. The laser system according to any of the preceding items, wherein the laser system is configured to deliver the frequency-converted optical signal to a quantum system, such as a quantum computing system, an atomic clock, an atomic interferometer, or a quantum gravimeter.

[0120] 63. A quantum computing system, comprising

[0121] - a plurality of qubits; and

[0122] - the laser system according to any one of the items 1 to 62, wherein the laser system is optically coupled to the plurality of qubits.

[0123] 64. The quantum computing system according to item 63, wherein the laser system is configured to deliver the frequency-converted optical signal to one or more of the qubits.

[0124] 65. The quantum computing system according to any of the items 63 to 64, wherein the frequency-converted optical signal is configured for controlling and / or reading out a state of the plurality of qubits.

[0125] 66. The quantum computing system according to any of the items 63 to 65, wherein the frequency-converted optical signal is configured for laser cooling, trapping, qubit initialization, state manipulation of quantum logic gates, and / or combinations thereof.67. The quantum computing system according to any of the items 63 to 66, wherein the plurality of qubits comprises trapped ions.

[0126] 68. The quantum computing system according to item 67, wherein the trapped ions are selected from the group of: Ytterbium ions (Yb+), Barium ions (Ba+), or Rubidium ions (Rb+).

[0127] 69. The quantum computing system according to any of the items 63 to 66, wherein the plurality of qubits comprises neutral atoms.

[0128] 70. The quantum computing system according to item 69, wherein the neutral atoms are selected from the group of: Rubidium (Rb) or Strontium (Sr).

[0129] 71. The quantum computing system according to any of the items 63 to 70, wherein the frequency-converted optical signal is configured for laser cooling of neutral atoms, such as laser cooling of strontium atoms.

[0130] 72. The quantum computing system according to any of the items 63 to 71 , wherein the frequency-converted optical signal is configured for magneto optical trapping (MOT) of neutral atoms, such as optical trapping of strontium atoms.

[0131] 73. An optical atomic clock, comprising: an atomic frequency reference comprising one or more atoms defining a clock transition; the laser system according to any one of the items 1 to 62, wherein the laser system is configured to generate the frequency- converted optical signal as a probe signal to interrogate the clock transition.

[0132] 74. The optical atomic clock according to item 73, further comprising a feedback control system configured to lock a frequency of the probe signal to the clock transition, thereby providing a stabilized frequency output.

[0133] 75. The optical atomic clock according to any of the items 73 to 74, wherein the atomic frequency reference comprises a plurality of neutral atoms confined in an optical lattice.

[0134] 76. The optical atomic clock according to any of the items 73 to 75, wherein the one or more atoms, or neutral atoms, are Strontium (Sr) atoms.77. The optical atomic clock according to any of the items 73 to 76, wherein the clock transition is the1S0— >3Pi intercombination line in Strontium.

[0135] 78. The optical atomic clock according to any of the items 73 to 77, wherein the frequency-converted optical signal has a wavelength of approximately 689 nm.

[0136] 79. A method of reducing the relative intensity noise (RIN) in a frequency conversion process, the method comprising the steps of:

[0137] providing a first pump signal at a first pump power;

[0138] providing a second pump signal at a second pump power; and

[0139] generating a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals,

[0140] wherein the first pump power is sufficiently high that the frequency conversion process operates in saturation with respect to the first pump signal, whereby the RIN of the frequency-converted optical signal is reduced compared to a non-saturated situation.

[0141] 80. The method according to item 79, wherein the method is carried out by the laser system of any of the items 1-62.

[0142] 81. A method comprising the steps of:

[0143] - providing at least two pump signals having different frequencies, said pump signals comprising at least a first pump signal at a first pump power and a second pump signal at a second pump power; and

[0144] - generating a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals;

[0145] 82. The method according to any of the items 79-81 , wherein the first pump signal has a higher relative intensity noise (RIN) than the second pump signal.

[0146] 83. The method according to any of the items 79-82, wherein the power of the first pump signal is higher than the power of the second pump signal.84. The method according to any of the items 79-83, wherein the frequency conversion operates in saturation in regard to the first or the second pump signal.

[0147] 85. The method according to any of the items 79-84, wherein the power of the frequency- converted optical signal is substantially insensitive to fluctuations in the power of the first pump signal and / or the second pump signal.

[0148] 86. Use of a laser system in accordance with any of the items 1-62, wherein the laser system is configured, and used, to reduce the relative intensity noise (RIN) of a frequency-converted optical signal in a frequency conversion process, such as in a sum-frequency generation (SFG) conversion process.

[0149] Although some embodiments have been described and shown in detail, the disclosure is not restricted to such details, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present disclosure. Furthermore, the skilled person would find it apparent that unless an embodiment is specifically presented only as an alternative, different disclosed embodiments may be combined to achieve a specific implementation, and such specific implementation is within the scope of the disclosure.

Claims

24Claims1. A laser system, comprising:a first pump source configured to provide a first pump signal at a first pump power;a second pump source configured to provide a second pump signal at a second pump power; anda frequency conversion module configured to generate a frequency-converted optical signal through sum-frequency generation (SFG) of the first and second pump signals;wherein the laser system is configured to operate the frequency conversion module in a saturation regime with respect to the first pump signal.

2. The laser system according to claim 1 , wherein the frequency conversion module is configured to operate outside a quadratic regime, such that the power of the frequency-converted optical signal does not scale quadratically with the first pump power.

3. The laser system according to any of the preceding claims, wherein the frequency conversion module is configured to operate outside a linear regime, such that the power of the frequency-converted optical signal does not scale linearly with the first pump power.

4. The laser system according to any of the preceding claims, wherein the frequency conversion module is operated in a depletion regime, such that a variation in the first pump power causes only a slight variation in the power of the frequency-converted optical signal.

5. The laser system according to any of the preceding claims, wherein the saturation with respect to the first pump signal causes a reduction in the relative intensity noise (RIN) of the frequency-converted optical signal.

6. The laser system according to any of the preceding claims, wherein the output power from the frequency conversion module is substantially insensitive to fluctuations in the first and / or the second pump power.

7. The laser system according to any of the preceding claims, wherein the first pump signal has a higher relative intensity noise (RIN) than the second pump signal, or vice versa.

8. The laser system according to any of the preceding claims, wherein the reduction in RIN is at least 10 dB, such as at least 15 dB, such as at least 20 dB, at least in a range of noise frequencies.

9. The laser system according to claim 8, wherein the range of noise frequencies covers at least a 10 Hz bandwidth, such as at least a 100 Hz bandwidth.

10. The laser system according to any of the preceding claims, wherein the number of photons is higher in the first pump signal than in the second pump signal at a given pump power, or vice versa.

11. The laser system according to any of the preceding claims, wherein the first pump power is higher than the second pump power, or vice versa.

12. The laser system according to any of the preceding claims, wherein the first pump power is at least 25 % higher than the second pump power, such as at least 33 % higher, such as at least 65 % higher, or vice versa.

13. The laser system according to any of the preceding claims, wherein the first pump power is at least 1.5 times higher than the second pump power, such as at least 2 times higher such as at least 3 times higher, or vice versa.

14. The laser system according to any of the preceding claims, wherein the first pump signal is provided at a first wavelength selected in a range from about 1.4 pm to about 1.6 pm, and the second pump signal is provided at a second wavelength selected in a range from about 0.9 pm to about 1.1 pm.

15. The laser system according to any of the preceding claims, wherein operation in the saturation regime causes a reduction in the relative intensity noise (RIN) of the frequency-converted optical signal of at least 10 dB, at least in a range of noise frequencies, compared to operation in a non-saturated regime.

16. The laser system according to any of the preceding claims, wherein the number of photons in the first pump signal is at least two times higher than the number of photons in the second pump signal.

17. The laser system according to any of the preceding claims, wherein the laser system is suitable for quantum applications, such as quantum computing systems, atomic clocks, quantum sensing or metrology, cold-atom interferometry, and / or quantum gravimeters.

18. The laser system according to any of the preceding claims, wherein the frequency- converted optical signal is suitable for: atom cooling and / or trapping, state preparation of qubits, or precision spectroscopy of atomic / molecular lines.

19. A quantum computing system, comprising:- a plurality of qubits, such as trapped ions or neutral atoms; and- the laser system according to any of the claims 1 to 18, wherein the laser system is optically coupled to the qubits, and wherein the laser system is configured to deliver the frequency-converted optical signal to one or more of the qubits.

20. The quantum computing system according to claim 19, wherein the plurality of qubits are neutral atoms, and wherein the frequency-converted optical signal is configured for magneto optical trapping of the neutral atoms.

21. The quantum computing system according to any of claims 19 to 20, wherein the frequency-converted optical signal is configured to control and / or read out a quantum state of one or more of the qubits.

22. The quantum computing system according to any of claims 19 to 21, wherein the plurality of qubits include a plurality of trapped ions selected from the group of Ytterbium (Yb+), Barium (Ba+), or Rubidium (Rb+).

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