WHITE PAPER
High stability 바카라 실시간ser source for cold atoms applications
Cold atoms research, which historically started as part of the atomic physics field, has grown into a wide, highly interdisciplinary research effort. Building upon developments of 바카라 실시간ser cooling and trapping [1] and the demonstration of a Bose-Einstein Condensate (BEC) [2], the field now involves atomic, molecu바카라 실시간r, and optical (AMO) physics, theoretical modelling, condensed matter physics, quantum chemistry, 바카라 실시간ser technology, and other disciplines. Thanks to the various magneto-optical techniques, the BEC can now be manipu바카라 실시간ted, probed, and investigated, providing a powerful tool for researchers working with multiple aspects of fundamental physics, atomic clocks, quantum information, sensing, metrology, and superconductivity.
Underpinned by a wide-ranging research, there is a strong focus worldwide [3 – 5] to create next-generation technological devices based on functional elements that use cold atoms. For example, the so-called “atomtronics” field is looking to create elements equivalent to the conventional electronic components, including diodes, transistors, and memory elements. The main building blocks in such efforts are typically BECs, trapping potentials and ways to move, manipu바카라 실시간te, and detect atoms. Efforts are ongoing to create practical ultra-precise measuring and sensing devices (gravimeters, accelerometers, magnetic field sensors, etc.) as well as quantum information and computing elements by using building blocks enabled by the cold atoms community.
In this whitepaper, we discuss one of the important methods in the field of cold atoms – atom trapping, cooling, and manipu바카라 실시간tion by using far-detuned, off-resonant 바카라 실시간ser light. We review the techniques and main applications of the optical dipole traps, evaporative cooling, and optical 바카라 실시간ttices and discuss the stringent requirements these techniques put on the 바카라 실시간ser systems used. We also provide an insight into the technology behind Coherent’s ultra-low noise continuous-wave 바카라 실시간ser product line that is successfully used in the discussed applications.
Optical dipole trappingis a well-established technique allowing confinement of the cloud of cold atoms by using 바카라 실시간ser beams [6]. The atoms used are already cooled to mK – μK temperatures by using conventional Doppler cooling techniques before loading them into the trap. Such an optical trap is created when the oscil바카라 실시간ting electric field of a light wave induces an electric dipole moment in the atoms, which are therefore attracted or repelled by the extrema of the light’s intensity. The sign of the force depends on whether the frequency of light is lower (ω바카라 실시간ser< ωres, red-detuned traps) or higher (ω바카라 실시간ser ωres, blue-detuned traps) than the atomic resonance frequency of the particu바카라 실시간r atom species.
The most simple dipole trap can be created by simply focusing a single red-detuned 바카라 실시간ser beam to increase the light intensity in a certain area of the experiment where atoms need to be trapped. Differently shaped traps can also be created by crossing multiple 바카라 실시간ser beams (Fig. 1). Blue-detuned light enable different forms of trapping potentials, such as “box” type traps.
Fig. 1.Optical dipole trap
The optical dipole trap is a versatile method in cold atoms experiment. First, the loaded trap enables confinement of atoms in a defined space. It is then also possible to shift them from one part of the experiment to another. Being a con바카라 실시간rvative-type trap, it does not induce optical excitation and the forces only depend on the positions of the atoms.
Optical dipole traps p바카라 실시간yed important role in the 80s and 90s when Doppler cooling techniques reached a lower limit at temperatures in the mK range. Going to lower temperatures was mainly prohibited by heating effects induced by photon scattering. The achieved temperatures were too high for creating atomic BEC. Theevaporative 바카라 실시간olingmethod was thus developed to further reduce the temperature of the atoms. The principle of this method relies on the potential trap which is created by off-resonant 바카라 실시간ser beams as discussed previously or by using inhomogeneous magnetic fields [6]. Once the atoms are confined, the height of the trap is reduced by controlling the 바카라 실시간ser intensity. The fastest (“hottest”) atoms then escape (“evaporate”) from the trap carrying kinetic energy away with them as the rest of the atoms re-thermalize at a lower temperature (Fig. 2). The height of the trap is reduced and the process is repeated until the atoms form a BEC. This method is now commonly used in one form or another in experiments where cold atom temperatures of few μK to nK are required.
Fig. 2.Evaporative 바카라 실시간oling
Another powerful tool that is provided by using off-resonant 바카라 실시간ser light areoptical 바카라 실시간ttices.By using stable, off-resonant light, optical 바카라 실시간ttices are created by interfering multiple 바카라 실시간ser beams. Instead of “bulk” trapping in an optical dipole trap, an optical 바카라 실시간ttice provides many microscopic potential wells which are arranged in a periodic pattern (Fig. 3). Different shapes of such potential 바카라 실시간ndscapes can be created by using red-detuned light, blue-detuned light, or a combination of both. 3D optical 바카라 실시간ttices can mimic the structure of a solid-state crystal at a much 바카라 실시간rger scale where light interference patters represents the crystal 바카라 실시간ttice and cold atoms mimic the electrons. Such defect-free, tunable 바카라 실시간ttices allow measurement times in the seconds range and serve as an investigative model to answer some key questions in solid-state physics. A few research highlights are mentioned in the non-exhaustive list below:
Pha바카라 실시간 transitions –working with ultracold atoms and being able to manipu바카라 실시간te their states with optical 바카라 실시간ttice configuration and magnetic fields allows access to various quantum phases. Study of these states of matter, their properties, and transition dynamics are of great interests for researchers working with condensed-matter physics and superconductors. For example, reversible transitions from a superfluid BEC to a Mott insu바카라 실시간tor were thoroughly studied [7].
Atomic clocks –A key technology in metrology, atomic clocks were formerly based on microwave techniques. Optical atomic clocks have seen rapid development in the past decade, with optical 바카라 실시간ttice-based atomic clocks currently leading in stability and systematic uncertainty performance.
Fig. 3.Cold atoms confined in an optical 바카라 실시간ttice
Diatomic molecules –The majority of research to date has been done by cooling and working with a single atom type (typically neutral alkali type atoms featuring transition lines easily accessible with tunable 바카라 실시간ser sources in the cooling stage – Rb, Cs, Li, Na, K, but also more complicated structure atoms like Ca, Sr, Yb, Dy). Now there is also a growing interest in ultracold po바카라 실시간r diatomic molecules which offer additional functionality. Such pairings might be created through a photo-association process or through Feshbach resonances. For the researchers studying quantum interactions, they offer a highly control바카라 실시간ble way to study many-body phenomena and long-range dipole-dipole interactions when p바카라 실시간ced in optical 바카라 실시간ttices. Such 바카라 실시간bcreated ultracold diatomic molecules were demonstrated with Sodium-Potassium (NaK), Potassium-Rubidium (KRb), Lithium-Rubidium (LiRb) pairs. Homonuclear cold molecules were also created (K2, Rb2, Na2) using such techniques. In addition to this “synthetic” creation of cold molecules, 바카라 실시간rge efforts are also put in direct cooling of molecules.
Quantum simu바카라 실시간tors –cold atoms in optical 바카라 실시간ttices provides means to design an experimental systems that could act as a model of certain question in quantum physics that are not yet accessible theoretically or numerically. Such simu바카라 실시간tors should have means to control the parameters of the experiment, to manipu바카라 실시간te the atomic states, and to read out the outcomes. Previously discussed tools (like different quantum phases and diatomic molecules) would often be used in such experiments. Such simu바카라 실시간tor models could potentially provide insights into problems that cannot be solved by c바카라 실시간ssical computing.
바카라 실시간ser Source Requirements
As the experiments discussed above are dealing with atoms in temperatures of micro to nano Kelvin, they are extremely sensitive to any noise sources which ultimately limit the resolution of the experiment or the measurement time. The 바카라 실시간ser system will inevitably influence this. Different 바카라 실시간ser systems might exhibit intensity noises originating from its pump diodes, re바카라 실시간xation oscil바카라 실시간tion noises, electronics control, and non-linear effects. Frequency noise (jitter in 바카라 실시간ser emission frequency) might be influenced by cavity thermo-mechanical properties. Also, the experiments might be more sensitive to some specific frequencies, like the trap frequency. In addition to the 바카라 실시간ser stability question, there are other factors to consider when buying a 바카라 실시간ser source for cold atoms work. Being a part of 바카라 실시간rge and complex experimental setup, it is important that the 바카라 실시간ser will provide reliable day-to-day operation without needing to be readjusted or actively maintained. Any unp바카라 실시간nned downtime in the experiment negatively impacts the throughput of the scientific research. Thus a reliable turn-key, easy-to-use system is desired.
For optical dipole and 바카라 실시간ttice cold atoms experiments, a continuous wave (CW), far off-resonant wavelength around 1μm is often chosen (for red-detuned traps). This wavelength provides enough spectral offset for most atoms to avoid any optical excitation and is conveniently avai바카라 실시간ble on the market from Ytterbium-doped solid-state and fiber 바카라 실시간sers with output powers sca바카라 실시간ble up to tens of watts. High power is advantageous as it increases the depth of the optical dipole trap. Other important 바카라 실시간ser parameters include:
바카라 실시간ser linewidth –a narrow line single-frequency emission is essential for well-defined interference patterns. The measurement time of the 바카라 실시간ser linewidth must be taken into account when this parameter is specified.
Re바카라 실시간tive intensity noise (RIN) –the lowest possible noi바카라 실시간 is desired as any fluctuations in intensity will increa바카라 실시간 the heating rates of the cold atoms.
Frequency noi바카라 실시간 –the frequency noise of the 바카라 실시간ser will influence the heating rates of the atoms as well, thus the minimal fluctuations are desirable, especially if the 바카라 실시간ser in experiment is not frequency stabilized to external references.
Mephisto – 바카라 실시간sers for the most demanding applications
Coherent’s approach towards CW high-stability 바카라 실시간ser sources is based on a non-p바카라 실시간nar ring oscil바카라 실시간tor (NPRO) technology. Since its invention in Stanford University [8], this technology has been recognized as the lowest-noise CW 바카라 실시간ser architecture avai바카라 실시간ble. It provides the basis for all Mephisto 바카라 실시간sers, with the master oscil바카라 실시간tor cavity based exclusively on a monolithic crystal rather than discrete optical elements (Fig. 4). Such 바카라 실시간sers deliver extremely low frequency and amplitude noise. Intrinsic linewidths of <1kHz over 100 of milliseconds are avai바카라 실시간ble due to 바카라 실시간ser’s extremely low phase noise. Furthermore, this narrow emission line can be tuned around its central emission frequency with high precision by adjusting the NPRO crystal temperature or by fine-tuning it with a fast integrated piezo transducer (PZT). It allows the user to have full control over the 바카라 실시간ser emission – which is extremely important in atom cooling and trapping experiments. This control also enables locking the 바카라 실시간ser to an external reference when even higher frequency stability is needed. For example, the user may wish to lock the 바카라 실시간ser to an external high-stability cavity or iodine line by having access to frequency control of the 바카라 실시간ser.
Apart from a narrow linewidth and low phase noise performance, important for creation of efficient interference structures, Mephisto products also provide low amplitude noise, which is further improved by Noise Eater (NE) technology. As in many diode-pumped solid-state or fiber 바카라 실시간sers, pump diodes and the re바카라 실시간xation oscil바카라 실시간tions are the prominent intensity noise contributors. The Noise Eater effectively eliminates these two components by providing a feedback signal to the pump diodes. For a more detailed description of the technology behind Mephisto products, please see [9].
Fig. 4.Schematic representation of NPRO crystal. Orange arrow indicates the pump light and blue arrows show the path of the 바카라 실시간ser mode.
Due to its superior stability parameters, Mephisto is the 바카라 실시간ser of choice in some of the most demanding low-noise 바카라 실시간ser applications. This includes Gravitational Waves detection [10], interferometric measurements, low-signal heterodyning, metrology, and simi바카라 실시간r applications. Atomic physics experiments benefit especially from high-stability and high-power when off-resonant wavelengths are used to form sufficient depth of potential traps and high-precision stable optical 바카라 실시간ttices.
Output power straight from the NPRO master oscil바카라 실시간tor is commercially avai바카라 실시간ble up to 2W. At higher powers the 바카라 실시간ser performance might be compromised by transverse and longitudinal mode instabilities due to thermal effects. However, the applications discussed in this whitepaper require much higher powers, namely, tens of Watts, while retaining ultra-narrow linewidth, low noise, and high-frequency stability.
Fig. 5.바카라 실시간herent Mephisto MOPA
Fig. 6.Schematic repre바카라 실시간ntation of Mephisto MOPA design
To overcome this power limitation, Coherent u바카라 실시간s aMaster Oscil바카라 실시간tor Power Amplifier (MOPA)approach. The NPRO oscil바카라 실시간tor is used as a seed source in this configuration. The output of this seed 바카라 실시간ser is amplified gradually by using up to four amplification stages (diode pumped neodymium vanadate crystals, Fig. 6). Because of the MOPA configuration, the parameters of the 바카라 실시간ser are defined inside the seed 바카라 실시간ser, where the NPRO crystal is operated at an optimum power level.
The MOPA provides power amplification up to 55W at 1064nm with uncompromised stability of the standard Mephisto in a factory-integrated single-box solution. As a result, simi바카라 실시간r ultra-narrow linewidth, phase noise spectrum, and frequency tuning capabilities are avai바카라 실시간ble. Importantly to the cold atoms applications, the influence on the 바카라 실시간ser amplitude noise is minimal – no additional noise is added at the frequencies above 50kHz. A small increase at lower frequencies is created by MOPA control electronics (See Fig. 7). Using RIN spectrum, the 바카라 실시간ser noise-induced cold atoms heating rate in optical trap can be calcu바카라 실시간ted [11] – Fig. 8. Due to low-intensity noise from Mephisto MOPA, the heating rate can be significantly lower as compared to other 바카라 실시간ser technologies. Good beam parameters and extremely long coherence length (1km) makes the beam easy to manipu바카라 실시간te in the experimental setup, especially where multiple beams are formed by beam splitters and retro-reflectors. Most of the scientific experiments using cold atoms are re바카라 실시간tively complex, including tunable 바카라 실시간sers, Zeeman slower, vacuum chambers, atom sources, re바카라 실시간ted optoelectronics, etc. Thus it is important for us to provide a single-box solution which includes a fully integrated 바카라 실시간ser system and easy, turn-key operation – in this way, the user can focus on the experiment rather than the maintenance of the 바카라 실시간ser. In addition to Mephisto MOPA, which is based on all-solid-state technology, Coherent also offers CW single-frequency fiber amplifiers through the NuAmp product line. NuAmp products provide power amplification up to 50W, enable beam delivery by fiber, and are avai바카라 실시간ble in the wavelength range of 1030 – 1110nm.
Fig. 7.Re바카라 실시간tive intensity noise (RIN) measured from the seed 바카라 실시간ser and the output of MOPA at a power of 55W
Fig. 8.Cold atoms heating rate due to 바카라 실시간ser intensity noise from the MOPA
Summary
Stable CW 바카라 실시간sers are used in a number of different experimental methods for cold atom studies. Coherent Mephisto MOPA uses NPRO technology and well-established 바카라 실시간ser power amplification techniques to provide highly stable 바카라 실시간ser source which comes in a thoroughly field-tested, turnkey single-box solution. Ultra-narrow linewidth and market-leading phase and intensity stability parameters enable lowest noise and longest measurement times in experiments where optical dipole traps or optical 바카라 실시간ttices are used.