In-situ transfer device and method for ultra-cold atoms from optical trap to magnetic trap

Abstract

The invention discloses an in-situ transfer device and method for ultra-cold atoms from an optical trap to a magnetic trap, and belongs to the field of in-situ transfer of the ultra-cold atoms. The device comprises a crossed optical dipole trap, a quadrupole magnetic field, bias magnetic fields in x, y and z directions and a time sequence control system, by realizing high-efficiency and low-disturbance in-situ transfer of the ultra-cold atomic group, the key problems of heating de-coherence, atom loss and the like caused by atomic position change in a traditional method are successfully avoided; various problems caused by a mechanical atom transfer mode are eliminated through combination of dynamic magnetic field compensation and adiabatic transfer; through the technical means of atom pre-polarization and adiabatic transfer, the transfer efficiency is greatly improved; the system structure is greatly simplified, and the problem of coil heating is reduced; according to the method, the problems of vibration, efficiency, complexity and the like existing in a mechanical atom transfer mode are solved, and a solid foundation is laid for subsequent precise quantum experiments; the method is used for in-situ transfer of ultra-cold atoms.

Description

Technical Field

[0001] This invention relates to in-situ transfer technology of ultracold atoms, specifically to an in-situ transfer device and method for ultracold atoms from an optical trap to a magnetic trap. Background Technology

[0002] Ultracold atom systems (Bose-Einstein condensates) have become an ideal platform for studying quantum many-body physics, novel states of matter, and quantum information due to their purity, high manipulability, and significant quantum effects. In many quantum precision experiments, the environment in which ultracold atoms are prepared (such as magneto-optical trap cooling or evaporative cooling) often differs from the experimental environment. Ultracold atoms are prepared and confined in a specific region (e.g., an environment with extremely high vacuum and minimal background gas interference), while the atomic clusters are manipulated and measured in another region. The preparation region may require a high vacuum to prevent the atoms from being heated by background gas scattering, and requires specific magnetic and optical field environments, while the manipulation and measurement region may require specially designed optical windows to achieve high-resolution imaging and precise spectroscopic measurements.

[0003] To achieve atomic transfer in different regions to meet experimental needs, the following mechanical transfer schemes are commonly used to move atomic groups: Moving optical dipole trap (ODT) technology uses a focused, red-detuned laser beam to trap atoms. The laser focus is changed in space by mechanically moving a platform housing a focusing lens, thereby dragging the trapped atomic cluster. The lens is mounted on a high-precision linear translation stage, and its movement direction must be precisely aligned with the incident parallel laser beam. After a Bose-Einstein condensate (BEC) is obtained in the main cavity, the ODT laser power is slowly increased while the magnetic trap is reduced and shut off, adiabatically transferring the BEC to the static ODT. Then, by controlling the translation stage to move the lens in a predetermined manner, the focus of the ODT is moved, thus dragging the BEC into the scientific cavity.

[0004] Magnetic guided transport technology utilizes a series of anti-Helmholtz coil pairs. By sequentially controlling the current in each pair of coils, a moving magnetic field zero is generated. Atoms in a low-field seeking state at the initial position are trapped near this zero and guided to the target position as the zero moves. The coil system consists of multiple pairs of anti-Helmholtz coils arranged in a straight line along a predetermined transport path, mounted on a dedicated coil platform. The scientific cavity is located at the beginning of the transport path, surrounded by a coil cube for the evaporative cooling and confinement of atoms.

[0005] Mechanical translation magnetic trap transfer technology moves the magnetic trap by physically moving a coil using a linear translation stage. The track of the translation stage is parallel to the vacuum chamber, and a slender tube connects the magneto-optical trap (MOT) cavity and the vacuum chamber. Atoms are trapped in the magnetic trap within the tube, so as the coil slowly moves mechanically, the atoms are dragged along with it. This technique is used to transfer atomic clusters from the MOT cavity to the scientific cavity for evaporative cooling to prepare BECs and conduct subsequent measurement experiments.

[0006] Atom-on-a-chip transport technology utilizes microfabrication techniques to create photolithographic wire structures on the surface of a chip. By passing current through these wires, a magnetic field gradient is generated at a height of micrometers above their surface, thus forming magnetic traps to imprison atoms. By changing the magnitude and direction of the current in different wires, the position of the magnetic traps can be moved to achieve displacement control of the condensate.

[0007] While existing mechanical atom transfer techniques can achieve long-distance atom transport, they generally suffer from several significant drawbacks. First, vibration-induced heating and atom loss are the most prominent problems in moving optical dipole trap technology. This stems from the vibrations generated during the movement of the mechanical translation stage used to move the focusing lens. These vibrations are transmitted into the optical trap, causing heating of the trapped atoms.

[0008] Similarly, the physical moving magnetic trap coil transfer technology also suffers from low transmission efficiency and atom loss. The main reason is the conflict between mechanical alignment precision and the space constraints of the transfer channel: the atom cloud (approximately 2.2 mm in diameter) must pass through a connecting channel with an inner diameter of only 8 mm to maintain the vacuum difference. Any minute mechanical misalignment will cause the atom cloud to collide with the channel inlet, resulting in significant atom loss. This exposes the technology's lack of flexibility and low fault tolerance.

[0009] The problems with magnetic guidance transmission technology are its high system complexity and significant heat generation. This technology relies on the sequential operation of multiple sets of anti-Helmholtz coils to generate a moving magnetic trap. Although there are no moving parts, the coil system is large and complex, and it generates a large amount of Joule heat, requiring an integrated water-cooling system for cooling; otherwise, the magnetic field will become unstable. Furthermore, precisely controlling the timing of the currents in the multiple sets of coils to smoothly move the magnetic trap to its zero point places extremely high demands on the control system, increasing system complexity and failure rate.

[0010] In summary, the shortcomings of existing mechanical transfer technologies mainly stem from the inherent contradiction between "macroscopic mechanical motion" and "microscopic quantum state stability". Summary of the Invention

[0011] To address the issue in existing technologies where atomic positions cannot be moved, 87This invention addresses the critical challenge of efficiently and with minimal disturbance transferring ultracold Rb atoms from an initial trapped potential well (optical trap) to a target region in another type of trap (magnetic trap). It proposes an in-situ transfer device and method for ultracold atoms from an optical trap to a magnetic trap. By precisely controlling the potential trap parameters, it achieves accurate switching of the potential trap where the ultracold atom cluster is located, avoiding the heating decoherence and atom loss problems caused by changes in the position of the atom cluster during the atom transfer process in traditional techniques. This provides a crucial technical foundation for the serialization operation and high-precision measurement of complex quantum experiments.

[0012] One object of the present invention is to provide an in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap.

[0013] The in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap according to the present invention includes: a cross-optical dipole trap, a quadrupole magnetic field, bias magnetic fields in the x, y, and z directions, and a timing control system; wherein, two intersecting laser beams form a cross-optical dipole trap located in a horizontal plane, and the two laser beams are perpendicular to each other; the quadrupole magnetic field generates a linear magnetic field gradient along the axial direction, i.e., the z-direction; the light from the cross-optical dipole trap passes through the center of the bias magnetic fields in the x and y directions; the cross-optical dipole trap, the quadrupole magnetic field, and the bias magnetic fields in the x, y, and z directions are respectively connected to the timing control system; the timing control system controls the intensity of the cross-optical dipole trap and the quadrupole magnetic field, x, y, and z-direction bias magnetic fields in stages. The strength of the bias magnetic field is controlled; optical aggregates are bound at the center of the cross-optical dipole trap; in-situ transfer includes: the initial evaporation cooling and atomic polarization state preparation stage, the secondary evaporation cooling and atomic in-situ transfer stage, and the dynamic compensation and holding stage; in the initial evaporation cooling and atomic polarization state preparation stage, the timing control system continuously reduces the strength of the cross-optical dipole trap to evaporate and cool the atoms to a predetermined temperature. At the same time, the timing control system controls the strength of the quadrupole magnetic field in stages. The quadrupole magnetic field generates a quantization axis along the z-direction to pre-polarize the atoms, causing atoms of different magnetic level energy to split and be in an atomic polarization state m. FAtoms in the -1 state become the energy level retained during the evaporation and cooling process. As the evaporation and cooling process progresses, more atoms remain in the -1 state. During this process, the strength of the bias magnetic field in the z-direction changes synchronously with the strength of the quaternary magnetic field to prevent atomic cluster leakage caused by drastic changes in the magnetic field. In the initial evaporation and cooling and atomic polarization state preparation stage, atoms are evaporated and cooled to a predetermined temperature. The optically bonded clusters after evaporation and cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters. In the secondary evaporation and cooling and in-situ atomic transfer stage, the timing control system further reduces the strength of the cross optical dipole trap. Based on the adiabatic principle, the change in the strength of the cross optical dipole trap is set. The rate of change of the potential well parameters of the cross optical dipole trap is much smaller than the characteristic frequency of the system, adiabatically following the change in the potential energy surface, avoiding non-adiabatic transitions and energy excitation. At the same time, the timing control system controls the strength of the quaternary magnetic field and the strength of the bias magnetic fields in the x, y, and z directions in stages. The quaternary magnetic field generates a magnetic field zero point. The strength of the bias magnetic fields in the x and y directions controls the position of the magnetic field zero point in the x and y directions, respectively, while the strength of the bias magnetic field in the z direction controls the position of the magnetic field zero point along the z-axis. The radial and vertical potential energy distributions are calculated based on the magnetic field gradient along the z direction and the strengths of the bias magnetic fields in the x, y, and z directions. This yields the distance between the magnetic field zero point and the optical trap, determining the position of the magnetic field zero point and ensuring gravitational compensation and force balance for the atoms. Under the combined action of the bias magnetic fields in the x, y, and z directions and the quadrupole magnetic field, the potential trap generated by the magnetic field becomes the magnetic trap. The strength of the cross optical dipole trap is zero, and the magnetic field maintains its strength, keeping the ultracold atomic cluster suspended and stationary. This allows the ultracold atomic cluster to be transferred from the optical trap to the magnetic trap. During the dynamic compensation and maintenance phase, the Bose-Einstein condensate is only affected by the quadrupole magnetic field. By precisely adjusting the position of the magnetic field zero point, it is aligned directly above the center of the cross optical dipole trap, maintaining its vertical position without any optical constraints.

[0014] The quadrupole magnetic field includes a pair of coils, with the direction of the pair of coils along the z-direction; the bias magnetic fields in the x, y, and z directions each include a pair of coils along the x, y, and z directions, respectively; the strength of the corresponding magnetic field is controlled by controlling the magnitude of the current flowing through the coils.

[0015] In a gravitational field, an atom experiences a downward force mg. The quadrupole magnetic field is a non-harmonic oscillation, and its potential energy expression is:

[0016] in, B The magnetic field strength, μ B For Bohr magneton, r Let this be the radial position of the zero point of the magnetic field in the given cylindrical coordinate system. B ′ is the magnetic field gradient along the z-direction generated by the quadrupole magnetic field, where z-direction is the axial direction. B0 represents the strength of the bias magnetic field in the z-direction, used to adjust the position of the magnetic field zero point along the z-direction. and These are the radial and perpendicular distances between the Bose-Einstein condensate (BEC) and the zero point of the magnetic field, respectively. At any position in the quadrupole magnetic field, the perpendicular potential energy... U z The distribution is as follows:

[0017] exist In the ideal case where =0, the z-related terms in the square root can be extracted separately. In this case, the potential energy distribution in the z-direction is linear, and the atomic polarization state m... F The gradient force experienced by an atom in the -1 state is a constant force. μ B B '.when At that time, near the zero point of the magnetic field, U z It is approximately the potential energy of a simple harmonic oscillation, and the range of the potential energy of a simple harmonic oscillation is approximately Order of magnitude, radial potential energy U r The distribution is as follows:

[0018] By precisely adjusting the magnetic field gradient along the z-direction generated by the quadrupole magnetic field B 'And the position of the zero point of the magnetic field, based on the interaction between the atomic magnetic moment and the magnetic field gradient, and the total angular momentum quantum number of the atom.' F =1 and atomic polarization state m F The upward magnetic force generated by atoms in the -1 state F m = μ B B This is exactly equal to the gravity mg of the ultracold atomic group, where m is the mass of the ultracold atomic group and g is the gravitational acceleration. At this point, the net force on the atoms in the direction of gravity is almost zero, achieving an equivalent microgravity environment.

[0019] The zero-point position of the magnetic field generated by the quadrupole magnetic field is determined by the magnetic field gradient in the z-direction generated by the quadrupole magnetic field. B The strength of the bias magnetic field in the z-direction Jointly determined. The strength of the bias magnetic field in the z-direction is controlled independently and precisely. Without significantly changing the magnetic field gradient in the z-direction B That is, without violating the gravity compensation conditions, the position of the zero point of the magnetic field is moved in three dimensions.

[0020] In this invention, atoms are stabilized in an atomic polarization state. In this state, the magnetic force can counteract gravity. If an atom undergoes a Majorana transition, it will move from the atomic polarization state. State change to m F In the =0 or +1 states, the magnetic field force of atoms in these states cannot balance gravity, disrupting the gravitational compensation effect and leading to a loss of BEC atoms. This invention actively stabilizes atoms at a position set away from the zero point of the magnetic field, where the magnetic field strength is sufficiently large and the potential energy change is gradual, thereby effectively avoiding atomic loss caused by Majorana transitions.

[0021] The initial evaporation cooling and atomic polarization state preparation stages include five phases: In the first phase, lasting 15–25 ms, the intensity of the cross-optical dipole trap is continuously reduced under the control of a timing control system, and the laser intensity follows an exponential decrease curve; in the second phase, lasting 450–550 ms, the intensity of the quadrupole magnetic field increases linearly, and the quadrupole magnetic field generates a quantization axis along the z-axis, realizing the splitting of atoms at different magnetic level energy levels; the atom is split into atomic polarization states m. F =-1、m F =0 and m F =+1 state, in m F Atoms in the =0 state are unaffected by magnetic fields and are therefore only subject to gravity; while atoms in the m state... F The magnetic force on atoms in the +1 state is in the same direction as gravity; in contrast, m F The magnetic force on atoms in the -1 state can partially counteract gravity, causing more atoms to remain in the -1 state. In the second to fourth stages, the strength of the crossed optical dipole trap continues to decrease exponentially. The quadrupole magnetic field and the bias magnetic field in the z-direction increase linearly in the second stage, making... With a sufficient number of atoms in the quadrupole magnetic field (up to 90%), the strength of the quadrupole magnetic field remains constant for 2500-3500 ms in the third stage to ensure complete separation of atoms from different magnetic level energy levels and achieve pre-polarization. In the fourth stage, lasting 500-1500 ms, the magnetic field strength gradually decreases linearly to prevent drastic changes in the magnetic field environment caused by instantaneous magnetic field shutdown from affecting the atoms. From the second to the fourth stage, the quadrupole magnetic field pre-polarizes the atoms, and the strength of the bias magnetic field in the z-direction changes synchronously with the strength of the quadrupole magnetic field to prevent atomic cluster leakage caused by drastic changes in the magnetic field. The bias magnetic field strength in the y direction remains unchanged; the longer the quadrupole magnetic field is applied during the evaporation and cooling process, the higher the proportion of atoms in the -1 state will be; the fifth stage lasts for 40~60ms, during which the quadrupole magnetic field and the bias magnetic field strength in the x, y and z directions are all zero, the intensity of the cross-coupled optical trap continues to decrease with an e-exponential curve, and the atoms evaporate and cool to the predetermined temperature of 80~120nK. The optical clusters after evaporation and cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters; the fifth stage ends, at which point the optical power of the cross optical dipole trap is about 0.05~0.1W.

[0022] The secondary evaporation-cooling and in-situ atom transfer stage includes a sixth stage: During this stage, lasting 450–550 ms, the timing control system controls the intensity of the cross-optical dipole trap to decrease again in a linear fashion with a constant slope, reducing the light intensity of the cross-optical dipole trap to zero, thus initiating the second evaporation-cooling process. This process transfers the Bose-Einstein condensate from the cross-optical dipole trap to the quadrupole magnetic trap while minimizing the heating effect. Simultaneously, the quadrupole magnetic field is gradually strengthened, linearly increasing over approximately 500 ms, generating a force that gradually compensates for gravity. Meanwhile, the intensity of the cross-optical dipole trap continues to decrease slowly, effectively reducing the trap depth. This dual-control mechanism… This method allows the remaining atoms to undergo further evaporative cooling. Higher-energy atoms will escape from the trap, thereby lowering the temperature of the atomic cluster to 40-60 nK. The evaporative cooling process continues until the intensity of the cross-optical dipole trap drops to zero. At this point, the Bose-Einstein condensate has been completely transferred to the magnetic trap. Throughout the evaporative cooling process, the z-axis bias magnetic field increases and decreases proportionally with the quadrupole field to prevent drastic changes in the zero point of the magnetic field. At the end of this stage, the gradient of the quadrupole magnetic field is strong enough to exert a force on the atoms that can counteract the effect of gravity, thereby effectively suspending the atoms in a dynamic equilibrium state. At the same time, the ultracold atoms have completed the in-situ transfer from the optical trap to the magnetic trap.

[0023] Secondary evaporative cooling is crucial for maintaining the coherence of the BEC during transfer. Without this additional evaporative cooling step, the transfer from the optical trap to the magnetic trap would result in a significant heating effect, which could potentially compromise the integrity of the condensate. By gradually adjusting the magnetic field and the depth of the crossed optical dipole trap, it is possible to ensure that the BEC remains at a low temperature, providing suitable conditions for subsequent microgravity experiments.

[0024] Dynamic Compensation and Maintenance Phase: After the sixth phase, the Bose-Einstein condensate is only affected by a quadrupole magnetic field. By precisely adjusting the position of the magnetic field zero point to align it directly above the center of the crossed optical dipole trap, its vertical position is maintained without any optical constraints. In the experimental setup of this invention, two pairs of orthogonal bias magnetic fields in the x and y directions are placed in a horizontal plane. The bias magnetic fields in the x and y directions dynamically control the magnetic field zero point in the horizontal plane, continuously adjusting its position relative to the BEC. This dynamic adjustment ensures that the magnetic field zero point is always directly above the atom, guaranteeing that the atom is unaffected by gravity throughout the process. Precise measurements and quantum simulations can be performed under microgravity-like conditions, significantly expanding the experimental application possibilities of Bose-Einstein condensates.

[0025] Another objective of this invention is to propose an in-situ transfer method for ultracold atoms from an optical trap to a magnetic trap.

[0026] The in-situ transfer method of ultracold atoms from an optical trap to a magnetic trap according to the present invention includes the following steps: 1) The optical cluster is confined at the center of the cross optical dipole trap; 2) Initial evaporation cooling and atomic polarization state preparation stage: During the initial evaporation cooling and atomic polarization state preparation stage, the timing control system continuously reduces the intensity of the cross optical dipole trap to evaporate and cool the atoms to the predetermined temperature. Simultaneously, the timing control system controls the strength of the quadrupole magnetic field in stages. The quadrupole magnetic field generates a quantization axis along the z-direction, pre-polarizing the atoms and causing atoms at different magnetic level energy levels to split and enter an atomic polarization state m. F Atoms in the -1 state become the energy level that remains during the evaporative cooling process. As the evaporative cooling process continues, more atoms will remain in the -1 state. During this process, the strength of the bias magnetic field in the z direction changes synchronously with the strength of the fourth-order magnetic field to prevent the leakage of atomic clusters caused by drastic changes in the magnetic field; in the initial evaporation cooling and atomic polarization state preparation stage, the atoms are evaporated and cooled to the predetermined temperature, and the optical clusters after evaporation cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters. 3) Secondary evaporation cooling and in-situ atomic transfer stage: During the secondary evaporation cooling and in-situ atomic transfer stages, the timing control system further reduces the intensity of the cross-optical dipole trap. Based on the adiabatic principle, the intensity of the cross-optical dipole trap is set to change. The rate of change of the potential well parameters of the cross-optical dipole trap is much smaller than the characteristic frequency of the system. It adiabatically follows the change of the potential energy surface, avoiding non-adiabatic transitions and energy excitation. Simultaneously, the timing control system controls the intensity of the four-level magnetic field and the intensity of the bias magnetic field in the x, y and z directions in stages. The four-level magnetic field generates a magnetic field zero point. The intensity of the bias magnetic field in the x and y directions controls the position of the magnetic field zero point in the x and y directions, respectively, and the intensity of the bias magnetic field in the z direction controls the position of the magnetic field zero point along the z axis. The radial and vertical potential energy distributions are calculated based on the magnetic field gradient along the z-direction and the strengths of the bias magnetic fields in the x, y, and z directions. The distance between the magnetic field zero point and the optical trap is obtained, and the position of the magnetic field zero point is determined so that the atoms are compensated for by gravity and are in force balance. Under the combined action of the bias magnetic fields in the x, y, and z directions and the quadrupole magnetic field, the potential trap generated by the magnetic field is the magnetic trap. The strength of the cross optical dipole trap is zero, and the magnetic field maintains its strength, so that the ultracold atom cluster remains suspended in place, and the ultracold atom cluster is transferred from the optical trap to the magnetic trap in situ. 4): Dynamic compensation and maintenance phase: During the dynamic compensation and maintenance phase, the Bose-Einstein condensate is only subjected to a quadrupole magnetic field; by precisely adjusting the position of the magnetic field zero point, the magnetic field zero point is aligned directly above the center of the cross optical dipole trap; its vertical position is maintained without any optical constraints.

[0027] This invention eliminates all mechanical movement of components, achieving in-situ atomic transfer through the manipulation of magnetic and optical fields, thus fundamentally eliminating heating and disturbances caused by mechanical vibrations. It directly utilizes the coil framework of existing magneto-optical trap systems, eliminating the need for complex multi-coil magnetic guidance systems or moving platforms, significantly simplifying the system structure and reducing power consumption and thermal management requirements. By adjusting the position of the optical trap center relative to the magnetic field zero point, actively moving it away from the zero point, Majorana transitions of atoms are avoided, significantly extending the lifetime of Bose-Einstein condensates. Dynamic magnetic field zero-point control enables this technology not only for static levitation but also for unprecedented dynamic compensation capabilities, opening up new possibilities for studying quantum effects in moving ultracold atomic systems.

[0028] Advantages of this invention: This invention achieves highly efficient and low-disturbance in-situ transfer of ultracold atomic groups, successfully avoiding key problems such as heating decoherence and atomic loss caused by changes in atomic position in traditional methods. This significantly improves the purity and efficiency of quantum state preparation, providing crucial technical support for multi-step quantum simulation, quantum precision measurement, and quantum computing experiments, and greatly expanding the operability and research potential of the ultracold atom platform.

[0029] This invention eliminates the various problems associated with mechanical atom transfer by combining dynamic magnetic field compensation with adiabatic transfer. Unlike moving optical trap technology which physically moves lenses or moving magnetic trap technology which physically moves coils, this invention requires no movement of any macroscopic components throughout the process. By precisely controlling the strength of the quadrupole magnetic field and the bias magnetic field in the z-direction, and dynamically adjusting the magnetic field gradient and zero-point position, atoms can complete the potential well change in an extremely stable environment to achieve in-situ atom transfer, perfectly maintaining their phase space density and quantum coherence.

[0030] Secondly, this invention significantly improves transfer efficiency through atomic pre-polarization and adiabatic transfer techniques. This technology pre-prepares atoms to a specific m-shape during the initial stage of optical trap evaporation and cooling by briefly applying a magnetic field. F The -1 magnetic level ensures that nearly 100% of the atoms are in the target state where they can be levitated. Then, over a period of 500 milliseconds, the magnetic field gradient is slowly increased while the strength of the crossed optical dipole trap is decreased, achieving a smooth atomic transfer. This design avoids drastic kinetic processes, minimizing heating and atomic loss.

[0031] Finally, this invention significantly simplifies the system structure and reduces coil heating. Unlike magnetic transfer technology, which requires multiple high-current coils and a complex water-cooling system, this technology achieves in-situ transfer of ultracold atoms by controlling a quadrupole magnetic field, three pairs of bias magnetic fields, and a pair of cross-optical couplers within the magneto-optical trap system. Therefore, the magnetic field coil generates little heat, eliminating the need for an additional complex cooling system and reducing system complexity and power consumption. Simultaneously, the atomic gravity compensation technology stabilizes atoms in a specific state at a position approximately 500 micrometers from the magnetic field zero point, effectively avoiding atomic loss caused by Majorana transitions and enhancing the ability to manipulate atomic states.

[0032] This invention, by transforming the "mechanical movement" of atoms into "in-situ transfer," not only solves the problems of vibration, efficiency, and complexity inherent in mechanical atom transfer methods, but also creates a highly stable, controllable, and efficient transfer process, laying a solid foundation for subsequent precision quantum experiments. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of an embodiment of the in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap according to the present invention; Figure 2 This is a timing control diagram of an embodiment of the in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap according to the present invention; Figure 3 This diagram illustrates the effect of the duration of the quadrupole magnetic field on the particle number distribution of different magnetic level in the in-situ transfer device of the ultracold atom from the optical trap to the magnetic trap according to the present invention. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0035] like Figure 1 As shown, the in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap in this embodiment includes: a cross-optical dipole trap, a quadrupole magnetic field, bias magnetic fields in the x, y, and z directions, and a timing control system. The cross-optical dipole trap, formed by two intersecting laser beams, is located in a horizontal plane and is perpendicular to each other. The quadrupole magnetic field generates a linear magnetic field gradient along the axial direction (z). The light from the cross-optical dipole trap passes through the center of the bias magnetic fields in the x and y directions. The cross-optical dipole trap, the quadrupole magnetic field, and the bias magnetic fields in the x, y, and z directions are respectively connected to the timing control system. The timing control system controls the intensity of the cross-optical dipole trap and the intensity of the quadrupole magnetic field and the bias magnetic fields in the x, y, and z directions in stages. A camera images the atoms through an imaging optical path, capturing an atomic state diagram.

[0036] The quadrupole magnetic field includes a pair of coils, with the direction of the pair of coils along the z-direction; the bias magnetic fields in the x, y, and z directions each include a pair of coils along the x, y, and z directions, respectively; the strength of the corresponding magnetic field is controlled by controlling the magnitude of the current flowing through the coils.

[0037] In the initial stage, optical molases provided by the pre-cooling of the preceding two-dimensional magneto-optical trap (2D MOT) or three-dimensional magneto-optical trap (3DMOT) are loaded into the cross-optical dipole trap to initially form Bose-Einstein condensates through evaporative cooling.

[0038] The timing control system is used for precise programming and real-time adjustment and control of the magnitude and timing sequence of the current in the coils of the quadrupole magnetic field and the bias magnetic fields in the x, y and z directions, so as to realize feedback management of the entire transfer and compensation process. Figure 2 The timing control curves for each structure are shown.

[0039] In a gravitational field, an atom experiences a downward force mg. The quadrupole magnetic field is a non-harmonic oscillation, and its potential energy expression is:

[0040] in, B The magnetic field strength, μ B For Bohr magneton, r Let this be the radial position of the zero point of the magnetic field in the given cylindrical coordinate system. B ′ is the magnetic field gradient along the z-direction generated by the quadrupole magnetic field, where z-direction is the axial direction. B 0 represents the strength of the bias magnetic field in the z-direction, used to adjust the position of the magnetic field zero point along the z-direction. and These are the radial and perpendicular distances between the Bose-Einstein condensate (BEC) and the zero point of the magnetic field, respectively. At any position in the quadrupole magnetic field, the perpendicular potential energy... U z The distribution is as follows:

[0041] exist In the ideal case where =0, the z-related terms in the square root can be extracted separately. In this case, the potential energy distribution in the z-direction is linear, and the atomic polarization state m... F The gradient force experienced by an atom in the -1 state is a constant force. μ B B '.when At that time, near the zero point of the magnetic field, U z It is approximately the potential energy of a simple harmonic oscillation, and the range of the potential energy of a simple harmonic oscillation is approximately Order of magnitude, radial potential energy Ur The distribution is as follows:

[0042] By precisely adjusting the magnetic field gradient along the z-direction generated by the quadrupole magnetic field B 'And the position of the zero point of the magnetic field, based on the interaction between the atomic magnetic moment and the magnetic field gradient, for a specific state (the total angular momentum quantum number of the atom is used in this invention). F =1, atomic polarization state m F The upward magnetic force generated by atoms in the -1 state F m = μ B B This is exactly equal to the gravity mg of the ultracold atomic group, where m is the mass of the ultracold atomic group and g is the gravitational acceleration. At this point, the net force on the atoms in the direction of gravity is almost zero, achieving an equivalent microgravity environment.

[0043] The zero-point position of the magnetic field generated by the quadrupole magnetic field is determined by the magnetic field gradient in the z-direction generated by the quadrupole magnetic field. B The strength of the bias magnetic field in the z-direction Jointly determined. The strength of the bias magnetic field in the z-direction is controlled independently and precisely. Without significantly changing the magnetic field gradient in the z-direction B That is, without violating the gravity compensation conditions, the position of the zero point of the magnetic field is moved in three dimensions.

[0044] When a neutral atom is near the zero point of the magnetic field, it is prone to Majorana transitions, causing the atom to escape from the target bound state and resulting in atomic loss. In the configuration of this invention, the atom needs to be stabilized in an atomic polarization state. In this state, the magnetic force can counteract gravity. If an atom undergoes a Majorana transition, it will move from the atomic polarization state. State change to m F In the =0 or +1 states, the magnetic field force of atoms in these states cannot balance gravity, disrupting the gravitational compensation effect and leading to a loss of BEC atoms. This invention actively stabilizes atoms at a position approximately 500 μm from the zero point of the magnetic field, where the magnetic field strength is sufficiently large and the potential energy change is gradual, thereby effectively avoiding atomic loss caused by Majorana transitions.

[0045] The initial evaporation cooling and atomic polarization state preparation stages include five phases: In the first phase, t1, lasting approximately 20 ms, the timing control system continuously reduces the intensity of the cross-optical dipole trap, and the laser intensity follows an exponential decrease curve. In the second phase, t2, lasting 500 ms, the intensity of the quadrupole magnetic field increases linearly, generating a quantization axis along the z-axis, thus splitting atoms into different magnetic level energy states (m). F =-1、m F =0 and m F In the +1 state, because the magnetic field zero point is tuned directly above the optical trap, atoms at each energy level experience different trap depths in the magnetic trap and the optical trap, and are in the m state. F Atoms in the =0 state are unaffected by a magnetic field and are therefore only subject to gravity. Gravity causes the optical trap potential energy to tilt, significantly reducing the actual trap depth compared to the nominal trap depth; while atoms in the m state... F The magnetic force on atoms in the +1 state is aligned with the direction of gravity, which further exacerbates the tilt of the optical trap, resulting in the shallowest effective trap depth; in contrast, m F The magnetic force on atoms in the -1 state can partially counteract gravity. This counteracting effect increases the effective trap depth, making the -1 state the most favorable energy level for atomic retention during evaporation and cooling. Therefore, as the evaporation and cooling process progresses, more atoms will remain in the -1 state; Figure 2 As shown, this process in the time series is manifested as follows: in the second to fourth stages (t2-t4), the intensity of the optical trap continuously decreases with an exponential e-curve; in the third stage, the magnetic field gradient B2′ of the quadrupole magnetic field increases to 75 Gs / cm; in the third stage (t3), the intensity of the quadrupole magnetic field remains constant for 3000 ms, ensuring complete separation of atoms from different magnetic level magnetic fields; in the third stage, the control voltage V2(z) of the z-direction bias magnetic field remains constant, assisting the quadrupole magnetic field; in the fourth stage (t4), the magnetic field intensity gradually decreases linearly for 1000 ms, preventing drastic changes in the magnetic field environment caused by instantaneous magnetic field shutdown from affecting the atoms; in the second to fourth stages, the quadrupole magnetic field pre-polarizes the atoms, ultimately making them... The proportion of atoms in the state reaches more than 95%. The strength of the bias magnetic field in the z direction changes synchronously with the strength of the fourth magnetic field to prevent the leakage of atomic clusters caused by drastic changes in the magnetic field. The strength of the bias magnetic field in the x and y directions remains unchanged. Figure 3 The image above is m F The proportion of atoms in the -1 state n The graph showing the effect of the quadrupole magnetic field duration on the particle number distribution of different magnetic level energy levels is as follows: the longer the quadrupole magnetic field is applied during the evaporation and cooling process, the higher the proportion of atoms ultimately in the -1 state. As the proportion of the duration during which the quadrupole magnetic field strength remains constant in the third stage relative to the initial evaporation and cooling and atomic polarization state preparation stages (t1-t5) increases, the magnetic field duration also increases.F The proportion of atoms in the -1 state increases, from Figure 3 As shown in the figure above, at points a (10%), b (70%), and c (100%), the proportion of atoms in the -1 state gradually increases. Figure 3 The following figure shows the atomic state diagrams captured by the camera at points a, b, and c, respectively. In the fifth stage t5, which lasts for 50 ms, the quadrupole magnetic field and the bias magnetic fields in the x, y, and z directions are all zero. The intensity of the cross-coupled optical trap continues to decrease with an e-exponential curve. The atoms evaporate and cool to the predetermined temperature of 100 nK. The optical clusters after evaporation and cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters.

[0046] The secondary evaporation-cooling and in-situ atom transfer stage includes a sixth stage: In stage t6, lasting 500 ms, the timing control system controls the intensity of the crossed optical dipole trap to decrease again in a linear fashion with a constant slope, performing a second evaporation-cooling process to transfer the Bose-Einstein condensate from the crossed optical dipole trap to the quadrupole magnetic trap, while ensuring minimal heating effects; simultaneously, the quadrupole magnetic field is gradually increased, such as... Figure 2 As shown in the timing diagram, the quadrupole magnetic field linearly increases within approximately 500 milliseconds, and the force generated gradually compensates for gravity. Ultimately, the magnetic field gradient B1′ of the quadrupole magnetic field in the fifth stage is 30.5 Gs / cm. Simultaneously, the strength of the cross-optical dipole trap continues to decrease slowly, effectively reducing the depth of the optical trap. This dual-control method allows the remaining atoms to undergo further evaporative cooling, with higher-energy atoms escaping from the trap, thereby further reducing the temperature of the atomic cluster to 50 nK. The evaporative cooling process continues until the strength of the cross-optical dipole trap drops to zero. At this point, the Bose-Einstein condensate has been completely transferred to the magnetic trap. Throughout the evaporative cooling process, the z-axis bias magnetic field increases and decreases proportionally with the quadrupole field to prevent drastic changes in the magnetic field zero point. At the end of this stage, the gradient of the quadrupole magnetic field is strong enough to exert a force on the atoms that can counteract gravity, thereby effectively suspending the atoms in a dynamic equilibrium state. At the same time, the ultracold atoms have completed the in-situ transfer from the optical trap to the magnetic trap.

[0047] The second stage of evaporative cooling is crucial for maintaining the coherence of the BEC during transfer. Without this additional evaporative cooling step, the transfer from the optical trap to the magnetic trap would result in a significant heating effect, which could potentially compromise the integrity of the condensate. By gradually adjusting the magnetic field and the depth of the cross-optical dipole trap, the BEC can be kept at a low temperature, providing suitable conditions for subsequent microgravity experiments.

[0048] Dynamic compensation and holding stage: After the sixth stage t6, the Bose-Einstein condensate is only subjected to the quadrupole magnetic field; by precisely adjusting the position of the magnetic field zero point to align it directly above the center of the cross optical dipole trap, the atoms are stabilized at a position about 500 μm away from the magnetic field zero point; maintaining its vertical position without any optical restraint, the control voltage V1(z) of the bias magnetic field in the z direction remains unchanged during the dynamic compensation and holding stage, assisting the quadrupole magnetic field; the control voltages V(x) and V(y) of the bias magnetic fields in the x and y directions adjust the position of the atoms in the x and y directions.

[0049] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. An in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap, characterized in that, The in-situ transfer device includes: a cross-optical dipole trap, a quadrupole magnetic field, bias magnetic fields in the x, y, and z directions, and a timing control system; wherein, the two intersecting lasers form the cross-optical dipole trap located in a horizontal plane, and the two lasers are perpendicular to each other; the quadrupole magnetic field generates a linear magnetic field gradient along the axial direction, i.e., the z-direction; the light from the cross-optical dipole trap passes through the center of the bias magnetic fields in the x and y directions; the cross-optical dipole trap, the quadrupole magnetic field, and the bias magnetic fields in the x, y, and z directions are respectively connected to the timing control system; the timing control system controls the intensity of the cross-optical dipole trap and the intensity of the quadrupole magnetic field and the bias magnetic fields in the x, y, and z directions in stages; The optical clusters are confined at the center of the cross optical dipole trap; the in-situ transfer includes: the initial evaporation cooling and atomic polarization state preparation stage, the secondary evaporation cooling and atomic in-situ transfer stage, and the dynamic compensation and holding stage; in the initial evaporation cooling and atomic polarization state preparation stage, the optical clusters after evaporation cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters; in the secondary evaporation cooling and atomic in-situ transfer stage, the ultracold atomic clusters are transferred from the optical trap to the magnetic trap in situ; in the dynamic compensation and holding stage, the Bose-Einstein condensate is only affected by the quadrupole magnetic field and maintains its vertical position without any optical confinement.

2. The in-situ transfer device according to claim 1, characterized in that, During the initial evaporation-cooling and atomic polarization state preparation stage, the timing control system continuously reduces the intensity of the cross-optical dipole trap to evaporate and cool the atoms. Simultaneously, the timing control system controls the intensity of the quadrupole magnetic field in stages. This quadrupole magnetic field generates a quantization axis along the z-direction, pre-polarizing the atoms and causing the atoms at different magnetic level energy levels to split and enter an atomic polarization state. F Atoms in the -1 state become the energy levels retained during the evaporation and cooling process. As the evaporation and cooling process progresses, more atoms will remain in the -1 state. During this process, the strength of the bias magnetic field in the z direction changes synchronously with the strength of the fourth magnetic field. The optical clusters after evaporation and cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters.

3. The in-situ transfer device according to claim 2, characterized in that, During the secondary evaporation cooling and in-situ atomic transfer stage, the timing control system further reduces the intensity of the cross-optical dipole trap, setting the intensity variation based on the adiabatic principle. Simultaneously, the timing control system controls the intensity of the quadrupole magnetic field and the intensity of the bias magnetic fields in the x, y, and z directions in stages. The quadrupole magnetic field generates a magnetic field zero point, and the position of the magnetic field zero point is adjusted by controlling the intensity of the bias magnetic field. The distance between the magnetic field zero point and the optical trap is calculated, and the position of the magnetic field zero point is determined, so that the atoms are compensated by gravity and balanced by forces. Under the combined action of the bias magnetic fields in the x, y, and z directions and the quadrupole magnetic field, the potential trap generated by the magnetic field is the magnetic trap. The intensity of the cross-optical dipole trap is zero, and the magnetic field maintains its intensity, keeping the ultracold atomic cluster suspended and stationary, transferring the ultracold atomic cluster from the optical trap to the magnetic trap in situ.

4. The in-situ transfer device according to claim 3, characterized in that, The radial and vertical potential energy distributions are calculated based on the magnetic field gradient along the z-direction and the intensity of the bias magnetic field along the z-direction. The distance between the magnetic field zero point and the optical trap is obtained, and the position of the magnetic field zero point is determined so that the atoms can be compensated for by gravity.

5. An in-situ transfer method for the in-situ transfer device for ultracold atoms from an optical trap to a magnetic trap according to claim 1, characterized in that, The in-situ transfer method includes the following steps: 1) The optical cluster is confined at the center of the cross optical dipole trap; 2) Initial evaporation cooling and atomic polarization state preparation stage: During the initial evaporation-cooling and atomic polarization state preparation stages, the timing control system continuously reduces the intensity of the cross-optical dipole trap to evaporate and cool the atoms. Simultaneously, the timing control system controls the intensity of the quadrupole magnetic field in stages to pre-polarize the atoms, causing atoms at different magnetic level energy levels to split and enter an atomic polarization state. F Atoms in the -1 state become the energy levels remaining during the evaporation and cooling process. During this process, the strength of the bias magnetic field in the z direction changes synchronously with the strength of the fourth magnetic field. The optical aggregates after evaporation and cooling form Bose-Einstein condensates, i.e., ultracold atomic clusters. 3) Secondary evaporation cooling and in-situ atomic transfer stage: During the secondary evaporation cooling and in-situ atomic transfer stages, the timing control system further reduces the intensity of the cross-optical dipole trap, setting the intensity variation based on the adiabatic principle. Simultaneously, the timing control system controls the intensity of the quadrupole magnetic field and the bias magnetic fields in the x, y, and z directions in stages. The quadrupole magnetic field generates a magnetic field zero point, and the position of the magnetic field zero point is adjusted by controlling the intensity of the bias magnetic field. The distance between the magnetic field zero point and the optical trap is calculated, and the position of the magnetic field zero point is determined, so that the atoms are compensated by gravity and balanced by forces. Under the combined action of the bias magnetic fields in the x, y, and z directions and the quadrupole magnetic field, the potential trap generated by the magnetic field is the magnetic trap. The intensity of the cross-optical dipole trap is zero, and the magnetic field maintains its intensity, keeping the ultracold atomic cluster suspended and stationary, transferring the ultracold atomic cluster from the optical trap to the magnetic trap in situ. 4) Dynamic compensation and maintenance phase: During the dynamic compensation and maintenance phase, the Bose-Einstein condensate is only affected by a quadrupole magnetic field and maintains its vertical position without any optical constraints.

6. The in-situ transfer method according to claim 5, characterized in that, In step 2), during the initial evaporation cooling and atomic polarization state preparation stage, the timing control system controls the intensity of the cross optical dipole trap to satisfy the e-exponential decrease curve.

7. The in-situ transfer method according to claim 5, characterized in that, In step 3), during the secondary evaporation cooling and in-situ atomic transfer stage, the timing control system controls the intensity of the cross optical dipole trap to decrease linearly.

8. The in-situ transfer method according to claim 5, characterized in that, In step 3), the radial potential energy distribution and vertical potential energy distribution are calculated based on the magnetic field gradient along the z direction and the strength of the bias magnetic field in the x, y and z directions. The distance between the magnetic field zero point and the optical trap is obtained, and the position of the magnetic field zero point is determined so that the atoms can be compensated for by gravity.

9. The in-situ transfer method according to claim 5, characterized in that, In step 3), the strength of the bias magnetic field in the x and y directions controls the position of the magnetic field zero point in the x and y directions, respectively, and the strength of the bias magnetic field in the z direction controls the position of the magnetic field zero point along the z axis.

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