The possibility of constructing scalable quantum computation systems in the near future is quite intriguing. Emergent systems based on polar molecules as qubits and quantum gates are considered among the most promising candidates. Controlled loading of the microtraps in a quantum-state-selective manner is a critical precursor to the formation and manipulation of qubits. This thesis details a design of a microchip capable of controlling the motion of molecules in high-field-seeking and low-field-seeking quantum states. The design is based on an alternative type of a Stark decelerator/accelerator, the so-called type-B, in which the electrode separation distance changes along the beam axis while the electric field switching time remains constant. Monte-Carlo simulation method shows that a 2-cm long device consisting of 100 stages can decelerate HCN-like polar molecules, in a phase-stable manner, from 200 m/s to a near standstill in about 150 microseconds. The same device can be operated `in reverse' to accelerate stationary or slow moving molecules from microtraps. Two different types of geometries for alternating-gradient (AG) focusing of molecular motion are proposed. Comparison of the electric field distribution to the ideal harmonic field, as well as an analysis of the field magnitudes and gradients show that both geometries should be able to effectively decelerate or accelerate molecules while maintaining their transverse stability and focus. Finally, we propose a new technique for achieving longitudinal and transverse stability using only the accelerating fields. This new method is similar to the alternating phase focusing (APF) used in charged particle accelerators but applied to the case of polar molecules. We showed using 1D trajectory simulations that this technique is capable of decelerating molecules in a phase-stable manner, but were unable to confirm transverse focusing.
The possibility of constructing scalable quantum computation systems in the near future is quite intriguing. Emergent systems based on polar molecules as qubits and quantum gates are considered among the most promising candidates. Controlled loading of the microtraps in a quantum-state-selective manner is a critical precursor to the formation and manipulation of qubits. This thesis details a design of a microchip capable of controlling the motion of molecules in high-field-seeking and low-field-seeking quantum states. The design is based on an alternative type of a Stark decelerator/accelerator, the so-called type-B, in which the electrode separation distance changes along the beam axis while the electric field switching time remains constant. Monte-Carlo simulation method shows that a 2-cm long device consisting of 100 stages can decelerate HCN-like polar molecules, in a phase-stable manner, from 200 m/s to a near standstill in about 150 microseconds. The same device can be operated `in reverse' to accelerate stationary or slow moving molecules from microtraps. Two different types of geometries for alternating-gradient (AG) focusing of molecular motion are proposed. Comparison of the electric field distribution to the ideal harmonic field, as well as an analysis of the field magnitudes and gradients show that both geometries should be able to effectively decelerate or accelerate molecules while maintaining their transverse stability and focus. Finally, we propose a new technique for achieving longitudinal and transverse stability using only the accelerating fields. This new method is similar to the alternating phase focusing (APF) used in charged particle accelerators but applied to the case of polar molecules. We showed using 1D trajectory simulations that this technique is capable of decelerating molecules in a phase-stable manner, but were unable to confirm transverse focusing.
A Stark decelerator is an effective tool for controlling motional degrees of freedom of polar molecules. Due to technical limitations, many of the current Stark decelerators focus on molecules in low-field-seeking quantum states and are built based on a fixed electrode size and spacing (type-A architecture). Here, we present an alternative method based on a different architecture, so-called type-B, with microstructured electrodes and a simpler electric field pulse timing with the prospect of producing cold, quasi-CW molecular beams. We demonstrate with simulations the feasibility of this method and show that a 2 cm decelerator consisting of 100 stages can bring molecules from 200 m/s to a near standstill in about 150 microseconds. Subsequently, the molecules can be reaccelerated to 200 m/s in a phase-stable manner. Our simulations focus on the longitudinal transmission leaving the problem of a transverse confinement for a future study. This work points to a possible way of integrating polar molecule accelerators with solid-state electronics.
Beams of polar molecules can be focused using an array of electrostatic lenses in alternating gradient (AG) configuration. They can also be accelerated or decelerated by applying an appropriate high-voltage switching sequence to the lenses. AG focusing is applicable to molecules in both low-field- and high-field-seeking states and is particularly well suited to the problem of decelerating heavy molecules and those in their ground rotational state. We describe the principles of AG deceleration and set out criteria to be followed in decelerator design, construction and operation. We calculate the longitudinal and transverse focusing properties of a decelerator, and exemplify this by 2D-imaging studies of a decelerated beam of metastable CO molecules.
A Stark decelerator can slow down polar molecules to very low velocities. When the velocities are very low, the number of cold molecules obtained is very small. In order to obtain a higher quantity of cold molecules, inspired by the work of Reens et al. [Phys. Rev. Res. 2 (2020) 033 095], we propose an alternative method of operating a Stark decelerator. Through the trajectory simulation of OH molecules in the decelerator, we find that the number of cold molecules can be greatly increased by one order of magnitude at both low and high final velocities on a Stark decelerator consisting of around 150 electrodes. This development is due to the improved longitudinal and the transverse focusing property provided by the new switching schemes and the high-voltage configurations on the decelerator unit.
The subject of this thesis’s work is research of cold molecules on a microchip. On the one hand, cold and ultracold molecules might allow high- precision measurements of molecular properties and the determination of their behavior at the quantum level. On the other hand, one could try to study highly controlled chemical reactions or to build a quantum computer with their help. In the experiments presented here, electrically neutral, polar molecules in low-field-seeking states are trapped and decelerated on a microchip with the help of electric fields. This exploits the Stark effect: the energy level shift of atoms and molecules in an electric field. After manipulation on the chip the molecules can be detected quantum-state specifically via the REMPI method. At first, CO molecules were prepared in the upper Λ-doublet component of the a3Π1, v = 0, J = 1 state, trapped on the chip and get vibrationally excited by an infrared laser in the middle of the chip to the J = 1 or J = 2 rotational state of the vibrational state v = 1. This could be shown experimentally and was analyzed theoretically for molecules which were trapped at all times. Secondly, CO molecules were detected on the chip and the structure of previously trapped molecule clouds of several of the only 120 μm spaced traps was spatially resolved. For this purpose the chip was extended with an ion lens setup, which enabled the determination of the temperature of the clouds by following the free ballistic expansion of the molecules in time; either by comparing the data to trajectory simulations (T = 11 ± 1 mK) or by applying a ballistic expansion model (T = 13 ± 3.5 mK), comparable to the method used by the ultracold atom community. The spatial imaging enabled the observation of a phasespace manipulation scheme, which adiabatically expanded the molecules in the trap and significantly reduces their temperature to approximately one third of their initial value. The third goal was to trap ground state ammonia molecules, which should extend the trapping time and would show that also other molecules could be trapped on the chip. The ammonia isotopologues 14NH3, 14ND3, and 15ND3 in the |vInv = 1, J = 1,K = 1> state of the electronic ground state X were used. No signal could be measured from trapped molecules, which is attributed to large nonadiabatic transitions to untrappable quantum states, leading to severe trap loss. At last, a new chip- based Stark decelerator is presented, which is a miniaturized version of the so-called “traveling-wave” ring decelerator. This chip-based decelerator should be able to trap more molecules compared to the chip that was used so far, and due to improved electronics it should be able to decelerate molecules from molecular beam velocities of approximately 360 m/s down to a standstill, in contrast to large-scale traveling-wave decelerators so far. The chip is still in development to achieve operation with the full specification (voltage). Performance simulations show the usefulness of this design, which needs to be tested in experiments.
Neutral dipolar molecules can be decelerated and trapped using time-varying inhomogeneous electric fields. This has been demonstrated only for molecules in low-field seeking states, but can, in principle, be performed on molecules in high-field seeking states as well. Transverse stability is then much more difficult to obtain, however, since molecules in high-field seeking states always experience a force towards the electrodes. Here we demonstrate that an array of dipole lenses in alternate gradient configuration can be used to maintain transverse stability. A pulsed beam of metastable CO in high-field seeking states is accelerated from 275 to 289 m/s as well as decelerated from 275 to 260 m/s.
The growing field of cold molecule physics has demonstrated many methods of quantum control for precision measurement and molecular interaction studies. One particular method is Stark deceleration, which uses electric field gradients to produce packets of slow polar molecules. Since Stark deceleration does not increase phase-space density, the efficiency of the deceleration process is critical in determining the final cold molecule density. This dissertation describes three different Stark deceleration techniques. The first type of deceleration is pulsed-pin deceleration. We use this technique to produce slow OH molecules that can be trapped for cold collision studies. A two photon state-selective detection system, which includes a 118 nm photon, is used to determine the density distribution of OH in the trap. We explore the advantages and limitations to creating 118 nm light. The second and third types of Stark deceleration are pulsed-ring and traveling-wave methods of running a ring-geometry decelerator. Ring-geometry decelerators present improved deceleration efficiencies due to the cylindrical symmetry of the electrodes and true three-dimensional confinement. The pulsed-ring mode of the ring decelerator uses commercial high-voltage switches to apply discrete pulses to the ring electrodes. The continuous operation mode of the ring decelerator uses varying sine-wave voltages to create a traveling Stark potential well and has proven to be incredibly challenging due to the analog high voltage requirements of the system. In order to bring ND3 seeded in krypton (∼415 m/s) down to rest in the laboratory, sine wave voltages with amplitudes up to ±10 kV, currents up to 500 mA, and a bandwidth of 30 kHz down to DC are required. We describe a high-voltage amplifier that can be used for traveling-wave deceleration. The traveling-wave Stark deceleration mode is then compared to the pulsed-ring Stark deceleration mode and cases where one mode is superior to another are discussed.