Cryogenic trapped-ion system for large scale quantum simulation
Guido Pagano, Paul Hess, Harvey Kaplan, Wen Lin Tan, Philip Richerme, Patrick Becker, A. Kyprianidis, J. Zhang, Eric Birckelbaw, Micah Hernandez, Yukai Wu, Christopher R. Monroe
Trapped-ion systems are among the most promising hardware candidates for large scale quantum computing and quantum simulation. In order to scale up such devices, it is necessary to engineer extreme-high vacuum (XHV) environments to prevent background gas from disrupting the ion crystal. Here we present a new cryogenic ion trapping system designed for long time storage of large ion chains. Our apparatus is based on a segmented-blade ion trap enclosed in a 4 K cryostat, which enables us to routinely trap and hold over 100 171 Yb + ions for hours in a linear configuration, due to low background gas pressure from differential cryo-pumping. We characterize the XHV cryogenic environment measuring pressures below by recording both inelastic and elastic collisions between the ion chain and the molecular background gas. We also demonstrate coherent one and two-qubit operations and nearly equidistant ion spacing for chains of up to 44 ions using anharmonic axial potentials, in order to enable better detection and single ion addressing in large ion arrays. We anticipate that this reliable production and lifetime enhancement of large linear ion chains will enable quantum simulation of models that are intractable with classical computer modeling.
Trapped-ion systems are among the most promising hardware candidates for large scale quantum computing and quantum simulation. In order to scale up such devices, it is necessary to engineer extreme-high vacuum (XHV) environments to prevent background gas from disrupting the ion crystal. Here we present a new cryogenic ion trapping system designed for long time storage of large ion chains. Our apparatus is based on a segmented-blade ion trap enclosed in a 4 K cryostat, which enables us to routinely trap and hold over 100 171 Yb + ions for hours in a linear configuration, due to low background gas pressure from differential cryo-pumping. We characterize the XHV cryogenic environment measuring pressures below by recording both inelastic and elastic collisions between the ion chain and the molecular background gas. We also demonstrate coherent one and two-qubit operations and nearly equidistant ion spacing for chains of up to 44 ions using anharmonic axial potentials, in order to enable better detection and single ion addressing in large ion arrays. We anticipate that this reliable production and lifetime enhancement of large linear ion chains will enable quantum simulation of models that are intractable with classical computer modeling.
We present a room-temperature Extreme High Vacuum (XHV) system engineered to support the long-duration operation of a trapped-ion quantum processor. Background-gas collisions impose limitations on trapped-ion performance and scalability by interrupting algorithmic execution and, in some cases, ejecting ions from the trap. Using molecular-flow simulations, we optimize the chamber geometry, conductance pathways, and pumping configuration to maximize the effective pumping speed at the ion location. We perform high-temperature heat treatment of stainless steel vacuum components to achieve the desired outgassing rate, guided by quantitative relations of bulk diffusive processes, allowing us to reduce the H2 outgassing load to the 10−15mbarls−1cm−2 level. The final pressure in our chamber, measured by a hot cathode gauge, is 1.5×10−12mbar, corresponding to the gauge's measurement limit. We measure the local pressure at the ion location by observing collision-induced reordering events in a long ion chain of mixed-isotope Yb+. From the observed reordering frequency, we extract the average interval between collisions to be (1.9±0.1)hrs/ion. This corresponds to a local pressure of (3.9±0.3)×10−12mbar at the ion location, assuming that all collisions arise from background H2 molecules at room temperature. Our demonstration extends the continuous operation time of a quantum processor while maintaining the simplicity of a room-temperature system that does not require cryogenic apparatus.
One of the useful applications of a quantum computer is quantum simulation. While the quest for a universal quantum computer is still undergoing research, analog quantum simulators can study specific quantum models that are classically challenging or even intractable. These quantum simulators provide the opportunity to test particular quantum models and scale up the system size to gain insight into more exciting physics. The analog quantum simulator featured in this thesis is a cryogenic trapped-ion system. It serves the purpose of a large-scale quantum simulation by reducing the background pressure for storing a large ion chain with a long lifetime. This work presents the construction and characterization of this cryogenic apparatus and its performance as a trapped-ion quantum simulator. Quantum information is encoded in the atomic state of the ion chain. The entangling operation in trapped ions uses the collective motion of the ion chain for quantum simulation. Therefore, it is imperative to develop a cooling mechanism to prepare the ion chain to near motional ground-state for achieving high fidelity operations. Here, with this system, we explore another ground-state cooling mechanism with electromagnetically induced transparency (EIT) in a four-level system (171Yb+). EIT cooling allows simultaneous ground-state cooling across a bandwidth of motional modes, which it is useful in a large ion chain. Finally, we report the observation of magnetic domain-wall confinement in interacting spins chains. Such confinement is analogous to the color confinement in quantum chromodynamics (QCD), where hadrons are produced by quark confinement. We study the implications of such confinement in many-body spin system by observing the information propagation after applying a quantum quench of the confinement Hamiltonian. We also measure the excitation energy of domain-wall bound states from non-equilibrium quench dynamics. At the end of this experiment, we explore the number of domain wall excitations created with different quench parameters, which can be challenging to model with classical computers.
Cryogenic environments benefit ion trapping experiments by offering lower motional heating rates, collision energies, and an ultrahigh vacuum (UHV) environment for maintaining long ion chains for extended periods of time. Mechanical vibrations caused by compressors in closed-cycle cryostats can introduce relative motion between the ion and the wavefronts of lasers used to manipulate the ions. Here, we present a novel ion trapping system where a commercial low-vibration closed-cycle cryostat is used in a custom monolithic enclosure. We measure mechanical vibrations of the sample stage using an optical interferometer, and observe a root-mean-square relative displacement of 2.4 nm and a peak-to-peak displacement of 17 nm between free-space beams and the trapping location. We packaged a surface ion trap in a cryopackage assembly that enables easy handling while creating a UHV environment for the ions. The trap cryopackage contains activated carbon getter material for enhanced sorption pumping near the trapping location, and source material for ablation loading. Using171Yb+as our ion, we estimate the operating pressure of the trap as a function of package temperature using phase transitions of zig-zag ion chains as a probe. We measured the radial mode heating rate of a single ion to be 13 quanta/s on average. The Ramsey coherence measurements yield 330-ms coherence time for counter-propagating Raman carrier transitions using a 355-nm mode-locked pulse laser, demonstrating the high optical stability.
statistics. We present progress towards the construction of a cryogenic ion trap apparatus, designed to cryopump background gas within the 4K chamber. The resulting reduction in pressure will allow robust trapping of up to 100 ions in a single chain. Cooling is provided by a closed cycle cryostat with a gas mediated thermal linkage which mechanically decouples the ion trap from the vibrating cold head. A spherical octagon surrounding the ion trap allows optical access for global and individual addressing beams and high numerical aperture uorescence collection.
The trapped ions platform represents an excellent framework for Quantum information science experiments. Long coherence times, extremely high state initialization and detection fidelity, inherent full-connectivity between qubits are some features that make trapped ions the ideal qubits. It is the same features that make this platform extremely suitable for quantum simulation of various physical phenomenon, particularly quantum spin models. In this thesis, I present the design and construction of an ion trapping apparatus for quantum simulation experiments. This apparatus is operational and is used for the trapping of ionized Yb atoms. The 6 electrodes of the trap, two of which are needle electrodes, are made out of tungsten. I discuss the unique technique we use to make tungsten needle electrodes. The design, construction, and testing of the Yb source, used to produce a thermal beam of Yb atoms, is also discussed. The apparatus discussed above needs to be housed inside an ultra-high vacuum environment to keep the ions free from background collisions. I have discussed the design and construction of the vacuum system. In the end, I have provided a brief overview of the system in the lab and presented results in the form of pictures of single and multiple trapped ions.
Trapped ions are one of the leading platforms for building scalable quantum information processors. Current ion traps run on less than 100 qubits. A useful quantum computer needs to operate thousands or even millions of qubits. If systems reach gate fidelities that allow for quantum error correction, gate errors below the physical fidelity can be reached, which is important for scaling. The most promising platform for scaling up ion-based quantum computers are surface electrode traps in connection with the so-called QCCD (Quantum Charge Coupled Device) architecture. In an evolution of the standard (laser-based) approach to quantum logic gates, gates can be performed by applying an oscillating magnetic near-field gradient to the ions. Current fidelities are about one order of magnitude away from reaching the fault tolerant threshold with no fundamental limitations in sight. Scaling QCCDs is impeded by heating rates of the ions as well as vacuum quality. Ions have to be stored for at least as long as it takes for an algorithm to run and subsequent readout. Collisions with background gas and heating can expel ions from the trap. The microwave near-field approach of driving gates benefits from ions that are trapped close to the trap surface. Unfortunately, heating becomes more prominent near the surface. A cryogenic environment reduces the background pressure by several orders of magnitude and the kinetic energy of gas molecules by two orders of magnitude. Heating rates in a cryogenic trap are about two orders of magnitude lower than in a room temperature trap with otherwise equal physical properties. To operate a QCCD processor, it is required to employ phase-stable Raman laser beams. The cryogenic design of the apparatus needs to take into account the resulting requirements for ultra-low vibration amplitudes. This is particularly relevant for closed-cycle cooling systems, which are desirable from an economic point of view compared to liquid cryostats that boil off helium, but which also feature moving mechanical parts that can introduce a serious amount of vibrations. Here, we present a vibration isolated closed cycle Gifford-McMahon cryocooler with record low vibration amplitudes of 29 nm (RMS=7.8 nm) in the vertical and 51 nm (RMS=13.5 nm) in the horizontal direction. It contains a self-sustained inner vacuum chamber that houses the trap. It features 100 DC connections, 10 of which are able to carry up to 1 A of current and 8 high frequency lines. At 5 K, we can apply continuous 3.4 W of microwave power continuously at 1 GHz to the trap electrodes without heating the system. This is about 5.8 times more than state of the art near-field gates require. We also present the laser systems required to for single-shot ablation loading and Doppler cooling of Be ions: a 70 mW 313 nm Dopper cooling laser, a 1064 nm ns-pulse laser for ablation of neutral beryllium atoms and a 235 nm laser for ionizing them. The detection system features an in vacuum imaging system on a cryogenic 3D translation stage that is able to scan and image the complete surface of an ion trap. The light can be detected on a photomultiplier tube or spatially resolved on an EMCCD camera. Our trap features a meander structure for driving gates and can trap up to 5 ions of which we, so far, did not lose any due to background gas collisions. This hints at a vacuum pressure of 1e-12 mbar or lower. The demonstrated system could hold a cryogenic QCCD chip with a few 10 qubits