Over the past 15 years ion traps have demonstrated all the building blocks required of a quantum computer. Despite this success, trapping ions remains a challenging task, with the requirement for extensive laser systems and vacuum systems to perform operations on only a handful of qubits. To scale these proof of principle experiments into something that can outperform a classical computer requires an advancement in the trap technologies that will allow multiple trapping zones, junctions and utilize scalable fabrication technologies. I will discuss the construction of an ion trapping experiment, focussing on my work towards the laser stabilization and ion trap design but also covering the experimental setup as a whole. The vacuum system that I designed allows the mounting and testing of a variety of ion trap chips, with versatile optical access and a fast turn around time. I will also present the design and fabrication of a microfabricated Y junction and a 2- dimensional ion trap lattice. I achieve a suppression of barrier height and small variation of secular frequency through the Y junction, aiding to the junctions applicability to adiabatic shuttling operations. I also report the design and fabrication of a 2-D ion trap lattice. Such structures have been proposed as a means to implement quantum simulators and to my knowledge is the first microfabricated lattice trap. Electrical testing of the trap structures was undertaken to investigate the breakdown voltage of microfabricated structures with both static and radio frequency voltages. The results from these tests negate the concern over reduced rf voltage breakdown and in fact demonstrates breakdown voltages significantly above that typically required for ion trapping. This may allow ion traps to be designed to operate with higher voltages and greater ion-electrode separations, reducing anomalous heating. Lastly I present my work towards the implementation of magnetic fields gradients and microwaves on chip. This may allow coupling of the ions internal state to its motion using microwaves, thus reducing the requirements for the use of laser systems.
Abstract Numerical simulations are used to study unsharp measurements in a system of trapped ytterbium ions to monitor resonantly driven Rabi oscillations of a target ion by weak coupling to an auxiliary ion. The simulation combines the positive operator-valued measure formalism and optical Bloch equations to model the state of the target ion during the implementation of the unsharp measurement protocol. State monitoring is done using an estimation scheme that is updated with the unsharp measurement results. The model expands on previous work by explicitly including spontaneous emission into the calculations. We propose a method to determine convergence of the estimation scheme, useful in an experimental setting, which does not rely on using the target state. The simulation results show the feasibility conditions for implementing unsharp measurements experimentally and demonstrate the robustness of the estimation scheme in the presence of experimental noise and when the qubit Rabi frequency is not precisely known. We find that the timescale of transitions that change the motional state is the main technical challenge for experimental implementation.
A fault-tolerant quantum computer is expected to require thousands of qubits. Trapped ion architectures provide a modular approach where the quantum register is divided into multiple subregisters connected by physically moving the corresponding ions. Transporting ions at scale comes with several challenges such as the need to connect thousands of control lines to an ion trap chip. Multiplexing the required control voltages from few input signals to multiple electrodes offers a solution to this wiring challenge. Here we demonstrate a quantum processing unit that combines a surface ion trap with a time multiplexer via a sample-and-hold technique that initially charges electrodes to fixed voltages and disconnects them during qubit operations. We characterize the unit's performance by measuring motional heating rates below one phonon per second in both open and closed switch configurations. We further characterize the sample and hold process and find that sampling intervals below 50 ms are sufficient to keep expected gate errors from decaying charges during the hold phase below
10−4. Our results indicate that the multiplexing scheme is compatible with high-fidelity operations.
The highly popular ytterbium-171 (
171mathrmYb+) ion is commonly employed in quantum information research as a qubit whose excellent coherence time and fast, simple state preparation has allowed cutting edge work in quantum computation and simulation. Despite these large benefits, the demonstrated measurement fidelity of this ion has lagged the state preparation and gate fidelity achieved to date.In this thesis we investigate and realize methods of increasing the measurement fidelity of
171mathrmYb+ in a scaleable way for large quantum systems. Using methods of coherent control, we implement a pulsed state detection scheme using a mode-locked laser to perform background-free spectroscopy of the ``bright'' state of the qubit. The small hyperfine splitting of the ion necessitates the use of multiple (two) pulses to manipulate time dynamics of the ion to excite a single transition. A Mach-Zehnder interferometer is constructed to control these pulse separations both coarsely (
sim 237 ps) and on a fine sub-femtosecond scale. These pulses cause destructive/constructive interference of the electron wave packet of a single ion levitated in vacuum and are engineered to state-selectively excite the qubit. This allows measurement of the qubit whose transition frequency is much smaller than the bandwidth of the interrogation laser.During this spectroscopy, mechanical forces from the mode-locked laser frequency comb can drive the ion into large coherent states of motion. This motion has been dubbed ``phonon lasing''. We investigate the phonon lasing affect and how the ion interacts with multiple comb teeth. The large number of teeth leads to a protection mechanism from runaway energy gain by near-by blue detuned teeth, allowing ions to be trapped and cooled by the mode-locked laser, regardless of its detuning. We further explore these discrete amplitude coherent states by injecting energy into the ion's motion and exciting higher-order oscillations.We, for the first time, implement an ``electron shelving'' of the hyperfine qubit, and incoherently transfer the bright state population in the extremely long-lived ($\\approx$5 yr)
2F
7/2 state of
171Yb
+, functionally disconnected state. This is accomplished via narrow-band optical pumping on the
2S
1/2 to the
2D
5/2 quadrupole which has a leaky dipole channel into the
2F
7/2. Narrow-band optical pumping is again used to rescue the ion at the end of the experiment with the aid of a 760 nm E2 transition back into the cooling cycle. Measurement with this scheme is no longer limited by off-resonant effects from the main cycling transition. Limits of this novel technique, as well as further directions using the F state as a utility for quantum information are explored. Finally, we combine the pulsed background-free spectroscopy with shelving and demonstrate high-fidelity, background free detection of a single trapped
171mathrmYb+ qubit.
Quantum computation is a promising technology owing to the unique physical nature of quantum systems. Quantum computers have the potential to be as much as exponentially more efficient than today’s most powerful classical computer at certain tasks. One of the major challenges the scientific community must overcome, arises due to the intrinsic tendency of quantum information to be highly susceptible to environmental noise. This thesis presents work in developing and characterizing methods for precision quantum control in the presence of decoherence inducing noise, using trapped 171Yb+ ions as a model experimental platform. A flexible , robust microwave system is used to access the 12.6 GHz hyperfine qubit manifold in the outer S-shell orbital. The low phase noise characteristics of a microwave oscillator allow free-evolution coherence times in excess of three seconds, and operational fidelities F > 99.99%, characterized by randomized benchmarking. Starting from this relatively clean baseline, control techniques may be implemented in order to simulate a variety of realistic, time-dependent noise models, via arbitrary IQ modulation of the microwave source. A formalism for calculating the spectral noise filter functions of piece-wise constant control sequences is developed. The efficacy of this formalism is demonstrated by experimentally characterizing a variety of control protocols in the presence of engineered noise.