Session U6: Quantum/Coherent Control II

Electric-field noise from carbon-adatom di↵usion on a Au(110) surface: first-principles calculations and experiments

The decoherence of trapped-ion quantum gates due to heating of their motional modes is a fundamental science and engineering challenge. Mitigating this noise, is fundamental to efficient and scalable operations in ion microtraps. To understand heating at the trap-electrode surfaces, we investigate the possible source of noise by focusing on the diffusion of carbon-containing adsorbates onto the Au(110) surface. Using density functional theory and detailed scanning probe microscopy, we show that the diffusive motion of carbon adatom on gold surface significantly affect the energy landscape and adatom dipole moment variation. A model for the diffusion noise, which varies quadratically with the variation of the dipole moment, qualitatively reproduces the measured noise spectrum, and the estimate of the noise spectral density is in accord with measured values.   

Demonstration of two-qubit entanglement with ultrafast laser pulses.

One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits.~In trapped ions, the use of~electromagnetic fields for quantum~control and entanglement generation has proven successful~for small systems, but long~interaction times relative to the trap oscillation period make the~operation~sensitive to a noisy background environment.~In contrast, ultrafast pulses should enable interaction times much~faster than external noise sources.~Here we demonstrate a fully entangling phase gate between two trapped 171Yb$+$ ions using a sequence of spin-dependent ultrafast momentum kicks. Due to its fast nature, the demonstrated entangling gate is temperature independent and does not require ground state cooling. We verify entanglement by applying the entangling pulse sequence within a Ramsey interferometer and measuring the resulting parity oscillation. The best achieved fidelity is \textasciitilde 60{\%} in about 20 $\mu $s, which is limited by the spin-dependent kick fidelity. Future work includes systematically studying the limitations of the spin-dependent kick fidelity and speeding up the gate sequences to less than a harmonic trap evolution period.   

Loading and loss of ions in the trapping region of a linear Paul trap

The trap depth of a radio-frequency (rf) Paul ion trap is derived using a pseudopotential approximation, which models the time-dependent saddle potential as a time independent potential well. The depth depends on the physical spacing of the trap electrodes, the species trapped, the amplitude and frequency of the applied rf fields, and the furthest extent of the ion's motion before being lost from the trap, $\hat{R}_\mathrm{cut}$. Traditionally, $\hat{R}_\mathrm{cut}$ is taken to be equal to the shortest distance from the center of the trap to the electrodes, though in a physical trap, $\hat{R}_\mathrm{cut}$ occurs at a smaller radius. We show that $\hat{R}_\mathrm{cut}$ is a chaos border, which arises due to the quartic admixture in the $z$ potential of the trap. Once an ion moves beyond $\hat{R}_\mathrm{cut}$, the regular periodic motion of the ion becomes chaotic and it is no longer trapped. Further, using experiment, simulation, and analytical theory, we show that $\hat{R}_\mathrm{cut}$ can be used to predict the saturated ion number in the trap at loading rates achievable in many atom-ion hybrid trap systems, which can be used to measure cold ion-atom collision rates.   

Complete 3-Qubit Grover Search with Trapped Ions

We present experimental results on a complete 3-qubit Grover’s search. The algorithm is performed for all 8 possible single-result oracles and all 28 possible two-result oracles. Two methods of state marking, with and without an ancilla, are used for the oracles. All quantum solutions are shown to outperform their classical counterparts. The algorithm’s constituent gates include Toffoli-3 and Toffoli-4 gates, with process fidelities 89.6\% and 70.5\%, respectively. The experiments are performed on a programmable quantum computer consisting of a linear chain of five trapped $^{171}$Yb$^+$ ions. We execute modular one- and two-qubit gates through Raman transitions driven by a beat note between counter-propagating beams from a pulsed laser [1]. The system’s individual addressing capability [2] provides arbitrary single-qubit rotations as well as any two-qubit XX-entangling gate, which are implemented using a pulse-segmentation scheme [3]. [1] PRL 104, 140501 (2010), [2] Nature 536, 63 (2016), [3] PRL 112, 19502 (2014).   

The use and development of ion dispensers for laser-cooled atomic ion experiments

Fast, reliable, efficient loading of ions in ion traps is important for laser cooled ion trapping experiments. We utilize a simple surface ionization technique where ions are directly emitted from a platinum surface upon sublimation. This technique of direct ion production has wide applicability to ion trapping experiments and should apply to the direct production of positively charged atomic and molecular species as well as molecular anions. We experimentally demonstrate the ease and flexibility of this technique by directly producing calcium, strontium, cesium, barium, and potassium ions from a heated platinum surface. In addition, this technique is useful for loading rare isotopes into an ion trap. We experimentally demonstrate this by loading large numbers barium ions into an ion trap and distilling rare, isotopically pure ion chains through voltage control and laser heating and cooling. These techniques are directly applicable to the loading of $^{\mathrm{133}}$Ba$^{\mathrm{+}}$ ions, a candidate qubit that combines the favorable atomic structure of $^{\mathrm{171}}$Yb$^{\mathrm{+}}$, long-lived metastable states to ensure high fidelity detection, and visible optical transitions to leverage existing optical technologies.   

Ultrafast time scale X-rotation of cold atom storage qubit using Rubidium clock states

Ultrafast-time-scale optical interaction is a local operation on the electronic subspace of an atom, thus leaving its nuclear state intact. However, because atomic clock states are maximally entangled states of the electronic and nuclear degrees of freedom, their entire Hilbert space should be accessible only with local operations and classical communications (LOCC) [1]. Therefore, it may be possible to achieve hyperfine qubit gates only with electronic transitions. Here we show an experimental implementation of ultrafast X-rotation of atomic hyperfine qubits, in which an optical Rabi oscillation induces a geometric phase [2] between the constituent fine-structure states, thus bringing about the X-rotation between the two ground hyperfine levels. In experiments, cold atoms in a magneto-optical trap were controlled with a femtosecond laser pulse from a Ti:sapphire laser amplifier [3]. Absorption imaging of the as-controlled atoms initially in the ground hyperfine state manifested polarization dependence, strongly agreeing with the theory. The result indicates that single laser pulse implementations of THz clock speed qubit controls are feasible for atomic storage qubits. [1] Quantum Inf. Comput. 7, 1 (2006). [2] Phys. Rev. B 74, 205415 (2006). [3] Phys. Rev. A 91, 053421 (2015).   

Picosecond Ramsey spectroscopy of a short-lifetime dipole transition

Ramsey spectroscopy is a powerful technique that continues to evolve and prove useful for a wide variety of applications in quantum information and precision spectroscopy. Using a single trapped atomic ion, we demonstrate Ramsey spectroscopy of a short-lived, electric dipole allowed transition via picosecond pulses from a frequency-doubled mode-locked laser. The first pulse localizes the valence electron to one side of the nucleus and the arrival of the second pulse either adds constructively to this polarization or depolarizes the atom, depending upon its arrival time relative to the electron's motion. By scanning the time delay, we measure Ramsey fringes with a period of about 1 fs that show the valence electron of this single trapped atomic ion orbiting its nucleus. We discuss how this process can be used to execute high-speed single qubit gates, despite the short T1 time of the excited state. This work is supported by the US Army Research Office.   

Investigation of Ion Motional Heating in the Presence of Technical Noise

Surface-electrode ion traps show tremendous promise for large-scale quantum information processing. However, motional heating of ions has a detrimental effect on the fidelity of quantum logic operations. Despite the extensive study of motional heating in recent years, the underlying mechanisms are not completely understood. In these experiments however, contributions due to technical noise present on the DC and RF electrodes are often overlooked. We present a method for determining if the motional heating is dominated by residual voltage noise on the DC or RF electrodes. Also, we have found that stray DC electric fields can shift the ion position such that technical noise on the RF electrodes can significantly contribute to the motional heating rate. After minimizing the pseudopotential gradient by using parametric excitation, the motional heating due to RF technical noise can be significantly reduced.   

Shaping single photons and biphotons by inherent losses and grating defects

Inherent loss is always to be avoided in generating single photons or biphotons, but interestingly it provides opportunities for manipulating the photon wave packet. In this talk we show how inherent loss in parametric down-conversion can be employed to tailor the wave packets of single photons and biphotons. As an example, we propose a scheme to realize a single photon in a single cycle using inherent loss. Our work has potential applications in quantum communication, quantum computation, and quantum interface.   

Directed Field Ionization

Selective field ionization is an important experimental technique used to study the state distribution of Rydberg atoms. This is achieved by applying a steadily increasing electric field, which successively ionizes more tightly bound states. An atom prepared in an energy eigenstate encounters many avoided Stark level crossings on the way to ionization. As it traverses these avoided crossings, its amplitude is split among multiple different states, spreading out the time resolved electron ionization signal. By perturbing the electric field ramp, we can change how the atoms traverse the avoided crossings, and thus alter the shape of the ionization signal. We have used a genetic algorithm to evolve these perturbations in real time in order to arrive at a target ionization signal shape. This process is robust to large fluctuations in experimental conditions.