On Tuesday, April 16th, 2019 the Institute for Quantum Computing will host the Toronto Ultracold Atom Network for its second yearly meeting.
The one-day meeting aims to both share knowledge and strengthen ties between local ultracold atom groups. The day will consist of talks and posters on topics including trapped ions, optical lattices, Bose-Einstein condensates and optical techniques for atomic state manipulation.
Organisers
Matthew Day, Institute for Quantum Computing
Crystal Senko, Institute for Quantum Computing
Rajibul Islam, Institute for Quantum Computing
Schedule
Abstracts
Experimental demonstration of strong cross-Kerr nonlinearity in ultracold atoms using Rydberg-Rydberg interactions and EIT
Josiah Sinclair
Last year, I discussed an experimental proposal for combining Rydberg-Rydberg interactions and electromagnetically induced transparency in order to create a strong cross-Kerr nonlinearity. This year, I am happy to report the experimental observation of this cross-Kerr nonlinearity, which we used to implement cross phase modulation between two weak coherent states. The nonlinear phase shifts measured were as large as 8mrad/nW corresponding to a large χ(3)of 10-8 m2V2and the effect scales with the adjusted principal quantum number like n5.6+/-0.4, consistent with our expectations for a van der Waals based nonlinearity. I will conclude by discussing progress towards our next experiment, where we intend to harness this large cross-Kerr nonlinearity to implement photon number squeezing.
Single-photon switching in a polarization-selective fiber-integrated cavity
Jeremy Flannery
Hollow-core fibers loaded with atomic ensembles offer a platform for strong light-matter interactions, with experimental demonstrations in recent years including all-optical switching, cross-phase modulation with few photons, and single-photon broadband quantum memory. Further enhancement of these light-matter interactions can be accomplished by incorporating a cavity into the fiber and we realized such cavities by attaching photonic crystal membranes acting as mirrors to the ends of a hollow-core photonic crystal fiber (HCPCF) segment [1]. More recently, we designed and fabricated our photonics-crystal mirrors to be polarization dependent — highly reflective for one linear polarization but with almost full transmission for the orthogonal polarization. This effect is achieved by designing the photonic crystal pattern to have broken Cartesian symmetries to allow for linear polarization dependence. Such a unique type of polarization-dependent fiber-integrated cavity can allow for experimental schemes in which light of one polarization interacts with an atomic cloud in a single pass as if the cloud was in free space, while the interaction of co-propagating but perpendicularly polarized light with the atoms is enhanced by the cavity.
Based on such fiber-integrated cavities with cold atoms loaded into a hollow-core optical fiber, we propose a novel scheme for an all-optical switch for single photons. Our proposal employs the phenomena of vacuum induced transparency (VIT). The weak probe field resonant with one of the atomic transitions is transmitted though an otherwise highly absorptive atomic media by the presence of the cavity vacuum mode that is coupled to an unpopulated transition in the atoms.
In ours scheme, we set the cavity mode and probe field far detuned from the excited state. This is the regime of a Raman two-photon absorption (TPA), however because we use a vacuum cavity mode as a coupling field with only the weak source field present, this may be referred to as a vacuum induced Raman absorption (VIRA). The benefit of this regime is that the resonance condition for the VIT, allowing for full transmission, and the VIRA, resulting in a large absorption, can be very close in frequency. We exploit this by setting the source photon on the VIRA resonance. The optical switch can then be activated by injecting a single gate photon into the cavity mode, shifting the VIRA resonance such that the probe field is instead at the VIT resonance, allowing for large transmission.
Realistic high-fidelity protocols for qudit-based quantum computing
Brendan Bramman
We present on the feasibility of implementing quantum information processing using multi-level qudits encoded within trapped ions. We describe protocols for how current technology may be used to implement high-fidelity state preparation, measurement, and single- and two-qudit gates in a trapped ion framework. A scalable measurement scheme using rapid adiabatic passage to a meta-stable state is presented, along with a discussion of single-qudit gate implementation, and a practical method for implementing two-qudit entangling gates (mediated by collective phonon modes) using a geometric phase approach. From our error estimations, we can achieve better than 99% fidelity for three-level qudit operations and measurement, which will allow us to perform fault-tolerant surface code quantum computing on these platforms. We anticipate that further improvements to the measurement technique and the qudit manipulations could be made to push these fidelities higher.
Towards position measurements of an impurity atom in a double well BEC
Matthew Richards
We consider a toy-model for quantum measurements: an impurity atom sharing a double well potential with a BEC. If there is a macroscopic number of bosons, and they have a strong repulsive interaction with the impurity, the BEC can act as a measurement device showing which well the impurity isn’t in. This system can be mapped onto the Dicke model which has a well-known Z2 symmetry breaking phase transition. We investigate the dynamics as the interaction is swept from zero, paying particular attention to the influence of the phase transition.
Holographic optical engineering for manipulating individual qubits in a trapped ion quantum simulator
Chung-You (Gilbert) Shih
The ability to individually manipulate trapped ion qubits in a programmable way opens the possibility to simulate more complex Hamiltonians than what is possible with global quantum gates alone. Ion qubits can be individually manipulated by laser beams with programmable spatial intensity profile. However, mitigating optical aberrations and cross-talk can be challenging, especially in a high numerical aperture optical system. Here, we will report on a novel high precision qubit addressing system based on a digital micro-mirror device acting as a Fourier hologram. The holographic control of laser beams enable the compensation of aberrations and the creation of arbitrary diffraction-limited intensity profile at ions.
Magic polarization for light shift cancellation in two-photon optical clocks
Shira Jackson
Probe laser light shifts represent an important source of uncertainty in optical clocks, especially for clocks operating on highly forbidden transitions or multi-photon transitions. We present a robust and simple scheme to cancel probe laser light shifts in two-photon clocks by finding a magic polarization angle for the probe laser at which the differential polarizability of the clock states is zero. Two-photon transitions are inherently Doppler and recoil free which eliminates the need for tight confinement of the atoms during interrogation. This allows for a simplified design that can be made compact and portable. We report on experimental progress towards the construction of an optical clock based on the 4s2 1S0→ 4s3d 1D2 two-photon transition in calcium atoms.
A Real-Time Control System for Multi-User Ion Trapped Quantum Computing
Richard Rademacher
Some major barriers in the use of ion traps for quantum computation and simulation are the expense of the apparatus, and the technical knowledge necessary to convert circuit-level descriptions of quantum algorithms into the laser timing pulses and associated controls. We present the design for a multi-user, 10-qubit quantum computer that brings useability closer to the general research community. A new, custom control system provides users with remote control capability at various levels of abstraction: timing, gate, and circuit. Provisions for control of all hardware is provided along with built-in calibration, safety interlocks, advanced timing control and arbitrary pulse generation. The combination of multi-user control on a modern ion trap platform brings performance, and useability to both the experimentalist and theorist.
Dichroic mirrors for circularly polarized light
Behrooz Semnani
Since circularly polarized light interacts more strongly with atoms than linearly polarized light, circularly dichroic mirrors, as well as cavities based on such mirrors, open tantalizing possibilities for engineering novel types of light matter interactions. However, circular dichroism is not present in conventional, metallic or dielectric-stack, mirrors. Here, we present design, fabrication, and characterization of a chiral photonic-crystal structure based on a thin dielectric slab that acts as a nearly perfect mirror for either Right-Handed or Left-Handed circularly polarized light but not for both.
Measurement of Tunnelling Time for a Rb-87 Condensate through a Laser-induced Potential
Joseph McGowan
The question of how long a tunnelling particle spends in the classically forbidden region is a controversial one, with uncertainty as to how one even defines this quantity. The Larmor time provides an intuitive interpretation of the tunnelling time by effectively attaching a stopwatch to the particle which only ticks inside the barrier. We have performed a first measurement of this Larmor time using a BEC of rubidium-87 tunnelling through a blue-detuned laser and compared to theoretical predictions. We find that, as the atomic energy drops below the barrier height, the time spent in the barrier region decreases, with tunnelling times on the order of 1 ms for a barrier width of about 1 μm. Future experiments plan to investigate the role that atomic interactions play in the tunneling event, as well as the impact of strong Larmor measurements.
Current dissipation of ultracold atoms in an optical lattice
Rhys Anderson
We measure the current dissipation rate of fermionic ultracold atoms in an optical lattice. A quantum gas microscope enables high-resolution uorescence imaging of atoms pinned to lattice sites. Using micron-scale periodic displacements of an underlying harmonic potential to provide an oscillating uniform force, we measure the global current response of the atoms for multiple frequencies within the lowest band. We observe that the current response scales linearly with the forcing, providing experimental verification that data is taken in the linear response regime. Broadening of the current response spectrum for increasing lattice depth, interaction strength, and density provides a measure of the rate of dissipation. This dissipation occurs purely due to fermion-fermion collisions, given the absence of phonons or impurities in our potentials. It is observed to require a _nite lattice depth in order to break Galilean invariance, as well as to enable Umklapp scattering events, which play a significant role in the dynamics. Measured dissipation rates collapse onto the predictions of a kinetic theory under the wide range of conditions studied.
Machine learning methods to program a linear chain of ions for simulating two and three dimensional spin models
Yi Hong Teoh
Trapped ions are a versatile platform for quantum simulation. Phonon-mediated long range spin-spin interactions enable the simulation of many types of Hamiltonians. However, ions are most readily trapped in a linear chain, limiting their usefulness in simulating higher dimensional spin models that can show qualitatively different phenomena than 1D models. Here we will discuss a protocol to engineer the spin-spin interaction graph of arbitrary 2D and 3D lattices in a linear chain of ions. The protocol relies on optical manipulation of individual spin and phonon modes, which is feasible with current technology with a moderate number (a dozen or so) of qubits or spins. We employ a machine learning neural network to extract the experimental parameters (intensity and frequency of multiple laser beams) such that a trapped ion quantum simulator can be programmed to simulate arbitrary 2D and 3D lattices, and switch between various lattice geometries dynamically. The capability to engineer re-programmable lattice geometries in the same physical system will significantly enhance the capability of quantum simulator beyond the state-of-the-art.
Phonon-phonon interactions in a one-dimensional Bose gas far from equilibrium
Ryan Plestid
In this talk I will motivate and discuss a classical field model for one-dimensional Bose gases such as those realized by the atom chip groups of Schmiedmayer and Bouchoule. These systems can serve as an analog quantum simulator of the Lieb-Liniger model, and are a fertile testing ground for our understanding of many-body physics. After reviewing some recent experimental work from each group, I will discuss non-linear phonon-phonon interactions and their role in the dynamics after a quantum quench. I will contrast the studies being undertaken by both the Schmiedmayer and Bouchoule groups, and discuss how their results compare to my own classical field simulations of a homogeneous 1D Bose gas.
The 23P1-to-23P2fine-structure interval in atomic helium is measured using the frequency-offset separated-oscillatory-fields (FOSOF) technique. Two temporally separated microwave fields set up excitation paths that accumulate different quantum-mechanical phases. To detect the atoms that have changed states due to the microwaves, these atoms are excited to a Rydberg state and Stark ionized. The number of resulting ions is counted on a channel electron multiplier. In a typical SOF experiment, the relative phase between the two microwave pulses is toggled between 0° and 180°, and the change in the signal amplitude between the two phases is detected as a function of applied microwave frequency. In the FOSOF technique, two microwave pulses with a slight frequency offset are applied to the atoms. The relative phase seen by the atoms changes continuously due to the frequency offset, leading to a sinusoidally oscillating atomic signal. The phase of the oscillating signal is measured with respect to the phase of a reference generated by combining the frequency-offset microwaves. The phase difference between the oscillating atomic signal and reference signal crosses zero at resonance and changes linearly as a function of applied microwave frequency.
Major signal-to-noise ratio (SNR) enhancement has been achieved by employing a two-dimensional magneto-optical trap and by using Stark-ionization detection. The excellent SNR allows for a very extensive study of systematic effects. A wide range of experiment parameters has been investigated. The final measured result is 2291176590(25)Hz. This is the most precise measurement of the interval to date and thus the most precise test of the two-electron quantum-electrodynamics theory. When the 23P0-to-23P1transition is measured at the same level of precision and the combined result of the 23P0-to-23P2fine-structure interval is compared with a sufficiently precise theory, a sub-part-per-billion determination of the fine-structure constant using a two-electron system will become possible for the first time. Comparison with other fine-structure constant measurements could lead to tests of possible beyond-the-Standard-Model physics.