Session Q6: Quantum Applications and Enabling Technology

Classical and quantum bounds for remote optical sensing

We investigate resolution in a remote sensing laser radar system and map out the limits of classical versus quantum enhanced sensing. We here define resolution to be the ability to distinguish between two targets using binary hypothesis decision theory. In particular we derive the optical quantum limit of this resolution given reception of a coherent state. For targets with well defined phase, we find the minimum resolvable target separation scales with signal-to-noise (SNR) as SNR$^{-1/2}$. This scaling matches homodyne and heterodyne detection methods, though with a different front factor. It also beats direct photon detection, which is found to scale as SNR$^{-1/4}$. We then go on to compare with cases where the targets exhibit random and unknown phase fluctuations (speckle).   

Progress Toward Atomic Magnetometry Beyond the Conventional Heisenberg Scaling

We describe an atomic magnetometer whose field estimation uncertainty is expected to decrease faster than the conventional Heisenberg (1/N) scaling with the number of atoms in the atomic sample. Our procedure makes use of the effective two-body atomic interactions obtained by double-passing an off-resonant probe laser through the atomic sample during atomic Larmor precession. Performing balanced polarimetry on the transmitted probe field provides a continuous measurement signal that can be used to estimate the value of the magnetic field. We report on numerical simulations of our proposed quantum parameter estimation procedure and describe our ongoing efforts to implement our proposal using room-temperature Cs atoms.   

Coherent control of single atom Rydberg excitation

We demonstrate Rabi flopping of cold Rb atoms between ground state and Rydberg levels as high as n=43 with a Rabi frequency of 0.5 MHz. A double pulse experiment is used to demonstrate coherence of an atom left for a short time in a Rydberg level, which is an important step towards a neutral atom quantum gate. Rabi flopping with more than one atom is shown to dephase rapidly due to dipole-dipole interactions between atoms.   

Consequences of Zeeman Degeneracy for van der Waals Blockade between Rydberg Atoms

We analyze the effects of Zeeman degeneracies on the long-range interactions between like Rydberg atoms, with particular emphasis on applications to quantum information processing using van der Waals blockade. We show how degeneracies affect the primary error sources in blockade experiments, in particular that blockade errors are sensitive primarily to the weakest possible atom-atom interactions between the degenerate states, not the mean interaction strength. We present van der Waals potentials in the limit where the fine-structure interaction is large compared to the atom-atom interactions. For most angular momentum channels there are one or more superpositions of Zeeman levels that have extremely small dipole-dipole interactions and are therefore poor candidates for effective blockade experiments. Other channels with promising properties are identified and discussed. We combine the blockade and van der Waals results to quantitatively analyze the angular distribution of the blockade shift and its consequence for blockade experiment geometries of particular interest.   

Simulation of a strong van der Waals blockade in a dense ultracold gas

We report on simulations involving the blockade effect on a dense ultracold gas. The blockade effect is seen when the interaction energy between two excited Rydberg atoms is large enough to shift the two-excitation state out of resonance. In this paper we investigate a system that exhibits a strong van der Waals blockade, where only one out of thousands of atoms can be excited per blockade volume. With such a high number of atoms blockaded, the collective oscillation rate of an ensemble of atoms is much faster than the single atom oscillation rate. We examine the effects of this high density and the effects of a non-uniform density distribution as commonly seen in a magneto-optical trap (MOT). We use three different models and compare them to recent experimental data. The agreement between theory and experiment, although qualitative, suggests the non-uniformity of the density within a blockade region presents a new challenge to theoretical models.   

MEMS-based Flexible Optical Beam Steering for Quantum Information Processing in Atomic Arrays

The main attraction of quantum computation utilizing arrays of atoms trapped in optical lattice is their potential scalability where a large array of individual atoms is made available. The main bottleneck in this vision is the availability of appropriate technology to realize flexible manipulation of the laser beams that control the individual qubits. Micro-electromechanical systems (MEMS) technology provides a very flexible technology platform for steering of multiple simultaneous laser beams in one or two dimensions over a very wide range of spectrum. Since the nature of the steering mechanism is based on mechanical motion of mirrors, limited operational speed has been the major bottleneck of the MEMS approach. In this paper we report a two-spot beam steering system based on MEMS technology that is capable of simultaneously and independently illuminating any of 25 different locations within a 5x5 array with 2 laser beams of different wavelengths. Mirrors with settling times of $<$ 3$\mu $s have been fabricated allowing fast access times between qubits. Such systems can be used to implement two-qubit gates in a 1D or 2D array of qubits.   

Single photon generation by corporative spontaneous emission of atomic ensembles

The ability to generate single photons on demand is of key importance in a variety of quantum optical applications including quantum key distribution and quantum computation. Recent advances in ion trap technology allow complete control over all degrees of freedom of the trapped ions which in turn permits the creation of large entangled states with long coherence times. In this work, we investigate the feasibility of this technology for deterministic generation of single photons by harnessing the corporative spontaneous emission of the ions. We consider different geometric arrangements of entangled ions and aim to reduce the allowed decay modes into a narrow solid angle such that the emitted photon can be coupled to an optical fiber. For lattice spacings of the order of a wavelength or so, corporative radiative effects must be accounted for. Rather counterintuitively perhaps, superradiance tends to be undesirable since it can drastically alter the lifetime of different decay channels. The ion arrangement must therefore be optimized to ensure not only emission into a narrow solid angle but also that all the decay modes have approximately the same lifetime.   

Atomtronics and basic logic: Constructing AND and OR gates from atomtronic transistors

Our atomtronics research focuses on creating an analogy of electronic devices and circuits with ultracold atoms. Such an analogy arises from the highly tunable band structure of ultracold neutral atoms trapped in optical lattices. In previous work it has been demonstrated that the electronic behavior of a diode, field effect transistor (FET), and bipolar junction transistor (BJT) can all be realized in systems composed of optical lattices connected to reservoirs of neutral, ultracold atoms. We demonstrate that the behavior of simple logic gates namely, the AND and OR gates, can be realized by connecting the BJTs in the traditional electronic manner.