Although the quality of quantum bits (qubits) and quantum gates has been steadily improving, the available quantity of qubits has increased quite slowly. To address this important issue in quantum computing, we have demonstrated arbitrary single qubit gates based on targeted phase shifts, an approach that can be applied to atom, ion or other atom-like systems. These gates are highly insensitive to addressing beam imperfections and have little crosstalk, allowing for a dramatic scaling up of qubit number. We have performed gates in series on 48 individually targeted sites in a 40% full 5 × 5 × 5 3D array created by an optical lattice. Using randomized benchmarking, we demonstrate an average gate fidelity of 0.9962(16), with an average crosstalk fidelity of 0.9979(2).PACS numbers: 03.67.Lx, 37.10.JkThe performance of isolated quantum gates has recently been improved for several types of qubits, including trapped ions [1][2][3], Josephson junctions [4], quantum dots [5], and neutral atoms [6]. Single qubit gate errors now approach or, in the case of ions, surpass the commonly accepted error-threshold [7,8] (error per gate < 10 −4 ), for fault-tolerant quantum computation [9][10][11][12]. It remains a challenge in all these systems to execute targeted gates on many qubits with fidelities comparable to those for isolated qubits [13,14]. Neutral atom and ion experiments have to date demonstrated the most qubits in the same system, 50 and 18 respectively [15,16]. The highest fidelity gates in these systems are based on microwave transitions, but addressing schemes typically depend on either addressing light beams [6,15,[17][18][19] which are difficult to make as stable as microwaves, or magnetic field gradients [2, 20] which limit the number of addressed qubits. In this report, we present a way to induce phase shifts on atoms at targeted sites in a 5 × 5 × 5 optical lattice that is highly insensitive to addressing laser beam fluctuations. We further show how to convert targeted phase shifts into arbitrary single qubit gates. These high fidelity gates are only sensitive to laser fluctuations at second order in intensity and fourth order in beam pointing. We demonstrate average single gate errors across our array that are below 0.004, and present a path towards reaching the fault-tolerant threshold.In previous work [15] we performed single site addressing in a 3D lattice using crossed laser beams to selectively ac Stark shift target atoms, and microwaves to temporarily map quantum states from a field insensitive storage basis to the Stark-shifted computational basis. While we use most of the same physical elements here, the crucial difference is that these new gates are based on phase shifts in the storage basis, and do not require transitions out of it. Non-resonant microwaves are applied that give opposite-sign ac Zeeman shifts for different atoms. A specific series of non-resonant pulses and global π-pulses on the qubit transition gives a zero net phase shift for nontarget atoms and a controllable net phase shift for ...

We report on the experimental realization of a conservative optical lattice for cold atoms with a subwavelength spatial structure. The potential is based on the nonlinear optical response of three-level atoms in laser-dressed dark states, which is not constrained by the diffraction limit of the light generating the potential. The lattice consists of a one-dimensional array of ultranarrow barriers with widths less than 10 nm, well below the wavelength of the lattice light, physically realizing a Kronig-Penney potential. We study the band structure and dissipation of this lattice and find good agreement with theoretical predictions. Even on resonance, the observed lifetimes of atoms trapped in the lattice are as long as 44 ms, nearly 10^{5} times the excited state lifetime, and could be further improved with more laser intensity. The potential is readily generalizable to higher dimensions and different geometries, allowing, for example, nearly perfect box traps, narrow tunnel junctions for atomtronics applications, and dynamically generated lattices with subwavelength spacings.

We demonstrate arbitrary coherent addressing of individual neutral atoms in a 5×5×5 array formed by an optical lattice. Addressing is accomplished using rapidly reconfigurable crossed laser beams to selectively ac Stark shift target atoms, so that only target atoms are resonant with state-changing microwaves. The effect of these targeted single qubit gates on the quantum information stored in nontargeted atoms is smaller than 3×10^{-3} in state fidelity. This is an important step along the path of converting the scalability promise of neutral atoms into reality.