Aishwarya Kumar

Coherent Addressing of Individual Neutral Atoms in a 3D Optical Lattice.

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.

Single-qubit gates based on targeted phase shifts in a 3D neutral atom array

How to single out the right atoms For a quantum computer to be useful, its qubits have to be able to change their state in response to external stimuli. But when a large number of qubits are packed in a three-dimensional (3D) structure to optimize the use of space, altering one qubit can unintentionally change the state of others. Wang et al. devised a clever way to perform high-fidelity quantum gates only on intended qubits in a 3D array of Cs atoms. Although the operation initially changed the state of some of the other atoms, additional manipulation recovered their original state. The technique may be applicable to other quantum computing implementations. Science, this issue p. 1562 Quantum gates with high fidelity are demonstrated in a densely packed three-dimensional lattice of cesium atoms. Although the quality of individual quantum bits (qubits) and quantum gates has been steadily improving, the number of qubits in a single system has increased quite slowly. Here, we demonstrate 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 cross-talk, 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 by 5 by 5 three-dimensional array created by an optical lattice. Using randomized benchmarking, we demonstrate an average gate fidelity of 0.9962(16), with an average cross-talk fidelity of 0.9979(2) (numbers in parentheses indicate the one standard deviation uncertainty in the final digits).

Sorting ultracold atoms in a three-dimensional optical lattice in a realization of Maxwell’s demon

In 1872, Maxwell proposed his famous demon gedanken experiment. By discerning which particles in a gas are hot and which are cold, and then performing a series of reversible actions, Maxwell’s demon could rearrange the particles into a manifestly lower entropy state. The apparent violation of the second law of thermodynamics was resolved in the twentieth century: Maxwell’s demon often increases the entropy of the universe while gathering his information, and there is an unavoidable entropy increase associated with the demon’s memory. Despite its theoretical resolution, the appeal of the demon construct has led many experiments to be framed as demon-like. However, past experiments have either had no intermediate information storage, negligible change in the system entropy, or have involved systems of four or fewer particles. Here, we present an experiment that realizes the full essence of Maxwell’s demon. We start with a randomly half-filled 3D optical lattice with ~60 atoms. We can make the atoms sufficiently vibrationally cold that the initial disorder is the dominant entropy. After determining where the atoms are, we execute a series of reversible operations to create a fully filled sub-lattice, a manifestly low entropy state. Our Maxwell demon lowers the total entropy by a factor of 2.44, enough to cool the ensemble below the quantum degeneracy threshold. We plan to use this Maxwell demon to initialize a neutral atom quantum computer.In 1872, Maxwell proposed his famous ‘demon’ thought experiment1. By discerning which particles in a gas are hot and which are cold, and then performing a series of reversible actions, Maxwell’s demon could rearrange the particles into a manifestly lower-entropy state. This apparent violation of the second law of thermodynamics was resolved by twentieth-century theoretical work2: the entropy of the Universe is often increased while gathering information3, and there is an unavoidable entropy increase associated with the demon’s memory4. The appeal of the thought experiment has led many real experiments to be framed as demon-like. However, past experiments had no intermediate information storage5, yielded only a small change in the system entropy6,7 or involved systems of four or fewer particles8–10. Here we present an experiment that captures the full essence of Maxwell’s thought experiment. We start with a randomly half-filled three-dimensional optical lattice with about 60 atoms. We make the atoms sufficiently vibrationally cold so that the initial disorder is the dominant entropy. After determining where the atoms are, we execute a series of reversible operations to create a fully filled sublattice, which is a manifestly low-entropy state. Our sorting process lowers the total entropy of the system by a factor of 2.44. This highly filled ultracold array could be used as the starting point for a neutral-atom quantum computer.An experiment inspired by Maxwell’s ‘demon’ thought experiment uses a series of reversible operations to fully fill a three-dimensional optical lattice with ultracold atoms and realize a low-entropy state.