A nuclear spin can act as a quantum switch that turns on or off ultracold collisions between atoms even when there is neither interaction between nuclear spins nor between the nuclear and electron spins. This "exchange blockade" is a new mechanism for implementing quantum logic gates that arises from the symmetry of composite identical particles, rather than direct coupling between qubits. We study the implementation of the entangling sqrt SWAP gate based on this mechanism for a model system of two atoms, each with ground electronic configuration 1S0, spin 1/2 nuclei, and trapped in optical tweezers. We evaluate a proof-of-principle protocol based on adiabatic evolution of a one-dimensional double Gaussian well, calculating fidelities of operation as a function of interaction strength, gate time, and temperature.A nuclear spin can act as a quantum switch that turns on or off ultracold collisions between atoms even when there is neither interaction between nuclear spins nor between the nuclear and electron spins. This "exchange blockade" is a new mechanism for implementing quantum logic gates that arises from the symmetry of composite identical particles, rather than direct coupling between qubits. We study the implementation of the entangling sqrt SWAP gate based on this mechanism for a model system of two atoms, each with ground electronic configuration 1S0, spin 1/2 nuclei, and trapped in optical tweezers. We evaluate a proof-of-principle protocol based on adiabatic evolution of a one-dimensional double Gaussian well, calculating fidelities of operation as a function of interaction strength, gate time, and temperature.

arXiv:quant-ph/0609111Physical Review LettersFeb 14, 2007116 citations

Controlling nuclear spin exchange via optical Feshbach resonances in Y171b

Iris Reichenbach, Paul S. Julienne, Ivan Deutsch

Nuclear spin exchange occurs in ultracold collisions of fermionic alkaline-earth-metal-like atoms due to a difference between $s$ - and $p$ -wave phase shifts. We study the use of an optical Feshbach resonance, excited on the ${^{1}S}_{0}\ensuremath{\rightarrow}{^{3}P}_{1}$ intercombination line of $^{171}\text{Y}\text{b}$ , to affect a large modification of the $s$ -wave scattering phase shift and thereby optically mediate nuclear exchange forces. We perform a full multichannel calculation of the photoassociation resonances and wave functions and from these calculate the real and imaginary parts of the scattering length. As a figure of merit of this interaction, we estimate the fidelity to implement a $\sqrt{\text{SWAP}}$ entangling quantum logic gate for two atoms trapped in the same well of an optical lattice. For moderate parameters one can achieve a gate fidelity of $\ensuremath{\sim}95%$ in a time of $\ensuremath{\sim}50\text{ }\ensuremath{\mu}\text{s}$.Nuclear spin exchange occurs in ultracold collisions of fermionic alkaline-earth-metal-like atoms due to a difference between $s$- and $p$-wave phase shifts. We study the use of an optical Feshbach resonance, excited on the ${^{1}S}_{0}\ensuremath{\rightarrow}{^{3}P}_{1}$ intercombination line of $^{171}\text{Y}\text{b}$, to affect a large modification of the $s$-wave scattering phase shift and thereby optically mediate nuclear exchange forces. We perform a full multichannel calculation of the photoassociation resonances and wave functions and from these calculate the real and imaginary parts of the scattering length. As a figure of merit of this interaction, we estimate the fidelity to implement a $\sqrt{\text{SWAP}}$ entangling quantum logic gate for two atoms trapped in the same well of an optical lattice. For moderate parameters one can achieve a gate fidelity of $\ensuremath{\sim}95%$ in a time of $\ensuremath{\sim}50\text{ }\ensuremath{\mu}\text{s}$.

arXiv:0905.4556Physical Review AAug 7, 200927 citations

Quantum Control with Ultracold Atoms

Ivan Deutsch

Bulletin of the American Physical SocietyMar 10, 20080 citations

Atom-ion quantum gate

H. Doerk, Zbigniew Idziaszek, Tommaso Calarco

Ultracold collisions of ions with neutral atoms in traps are studied. Recently, ultracold atom-ion systems have become available in experimental setups, where their quantum states can be coherently controlled. This control allows for an implementation of quantum information processing, combining the advantages of charged and neutral particles. The state-dependent dynamics that is a necessary ingredient for quantum computation schemes is provided in this case by the short-range interaction forces that depend on the hyperfine states of both particles. In this work, a theoretical description of spin-state-dependent trapped atom-ion collisions is developed in the framework of a multichannel quantum-defect theory and an effective single-channel model is formulated that reduces the complexity of the problem. Based on this description, a two-qubit phase gate between a $^{135}\mathrm{Ba}$ ${}^{+}$ ion and a $^{87}\mathrm{Rb}$ atom is simulated using a realistic combination of the singlet and triplet scattering lengths. The gate process is optimized and accelerated with the help of optimal control techniques. The result is a gate fidelity of $1\ensuremath{-}{10}^{\ensuremath{-}3}$ within $350$ $\ensuremath{\mu}$s.Ultracold collisions of ions with neutral atoms in traps are studied. Recently, ultracold atom-ion systems have become available in experimental setups, where their quantum states can be coherently controlled. This control allows for an implementation of quantum information processing, combining the advantages of charged and neutral particles. The state-dependent dynamics that is a necessary ingredient for quantum computation schemes is provided in this case by the short-range interaction forces that depend on the hyperfine states of both particles. In this work, a theoretical description of spin-state-dependent trapped atom-ion collisions is developed in the framework of a multichannel quantum-defect theory and an effective single-channel model is formulated that reduces the complexity of the problem. Based on this description, a two-qubit phase gate between a $^{135}\mathrm{Ba}$${}^{+}$ ion and a $^{87}\mathrm{Rb}$ atom is simulated using a realistic combination of the singlet and triplet scattering lengths. The gate process is optimized and accelerated with the help of optimal control techniques. The result is a gate fidelity of $1\ensuremath{-}{10}^{\ensuremath{-}3}$ within $350$ $\ensuremath{\mu}$s.

arXiv:0906.3779Physical Review AJan 26, 2010101 citations

Nuclear spin exchange occurs in ultracold collisions of fermionic alkaline-earth-metal-like atoms due to a difference between

s

- and

p

-wave phase shifts. We study the use of an optical Feshbach resonance, excited on the ${^{1}S}_{0}\ensuremath{\rightarrow}{^{3}P}_{1}$ intercombination line of

^{171}\text{Y}\text{b}

, to affect a large modification of the

s

-wave scattering phase shift and thereby optically mediate nuclear exchange forces. We perform a full multichannel calculation of the photoassociation resonances and wave functions and from these calculate the real and imaginary parts of the scattering length. As a figure of merit of this interaction, we estimate the fidelity to implement a

\sqrt{\text{SWAP}}

entangling quantum logic gate for two atoms trapped in the same well of an optical lattice. For moderate parameters one can achieve a gate fidelity of $\ensuremath{\sim}95%$ in a time of $\ensuremath{\sim}50\text{ }\ensuremath{\mu}\text{s}$.

^{135}\mathrm{Ba}

{}^{+}

ion and a

^{87}\mathrm{Rb}

atom is simulated using a realistic combination of the singlet and triplet scattering lengths. The gate process is optimized and accelerated with the help of optimal control techniques. The result is a gate fidelity of $1\ensuremath{-}{10}^{\ensuremath{-}3}$ within

350

$\ensuremath{\mu}$s.