Neutral atom quantum computer

Initialization and operation of the computer is performed via the application of lasers on the qubits.

gate is carried out by leveraging the Rydberg blockade which leads to strong interactions when the qubits are physically close to each other.

[6] Neutral atom quantum computing makes use of several technological advancements in the field laser cooling, magneto-optical trapping and optical tweezers.

In one example of the architecture,[9] an array of atoms is loaded into a laser cooled at micro-kelvin temperatures.

Logic gates are performed using optical or microwave frequency fields and the measurements are done using resonance fluorescence.

Most of these architecture are based on rubidium,[10] caesium,[11] ytterbium,[12][13] and strontium[14] atoms.

Focused laser beams can be used to do single-site one qubit rotation using a lambda-type three level Raman scheme (see figure).

Single qubit gate fidelities have been shown to be as high as .999 in state-of-the-art experiments.

[15][13][16] To do universal quantum computation, we need at least one two-qubit entangling gate.

[citation needed] Atoms that have been excited to very large principal quantum number

When they are close to each other, their interaction potential is dominated by van Der Waals force

When one of the atoms is put into a Rydberg state (state with very high principal quantum number), the interaction between the two atoms is dominated by second order dipole-dipole interaction which is also weak.

When both of the atoms are excited to a Rydberg state, then the resonant dipole-dipole interaction becomes

Ignoring the coupling of hyperfine levels that make the qubit and motional degrees of freedom, the Hamiltonian of this system can be written as:

state is highly detuned from the rest of the system and thus is effectively decoupled.

The physics of this Hamiltonian can be divided into several subspaces depending on the initial state.

Thus we can ignore it and the effective evolution reduces to a two-level system consisting of the bright state and

We can use the Rydberg blockade to implement a controlled-phase gate by applying standard Rabi pulses between the

This is equivalent to a controlled-z gate up-to a local rotation to the hyperfine levels.

Moreover, the Adiabatic Gate prevents the problem of spurious phase accumulation when the atom is in Rydberg state.

In the Adiabatic Gate, instead of doing fast pulses, we dress the atom with an adiabatic pulse sequence that takes the atom on a trajectory around the Bloch sphere and back.

The levels pick up a phase on this trip due to the so-called "light shift" induced by the lasers.

state, the other atom picks up a phase due to light shift:

states, the atoms pick up a phase due to the two-atom light shift as seen by the eigenvalues of Hamiltonian above, then

The single atom light-shifts are then cancelled by a global pulse that implements

An extension to this gate was introduced to make it robust against errors in reference.

In this protocol, we apply the following pulse sequence: The intuition of this gate is best understood in terms of the picture given above.

, the pulses send the state around the Bloch sphere twice and accumulates a net phase

state, the other atom does not go around the Bloch sphere fully after the first pulse due to the mismatch in Rabi frequency.

This gate has been improved using the methods of quantum optimal controls recently.