Existing 51 qubit quantum simulator using rubidium atoms could scale to hundreds of qubits

In Juy, Mikhail Lukin at Harvard University announced they had a 51 quantum bit simulator. Quantum simulators are used to model the minute behavior of molecules, and could help study how drugs act within the human body. They aren’t full-blown quantum computers, though, says Simon Devitt at Macquarie University in Sydney.

Lukin and his team use of atom-by-atom assembly to deterministically prepare arrays of individually trapped
cold neutral 87Rb atoms in optical tweezers (lasers hold and manipulate the atoms). Controlled, coherent interactions between these atoms are introduced by coupling them to Rydberg states. Such interactions have already been used for realizing quantum gates, implementing strong photon-photon interactions and studying many-body physics.

The protocol that they implement is depicted.

First, atoms are loaded from a magneto-optical trap into a tweezer array created by an acousto-optic deflector (AOD). They then use a measurement and feedback procedure that eliminates the entropy associated with the probabilistic trap loading and results in the rapid production of defect-free arrays with over 50 laser cooled atoms as described previously. These atoms are prepared in a preprogrammed spatial configuration in a well-defined internal ground state. They then turn off the traps and let the system evolve under the unitary time evolution.

The final states of individual atoms are detected by turning the traps back on, and imaging the recaptured ground state atoms via atomic fluorescence, while the anti-trapped Rydberg atoms are ejected.

Lukin’s system was specifically built to solve one equation that models the interactions between certain atoms. If you wanted to solve a different equation, you’d have to rebuild the system from scratch.

Lukin’s work takes a different approach. His qubits are each made from a single rubidium atom, trapped in place using lasers and programmed via fluctuations in the laser beam.

Although less complex than quantum computers, simulators are still extremely expensive to build, says Devitt, so it’s unlikely that they will find many practical applications beyond physics departments for the time being.

And while this experiment show that it is possible to create large-scale quantum systems, we’ve still got a long way to go before we create universal quantum computers. “The full-blown quantum computer is the hardest system to get right,” Devitt says.

Outlook

Our observations demonstrate that Rydberg excitation of neutral atom arrays constitutes an exceptionally promising platform for studying quantum dynamics and quantum simulations in large systems. Our method can be extended and improved in a number of ways. Coherence properties of atoms can be improved by increasing intermediate state detuning to further suppress spontaneous emission and by Raman sideband cooling the atomic motion to the ground state to eliminate the residual Doppler shifts. Individual qubit rotations around the z-axis can be implemented using light shifts associated with trap light, while a second AOD can be used for individual control of coherent rotations around other directions. Further improvement in coherence and controllability can be obtained by encoding qubits into hyperfine sublevels of the electronic ground state and using state-selective Rydberg excitation. Implementing two-dimensional (2d) arrays could provide a path towards realizing thousands of traps. Such 2d configurations could be realized by directly using a 2d-AOD or by creating a static 2d lattice of traps and sorting atoms with an independent AOD, as demonstrated recently. With increased loading efficiencies, the robust creation and control of arrays composed of hundreds of atoms is feasible.

While their current observations already allow them to gain unprecedented insights into the physics associated with transitions into ordered phases and to explore novel many-body phenomena in quantum dynamics, they can
be directly extended along several directions. These include studies of entanglement in large arrays and the generation of many-particle quantum superposition states, investigation of quantum critical dynamics and tests of the quantum Kibble-Zurek hypothesis, and the exploration of stable non-equilibrium phases of matter. Further extension may allow for studies of the interplay between long-range interactions and disorder, quantum scrambling, topological states in spin systems and investigation of chiral clock models associated with transitions into exotic Z3 and Z4 states. Finally, we note that our approach is exceptionally well suited for the realization and testing of quantum optimization algorithms with systems that are well beyond the reach of modern classical machines. The latter may have broad potential applications ranging from modeling and optimization of chemical reactions to quantum machine learning.

Arxiv – Probing many-body dynamics on a 51-atom quantum simulator

Controllable, coherent many-body systems provide unique insights into fundamental properties of quantum matter, allow for the realization of novel quantum phases, and may ultimately lead to computational systems that are exponentially superior to existing classical approaches. Here, we demonstrate a novel platform for the creation of controlled many-body quantum matter. Our approach makes use of deterministically prepared, reconfigurable arrays of individually controlled, cold atoms. Strong, coherent interactions are enabled by coupling to atomic Rydberg states. We realize a programmable Ising-type quantum spin model with tunable interactions and system sizes of up to 51 qubits. Within this model we observe transitions into ordered states (Rydberg crystals) that break various discrete symmetries, verify high-fidelity preparation of ordered states, and investigate dynamics across the phase transition in large arrays of atoms. In particular, we observe a novel type of robust many-body dynamics corresponding to persistent oscillations of crystalline order after a sudden quantum quench. These observations enable new approaches for exploring many-body phenomena and open the door for realizations of novel quantum algorithms.

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