Quantum computers promise to perform certain types of operations much more quickly than conventional digital computers. The loss of order in quantum systems is a problem known as quantum decoherence, which worsens as the number of bits in a quantum computer increases. One proposed solution is to divide the computing among multiple small quantum computers that would work together much as today’s multi-core supercomputers team up to tackle big digital operations. The individual computers in such a system could communicate quantum information using Bose-Einstein condensates (BECs) – clouds of ultra-cold atoms that all exist in exactly the same quantum state. The approach could address the decoherence problem by reducing the number of bits necessary for a single computer.
A team of physicists at the Georgia Institute of Technology has examined how this Bose-Einstein communication might work. The researchers determined the amount of time needed for quantum information to propagate across their BEC, essentially establishing the top speed at which such quantum computers could communicate.
The researchers first assembled a gaseous Bose-Einstein condensate that consisted of as many as three million sodium atoms cooled to nearly absolute zero. To begin the experiment, they switched on a magnetic field applied to the BEC that instantly placed the system out of equilibrium. That triggered spin-exchange collisions as the atoms attempted to transition from one ground state to a new one. Atoms near one another became entangled, pairing up with one atom’s spin pointing up, and the other’s pointing down. This pairing of opposite spins created a correlation between pairs of atoms that moved through the entire BEC as it established a new equilibrium.
The research could help scientists anticipate the operating speed for a quantum computing system composed of many cores communicating through a BEC.
“This propagation takes place on the time scale of ten to a hundred milliseconds,” Raman said. “This is the speed at which quantum information naturally flows through this kind of system. If you were to use this medium for quantum communication, that would be its natural time scale, and that would set the timing for other processes.”
Though relevant to communication of quantum information, the process also showed how a large system undergoing a phase transition does so in localized patches that expand to attempt to incorporate the entire system.
ABSTRACT – We have experimentally observed the emergence of spontaneous antiferromagnetic spatial order in
a sodium spinor Bose-Einstein condensate that was quenched through a magnetic phase transition.
For negative values of the quadratic Zeeman shift, a gas initially prepared in the F = 1, mF = 0
state collapsed into a dynamically evolving superposition of all 3 spin projections, mF = 0,±1. The
quench gave rise to rich, nonequilibrium behavior where both nematic and magnetic spin waves were
generated. We characterized the spatiotemporal evolution through two particle correlations between
atoms in each pair of spin states. These revealed dramatic diﬀerences between the dynamics of the
spin correlations and those of the spin populations.
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