This article was updated on the 11th September 2019.
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Physicists at Harvard and MIT universities created one of the world’s largest quantum simulators using a new laser technique. The new technique manipulated atoms to create a system that brought super speed quantum computing a step closer to reality.
The team, published in the journal Nature, were able to manipulate 51 individual atoms, or quantum bits (qubits), into an array that they can individually control. By changing the way these atoms interact with each other, it could be possible to carry out superfast simulations.
This breakthrough has the potential to help solve real-world problems which current supercomputers would struggle to handle. The traveling salesman is the prime example. This is the problem of finding the most efficient route to visit all the points of sale. While a computer could work out some routes, when more addresses are added it begins to struggle. Quantum simulation is a far better way to solve this.
This problem is exponentially hard for a classical computer, meaning it could solve this for a certain number of cities, but if I wanted to add more cities, it would get much harder, very quickly.
For this kind of problem, you don’t need a quantum computer. A simulator is good enough to simulate the correct system. So we think these optimisation algorithms are the most straightforward tasks to achieve.
Vladan Vuletić, Co-author and Professor, MIT
This method of calculation also applies to many optimization problems like DNA sequencing, moving an automated soldering tip to different points, or routing packets of data through processing nodes across the internet. Essentially this could mean factory machines that run faster, safer airplanes and real-world DNA sequencing to better fight health problems.
While classic computers use binary “ones” and “zeros” to work out calculations through data processing, a quantum simulator uses qubits. These quantum bits are both in a state of “one” and “zero” allowing each qubit to process two lines of computation. This is what allows for super-fast computations.
The MIT and Harvard researchers’ computations were achieved by generating a chain of 51 atoms. These were programmed using a laser with varying frequencies and colors to undergo a quantum phase transition in which every other atom was excited.
This breakthrough for quantum simulations of specific situations still doesn’t solve the problem of creating a quantum computer that can approach multiple problems. The issue there lies in getting qubits to interact with each other while not engaging with their surrounding environment.
Vuletić, who is a member of the Research Laboratory of Electronics and the MIT-Harvard Center for Ultracold Atoms commented on this, “We know things turn classical very easily when they interact with the environment, so you need [qubits] to be super isolated. On the other hand, they need to strongly interact with another qubit.”
Previous methods that used charged ions suffered due to strong repelling forces like magnets. Artificial atoms also faced problems; as Vuletić points out, “By definition, every atom is the same as every other atom of the same species. But when you build them by hand, then you have fabrication influences, such as slightly different transition frequencies, couplings, et cetera.”
This 51 atom setup by Vuletić et al uses neutral atoms without a charge as qubits. That means they don’t repel each other like ions, but also have inherently identical properties, unlike fabricated qubits. This was done by trapping atoms using laser beams to first cool a cloud of rubidium atoms to near absolute zero, slowing them almost to a standstill.
Then, using a second laser split into over 100 beams, the atoms are trapped in place. These can then be rearranged to create the ordered array of qubits.
This array was then manipulated by temporarily turning off the laser frequencies of the array used to trap the atoms to allow them to naturally evolve. Then a third laser was used to excite atoms to a Rydberg state - one of very high energy. After this, the atom trapping laser was reactivated to detect the final stages of the atoms.
We think we can scale it up to a few hundred. If you want to use this system as a quantum computer, it becomes interesting on the order of 100 atoms, depending on what system you’re trying to simulate.
Vladan Vuletić, Co-author and Professor, MIT
Other co-authors of the study include visiting scientist Sylvain Schwartz, Harvard graduate students Harry Levine and Soonwon Choi, research associate Alexander S. Zibrov, and Professor Manuel Endres.
The National Science Foundation, the Center for Ultracold Atoms, the Army Research Office, and the Vannevar Bush Faculty Fellowship funded the research.