Physicist Adam Kaufman and his team at the University of Colorado Boulder, along with collaborators at the National Institute of Standards and Technology (NIST), have developed a new method of boson sampling using ultracold atoms in a two-dimensional optical lattice. This method, which involves using optical tweezers and advanced cooling techniques, allows for the preparation of specific patterns of identical atoms and their propagation through the lattice with minimal loss. The team’s work, which marks a significant advancement in quantum computing, was recently published in the journal Nature.
Quantum Phenomena and the Indistinguishability of Atoms
In the realm of quantum physics, indistinguishability plays a crucial role. Unlike in our everyday experiences, where objects can be differentiated, even the most skilled observer cannot distinguish one particle from another at the quantum level. This fundamental indistinguishability can significantly alter the behavior of particles. For instance, in a classic experiment by Hong, Ou, and Mandel, two identical photons striking opposite sides of a half-reflective mirror always exit from the same side. This behavior results from a unique kind of interference, not any interaction between the photons. As the number of photons and mirrors increases, this interference becomes exceedingly complex.
The pattern of photons emerging from a maze of mirrors is known as “boson sampling.” Simulating boson sampling on a classical computer is believed to be infeasible for more than a few tens of photons. Consequently, there has been a significant effort to perform such experiments with actual photons to demonstrate that a quantum device can perform a specific computational task that cannot be performed classically. This effort has led to recent claims of quantum advantage using photons.
A Novel Approach to Boson Sampling
In a recent Nature paper, a team led by JILA Fellow, NIST Physicist, and University of Colorado Boulder Physics Professor Adam Kaufman, in collaboration with the National Institute of Standards and Technology (NIST), demonstrated a novel method of boson sampling. They used ultracold atoms (specifically, bosonic atoms) in a two-dimensional optical lattice of intersecting laser beams.
The team used tools such as optical tweezers to prepare specific patterns of identical atoms. These atoms could be propagated through the lattice with minimal loss, and their positions detected with nearly perfect accuracy after their journey. This method of boson sampling represents a significant leap beyond what has been achieved before, either in computer simulations or with photons.
Techniques for Better Control
To achieve these results, the researchers employed several cutting-edge techniques, including optical tweezers and advanced cooling methods. Optical tweezers, highly focused lasers, can move individual atoms with exquisite precision. The researchers prepared specific patterns of up to 180 strontium atoms in a 1,000-site lattice, formed by intersecting laser beams that create a grid-like pattern of potential energy wells to trap the atoms.
The researchers also used sophisticated laser cooling techniques to prepare the atoms, ensuring they remained in their lowest energy state, thereby reducing noise and decoherence—common challenges in quantum experiments. The cooling and preparation ensured that the atoms were as identical as possible, removing any labels that could make a given atom different from the others.
A Matter of Scaling
Directly verifying that the correct sampling task has been performed is not feasible for the experiments with 180 atoms due to the complexity of boson sampling. To overcome this issue, the researchers sampled their atoms at various scales. They performed tests with two atoms, where the behavior is well understood, and at an intermediate scale where measurements could be compared to simulations involving reasonable error models for the experiment. At large scale, they could continuously vary how distinguishable the atoms are and confirm that nothing dramatic is going wrong.
Future Applications and Implications
The high-quality and programmable preparation, evolution, and detection of atoms in a lattice demonstrated in this work can be applied in situations where the atoms interact. This opens new approaches to simulating and studying the behavior of real and otherwise poorly understood quantum materials.
The researchers believe that applying these new tools to systems of many interacting particles will open the door to many exciting experiments. This research was supported by the Air Force Office of Scientific Research (AFOSR) and the National Science Foundation (NSF) Physics Frontier Center (PFC) at JILA.
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