Conceptual illustration of a new boson sampling method. Credit: Steven Burrows/Kaufman Group, edited
Researchers have demonstrated a new method for sampling bosons using ultracold atoms, marking a significant advance over previous techniques. Using optical tweezers and advanced cooling, the method enables precise control of the atoms in the lattice, facilitating complex quantum calculations that are impractical for classical computers.
When two subjects in everyday life “do not differ”, it is due to an imperfect state of knowledge. As the street magician mixes cups and balls, you can basically tell which ball is which by how they pass between the cups. However, on the smallest scales of nature, even a magician cannot distinguish one ball from another. True indistinguishability of this kind can fundamentally change the behavior of bullets.
For example, in the classic experiment by Hong, Ou and Mandel, two identical photons (balls) hitting opposite sides of a semi-reflective mirror always exit on the same side of the mirror (in the same glass). This is due to a certain type of interference, not an interaction between photons. For more photons and more mirrors, this interference becomes extremely complicated.
Advances in boson sampling
Measuring the pattern of photons emerging from a particular mirror maze is known as “boson sampling”. It is believed that it is impossible to simulate boson sampling on a classical computer for more than a few tens of photons. As a result, considerable efforts have been made to perform such photon experiments and show that a quantum device performs a (non-universal) computational task that cannot be performed classically. This effort has culminated in recent claims of quantum advantage using photons.
Now, in a recently published article Nature article, JILA Fellow, National Institute of Standards and Technology (NIST) physicist and University of Colorado Boulder physics professor Adam Kaufman and his team, along with NIST collaborators, demonstrated a new method of sampling bosons using ultracold atoms (especially bosonic atoms) in two-dimensional optical array of intersecting laser beams.
Using tools such as optical tweezers, specific models of identical atoms can be prepared. Atoms can propagate through the lattice with minimal loss and their positions are revealed with almost perfect precision. precision after their journey. The result is an implementation of boson sampling that is a big step beyond what has been achieved so far, either in computer simulations or with photons.
“Optical tweezers have enabled groundbreaking experiments in many-body physics, often for studies of interacting atoms, where atoms are stuck in space and interact over large distances,” Kaufman said. “However, a large class of fundamental many-body problems – called “Hubbard” systems – arise when particles can both interact and tunnel, quantum mechanically propagating through space. From the beginning of this experiment, we intended to apply this tweezer paradigm to large Hubbard systems – this publication marks the first realization of that vision.
Techniques for better control
To achieve these results, the researchers used several state-of-the-art techniques, including optical tweezers (highly focused lasers capable of moving individual atoms with extreme precision) and advanced cooling methods that bring the atoms closer together. absolute zero temperatures, minimizing their movement and enabling precise control and measurement.
In the same way that a magnifying glass creates a point of light when focused, optical tweezers can hold individual atoms in powerful beams of light, allowing them to move with greater precision. Using these tweezers, the researchers prepared specific patterns consisting of up to 180 strontium atoms in an array of 1,000 sites, formed by intersecting laser beams that create a lattice pattern of energy wells potential to trap the atoms. The researchers also used sophisticated laser cooling techniques to prepare the atoms, ensuring they remain in their lowest energy state, thus reducing noise and decoherence, common challenges in quantum experiments.
NIST physicist Shawn Geller explained that the cooling and preparation ensures that the atoms are as identical as possible, removing any markers, such as individualized internal states or states of motion, that might make up a particular element. atom different from the others. “Adding an extra label means the universe can detect which atom is which, even if you can’t see the label as an experimenter,” said first author and JILA alumnus Aaron Young. “The presence of such a label would make this absurdly difficult sampling problem completely trivial. »
Challenges and innovations in quantum scaling
For the same reason that boson sampling is difficult to simulate, it is not possible to directly confirm that the correct sampling task was performed for the 180-atom experiments. To overcome this problem, the researchers sampled their atoms at different scales.
According to Young, “We’re doing tests with two atoms, where we understand very well what’s going on. Then, at an intermediate level where we can still simulate things, we can compare our measurements with simulations that include reasonable error models for our experiment. On a large scale, we can continuously vary the difficulty of the sampling task by monitoring how different the atoms are and confirm that it’s not a bad thing.
Geller added: “What we’ve done is we’ve developed tests that use the physics we know to explain what we think is going on. »
Through this process, the researchers were able to confirm the high fidelity of atom preparation and evolution compared to previous demonstrations of boson sampling. In particular, the very small loss of atoms compared to photons as they evolve precludes modern computational techniques that challenge previous demonstrations of quantum advantages.
The high-quality, programmable preparation, evolution, and detection of lattice atoms presented in this paper can be applied to the situation where atoms interact with each other. This opens up new approaches to the simulation and study of the behavior of real and otherwise poorly understood quantum materials.
“Using non-interacting particles allowed us to take this specific boson sampling problem to a new regime,” Kaufman said. “Nevertheless, many of the most physically interesting and computationally demanding problems arise with systems of many interacting particles. In the future, we hope that applying these new tools to such systems will open the door to many exciting experiments. »