Quantum science promises a range of technological breakthroughs, such as quantum computers that can help discover new pharmaceuticals or quantum sensors for navigation. These capabilities rest on two unusual properties of quantum systems, superposition and entanglement. Just as a computer register stores information in the zeros or ones of classical bits, quantum bits, or qubits, store quantum information—but in the quantum world, superposition allows the qubit to be both a zero and a one at the same time. Furthermore, multiple qubits can be bizarrely correlated through a process called entanglement. When two qubits are entangled with each other, each qubit individually looks to be in a random state, but measuring one qubit reveals perfect information about its entangled partner. These properties of superposition and entanglement make qubits quite special, as they can work more efficiently than a classical computer’s bits.
However, a common challenge in actually using these quantum systems arises due to their limited memory time, or “coherence” time, which is often measured in milliseconds. Many researchers at JILA study and use superposition and entanglement of quantum systems, including JILA fellow Adam Kaufman. Previously, Kaufman and his research team focused on improving the coherence time of the strontium atoms’ superposition between the ground state and the “clock” state, so named because these two states form the basis for state-of-the-art atomic clocks. As reported in two new papers, researchers from this lab have extended these studies to much larger system sizes, with an atom in a superposition of hundreds of locations, and separately, demonstrating optical clock entanglement with seconds-scale coherence time.
In a new article published in Nature Physics, this team achieved two major breakthroughs creating entanglement on the clock transition and demonstrating how to preserve that entanglement for up to four seconds. “These entangled states are called Bell states, and are useful for quantum-enhanced atomic clocks,” said Kaufman, referring to a particular quantum state of two atoms in which the atoms are both in the ground state and both in the clock state at the same time. Kaufman continued, “But to be useful for improving clock performance, the Bell states need to persist on long timescales, hopefully, seconds or even minutes.” Having entangled states live for so long is a notorious challenge as any small environmental noise can “collapse” or destroy the state. This is one of the reasons why qubits are so fragile.
To create these Bell states, the researchers used a tweezer array, a two-dimensional grid of tightly focused spots of laser light. In each of the spots, a single atom can be trapped and manipulated while inside of an ultrahigh vacuum chamber. The team then used additional lasers in a configuration known as an optical lattice to more tightly confine the atoms, and yet another laser to turn the clock state atoms into so-called Rydberg atoms. “A Rydberg atom is when one of the atom’s electrons is very highly excited, far from the core…like popping popcorn, the electron goes from small and tightly bound to relatively big and squishy so it can be affected easily by its environment,” explained Nathan Schine, co-first author on this study. Such atoms interact with each other very strongly and were the key to making entangled states. Schine added: “We implemented a new type of entangling operation based on Rydberg atoms and used this to make the Bell states with high fidelity.”
The Bell states are useful for improving clock performance because pairs of atoms “team-up” to tick faster. “You can think of this 'ticking' as being like a pendulum in a grandfather clock,” said co-first author Aaron Young, describing this process. “Literally, what we do is give the electrons in the atoms a little kick and they oscillate. How quickly they oscillate and how long it takes them to wind down sets the precision of your clock. What's exciting about this experiment is that we're giving pairs of atoms a much more complicated kick, such that these two little pendula oscillate in a way that's somehow even more coordinated than just having two independent pendulums that are oscillating in lockstep.”
This work fits into an ongoing collaboration between JILA fellows Adam Kaufman and Jun Ye, who operate record-setting atomic clocks. “Networking an entangled clock in our lab…with an entangled clock [in the Ye lab] across the hall could be very exciting for making better measurements of the world around us,” said Schine. Such quantum sensors have been proposed for observing gravitational waves or even new fundamental physical principles.
Kaufman and his team have also used this combined tweezer array and optical lattice apparatus to study a different kind of superposition and entanglement, namely, between the positions the atoms can occupy in the lattice. While individual atoms are normally trapped in single sites of the optical lattice, occasionally an atom can “quantum tunnel” through the confining barrier into a neighboring site. Over time the atom takes multiple steps in what is called a quantum random walk and ends up in a superposition of an exponentially increasing number of different paths through which the atom has wandered across the lattice.
In a recent article published in Science, the Kaufman group observed how a single atom coherently walks over several hundred lattice sites, traversing tens of thousands of different paths. “All those paths coherently interfere, giving us these beautiful interference patterns,” lead author Aaron Young explained. “What's more, by carefully controlling all these different paths, we can use the resulting interference to solve certain computational problems.” By preparing the atom in a special superposition state, and using an additional special type of tweezer called an “oracle” tweezer to modify the shape of the lattice, the scientists used quantum tunneling to run a database search algorithm that, one day, could run faster than any classical search algorithm. “The oracle tweezer points at a single lattice site. It’s like a magic wand that makes the atom tunnel to that location rather than anywhere else in the lattice,” said co-author William Eckner.
The scientists say that this work shows the core physics that one day could underpin a powerful quantum computer. They are already hard at work attacking previously intractable problems involving many atoms tunneling around simultaneously in the lattice.
Written by Kenna Castleberry, JILA Science Communicator