Researchers from the Universities of Amsterdam and New South Wales have answered a question that has been around for decades: whether ions of the metal ytterbium can enter certain long-lived and nearly stable states, and if so, for how long. The measured long-lived states may find applications in quantum computers and atomic clocks.
Many modern atomic clocks and quantum computers have so-called trapped ions at their core. Ions, electrically charged atoms, can be in many different states, all with different amounts of energy. Because of their charge, these ions can be suspended in empty space and kept in place using electromagnetic fields. The working of the clock or the computer then relies on precisely controlling which energy state such an ion occupies.
Researchers from the Universities of Amsterdam and New South Wales have now discovered that the ytterbium ion (Yb⁺), a leading candidate for both technologies, can remain in previously unexplored states for surprisingly long times. In the lab of Rene Gerritsma at the UvA-Institute of Physics, the team pumped a single Yb⁺ ion into a variety of so-called metastable (that is, nearly stable) states, by laser-exciting it to a high energy level. From there, the ion decayed to lower-lying states, some of which turned out to be metastable as well, such that it would take a long time for the ion to return to its ground state. In this way, the team clocked decay signals corresponding to lifetimes of about 1 second and 10 seconds and even found evidence for a state with a lifetime of more than 30 seconds. The results were recently published in the journal Physical Review A.
PhD student Zeger Ackerman, first author of the publication, explains: “To make the measurements possible, we did not just trap one single ion, but in fact we trapped two ions together. One of these was for spectroscopy measurements, and the other to continuously cool and stabilize the system without disturbing the metastable state being studied. This allowed us to watch the ion remain in a metastable state and precisely time its decay. Detailed atomic structure calculations then confirmed the results from the measurements.”
The newly characterized states may find applications in quantum computing and atomic clocks. The researchers are particularly excited about the state with a 1-second lifetime. Calculations show that it may be reached from the ground state relatively easily, using just a single laser pulse. The lifetime of 1 second and the predicted strength of the transition suggest that it could be used for significantly improving the detection of states of quantum bits (qubits) and their generalized analogues (so-called qudits) in ytterbium ions.
When it comes to this particular state, the research answers a thirty-five-year-old question by theoretical physicists Fawcett and Wilson, who predicted a lifetime of 5.2 seconds for the same state and asked for experimental confirmation. Given how complicated the energy level structure of Yb⁺ is, their prediction turned out to be pretty close!