Closed-cycle, low-vibration 4 K cryostat for ion traps and other applications

Abstract

In-vacuo cryogenic environments are ideal for applications requiring both low temperatures and extremely low particle densities. This enables reaching long storage and coherence times for example in ion traps, essential requirements for experiments with highly charged ions, quantum computation, and optical clocks. We have developed a novel cryostat continuously refrigerated with a pulse-tube cryocooler and providing the lowest vibration level reported for such a closed-cycle system with 1 W cooling power for a < 5 K experiment. A decoupling system suppresses vibrations from the cryocooler by three orders of magnitude down to a level of 10 nm peak amplitudes in the horizontal plane. Heat loads of about 40 W (at 45 K) and 1W (at 4 K) are transferred from an experimental chamber, mounted on an optical table, to the cryocooler through a vacuum-insulated massive 120 kg inertial copper pendulum. The 1.4 m long pendulum allows installation of the cryocooler in a separate, acoustically isolated machine room. In the laser laboratory, we measured the residual vibrations using an interferometric setup. The positioning of the 4 K elements is reproduced to better than a few µm after a full thermal cycle to room temperature. Extreme high vacuum on the 10 −15 mbar level is achieved. In collaboration with the Max-Planck-Intitut für Kernphysik (MPIK), such a setup is now in operation at the Physikalisch-Technische Bundesanstalt (PTB) for a next-generation optical clock experiment using highly charged ions.

A room-temperature ion trapping apparatus with hydrogen partial pressure below 10−11 mbar

“…We stress again, that these results are generally dependent on trapping and laser cooling parameters, and thus not directly comparable. Alternative approach for achieving very low vacuum levels employs cryogenic vacuum setups 6,7 . Besides simplifying and speeding up the vacuum construction due to suppressed desorption rates, the reduced kinetic energy of the background gases limits the observable heating of the ion crystal caused by collisions.…”

“…The provided partial pressure analysis assumes that all molecular association events correspond to reaction of 40 Ca + with H 2 . This assumption is based on the dominant residual content of hydrogen observable on RGA before the chamber closure and on the typical characteristics of the residual gas content in the vast majority of stainless steel UHV chambers 6,7,17,29,32 . Although it is unlikely that we see reactions with other vacuum contaminants, it is practically demanding to implement means to unambiguously prove it and we have to regard the presented values as an upper limit on the hydrogen partial pressure, as contribution of any other element to the observed dark ion creation would effectively correspond to smaller hydrogen content in the chamber.…”

“…The achieved vacuum pressure is proportional to the mean free path of residual atomic particles, which in turn determines the achievable lifetime of the trapped ionic system. The collisions with the background gases can lead to thermalization of ions and their crystalized spatial structures, chemical reactions, or even direct ejection of ions from the trapping potential 6,7 . Vacuum quality can thus directly limit performed experiments and impose requirements on frequent ion reloading in some of the most advanced branches of ion trapping applications which involve large numbers of ions 3,[8][9][10][11][12][13][14][15] .…”

A cryogenic radio-frequency ion trap for quantum logic spectroscopy of highly charged ions

“…Convenient access to the inside of the cold stage is provided by just removing three lids, allowing maintenance work without detaching the cryogenic setup. Symmetric arrangement of the stainless steel spokes, as in reference [26,30], minimises displacements during thermal cycling. After a complete thermal cycle, the trapped ions can be optically addressed without realignment of the lasers or imaging system.…”

“…Compared to [26] our much smaller trap size yields higher secular frequencies, which are required for high-fidelity quantum logic operations [29]. Furthermore, the trap is connected to a low-vibration cryogenic supply line [30] and a compact EBIT producing the desired HCI [31].…”