The 21st Century is now being dubbed the Quantum Age. Whether it be through financial modelling, cyber-security, drug modelling or artificial intelligence, the benefits of quantum computing are already being realised.
The qubits in quantum computers can dramatically outperform classical techniques by combining states of 1 and 0, rather than existing as one or the other, enabling computation at incredible speeds. They do this by exploiting the strange behaviour of matter at the atomic level – namely superposition and entanglement. Not surprisingly, the hardware used is extremely fragile and vulnerable to perturbations, so it is challenging to prepare and control precise quantum states, and some components need to be chilled to near absolute zero, whilst others must be stored in an ultra-high vacuum (UHV).
2020 has been a very challenging year for most, but there was a small silver lining for a number of quantum researchers who have been able to capitalise on the “ghost-town” state of university labs and therefore absence of noise and vibration – precisely what qubits need to avoid decoherence (loss of quantum behaviour to the environment). Any interaction between the qubit and its environment can knock it out of the superposition or entanglement state. Therefore, being able to create and sustain a suitable UHV is critical.
Related: If you're working with HV and UHV systems, there are more than a few unique considerations you'll need to make to ensure the safety of your team and efficiency of your process. Our blog post shares what you need to know about working under HV and UHV.
Superposition is the ability for atoms or ions to be in multiple states simultaneously and entanglement is a unique shared connection between 2 qubits – there are many ways to produce entanglement, including bringing two particles close together, perform an operation to entangle them and move them apart again. No matter how far apart they are, they will always yield the same outcome. The operations can be performed by cooling the atoms or ions to near absolute zero, and manipulating them, with precision lasers within a UHV chamber.
The internal volume of such UHV chambers can be as small as a few cubic centimetres, but no matter what the size, the vacuum technology exists to achieve the conditions necessary. Ion sputter pumps e.g. the “small profile” TiTan range by Gamma Vacuum, are an accepted method of producing and maintaining UHV conditions with pumping speeds ranging from the miniature 0.2 l/s up to 75 l/s. With such pumps, and optionally a titanium sublimation pump (TSP) used in conjunction to boost pumping, a vacuum level more rarefied than outer space can be achieved.
When it comes to scaling up a solution, the phenomena of ion traps for storing quantum information is not quite as proven as producing the qubits as part of a superconducting circuit, commonly utilising the superconductors niobium and aluminium as the capacitor and inductor respectively. The manufacture of these superconducting thin film circuits can only be achieved through vacuum techniques, such as atomic layer deposition, pulsed laser deposition, and physical vapour deposition through magnetron sputtering or e-beam evaporation.
Superconducting devices have unique properties in that they become conductive at a certain temperature and dilution refrigerators are an accepted method today for maintaining the extremely low temperatures, in the order of millikelvin (even colder than outer space) – yet another process, aside from cryogen-free systems, that requires vacuum pumps to recycle and compress the vapourised helium-3 before diluting it again with helium-4, as well as providing inner and outer vacuum insulation.
Modern Vacuum Techniques
A sputter ion pump and TSP need to be operated at pressures below around 5E-4 mbar (dependant on the type of element) because the electrical energy required to ionise the many particles at higher pressures would be too large for the power supply. Therefore, adequate rough pumping by forevacuum pumps and turbomolecular pumps is essential.
The oil-free ECODRY plus multi-stage roots pump by Leybold exhibits extremely low vibration and noise, without the need to maintain for a number of years on clean applications. Pairing this with a low-vibration turbomolecular pump with magnetically-levitated bearings (as well as vibration-absorbing bellows to further enhance the stability) makes the perfect setup for a clean, vibration-free system, and reduces the need to isolate the UHV system from a separate roughing system.
At the vacuum level required for the likes of ion traps, hydrogen is the main residual gas and plays the villain in maintaining UHV as it reveals itself from the internals of metal components. As such, long bakeout procedures, sometimes in the order of weeks, are required to remove excess hydrogen from inside the components used. In experimentation this can be a real pain point, particularly if the system is repeatedly open and closed to atmosphere. One way to improve the pumpdown to UHV conditions and minimising the effects of the desorption rate is to introduce cryo pumping using cold plates that have a higher adsorption rate. This allows for faster pumping to UHV and can be achieved with Gifford-McMahon closed-cycle coldheads, for instance the COOLPOWER range by Leybold, which expand and compress helium to achieve cryogenic temperatures.
Helium comes in very handy again when it comes to revealing leaks and their location. Helium leak checking UHV systems is the most advanced method of leak detection and is critical to ensure they will maintain vacuum integrity throughout their lifetime. Thus a trapped ion can remain unperturbed by anything other than the laser or microwave beam. The PHOENIX range by Leybold is the most innovative in leak detection technology, meeting with accuracy and speed the increasing quality requirements of quantum research today.
Leybold have been providing bespoke solutions for thin film deposition techniques for decades, via the UNIVEX range. High purity superconducting materials can be formed to an accurate degree of thickness suitable for the circuits in quantum computers. DC or RF magnetron sputter sources, e-beam gun and/or ion-assisted deposition can be specified along with substrate heating/cooling and rotation, and full system control.
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