April 13, 2024

Quantum computing recharged with electromagnetic ion trap innovation

The ETH researchers’ experimental setup. The trap chip is located inside the container below the silver dome, in which a lens captures the light emitted by the trapped ions. Credit: ETH Zurich / Pavel Hrmo

ETH researchers were able to capture ions using static electric and magnetic fields and perform quantum operations on them. In the future, these traps could be used to create quantum computers with many more quantum bits than has been possible until now.

  • The use of an oscillating electromagnetic field in ion traps limits the currently achievable number of qubits in quantum computers.
  • ETH researchers have now created an ion trap on a microfabricated chip using only static fields – an electric field and a magnetic field – in which quantum operations can be carried out.
  • In this trap, ions can be transported in arbitrary directions and several of these traps fit on a single chip.

The energy states of electrons in a Moving a single trapped ion in a two-dimensional plane

Moving a single trapped ion in a two-dimensional plane and illuminating it with a laser beam allows researchers to create the ETH logo. The image is formed by averaging many repetitions of the transport sequence. Credit: ETH Zurich/Institute for Quantum Electronics

Ion trap with magnetic field

A team of researchers at ETH Zurich led by Jonathan Home has now demonstrated that ion traps suitable for use in quantum computers can also be built using static magnetic fields instead of oscillating fields. In these static traps with an additional magnetic field, called Penning traps, both arbitrary transport and operations necessary for future supercomputers were carried out. The researchers recently published their results in the scientific journal Nature.

“Traditionally, Penning traps are used when one wants to capture many ions for precision experiments, but without having to control them individually,” says PhD student Shreyans Jain: “On the other hand, in smaller ion-based quantum computers, Paul’s traps are used.”

The ETH researchers’ idea of ​​building future quantum computers also using Penning traps was initially met with skepticism by their colleagues. For several reasons: Penning traps require extremely strong magnets, which are very expensive and quite bulky. Furthermore, all previous realizations of Penning traps have been very symmetric, something that the chip-scale structures used in ETH violate. Placing the experiment inside a large magnet makes it difficult to guide the laser beams needed to control the qubits into the trap, while strong magnetic fields increase the spacing between the qubits’ energy states. This in turn makes laser control systems much more complex: instead of a simple diode laser, multiple phase-locked lasers are required.

Penning trap scheme

Schematic showing the middle section of the Penning trap used. An ion (red) is captured through a combination of an electric field produced by different electrodes (yellow) and a magnetic field. Credit: ETH Zurich/Institute for Quantum Electronics

Transport in arbitrary directions

However, Home and his collaborators were not deterred by these difficulties and built a Penning trap based on a superconducting magnet and a microfabricated chip with multiple electrodes, which was produced at the Physikalisch-​Technische Bundesanstalt in Braunschweig. The magnet used provides a field of 3 Tesla, almost 100,000 times stronger than the Earth’s magnetic field. Using a system of cryogenically cooled mirrors, the Zurich researchers were able to channel the necessary laser light through the magnet into the ions.

The efforts paid off: a single trapped ion, which can remain in the trap for several days, can now be moved arbitrarily on the chip, connecting points “in a straight line”, controlling the different electrodes – something that was not previously possible with the old approach based on oscillating fields. Since no oscillating fields are required for trapping, many of these traps can be packaged on a single chip. “Once charged, we can even completely isolate the electrodes from the outside world and thus investigate how strongly the ions are disturbed by external influences,” says Tobias Sägesser, who was involved in the experiment when he was a PhD student.

Coherent Qubit Control

The researchers also demonstrated that the energy states of the trapped ion qubit could also be controlled while maintaining quantum mechanical superpositions. Coherent control worked with both the electronic (internal) states of the ion and the quantized oscillation (external) states, as well as to couple the internal and external quantum states. The latter is a prerequisite for creating entangled states, which are important for quantum computers.

As a next step, Home wants to capture two ions in neighboring Penning traps on the same chip and thus demonstrate that quantum operations with multiple qubits can also be performed. This would be definitive proof that quantum computers can be realized using ions in Penning traps. The professor also has other applications in mind. For example, because the ions in the new trap can be moved flexibly, they could be used to probe electric, magnetic or microwave fields near surfaces. This opens up the possibility of using these systems as atomic sensors of surface properties.

Reference: “Penning micro-trap for

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