Various start-ups are venturing into the area of developing quantum computers. Photonics plays an enabler role in their concepts.

The DLR Quantum Computing Initiative (DLR QCI), funded by the German Federal Ministry for Economic Affairs and Climate Action, has been inviting tenders for the construction of quantum computers and application-related software since 2021. Competitive tendering processes often involve contracts worth tens of millions of euros. “We have been excited to discover that most of the bids come from start-ups,” says Dr. Robert Axmann, Head of DLR QCI, in the latest PHOTONICS interview. As a rule, these are spin-offs from research institutes that bring high scientific and technical quality to the table, maintain close ties to their institutes and are very focused on bringing their respective hardware platform to market maturity. Consequently, the expert sees start-ups as the driving force behind the construction of quantum computers and the establishment of the corresponding supply chains.

Different technological approaches—but they all need photonics

DLR QCI specifically focuses on technological diversity in this young market. The concepts are based on ion traps, neutral atoms, photonic qubits, solid-state spins and nitrogen vacancies in artificially created diamonds – known as NV centers. The initiative has projects for each of these approaches. Five orders with a total volume of 208.5 million euros have been placed in the area of ion traps alone. For the past two years, teams from the Siegen University spin-off eleQtron, from the University of Sussex spin-off, Universal Quantum and from QUDORA, deep-tech spin-off from Physikalisch-Technische Bundesanstalt (PTB), the Technical University of Braunschweig and Leibniz University Hannover, have been working towards creating hardware platforms with at least 50 qubits. DLR QCI breaks down the basic technological principle of the highly complex ion trap approach into an easily understandable image: “Charged atoms cannot escape from an ion trap: an electromagnetic field holds them in position. A laser and radio waves or microwaves can then change the state of the charged atoms (ions) in a targeted manner. In this way, they become qubits, the computing components of quantum computers.”

Of the five technological approaches, ion trap technology is the one that differs most from photonics. “Our qubits can be controlled with microwaves using Magnetic Gradient Induced Coupling (MAGIC) technology. This is not only more precise but is also much more robust compared to laser control from qubits – and above all, it is extremely scalable,” is how eleQtron promotes its self-developed high-frequency control system. However, lasers are also used here to cool and read out the quantum bits. And the spin-off’s job ads make clear that photonics is also essential for ion trap technology. The company is looking for experts in atomic, molecular and optical physics who are familiar with lasers, laser cooling, laser spectroscopy, optics, linear and non-linear optics, optoelectronics, high-frequency electronics and atomic physics. A description of the process from DLR QCI helps to better understand this: “Ion traps capture individual ions in an electric field and cool them down to a few millikelvin with the help of lasers. The qubits are created by different energy states within the hyperfine structure of the ion, which makes very long coherence times possible. Gate operations are implemented either by targeted laser pulses or by global microwave and magnetic fields. The state of the qubits is then measured by optical transitions.”

Neutral atoms—always identical and just as nature created them

At first glance, a closely related approach is neutral atom technology, which the Munich-based Max Planck spin-off planqc is promoting in projects for DLR QCI, among others. planqc CEO Dr. Alexander Glätzle describes the basic principle in the current PHOTONICS interview: “Specifically, we use individual neutral atoms as qubits in which we can store and manipulate quantum information. The atoms are created by nature and are completely identical in construction. It is therefore a very coherent system, and the coherence times of the qubits, in which calculations are possible, are in the range of seconds. This is an eternity in the quantum world.” To achieve this, the atoms are trapped in an ultra-high vacuum chamber, where they enter a state of immobility at temperatures around absolute zero. Lasers are used for cooling and trapping as well as for manipulating the atoms. In order to manipulate them for calculations and change their logical state, low-noise lasers with precise frequency and wavelength are required. The approach also places the highest demands on the optics. This is because the laser has to excite the atoms individually, even though they are only a few micrometers apart. In addition, optical switches are used to ensure control of the atoms one nanosecond at a time. In fact, the laser pulses with high temporal and spatial resolution put them into the so-called Rydberg state: the electrons in the atomic shell are very strongly excited in this state—and can interact with other atoms across micrometers. This is precisely what allows their entanglement to form two- and multi-qubit gates for quantum computers. The approach is considered promising because many of these neutral atoms serving as qubits can be arranged in three dimensions—which is positive for the scalability of neutral atom quantum computers.

Universal photonic quantum computer with up to 64 qubits

DLR QCI’s contractors also include QuiX Quantum, a company established in 2019 in Enschede in the Netherlands, which develops photonic quantum processors. The aim of the 14-million-euro contract is to develop a system with at least 64 qubits. The start-up’s processors process information in an entangled quantum state, which is formed from many individual photons. The entanglement takes place in the waveguides, which guide the light through the photonic processor in the same way as electrical conductors. At the same time, the photons can be controlled using established methods in order to stimulate them to interact and change phase. Single photon detectors are used to read out the quantum states that are generated. Ultimately, the photons serve as qubits that can be encoded, processed and measured. The measurements are the real key to photonic quantum computing: QuiX Quantum refers to this as “measurement-based quantum computing”.