Quantum computing, a revolutionary field, leverages the principles of quantum mechanics to process information in ways that classical computers cannot. At the heart of quantum computing are qubits, the quantum analogs of classical bits. Unlike classical bits, which can be either 0 or 1, qubits can exist in a superposition of states, embodying both 0 and 1 simultaneously. This property, along with entanglement and quantum interference, enables quantum computers to solve complex problems more efficiently than classical computers.

Breaking down the science behind qubits

Quantum computers may one day rapidly find solutions to problems no regular computer might ever hope to solve. An ordinary computer chip uses bits. Those are like tiny switches that can be either on or off, representing 0s and 1s. Now, a qubit is like a supercharged version of these switches. Instead of just being on or off, a qubit can be both on and off at the same time! It’s a bit like having a coin spinning in the air where you can’t tell if it’s heads or tails until it lands.

In essence every app you use, website you visit and photograph you take is ultimately made up of millions of these bits in some combination of ones and zeroes. This works great for most things, but it doesn’t reflect the way the universe actually works. In nature, things aren’t just on or off. They’re uncertain. And even our best supercomputers aren’t very good at dealing with uncertainty.

Understanding the basics of quantum computers

The ability to be in multiple states simultaneously is called “superposition.” It’s as if the qubit can explore different options all at once. For example, think about finding your way through a maze. Instead of choosing one path at a time, a qubit could explore all paths simultaneously, making it incredibly powerful for certain kinds of problem-solving. A quantum computer can go down every path of the maze at once. It can hold uncertainty in its head. This means that they hold a future of exciting developments in many fields including from healthcare to finance to AI, and everything in between.

The choice of qubits which are currently being explored in quantum computing depends on many factors and therefore it is not surprising that there are several types which have been considered and researched.

What is the best type of qubit?

Several types of qubits exist, including superconducting qubits, ion trap qubits, cold atom-based qubits, nitrogen vacancy (NV) center-based qubits, as well as photon based qubits. Lasers are used to help generate these qubits for ion trap-based qubits, cold atom and NV center -based qubits. It remains to be determined which of the qubit candidates will be the best performing solution for future quantum computers.

Ion trap based qubits were one of the earliest and more advanced scalable methods for generating qubits with long lived internal levels of the ions serving as the qubits (Häffan et. al, 2008).

Cold atom-based qubits are particularly promising candidates for qubits due to their well-defined energy levels and long coherence times, which are crucial for maintaining quantum information. Some common cold atom qubit candidates are:

• Alkali Metals (e.g., Rubidium and Cesium):

Alkali atoms are popular for creating qubits because of their simple electronic structure, which makes them easier to manipulate with lasers and magnetic fields. Rubidium-87 and Cesium-133 are commonly used. Their hyperfine structure provides stable energy levels that can be precisely controlled for qubit operations.

• Group II Elements (e.g., Strontium and Calcium):

Atoms like strontium and calcium are used in optical lattice clocks and quantum computing. These atoms have narrow optical transitions suitable for high-precision measurements and manipulation.

• Rare Earth Elements (e.g., Ytterbium):

Ytterbium ions are used in trapped ion quantum computers due to their stable ground state and favorable transition wavelengths for laser cooling and manipulation.

Nitrogen Vacancy (NV) center-based qubits are based on using artificially grown defects in diamond crystals that can be used as qubits. They can have long coherence times at room temperature and are relatively stable.

Photon based qubits use a single photon of light as a qubit. Individual photons are easily polarized for encoding, are fast since they can travel at the speed of light and have long coherence.

The choice of the “best” qubits depends on the specific requirements of the quantum computing architecture being developed, including factors like coherence time, scalability, ease of manipulation, and integration with existing technologies. Different research groups are exploring various approaches to address these challenges and unlock the potential of quantum computing using different types of qubits

The role of lasers in generating qubits

Laser cooling for atomic qubits

Laser cooling is a pivotal technique in the preparation of cold atom-based qubits. It involves using the radiation pressure of light to slow down and cool atoms to temperatures close to absolute zero. This is essential for reducing thermal motion, thereby increasing the coherence times of qubits and improving their stability and control.

The Doppler cooling method is the most common laser cooling technique. Here, lasers tuned slightly below an atomic transition frequency cause atoms moving towards the laser to absorb photons, which slows them down. Subsequent spontaneous emission of photons occurs in random directions, resulting in a net reduction of the atom’s kinetic energy.

Other cooling techniques like Sisyphus cooling and evaporative cooling can bring atoms to even lower temperatures, reaching the microkelvin or nanokelvin range. These ultracold conditions are ideal for quantum computing, as they minimize decoherence and enhance the fidelity of quantum operations.

When it comes to the choice of which laser to use for cooling, it all depends on the cooling method and technique of choice. For example, our Cobolt Qu-T is a compact tunable laser perfect for Doppler cooling, with single-frequency CW emission in the 650-950 nm range, an inherently high level of flexibility in the center wavelength and a perfect TEM00 beam, 10’s kHz linewidth and powers up to 500 mW. Each emission wavelength can be coarsely tuned gap-free over several nm and actively locked to an external reference using a fast piezo control. This attribute is perfect for laser Doppler cooling since you can target the excitation frequency of the specific atom that you want to cool.

Laser trapping and cooling for trapped ions qubits

Trapped ions are a promising technology for quantum computing, serving as a foundation for creating atom qubits. In this approach, individual ions are confined or trapped being suspended in free space using electromagnetic fields in a vacuum chamber. Lasers for trapping therefore need to have extremely narrow linewidths (kHz) and ultra low noise in order to be able to hold the atoms and ions completely still.

The Ampheia™ ultra low noise fiber amplifier has exceptionally low RIN over a wide range of frequencies along with a linewidth in the kHz region and offers powers up to 50 W at 1064 nm, making is a perfect choice for trapping ions or atoms. The trapped ions can then be manipulated and controlled with laser pulses to perform quantum operations.

Laser cooling plays a crucial role in this process by reducing the ions’ thermal motion to near absolute zero, thereby minimizing decoherence and enhancing precision in quantum state manipulation. Each ion, thus cooled and trapped, acts as a qubit, with its electronic states representing the binary 0 and 1. The high degree of control over the ion’s quantum states and the ability to entangle multiple ions make trapped ions an ideal candidate for implementing scalable and highly accurate quantum computing systems.

Lasers for initiation and readout of NV center qubits

Nitrogen vacancy (NV) centers in diamond have been recently identified as suitable candidates because the point defect provides an isolated spin state which can be manipulated using microwaves. The other big advantage is that they can work over a wide range of temperatures (including room temperature) and do not need to be cooled to ultra cold temperatures.

Lasers in the visible region 510 nm – 560 nm can be used to excite these defects and populates a “bright” electron spin state that leads to strong fluorescence in the red. These lasers can be either direct diode or diode pumped lasers and need to be able to be modulated in the MHz region with fast rise and fall times (ns) and complete darkness when the laser is off. The Cobolt 06-MLD 515nm/520nm lasers are often the preferred choice for such applications.

For evaluation of other color centers candidates and not only in diamond, the widely tunable C-WAVE laser offers the flexibility of access to a wavelength range of 450 nm – 700 nm, together with single frequency performance and Watt level powers, is a perfect tool for characterizing the photoluminescence of novel colour centers.

Future trends for qubits

Research and development efforts are poised to transform quantum computing from a scientific curiosity to a practical and impactful technology. Bridging the gap between quantum and classical computing infrastructure is crucial for practical quantum applications. Future developments include building quantum networks for secure communication, developing quantum algorithms that leverage classical resources, and integrating quantum processors with classical computing environments.

At HÜBNER Photonics we supply high performance lasers for many areas of quantum research.

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