Lasers for Quantum Technologies

In the early 20th century, Max Planck and others developed the theory of quantum mechanics as a result of trying to understand the true nature of matter and light. This theory has revolutionized our understanding of the world at microscopic levels and led to the realization that both matter and light can simultaneously be seen as both particles and waves (“wave-particle duality”). The theory has enabled the development of a large number of technologies and devices that modern society relies on; such as lasers, semiconductor electronics, advanced medical equipment and many more.

E=hν

In 1900, Max Planck proposed that light from a black body is radiated in discrete energy elements (”quanta”), with an energy (E) that is proportional to the frequency (v) of the light. This was the starting point of the development of quantum mechanics. Planck’s constant (h) was later named after Max Planck himself.

Recent years have shown tremendous progress in a number of new technologies based on quantum mechanics, leading to innovations with a promise to bring transformative solutions to many of the challenges the world is facing today, such as climate change, need for more efficient healthcare and national security. This wave of scientific and industrial development in quantum technologies is often referred to as the Second Quantum Revolution or Quantum 2.0.

Quantum technologies are based on the creation, control and manipulation of microscopic objects that exhibit quantum properties. And many of these technologies rely heavily on the use of advanced laser technology.

Quantum key properties

Quantum objects, or systems, are associated with certain key properties, which make them fundamentally different from classical objects. These particular quantum properties are intriguing and often non-intuitive and difficult to understand, but they are central in the Second Quantum Revolution:

• Quantum states

Quantum systems can only exist in certain discrete or quantized “states” of energy, momentum or other quantities.

• Quantum superposition

However, quantum systems exist in a combination of all possible states simultaneously, each with a certain probability. This is called superposition. Only when measured, the quantum system “collapses” into one state.

• Quantum entanglement

Several quantum objects can be linked to one another, even over large distances. A change to or measure of the state of one object instantaneously influences the state of other linked quantum objects.

• Quantum interference

Quantum objects exhibit wave-like properties, and, in a probabilistic superposition state they interact and form constructive and destructive interference patterns. Quantum coherence is a measure of how long a quantum system preserves quantum properties and exhibits interference.

• Quantum tunneling

Due to the wave-like properties of quantum particles, they have the capability to pass through an energy barrier that they, according to classical mechanics, should not be able to.

Quantum technology application areas

New technologies developed in the Second Quantum Revolution, which are based on the creation of quantum objects, and the control, manipulation and observation of their quantum states can be divided into three main categories: Quantum sensing and imaging, quantum computing and quantum communication.

• Quantum sensing & imaging

Quantum states are normally extremely sensitive to their environment. This sensitivity often poses a challenge in handling quantum particles – but can also be used to construct extremely sensitive sensors, e.g. for electromagnetic fields, pressure, time, acceleration and gravity, which for instance enable the development of new navigation systems and gyroscopes. Quantum effects also enable the development of new types of imaging equipment with superior resolution and noise suppression («Quantum-enhanced imaging»), promising for use in clinical diagnosis.

• Quantum computing

The states of quantum objects can be used as quantum bits («qubits») to make computational operations, in analogy to the 1 or 0 bits created by transistors in a classical computer. Unlike in a classical computer, the qubits can exist in a combination of all states simultaneously via superposition and are linked to each other via entanglement. This allows quantum computers to solve certain computational challenges that classical computers or supercomputers are not able to handle, e.g in the fields of material science, drug discovery and cryptography.

• Quantum communication

Quantum technology is being used to develop secure, ”unhackable”, communication over data networks. In quantum communication, encrypted data is sent in standard classical bits (0 or 1), but the key for decryption is sent as qubits in the form of entangled photons distributed through fiber-optic cables. This process is called Quantum Key Distribution (QKD) and the polarization states of the photons represent the individual bit values of the key. Thanks to quantum entanglement and superposition, it is impossible for a hacker to intercept the key distribution without the sender or the receiver noticing it.

Common objects used in quantum technology

There is a variety of microscopic elements being developed and used as quantum objects, or qubits, in the Second Quantum Revolution. Each of these qubit alternatives has its specific advantages and challenges depending on the specific application, but many of them require advanced laser systems for creation, control and manipulation.

• Nanowires as qubits

Nanowires as qubits are made of a semiconducting nanowire sandwiched between two superconductors to create a controllable Josephson junction. Nanowire qubits are one of the main candidate technologies for building quantum computers, but they are very sensitive to noise which makes it a challenge to maintain quantum coherence. Elaborate schemes for cooling to temperatures close to absolute zero are required to achieve sufficient coherence. In addition, electrical wiring is becoming complex as the number of qubits is scaled. A special type of nanowire qubits are topological qubits. These are superconducting nanowires mounted on semiconductor substrates which under the influence of magnetic and electric fields transition into a new, toplogical, phase of matter.

• Crystal lattice defects as qubits

Crystal lattice defects or color centers are atomic-scale defects within a crystal lattice, such as missing atoms (vacancies) or impurities. The spin states of these defects have quantum properties and can be addressed with precisely controlled laser beams at specific wavelengths, usually in the visible range. Commonly used defects are vacancies in diamond such as Nitrogen (NV centers), Germanium (GeV centers) and Tin (SnV centers) as well as various defects in silicon carbide (SiC), such as Silicon vacancy centers. Color center qubits are very sensitive to the environment and are suitable for building ultra-sensitive sensors for electromagnetic fields, temperature and pressure.

• Photons as qubits

Photons are by nature quantum objects, and they are particularly useful for carrying quantum information over large distances. Entangled photons can be created through spontaneous parametric down-conversion (SPDC) in optical nonlinear materials and can be used for Quantum Key Distribution (QKD) in quantum communication networks and for quantum-enhanced imaging technologies (e.g. probing an object with one beam of photons but imaging it in the entangled beam at another wavelength). Photons can also be used to execute quantum computational operations by generating, controlling and detecting quantum properties of light, such as polarization and phase at single-photon levels, which can be achieved in Photonic Integrated Circuit (PIC) platforms.

• Neutral atoms and ions as qubits

Atoms and ions work well as quantum objects if trapped and cooled down to ultra-cold (cryogenic) temperatures. The creation and control of atoms and ions as quantum particles and the use of them in quantum technology applications rely heavily on the use of lasers:

1. Optical trapping

Tightly focused laser beams can be used to capture atoms in an optical dipole trap («atom trapping» or «optical tweezer»). If the laser wavelength is red shifted from the atomic resonance frequency, the atom will be drawn to the center of the beam focus where the intensity is highest. With multiple intersecting laser beams it is possible to create an interference pattern of intensity peaks and valleys and thereby form an «optical lattice» of atom traps in 1, 2 or 3 dimensions. On the other hand, electrically charged atoms (ions) can be trapped in (non-optical) electromagnetic fields.

2.  Laser cooling:

Clouds of atoms can also be trapped and cooled in a Magneto-Optical Trap (MOT), which consists of a combination of a quadrupole magnetic field and six lasers arranged in pairs that form a grid of three orthogonal counter-propagating laser beams intersecting in the center, where the magnetic field is zero. The magnetic field induces a restoring force depending on the atom’s position, i.e. taps the atom. At the same time, the laser beams are slowing down (cooling down) the atoms, either through exchange of momentum or by optical pumping of the atoms into specific quantum states with lower energy. If the laser beam frequency is fine-tuned to just below the atomic resonance, atoms moving towards the beam will more likely, due to the Doppler shift («Doppler cooling»), move into resonance and absorb photons and their momentum, which causes a slow-down effect (as re-emission of the photon occurs in a random direction). Other types of laser cooling mechanisms are used to achieve cooling beyond the limits of Doppler cooling, such as Raman cooling and side-band cooling. During the cooling process it is also often required to use a separate laser beam for «repumping» the atoms into states in which the cooling process works.

3. Atoms and ions used in quantum computing:

Once trapped and cooled into controllable qubits, the atoms or ions can be used to perform computational operations. The atoms are ionized into a Rydberg state which leads to strong, long-range quantum coherent interactions, which in turn enables implementation of quantum logic gating procedures. Lasers are then used to initialize state changes in the qubits (providing the logical inputs) and the outcome of the computations is measured by imaging the fluorescence emitted. Commonly used atoms and ions for quantum computing are Yb, Rb, Sr, Ba and Cs.

4. Atoms and ions used in optical quantum clocks:

The energy transitions of ultra-cold atoms or ions can provide extremely precise optical frequency references. By locking a narrow linewidth laser beam to such atomic transitions (trapped in an optical lattice) and translating the optical frequency of the laser (in THz) into a measurable frequency (in GHz) via an optical frequency comb (typically a femtosecond laser), it is possible to achieve an extremely precise measure of time, over 100 times more precise than standard atom clocks (which operate in the microwave regime). Common optical clock atoms and corresponding laser wavelengths are Yb (578 nm) and Sr (698 nm).

HÜBNER Photonics’ Lasers for Quantum