Lasers for Quantum Technologies

New technologies from quantum

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 electro-magnetic fields, pressure, time, acceleration and gravity, which for instance enable 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.

 

Example of color-center based instrumentation for measurement of electro-magnetic fields

Learn more about quantum sensing and metrology

• 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 for massively parallel computing and highly complex coordination of operations, which enable 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.

Quantum Objects

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 their specific advantages and challenges depending on the specific application, but many of them require advanced laser systems for creation, control and manipulation.

• Nanowires qubits

Nanowires 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.

Typical construction for integration and cooling of a nanowire-based quantum computer

• Crystal lattice defects or color centers

These 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 electro-magnetic fields, temperature and pressure.

• Photons

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

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

• Optical trapping:

Tightly focused laser beams can be used to capture atoms in 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) electro-magnetic fields.

       

Optical tweezer principle and 2D optical lattice with trapped atoms.

• 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/necessary to use a separate laser beam for «repumping» the atoms into states in which the cooling process works.

Magneto-Optical Trap (MOT) principle and set-up

Learn about the fundamentals of laser cooling and atom trapping.

• Ionization and gating:

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 are measured by imaging the fluorescence emitted. Commonly used atoms and ions for quantum computing are Yb, Rb, Sr, Ba and Cs.

• 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 a 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