## Communication

### Using lasers for quantum information

- Quantum computing
- Quantum teleportation
- Quantum cryptography
- Sources for single or entangled photons

It was only short time after the formulation and acceptance of quantum theory when scientists started to discuss possible benefits of this theory for mankind. The quantum computer, probably the most famous application of quantum theory, is expected to reach incredible computing speeds that enable calculations which were not possible before. Any coupled quantum mechanical system can be used for quantum computing. Solid state systems, trapped ions, atoms in optical lattices, and photons with linear optical elements are at the heart of quantum computer research. First quantum operations have been demonstrated with solid state systems and trapped ions but the race is still open.

The basis for quantum computing is “entanglement”, a quantum mechanical property of a system in which the state of one part of the system is fully linked to the state of another part. The famous “Schrödinger cat” example tries to visualize how strange entanglement is compared to experiences in daily life. Even Einstein doubted this property so much that he and his colleagues Podolski and Rosen published an article in 1935 in which they thought to proof that quantum theory cannot be complete and would have to be substituted by another theory including variables that in quantum theory are still “hidden”. Their “EPR paradox” argument was first theoretically falsified by Bell (“Bell’s theorem”) who showed that quantum mechanics is indeed complete. Until today, Bells theorem was experimentally supported many times. No hidden variables are needed to describe the quantum nature completely.

The strange property entanglement is also the basis for quantum teleportation – where one transfers a quantum mechanical state from one system at one place to another system at another place - and quantum cryptography. The goal of the latter is to send information from one place to another in a completely secure way. Obviously, a quantum cryptography apparatus would be a very powerful and important instrument. Quantum cryptography relies mostly on single on entangled photons and is already commercialized.

**Quantum computing** is expected to allow for calculations, simulations or operations at a speed that classical computing can never reach. For example, it was theoretically shown that a quantum computer would be able to perform database searches or factorization of large numbers much faster than classical computers. The enormous calculation power of a quantum computer is a consequence of two main ingredients. First of all, the fundamental piece of information is a quantum mechanical two state system (|0> and |1>) called QuBit that – unlike a classical bit which is either 0 or 1 – can be in any superposition (a|0> + b|1>) of the two states. Second, the basic calculations are coherent operations that act on such a superposition state. This way, all possible realizations of anything between |0> and |1> can be computed simultaneously and highly parallel computation is realized. Gate operations, the fundamental operations of computing, were shown with trapped ions and with photon based quantum computers. Using solid state systems (NMR), a proof of principle for quantum computed factorization of the number 15 was demonstrated.

**Quantum teleportation** is referring to a procedure in which the quantum mechanical state of one object is fully transferred to another object at a different place. It makes use of the non-locality of entanglement that confused not only Einstein. Using a clever sequence of measurements and entanglement operations on photons, the polarization state of one photon could be mapped to another photon completely. Just recently, quantum teleportation between distant matter QuBits was shown using two separate ion traps. Closely related to quantum teleportation and quantum computing is the so-called “quantum logic”. Here, depending on the quantum state of one object a specific state of another object is created. This controlled state preparation was used in metrology to realize one of the best atomic clocks in the world based on aluminum ions.

**Quantum cryptography** uses quantum physics properties like entanglement and back action of the measurement process on a quantum state to achieve secure communication between a sender (Alice) and a receiver (Bob). The standard approach is that Alice and Bob perform measurements on entangled quantum systems, usually entangled photons, in order to create a key for Alice and Bob. Since they can then use this code to encrypt and decrypt the real message, the quantum cryptography method is called quantum key distribution. The real message is encrypted by Alice according to her measurement results and sent through an open channel (so anyone is allowed to “listen”) to Bob who decrypts the message according to his measurements. Any eavesdropping, so any attempt of a third party to detect the quantum key, can be detected because according to quantum physics laws each measurement influences the quantum mechanical state itself. Eavesdropping would be noticed always. Due to its obvious significance, quantum cryptography research is pushed a lot and many results have been achieved so far. Quantum key distribution over hundreds of km in fiber or over a whole city in free space was shown already while satellite-links of entangled photons between earth stations are currently explored. To proof the usability, a quantum encrypted bank transaction was undertaken.

**Sources for single or entangled photons** are important tools for quantum computing and quantum cryptography. Single photon sources emitting exactly one photon at a triggered time can be realized in many ways incorporating e.g. color centers or ions in solids, single atoms in traps or optical cavities, trapped ions or quantum dot systems. The most common source for entangled photons is based on spontaneous parametric down conversion. A “blue” photon is converted into two red photons within a non-linear optical crystal. Polarization, momentum and energy of the two photons are strongly correlated. A lot of research on this topic is under way. Main efforts are focused on the development of efficient – ideally full deterministic – sources and realizations with mass production potential.

**TOPTICA’s added value**

TOPTICA is a highly appreciated supplier for quantum information experiments that involve trapped ions or atoms. Our lasers are successfully applied to cool, trap, optically pump or coherently manipulate ions and atoms. They are fabricated or tuned to the required wavelength such that they can be used to excite single photon emitters. To create entangled photon pairs by parametric down conversion one needs a fundamental laser at half the wavelength of the photon pair in order to initiate the conversion process. Frequently, entangled photons in the near infrared around 800 nm are used and hence violet lasers around 400 nm are required. The development and fabrication of lasers in the UV is TOPTICA’s core competence. We were the first company to produce diode laser systems in the UV and offer a variety of systems with different linewidth/coherence characteristics and power levels for scientific research and industry. No other company has a similar product portfolio. Please contact us to find the best laser for your application.

- Related Products
- Related Literature
- Brochure: Scientific Lasers
- Brochure: iBeam smart
- Article: Frequenzkonvertierte cw-Lasersysteme für Forschung und Industrie
- Application Notes: Trapping and quantum computing
- Book recommendation: Oliver Morsch: „Quantum Bits and Quantum Secrets“, Wiley

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