Quantum Electronics in the New Millennium
C17 Commission on Quantum Electronics
Hans-A. Bachor And
R. E Slusher
The laser opened a new area of physics and technology in the early 1960's. It gave access to new optical phenomena, all based on the process of stimulated emission. Initially a curiosity, the laser has rapidly led to many new applications ranging from precision measurements to communications, and from welding to surgery. Their many varied applications have become an integral part of our daily lives.
The laser itself developed rapidly. The early systems were mainly based on gas laser technology, especially the widespread He-Ne, Argon and Krypton lasers for scientific and medical work, and the CO2 laser for engineering applications. These lasers are reliable but very inefficient and costly in their operation. The last 10 years have seen a complete change in the technology based on all solid-state technology. The semiconductor diode laser appeared initially as an alternative for low power (milliwatt) operation in the red and near infrared regime. This compact laser combines simplicity, small size and low cost. More recently the quantum cascade laser has extended the availability of compact lasers further into the infrared, out to wavelengths as long as 100 microns. This extended wavelength range spans many of the interesting molecular vibrational modes, resulting in a major new area of quantum cascade laser applications in environmental gas monitoring. Compact, low cost lasers in the red, and more recently in the blue, have become essential components of all optical storage technologies, in particular CDs and DVDs for music and data storage. The requirements for all of these applications require the good beam quality, single mode characteristic, low cost and reliability that can now be provided by many types of semiconductor lasers.
Photonics and optical communications
Semiconductor lasers are also now widely used as the light source in communication systems. Just as electrical signals are controlled and manipulated by electronic devices, so also it is now possible to control and manipulate light signals with optical devices, giving rise to the field of photonics. Photonics technology forms the backbone of the extensive optical communications networks spreading around the globe. Its success initially derived from the development of low-loss single mode optical fibers that transmit optical signals over large distances, e.g. across the oceans. The modulated laser signal is transmitted through the optical fiber and manipulated by optically active signal splitters, combiners and amplifiers. A major breakthrough in the early 1990s was the development of erbium-doped fiber amplifier, used to compensate for the large losses over hundreds of kilometers of optical fiber. These amplifiers along with optical modulators with speeds as high as 40Gb/s, allow higher data rates over distances up to 4000km. The capacity of these systems has been increased hundredfold by using wavelength division multiplexing of hundreds of separate wavelength channels in order to achieve staggering total capacities well over a 1012 bits/second (Tb/s) on a single fiber. The clever use of the nonlinear properties of fibers through specially shaped pulses, called optical solitons, can increase the capacity and reach of these systems even further.
Information processing at large routers in the Internet and in networks of computers is being limited by the electrical interconnects that are presently used. The routers for communication networks that route billions of Internet information packets are approaching throughputs of a terabit/second. The electrical limits are fundamental in nature and limit the interconnect data rates to less than 20Gb/s. Optical interconnects do not suffer from these fundamental limits and will come into more frequent use as the data capacity needs intensify and the cost of integrated photonics circuits continue to decrease. Vertical Cavity Surface Emitting Lasers are now available in large arrays for interconnecting back planes of computers and large electronic routers.
High power lasers
Semiconductor lasers have been developed for high power continuous wave operation. The available power per laser increases every year. Diode lasers with powers of hundreds of watts are available. These lasers are ideally suited as optical pump sources for solid state laser systems such as the Nd:YAG laser. This laser operates at a wavelength of 1000 nm. It has an excellent beam quality, very high efficiency and small size. Its high power of more than 100 watts makes it perfectly suited for precision machining. Presently these lasers are replacing the old CO2 technology in many engineering applications. They are superior in their precision and versatility and more efficient. Semiconductor lasers are also being used to pump optical fibers to yield optical gain throughout the Raman shifted wavelengths in silica. This provides a tunable optical amplifier with distributed gain of great value to optical communications systems. These new amplifiers eliminate the restricted wavelength operating range of erbium doped fiber amplifiers and decrease the effective system noise.
The interaction of laser light with nonlinear optical materials leads to a wide variety of new phenomena. The interaction is nonlinear in that the material emits light at a frequency different from the incident signal. For example, frequency doublers emit light at twice the input frequency, and optical parametric oscillators emit light with a frequency equal to the sum or difference of two input signals. This effect can be used to amplify a very weak signal by mixing it with a strong signal. The quality and utility of nonlinear materials have improved dramatically in the last few years, greatly expanding the available wavelength range of solid-state lasers. For example, the frequency-doubled Nd:YAG laser at 500 nm is a now low cost alternative for the Ar ion laser. Combined with optical parametric oscillators, they provide continuously variable wavelengths both below and above 1000 nm. New materials, such as periodically poled lithium niobate (PPLN), have increased the efficiencies of these wavelength conversions and promise simple and easily tuneable systems.
Very high power lasers with intensities in the petawatt range are revolutionizing the study of nonlinear phenomena and providing new sources of X-rays through harmonic generation. At very high laser intensities the electric and magnetic fields of the light are stronger than atomic and molecular fields so that they dominate the dynamics of electrons in the light beam. This leads to interesting new physics and light generation phenomena. At the highest intensities quantum electrodynamics comes into play. Scattering of light from light, one of the weakest interactions in nature, can now be studied using high intensity lasers. There is also a wide range of plasma generation and laser accelerator physics that are being explored with high intensity lasers.
Ultra-short laser pulses
Enormous progress has been made in shortening the length of laser pulses. While in the 1980s pulses shorter than a nanosecond were rare, it is now possible routinely to reach the absolute limit of a few femtoseconds. This corresponds to only a few optical cycles. In order to achieve this limit, new laser media had to be developed that have an extremely broad gain spectrum. In particular the material Ti sapphire brought a breakthrough. In addition, nonlinear pulse compression techniques, such as nonlinear Kerr mirrors, were invented to further shorten the pulses. Femtosecond pulses are now readily available, allowing the probing of extremely fast biological and chemical processes. Reactions can be observed with an unprecedented time resolution. A parallel development will be the use of the pico- and femto-second pulses for the shaping and drilling of materials. Since the material has no time to melt, the fast pulses create perfect edges and holes. Damage centers and photoinduced chemistry may result in the ability to write patterns in glasses or photoresists. In this way ultra-short pulses will find their way into practical engineering. At the edge of research in ultra-short pulses is the generation of atto-second laser pulses using high power lasers that produce short UV and X-ray pulses.
Quantum optical processes
Even the simplest laser is based on quantum optical processes. It was known from the beginning that this leads to quantum fluctuations and thus limitations in the precision of the frequency of laser light. The quantum mechanical intensity fluctuations (shot noise) and the intrinsic laser linewidth are consequences of quantum optics and the uncertainty principle. Since the mid 1980s it has been possible to generate alternative types of light (called squeezed light) that allow an improvement of the measurement of one property, or quadrature, of a laser beam, while sacrificing a precise knowledge of the complementary property. Such squeezed light is now available routinely and, apart from fundamental tests of quantum optical principles, can be used to improve the sensitivity of optical sensors beyond the standard quantum limit. Related are ideas for quantum non-demolition measurements, such as counting the number of photons in a cavity without destroying them by absorption in a detector. The use of nonlinear processes can achieve optical measurements without the quantum mechanical disruption of the system, provided that information is lost about the complementary property of the system (in this case, the phase of the light waves).
Laser interferometers have recently been built for the detection of the gravitational waves resulting from cosmological events such as supernovas or the collapse of stars. To obtain the necessary sensitivity, these kilometer-sized instruments have to monitor the position of mirrors with the precision of optical wavelengths, and in addition measure displacements as small as 10-17 meters. The above techniques of quantum optics will give a sensitivity that will ultimately be limited only by the quantum noise of the light.
The quantum properties of individual photons continue to be interesting. Since photons have spin 1, they are bosons obeying Bose-Einstein statistics. Because of the quantum correlations this introduces amongst the photons, the detection of one photon affects the properties of the remaining light. As a consequence, the effect of a device that splits a photon beam into two branches is considerably different from that of a junction in a wire that carries fermionic electrons. This can be exploited by encoding information directly onto the properties of the photons, for example by modifying their state of polarisation. Any loss of photons from the signal then has a measurable effect on the remaining light. It therefore becomes impossible, at least in principle, for an intruder to tap information without leaving a trace. This idea for secure quantum cryptography has recently found a very rapid development from theoretical concepts to practical systems. Photon sources have now been demonstrated that can emit single photons on demand. These micro-resonator devices have relatively high efficiency and will further enhance quantum cryptographic technology.
Future quantum optics applications may include sytems to transmit data complete with all their quantum information (optical teleportation), and to use light as the transmitting medium in computers based on quantum states. Quantum computers could be used to simulate large quantum systems or as computers that exceed the performance of classical computers for some tasks. A number of physical systems are being explored to achieve quantum computation. An example from the field of quantum electronics is the development of quantum computational techniques using linear optical components and detection of light. The number of optical elements required increases only polynomially with the number of qubits being processed. Experimental implementation of these ideas is in progress, however the very high detection efficiencies, efficient single photon sources and very low optical losses required are a tremendous challenge for the future. It is possible to generate more complicated states of correlated photon beams using the optical parametric oscillator. Separate beams emerge, but the photons in the beams are strongly correlated, forming an “entangled state” of light. Because of quantum correlations between photons, measurements of photons in one beam allow conclusions to be drawn about photons in the other beam. These beams have been used to test some of the fundamental assumptions of quantum mechanics. For example, it may be that the indeterminacy of quantum mechanics is removed by a more fundamental underlying theory containing local “hidden variables” which determine uniquely the outcome of experiments. Based on an experiment originally proposed by Einstein, Podolski and Rosen, Bell derived a set of inequalities that must be satisfied if local hidden variables exist. However, measurements with photon beams show that Bell’s inequalities are violated, thereby ruling out local hidden variable theories. These optical experiments are now the best tests of such foundational principles of quantum mechanics.
Laser - atom interactions
Lasers allow the selective interaction with atoms in specific excited states since they can be tuned to a specific transition of a particular atom or molecule. This ability has led to many spectroscopic applications in chemistry. These include extremely sensitive analytical and diagnostic techniques, and the ability to monitor chemical concentrations. A great advantage is that the monitoring can be done remotely by analysing the light scattered back from a laser beam. This so-called LIDAR, in analogy to RADAR, is increasingly used as a tool for the detection of pollution in the atmosphere and for environmental monitoring.
Tuned laser light can also be used to influence or “coherently control” chemical reactions. Coherent control will undoubtedly have significant applications to the industrial processing of chemicals. Already the use of many-photon ionisation for isotope separation is a well-established technique in the current technology for the enrichment of nuclear fuel. The phase properties of light are also being used to control and optimise high harmonic generation and physical properties of semiconductors.
The use of coherent light has led to more complex interactions where the atom is coherently driven. This allows the use of special pulses to completely invert the atom system and to observe coherent phenomena such as photon echoes and optical nutation. The analogy to NMR is very strong and it is possible to built systems that show a variety of coherent effects.
In addition, the interaction between light and atoms changes the atomic momentum. Suitably detuned laser beams can be used to decelerate and even stop atoms. This is equivalent to cooling the atoms, and temperatures as low as micro Kelvin have been achieved. In combination with magnetic fields, it is possible to trap large number of cold atoms. These slow atoms exhibit clearly the wave-like nature associated with large deBroglie wavelengths. Many forms of atom diffraction by both material structures and periodic light fields have been demonstrated. Atom interferometry, which uses a coherent superposition of de Broglie waves, is now a mature technique that can be used for detailed measurements of atomic properties. A potential application is the precision measurement of variations in gravity from point to point on the earth. These gravity gradients are of great interest to geologists in locating mineral deposits and mapping other geological features of the earth.
Since 1995 it has been possible to cool atoms even further and to observe the influence of the bosonic nature of certain atoms. In particular, these atoms can form a Bose-Einstein condensate in which all the atoms enter the same coherent quantum state. The system is then represented by one macroscopic wave function for many atoms. The properties of these condensates are intriguing because they exhibit the quantum nature of matter on a macroscopic level. They have the properties of a superfluid, but with a density low enough that the interactions between the atoms are still weak. For this reason, it is possible to study theoretically the quantised vortices and quantised excitations of these novel systems. One of the most exciting recent advances is the formation of a coherent beam of atoms from the Bose-Einstein condensate to form what amounts to an atom laser—the atomic analogue of an optical laser. This field is still in its infancy, but the potential applications to microelectronics, micro-machining and atomic holography remain to be developed.
The interaction of light with atoms and molecules is making important contributions to biology and medicine. For example, single molecule fluorescence techniques are being used to observe conformal changes in biological molecules. Single living cells can be trapped and manipulated with infrared beams. The Raman spectra of these trapped cells is obtained to give biochemical composition for real time identification and analysis.
This overview shows that laser physics and optical technology have made great progress in many parallel directions. All properties of laser light have been dramatically improved and there is no end in sight. At the same time lasers have become simpler, cheaper and more reliable. Today’s applications affect every aspect of our lives and span many areas of technology. Optical technologies complement and sometimes replace conventional electronics. Fiber optics and semiconductor lasers have made a stunning transformation of the communications industry to form a ultra-high capacity global network, including of course, the Internet. Lasers will also play an increasing role in engineering and the chemical industry. Enhanced efficiency detectors may soon make solar energy a significant part of the economy. High efficiency organic light emitting diodes promise to revolutionize displays of all types and lighting in many applications.
The intrinsic and unique quantum properties of light will lead to new fundamental tests of physics and to further investigations of the macroscopic manifestations of quantum mechanics. The potential applications of just one very recent development, the atom laser, are as difficult to predict as they were for the optical laser when it was first invented, but it is certain to have an important impact in the future. One day we may develop quantum computers and simulation devices that will yet again transform our world.