1.2.- The tools of quantum technology.

Successful technologies are predicated on precise engineering, which in turn requires highprecision measurement. Quantum technology will thus require us to develop a quantum metrology. It is well known that measurement in quantum mechanics requires a radical reappraisal of traditional measurement concepts. The modern description of measurement, in terms of quantum open systems, suffices to develop the principles required for measurement in a technological context. While many questions remain, it is clear that quantum mechanics enables new types of high-precision measurement.

No complex technology can function without incorporating control systems; feedback, feedforward, error correction, etc. Control systems take as input a measurement record, perform complex signal analysis in the presence of noise, and use the information gained to adapt the dynamics of the system or measurement in some way. Some years ago a simple theory of quantum control and feedback was developed, which indicates that classical control theory is inadequate. The development of the general principles of quantum control theory is an essential task for a future quantum technology.

The recent discovery of quantum communication protocols that are more powerful than can be achieved classically suggests new ways for components of a large complex system to be interconnected. Quantum communication technologies require new principles of operation. A quantum internet based on quantum optical channels, for example, would require new protocols for communication (eg. distributed quantum computing, quantum packet switching) and might incorporate quantum key distribution and error correction as fundamental components. Already it is clear that there is a need to develop an understanding of quantum communication complexity.

Quantum mechanics enables, at times, exponentially more efficient algorithms than can be implemented on a classical computer. This discovery has led to the explosive growth of the field of quantum computation. Building a quantum computer is the greatest challenge for a future quantum technology, requiring the ability to manipulate quantum-entangled states for millions of sub-components. Such a technology will necessarily incorporate the previous three quantum applications for readout (quantum metrology), error correction (quantum control), and interconnects (quantum communication). A systematic development of the principles of measurement, control, and communication in a quantum world will facilitate the task of building a quantum computer.

* Quantum Information Theory:

In parallel to the developments in quantum computing and cryptography, there has evolved an entirely new science of quantum information theory that investigates the fundamental limits of how information can be processed in a quantum world. Due to nonlocal entanglements, which do not exist in classical information theory, all of the classical results have to be reworked to account for subtle quantum effects. There are now quantum versions of Shannon’s theorem for channel capacity, as well as many other fundamental laws of a field that was hitherto only classical at its foundation. Investigations are underway in the areas of quantum data compression and superdense coding.

* Single Spin Magnetic Resonance Force Microscopy:

The ability to detect a single spin at a spatially resolved location would have considerable implications for data storage, quantum metrology, and quantum computing. A number of proposals for solid state quantum computers require the ability to detect the state of a single spin (eg the Kane scheme). Magnetic resonance imaging (MRI) has been a significant technology for many decades and could even be called a quantum technology, based as it is on the intrinsic quantum degree of freedom called spin. The principal on which MRI is based is very simple: a small magnetic moment precessing in a magnetic field will emit a radio frequency field. The magnetic moment could be an electron or nuclear spin in a molecule or atom. If the precession frequency is driven on resonance by an external AC magnetic field, the resulting signal reveals something about the spin state of the molecule or atom and the external magnetic environment in which it is paced. Typically however it is impossible to detect a single spin and the signals obtained represent an ensemble average over a very large number of systems. A magnetic moment in an inhomogeneous magnetic field will experience a force. If this force can be detected and measured it can convey significant information about the source of the magnetic moment. The magnetic resonance force microscope (MRFM) combines the imaging ability of MRI and spatial resolution of scanning force microscopy. By using QEMS devices considerable improvements in sensitivity (minimum force detectable) and spatial resolution can be hoped for.

In the end, the objective is to image and measure the state of a single spin. This will require devices with atto-Newton-force sensitivity and mechanical resonance frequencies approaching the NMR Lamour frequency (10–100 MHz) and quality factors great than 10,000. At these frequencies and quality factors, and at the low temperatures at which such devices must operate, quantum mechanical noise in the mechanical oscillator will ultimately limit the performance of the device.

* Superconducting quantum circuits.

Superconducting Quantum Interference Devices (SQUIDs) are perhaps the best-known example of coherent quantum electronic devices. It is a superconducting ring that includes one or two Josephson junctions. A Josephson junction is an insulating barrier for current flow between two superconductors. The SQUID is a uniquely superconducting electronic device discovered in the mid-1960s that is most sensitive detector of magnetic fields. Commercial SQUIDs can measure magnetic fields 100 billion times smaller than the earth's magnetic field (which is very tiny to start with). With such sensors, signals as weak as the magnetic signature of electrical currents flowing in the human brain can be readily detected [Weinstock 96]. The Nobel-Prize- winning discovery of high temperature superconductors in the 1980s, which require only cheap liquid nitrogen rather than expensive liquid helium to cool, have paved the wave for many more practical applications. The high-temperature superconductors are brittle ceramics, which can be difficult to make into useful structures. However, intense investigation has led to the development of solid-state superconducting components and devices on a chip, using thin-film deposition technology and the same optical and electron-beam lithographic processing techniques that are at the core of the semiconductor industry. Compared to some of the other ideas in coherent quantum electronics, discussed above, the SQUID technology is already well developed. Still, there is much to be done, including the possible application to quantum computers. However, there is a serious roadblock that has yet to be overcome. A SQUID is normally found in a robust macroscopic quantum state with some phase, say 0 or Pi. However, to make a quantum computer it is required that the SQUID be put in a superposition state that has both phases 0 and Pi simultaneously. This is the signature of the elementary quantum logic bit or qubit, and it has just recently been observed in SQUIDs after years of effort. The problem was that the SQUID is a macroscopic quantum state that is not robust against certain types of decohering interactions with the environment. Any attempt to prepare a superposition state of 0 and Pi always ends up with the SQUID rapidly collapsing into either 0 or Pi.

* Quantum photonics.

When light of the appropriate frequency is directed onto a semiconductor an electron-hole pair can be created, bound loosely by the coulomb interaction. Such as pair is called an exciton. If the electron and hole are additionally confined in a quantum dot the exciton can behave very much like an artificial atom, exhibiting a complex spectra. Bound excitons can also be created electrically by injecting electrons and holes into a quantum dot. Quantum dot excitons enable the world of coherent quantum electronics to talk to the world of photons and quantum optics, and provide a path to quantum opto-electronics. Much of the activity in this field is currently directed at developing a single photon source, which produces pulses of light with each pulse containing one and only one photon. Such a technology will enable secure quantum key distribution and quantum optical computing. Currently the most promising paths to building a technology on excitonic quantum dots is based on self assembled interface dots and semiconductor nanocrystals. However the field is barely five years old.

* Spintronics.

The quantum transport properties of charge carriers, such as an electron or hole, are not only determined by the quantum motion of the particle as a whole, but can involve internal degrees of freedom such as spin. Indeed the field of spin-dependent transport, or spintronics, is the fastest growing area of quantum electronics due largely to the possible applications to conventional information processing and storage. The possible application to quantum computing of course has been noticed by many.

* Molecular coherent quantum electronics.

Molecular electronics is the end point of the quest for smaller electronic devices. The feasibility of using molecules as information storage and processing elements for classical computation was suggested long ago by Aviram and Ratner, but is only now having an impact on computer design, largely due to improvements in chemical synthesis. Both Hewlett Packard and IBM have research programs in molecular electronics. The implementation of computing with molecular electronics is forcing a reappraisal of computer architectures. Ultimately, molecular electronics may be able to utilise the self assembly that characterises biological systems, paving the way for a nanotechnology that mimics biological systems, a bio-mimetic nanotechnology. New materials such as carbon nanotubes and fullerenes provide a clear opportunity to engineer quantum nanodevices on a molecular scale, operating on either classical or quantum principles. The importance of this field was recognised by Science which nominated nanotube electronics as the most significant breakthrough of 2001. While much of the current interest in molecular electronics is not based on coherent quantum transport, this is beginning to change. Recently a electronic interferometer was demonstrated, which used on a carbon nanotube. In a more recent experiment still we can begin to see the merging of QEMS technology and molecular electronics. In this experiment charge was conducted trough a C60 molecule which formed an island of a single electron transistor. However the molecule could vibrate mechanically which modulated the conductance curves in such a way that the Terahertz vibration frequency of the molecule could be observed. It is not hard to imagine how such devices could approach a mesoscopic version of ion trap quantum computing experiments.

* Solid-State Quantum Computers.

While much of the interesting mesoscopic electronics is driven by the need to make smaller conventional computing devices, the possible application to quantum computing is obvious and compelling due to the potential robustness and scalability of solid state devices. Some of the earliest suggestions for quantum computing were based on quantum dots. There are now a very large number of schemes, some of which are being pursued in the laboratory. As mentioned above there has been considerable progress in harnessing superconducting quantum degrees of freedom for quantum information processing, and the first all solid-state qubit has been demonstrated in such a device. A solid state quantum computer is probably the most daunting quantum technological challenge of all and will require huge advances in almost all the areas of quantum technology we have discussed.

* Quantum Optical Interferometry:

Recently, several researchers have been able to demonstrate theoretically that quantum photon entanglement has the potential to also revolutionize the entire field of optical Interferometry - by providing many orders of magnitude improvement in interferometer sensitivity. The quantum entangled photon interferometer approach is very general and applies to many types of interferometers. In particular, without nonlocal entanglement, a generic classical interferometer has a statistical-sampling shot-noise limited sensitivity that scales like 1/√N , where N is the number of particles (photons, electrons, atoms, neutrons) passing through the interferometer per unit time. However, if carefully prepared quantum correlations are engineered between the particles, then the interferometer sensitivity improves by a factor of square-root-of-N, to scale like 1/N, which is the limit imposed by the Heisenberg Uncertainty Principle. For optical (laser) interferometers operating at milliwatts of optical power, this quantum sensitivity boost corresponds to an eight-order-of-magnitude improvement of signal to noise. This effect can translate into a tremendous science pay-off for NASA missions. For example, one application of this new effect is to fiber optical gyroscopes for deep-space inertial guidance and tests of General Relativity (Gravity Probe B). Another application is to ground and orbiting optical interferometers for gravity wave detection, Laser Interferometer-Gravity Observatory (LIGO) and the European Laser Interferometer Space Antenna (LISA), respectively. Other applications are to Satellite-to-Satellite laser Interferometry (SSI) proposed for the next generation Gravity Recovery And Climate Experiment (GRACE II). In particular, quantum correlations employed in SSI could improve the orbital sensitivity to gravity enough to give unprecedented accuracy in measuring Earth gravitational anomalies from space with a resolution of 1 km or less. Such a sensitivity could be used for orbital oil prospecting or measuring water content of aquifers. The use of quantum correlated optical interferometers in NASA missions such as these would give a quantum breakthrough in sensitivity and performance.

* Quantum Lithography and Microscopy:

In addition to these developments, it was recently shown that optical quantum entanglement effects can be used in Quantum Interferometric Lithography. This application brings about a breakthrough in lithographic resolution by overcoming the diffraction limit - allowing features to be etched that are several factors smaller than the optical wavelength. This application has tremendous commercial potential in the computer chip and semiconductor industries by allowing sub-100 nm fabrication resolution at low cost.

* Quantum Teleportation:

Quantum Teleportation is another nonintuitive idea that arose out of the field of Quantum Information Theory, discovered by several IBM researchers investigating the transmission of quantum information and quantum states. From the Heisenberg Uncertainty Principle, it is known that it is not physically possible to copy a quantum state exactly. Any attempt to copy a state involves making a measurement to see what you want to duplicate, but this destroys the original state to be copied. However, counter-intuitively, it is possible to “teleport” a quantum state. That is, Alice can destroy a quantum state in Atlanta - only to have an exact duplicate appear instantaneously in Bob’s lab in Boston, without Alice or Bob ever learning anything at all about this state. This phenomenon has been demonstrated experimentally by several groups.

* Atom Optics:

The advent of laser cooling techniques has created a new field of study analogous to the classical study of light optics, but here it is called atom optics. Atom optics allows us to treat the quantum wave nature of neutral atoms in a way that is parallel to the classical wave treatment of light. Using combinations of nanotechnology and laser cooling techniques, it is possible to make atom mirrors, atom lenses, atom diffraction gratings, atom fiber-optic wave guides, and atom traps.

In all cases, the devices guide, focus, diffract, or reflect atoms coherently, so that the phase of the atom wave function is preserved. Thus it is possible to make atom interferometers, atom lasers, atom trampolines, and other exotic matter-wave devices. Again, because of the relatively large mass of atoms, these devices are uniquely sensitive to gravitational and inertial effects. In addition to the inertial sensing aspects, Mara Prentiss at Harvard has a program under way to use atom interferometers as a new type of lithographic device for nanofabrication.

* Quantum Atomic Gravity Gradiometry:

For another application, recent experimental developments at Yale by Kasevich in the field of Quantum Atomic Gravity Gradiometers (QuAGG), have led to a new type of gravity gradiometer based on neutral-atom, matter-wave Interferometry. These gradiometers are, in prototype, as sensitive as current superconducting state-the- art gradiometers, but without the requisite need for liquid helium coolants that currently limit satellite lifetime or the superconductors. In QuAGG, the wave-nature of the atoms is being manipulated by techniques that are the signpost of the second quantum revolution. Quantum correlation effects have the potential of magnifying the sensitivity of the QuAGG by many orders of magnitude, providing incomparable precision in gravity-field mapping of Earth or the other planets from space. These gradiometers also have applications to geological surveying and tests of General Relativity. A fledgling JPL effort to develop a QuAGG for space observation of the Earth’s gravitational field are now underway.

* Atom Lasers:

An atom laser is to an incoherent atomic beam as an optical laser is to a light bulb. The analogy is very good. In an optical laser the photons are all of one frequency and move in a well-defined direction all with the same phase. In the atom laser the atoms all have the same velocity and phase, exploiting the coherent wave-like nature of matter that is dictated by quantum mechanics. Just as the real progress in quantum optics did not occur until the invention of the optical laser in the 60’s, so too is this the beginning of the coherent matter revolution with the atom laser now at our disposal.