Photo by Bill Wiegand
Research at Illinois
Quantum information science is the study of the often-bizarre-seeming features of quantum mechanics and their application to problems in information processing. These features include wave-particle duality, the principle of superposition, the intrinsic randomness of quantum mechanical measurement outcomes, and the phenomenon of entanglement. (Entanglement refers to the nonlocal correlation that can exist between quantum mechanical systems, even when the components are separated by large distances.) These phenomena are being applied to such tasks as quantum computation (which could allow an incredible speed-up over classical computation for certain types of problems); quantum cryptography (the only provably secure method of encryption, whose security is guaranteed by the laws of physics); and quantum metrology (by which measurements can be made with resolutions exceeding those allowed by classical physics).
The canonical building block of all quantum information processing is the quantum bit or "qubit." This is a two-level system (e.g., a spin-1/2 particle or a two-level atom), which like its classical counterpart, can be put into the state of "0" or "1," but can further be put into any superposition of the states | 0> and |1>. Here at the University of Illinois at Urbana-Champaign, we are investigating a number of different physical realizations of qubits involving photons, nuclear and electronic spins, and superconducting devices. The goal is to identify physical systems that are optimal for particular applications. For instance, photons are a natural choice for quantum communications, where it is desired to transmit quantum information over large distances. For quantum computation, on the other hand, it is desirable to have a system that can be readily mass-produced, such as a solid-state system.
One obstacle that must be overcome with any system is the phenomenon of decoherence, by which the quantum information is leaked away by unwanted interactions with the environment. We are experimentally and theoretically studying ways in which the deleterious effects of decoherence may be mitigated or avoided altogether. For example, by encoding a single logical qubit in a pair of entangled physical qubits, one can gain immunity to some forms of noise. We are also attempting to generalize existing arguments that allow us, to an extent, to infer the rate of decoherence in the quantum regime from the dissipation in the classical limit. In addition, we are exploring nonstandard methods of implementing quantum operations, e.g., using geometrical phases, and using the inherent nonlinearity of projective measurements.
The overall goal of our research is to gain a better understanding of basic quantum mechanical principles, how they are manifested in various physical systems, and how to apply them to push beyond the classical limits on information processing.
Brian DeMarco, experiment
James N. Eckstein, experiment
C. Peter Flynn, experiment
Paul M. Goldbart, theory
Anthony J. Leggett, theory
Myron B. Salamon, experiment
Foundations of Solid-State Quantum Information Processing*
Quantum information processing (QIP) lies at the forefront of revolutionary computing research, promising radically new powers to computation and communication, e.g. unconditionally secure quantum cryptography and quantum logic for greatly enhanced speed on certain computational problems. This project addresses the critical question of how to achieve a physical system capable of meeting the two most challenging requirements for building a quantum computer -- scalability, the fabrication and coupling of a large number of quantum bits ("qubits"), and quantum coherence, the control of noise and external coupling effects so that the exquisitely fragile quantum mechanical circuits will not be perturbed by unwanted influences.
An interdisciplinary research team at the University of Illinois at Urbana-Champaign is exploring a wide range of solid state systems based on the manipulation and measurement of magnetic moments to perform quantum logic operations. By studying the full range, from single spins to small clusters of spins (in quantum dots), to large current loops in superconductors, they are attempting to assess the relative merit of different techniques, and determine the physical size limits for magnetic systems acting as qubits. The ultimate goal is the physical realization of a small system for performing quantum logic operations. A key component is the integration of research and education via a highly interactive program involving undergraduates, graduate students, and postdoctoral investigators. This project is providing a crucial role for an explosive new field such as QIP, by providing general awareness of the issues involved and by training a pool of experienced researchers.
*This material is based upon work supported by the National Science Foundation under Grant No. 0121568. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Optical Investigations of Quantum Information
Central to most applications in quantum information processing is the phenomenon of entanglement. It is thus of great importance to fully understand how to produce, manipulate, and characterize entangled states. Our studies are focused on optical realizations of entanglement, e.g., in the correlated polarization states of photons produced via the process of spontaneous parametric down-conversion. We have already developed a tunable, ultrabright source of entangled photons and a method of quantum state tomography, which allows us to precisely measure the quantum mechanical state.
Having demonstrated the proof of principle of these techniques, the next step is to turn them into useful tools. As a first step, we are developing a fully-automated computer-controlled quantum tomography system. Such a system will permit us to characterize the various states we create much more quickly and reliably than is presently possible. We will also then be able to perform quantum process tomography, by which one can completely characterize an arbitrary "black box" affecting the qubits of interest; such a technique is critical for all quantum computing systems.
Simultaneously, we wish to further extend our ability to produce arbitrary states. In particular, a general state of two qubits is defined by 15 independent parameters. Using a realistic estimate of 10 resolvable values for each parameter, we should be able to produce 1015 states. For 4 qubits, this number becomes a staggering 10255. Our goal is to achieve state control and measurements at the 0.1-percent level, a factor of 20 beyond the current limits. The overall idea is to use the optical systems as a clean test bed, the results of which can then be applied to other physical realizations of qubits.
Optical Quantum Memory and "Delayed-Choice Quantum Cryptography"
In today's Information Age, the protection of electronic information is of critical importance for national security, industry, and private individuals. Quantum cryptography—more correctly described as quantum key distribution—is the only provably secure method of encryption. It uses single photon transmissions to generate a shared random secret key between the users, which they may then use in a variety of ways to encrypt and decrypt messages. Because there is no way to "tap" a single photon, and because it is known to be impossible to accurately copy the quantum state of a photon, the only strategy left to a potential eavesdropper is to make a measurement on the photons before sending them on to the intended recipient. However, this is known to introduce large and readily detectable errors, thus ensuring that no eavesdropping can go unnoticed. Thus, the security of these schemes is guaranteed by the laws of physics.
quantum cryptography schemes to date, there is a necessary upper limit
to the efficiency of the scheme, i.e., at most 50 percent of the photons
sent can contribute to the final key. By incorporating a novel delayed-choice
aspect, so that the receiver always makes an optimal measurement, we propose
to increase this limit to 100 percent, i.e., no photons need be wasted.
Another advantage is that more exotic quantum cryptography protocols,
which make an eavesdropper easier to detect, become practical. Finally,
the photon-storage hardware developed for this project will have great
utility in other aspects of quantum information processing. For example,
we will construct a heralded single-photon source, emitting pulses with
exactly one photon, at regularly timed intervals. Such a source improves
the security of all quantum cryptography protocols, and could form the
basis of all-optical quantum computers, in addition to being extremely
useful for precise calibrations of optical equipment.
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