# Information at the Quantum-Classical Frontier

Physicist Chris Monroe of the University of Michigan observes that:

"A quantum computer would process quantum superpositions of inputs, promising an exponential speed up over classical computers when applied to certain algorithms. Recent interest in quantum information science is largely driven by technology, as there is some hope that quantum computers might temper the imminent demise of Moore's Law of exponential circuit density growth. On a deeper level, this revolutionary form of computing calls for a re-examination of quantum mechanical foundations, as the thought experiments that proved so useful in the conceptual development of quantum mechanics are now being realized in the laboratory."

Elsewhere, he has wondered:

"Will a quantum computer ever be built? If so, it will probably not look anything like today's computers. Conventional computing relies on dissipation, or the stabilization of logic levels (e.g., voltage levels) through constant measurement and correction. Quantum computing, on the other hand, can only thrive in the near-absence of dissipation. Quantum memories cannot be monitored during the computation—even the mere possibility of distinguishing logic levels will lead to errors. So far, primitive quantum logic gate operation has only been demonstrated in exotic laboratory systems, such as individual atoms confined with electromagnetic fields in a vacuum tube, or individual photons bouncing around between 99.9999% reflecting mirrors. While we hope that quantum computers might someday be developed from the same technology that brought us the modern PC, it is not clear that such solid state systems will ever be sufficiently isolated for storing quantum information. If a quantum computer is ever built, it will likely involve technology radically different than anything out there now."

No doubt. For the move from the physical to the informative (and back again?) is tricky. How to go from it to bit, so to speak? But this it from bit notion from John Wheeler is being explored, developed, and will certainly crest the wave of the future. And for those interested in a little enjoyable history, Wheeler's student, Richard P. Feynman, wrote two related essays, "Computing Machines of the Future" and "There's Plenty of Room atthe Bottom," that are found in the 1999 book *The Pleasure of Finding Things Out* (Helix/Perseus, 1999).

To find out more about how quantum computing might be done, continue reading today's column which is part of a special series in anticipation of The Science & Ultimate Reality Symposium in Princeton, a symposium in honor of the 90th year of John Archibald Wheeler, a great physicist and teacher of physicists.

—Editor

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In recent days I have posted papers describing the how the subject of quantum information, and the related topic of quantum computation, is undergoing explosive growth worldwide. In the last decade physicists have realized that the rules for information processing—indeed the very conceptof information—is fundamentally different at the atomic and molecular level, where the familiar notions of cause and effect are replaced by the weird laws of quantum physics. The ability of a quantum particle such as a photon to be in many different states at once implies an exponential increase in information processing power.

Although the primary goal of this initiative is to build a functioning quantum computer, the research involved has probed deeply into the nature of information itself, and its physical realization in states of matter. This in turn has focused attention on the long-standing problems surrounding the interpretation of quantum mechanics and the nature of reality. A paper by Chris Monroe continues the theme. The summary is below.

—Paul Davies

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Summary:

A quantum computer would process quantum superpositions of inputs, promising an exponential speedup over classical computers when applied to certain algorithms. Recent interest in quantum information science is largely driven by technology, as there is some hope that quantum computers might temper the imminent demise of Moore's Law of exponential circuit density growth. On a deeper level, this revolutionary form of computing calls for a re-examination of quantum mechanical foundations, as the thought experiments that proved so useful in the conceptual development of quantum mechanics are now being realized in the laboratory.

The fundamental unit of quantum information is the quantum bit or qubit, which is simply a two-level quantum system such as a single photon, spin-1/2particle, or two resolved levels within an atom. The most general quantum state of N qubits is an entangled superposition of all 2^N basis states, and is not factorizable into individual qubit states. Roughly speaking, entangled qubits have hidden interconnects that are not available in classical devices, providing the power behind quantum computing.

Entanglement also aids a pragmatic description of the quantum measurement process and quantum decoherence. For example, when two orthogonal states of a pure quantum system |Q1> and |Q2> become entangled with distinct states of the environment |e1> and |e2> (where< 1), the resulting entangled superposition |Q1>|e1> + |Q2>|e2> loses coherence when we trace over the innumerable environmental degrees of freedom of the classical environment. This approach to the quantum measurement problem is conceptually lacking, as measurement itself (not to mention the observer) remains an integral part of the theory, implying an arbitrary boundary between the quantum and the classical. In the pursuit for large-scale entangled states for quantum computation, experiments will hopefully chip away at this boundary as the decoherence of larger and larger quantum systems gets documented. Moreover, we shouldn't rule out the possibility of testing alternative theories of quantum measurement (e.g., spontaneous wavefunction collapse) along the way.

Why do we not yet have a large-scale quantum computer? Quantum computer hardware requires the engineering and control of complex quantum states with many degrees of freedom, tantamount to building a mesoscopic version of Schrodinger's cat. The experimental requirements are therefore severe, with a degree of isolation never before demonstrated. This chapter will discuss several exotic physical systems where quantum computing appears feasible, including individual electromagnetically trapped atoms and photons confined in cavities. In these systems, the scale-up to useful quantum information processors appears to be hampered only by technical considerations. But perhaps the most exciting feature in the quest for a quantum computer is that the ultimate quantum hardware will probably be derived from a physical system that has not yet been discovered.

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