Number 250 (Story #1), December 1, 1995 by Phillip F. Schewe and Ben Stein TWO-BIT QUANTUM LOGIC GATES have been experimentally demonstrated for the first time. Analogous to conventional electronic logic gates in personal computers but different in that they follow the strange rules of quantum mechanics, a quantum logic gate, in its simplest form, consists of two "qubits." Each qubit is a quantum system (for example an atom or a photon) having two states corresponding to the 0 and 1 of a conventional gate. Unlike an ordinary digital bit, a qubit can be in a combination or "superposition" of 0 and 1, offering the potential for unique kinds of calculations. A NIST team (Chris Monroe, 303-497-7415) uses a single trapped beryllium ion to demonstrate a two-bit quantum logic gate. One bit, the control bit, is specified by the (quantized) external vibrations of the ion in the atom trap; the two lowest vibrational levels correspond to values 0 and 1. The other bit (the target bit) is specified by an internal state of one of the ion's electrons; it has a "spin-down" state (0) and a "spin-up" state (1). Shooting laser pulses at the single ion causes it to act as a two-bit "controlled NOT" gate. If the control bit is 0 then the target bit is left alone. If the control bit is 1 then the target bit flips its spin. Meanwhile, a Caltech group (Quentin Turchette, 818-395-8343) has demonstrated the feasibility of using a pair of electromagnetic fields (each representing a single photon or less) as a two-bit quantum gate. When the two fields interact with an atomic beam in between a narrow cavity, the first field, having one of two orientations, or "polarizations," can control the phase of the second field; switching the polarization prevents the first field from controlling the phase. Finally, in a paper submitted to Physical Review Letters, a team at the Ecole Normale Superieure (Serge Haroche, haroche@physique.ens.fr) reports a quantum logic gate in which a two-level electromagnetic field in a cavity changes the energy level of a Rydberg atom (an atom in a highly excited state) in the cavity. All groups are currently attempting to string together multiple gates, but this remains a major challenge. Performing the powerful calculations envisioned with quantum computers would probably require thousands of gates, but Haroche warns that systems of quantum gates are likely to become "decoherent," or lose their quantum properties, beyond several tens or hundreds of gates. While practical "quantum computers" might be difficult to realize with present concepts, physicists believe these two-bit experiments may pay off by opening possibilities for practical schemes of quantum teleportation and quantum cryptography and by bringing new insights into, as Haroche puts it, "the fuzzy boundary between the classical and quantum worlds." (C. Monroe et al. and Q. A. Turchette et al., Physical Review Letters, Dec. 18; journalists should contact AIP Public Information at physnews@aip.org)