The figure below shows the world's fastest digital IC, which was designed, fabricated and tested in the Department of Physics & Astronomy at Stony Brook. In this circuit, bits of information are transferred by moving the elementary unit of magnetic flux, the flux quantum F₀ = 2.07×10^-15 Web, through a sequence of superconducting loops containing Josephson junctions called SQUIDs (for Superconducting QUantum Interference Device). A simplified schematic of this divide-by-two circuit is seen in the upper left. The Xs represent the Josephson junctions, which are just tunnel junctions coupling the superconducting niobium electrodes. The green lines represent the Nb thin film wires that form the SQUID loops, since the ground points are also connected by Nb films. In operation, a pulse train of flux quanta enters the device on the left, passes through a series of SQUID loops (a Josephson transmission line) and enters the T-flip-flop, which consists of the 4 junctions on the right.

The quanta alternately exit either the top or bottom of the right most junctions. The output, which is taken across the top right junction then generates one output quantum for each two input quanta. The proper operation of the device can be verified by simply measuring the DC voltages at the input and output, since the Josephson equation provides a very fundamental relation between the number of flux quanta passing through a junction and the voltage across it: V = nF₀. So 1 Terabit/s give a voltage of approximately 2 mV. The comparison of the input and output voltages is shown in the lower right panel. To within our measurement accuracy, V_in = 2V_out for bit rates from DC to 750 Gbit/s. The lower left panel shows the actual device while the upper right panel compares our results to those of the fastest semiconductor digital ICs. In spite of being more than an order a magnitude faster, the superconductor circuit dissipates over 10⁵ times less power than the semiconductor version. This low dissipation is quite important, since the very dense packing of circuits required for, e.g. a petaflop computer, makes it essentially impossible to cool high dissipation circuits. More recent versions of our device are even more compact and operate to 770 GHz.

These results have recently been published:
"Superconductor Digital Frequency Divider Operating up to 750 GHz", W. Chen, A.V. Rylyakov, V. Patel, J. E. Lukens and K.K. Likharev, Appl. Phys. Lett., 73, 2817-2819, 1998 and
"Rapid Single Flux Quantum T-flip flop Operating up to 770 GHz",W. Chen, A.V. Rylyakov, V. Patel, J. E. Lukens and K.K. Likharev, IEEE Trans. Appl. Supercond., in press, 1999.

W. CHEN, A. V. RYLYAKOV, VIJAY PATEL, J. E. LUKENS, and K. K. LIKHAREV