Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Superconductive device
Reexamination Certificate
2001-08-22
2003-06-17
Elms, Richard (Department: 2824)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Superconductive device
C505S202000, C505S210000, C333S018000
Reexamination Certificate
active
06580310
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lowering bit error rates in superconductor integrated circuit devices, and, more particularly, to a superconductor means for producing useful double flux quantum pulses in response to received single flux quantum pulses.
BACKGROUND OF THE INVENTION
Metals, metal alloys and ceramics found to exhibit zero electrical resistance are commonly referred to as superconductors. Typically, those superconductors don't attain the superconductive state unless cooled to extremely low temperatures, referred to as cryogenic temperatures. Each such superconductor material possesses a unique cryogenic temperature, referred to as the transition temperature (“Tc”), at which the respective metal, metal alloy, or ceramic becomes superconducting, changing in electrical resistance from a measurable or relatively high value of resistance to zero. One known superconductor is niobium, a refractory metal, which transitions to a superconducting state at a temperature of 9.2 Kelvin.
Superconductor digital electronic devices have previously been constructed of superconductors and the functionality of such devices demonstrated. As example, with a zero-resistance characteristic during superconductivity, electrical current induced into a loop formed of the superconductor metal persists indefinitely. With appropriate drivers and sensors, the foregoing loop may serve as a digital memory. When the direction of the current induced in the loop is in a clockwise direction the memory state may represent a “1” digital bit; when the direction of induced current is counterclockwise, the memory state may represent the bit “0”.
Superconductor digital electronics devices have been fabricated as integrated circuits on a silicon wafer using the photo-lithographic mask and etch techniques or other known techniques most familiar to those in the semiconductor industry. Such superconductor integrated circuit devices provide the desired functionality in a very small package or chip. Superconductor devices operate at very high speeds, as example, 100 GHz to 770 GHz, and very low power, which is unattainable with present semiconductor devices. Because of the high speeds of operation and low power requirement, superconductor electronic devices remain attractive for many applications.
A principal element to the construction of a superconductor digital electronic device is the Josephson junction. The Josephson junction is formed, as example, of two layers of superconductors, such as niobium, separated by a very thin layer of electrical insulation, such as aluminum oxide. When cooled to the transition temperature and biased with DC current below a certain critical current, (“IC”) the Josephson junction is superconducting and the junction conducts current without developing a voltage drop there across and without dissipation of energy, exhibiting no electrical resistance. Consequently, the junction does not produce heat, which is a significant advantage for integrated circuits. If biased above the critical current, the Josephson junction produces an RF signal, consisting of a series of pulses at RF frequencies. Thus, the critical current is a boundary at which the electrical properties of the junction changes as described.
Superconductor circuits utilize the foregoing property of the junction to regenerate single flux quantum (“SFQ”) pulses. The time integral of the voltage of a single flux quantum pulse is physical constant approximately equal to 2.07 millivolt picoseconds or, in alternate terms, 2.07 milliamp picohenrys (e.g., h/2e, where h is Plank's constant and e is an electron charge). When a SFQ pulse is applied to a Josephson junction that is properly DC biased below the respective critical current, the current produced by the SFQ pulse when added to the DC bias current may cause the Josephson junction to brief exceed the critical current. The Josephson junction then undergoes a 360 degree shift in quantum phase or, as otherwise termed, electronically “flips-over.” In undergoing that shift the Josephson junction reproduces the single flux quantum pulse in response a the applied SFQ pulse.
In superconducting integrated circuit (“IC”) devices containing multiple Josephson junctions, the junctions are formed on a common superconductor metal layer, referred to as a ground plane, deposited over an insulator substrate, such as silicon. The multiple Josephson junction devices may be logically divided into groups of two or more junctions, the groups, sometimes referred to as “SQUIDs” (an acronym for superconducting quantum interference device). For example, a single flux quantum pulse transmission line, referred to as a Josephson transmission line, may be formed of a number of SQUIDs arranged in serial order, each SQUID containing two Josephson junctions connected electrically in parallel in a superconducting loop, the latter also sometimes referred to as a Josephson loop.
A single flux quantum pulse applied to the input of the Josephson transmission line (“JTL”), may be said to propagate along the transmission line to the output, moving from SQUID to SQUID in that line, and thence to the electrical load connected to the output of the transmission line. In fact, the SFQ pulse is regenerated at each Josephson junction (stage), which can produce current and power gain. The transmission line may in total contain two or more Josephson junctions, the number of Josephson junctions (and SQUIDs) that form the transmission line can be increased to traverse the desired distance.
Digital integrated circuits require superconductor chip-to-chip communication to convey Single Flux Quanta. The assignee of the present invention has internally demonstrated chip-to-chip communication of SFQ pulses at rates of up to 20 Gbps on a superconductor chip mounted on a passive superconductor carrier. However, the bit error rate (“BER”) was found to increase with frequency and exceeded a self-imposed limit of 10e
−16
at frequencies over five Gbps. The presence of attenuation and reflections (e.g. transients) on the chip-to-chip transmission line and other electronic noise which mask or obscure the digital bits at random is a principal factor causing bit errors. Those influences increase in adverse effect as the frequencies increase to 20 Gpbs, to 40 Gpbs, and more so at 100 Gpbs, the more desirable frequency regions for communication, and produce unacceptable bit error rates. The need to remove those influences, or to minimize the adverse affect of those influences or to otherwise lower the BER is apparent if superconductor chip-to-chip communication at those frequencies is ever to succeed.
When the foregoing kind of problem occurs in non-superconductor electronic devices commonly used at lower frequencies, such as semiconductor circuits, a known solution is to amplify the desired signal, that is, employ a driver to amplify and apply the amplified signal to the succeeding “noisy” stages of the communications equipment, thereby raising the level of signal relative to the electronic noise, the signal-to-noise ratio. By definition, a driver is a device that supplies a useful amount of signal energy to another device to insure the proper operation of the latter device. By upgrading the signal relative to the noise, the succeeding stages in the electronic apparatus more readily recognizes the signal, and, hence, the error rate is minimized or eliminated.
Although a Josephson junction is an active device, the junction cannot function as a signal amplifier in the customary sense. The only superconductor devices heretofore known in the superconductor art that are capable of obtaining operational speeds of twenty to one hundred Gbps with a negligible BER generate single flux quantum pulses. Otherwise, no superconductor drivers have been known previously. The unavailability of a useful superconductor driver would be expected to lead one to explore other alternatives for lowering the bit error rate. However, as an advantage, the present invention provides a new and useful superconductor driver.
An object of t
Goldman Ronald M.
Hur Jung H.
Northrop Grumman Corporation
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