How to Talk to Your Quantum Computer
No matter how exotic the innards of tomorrow’s quantum computers may be, users will still have to communicate with them using classical electronic circuits.
That deceptively simple statement conceals a host of very complicated problems. Every physical system under consideration as a quantum data bit (“qubit”) -- whether it consists of neutral atoms, ions, electron/ nuclear spins, quantum dots or Josephson junctions -- must be able to exist, at least temporarily, in a delicate quantum-mechanical condition called “superposition,” in which the bit has values of 0, 1 or both at once. Virtually any contact with the outside world destroys that condition.
But in order to operate the computer, researchers must be able to set and control the qubits, manipulate logic gates, correct errors, remove “noise” in processing, and record the computed values. All those functions will have to be controlled using some digital electronic technology.
The problem is notably acute for qubits constructed from Josephson junctions -- superconducting devices which are under intense study by several JQI Fellows including Chris Lobb, Fred Wellstood, Bob Anderson and Alex Dragt. Josephson junctions are extremely attractive candidates for qubits because the technology is well characterized and readily scalable, and junction response times are in the sub-nanosecond range (one billionth of a second).
That’s important because the JQI junctions’ coherence time (duration of sustained superposition before the system collapses into “decoherence”) is currently 50 to 100 nanoseconds. That’s pretty brief. But a Josephson junction qubit can knock out dozens of operations in that period.
However, Josephson junction qubits operate in ultra-cold conditions -- around 30 milliKelvin (thousandths of a degree above absolute zero) for the configurations that JQI is investigating. Even the chilliest of conventional electronic components would drastically overheat the system.
Moreover, no existing classical control/read-out system can react fast enough to take full advantage of the junctions ’ blazing speed. As a result, physicists have had to artificially slow their systems down in order to conduct experiments.
Obviously, some radically new technology is needed. And that’s where Anna Herr comes in.
From the University of Maryland, the JQI Associate Research Scientist coordinates an international collaboration that is designing and testing control and read-out circuits that are cold, fast and very sensitively coupled to Josephson junctions. Recently, the consortium produced the first working device in which classical and quantum circuits are integrated on the same chip.
Temperature was the main initial concern. Herr and colleagues overcame it by improving on an existing technology called Rapid Single Flux Quantum (RSFQ) logic, a cryogenic digital system that uses Josephson junctions instead of transistors to switch signals, and represents information as patterns of electrical pulses that travel on superconducting transmission lines. One major advantage of RSFQ is that its pulses are single quanta -- indivisible units of magnetic flux -- and thus cannot change form as they move.
The researchers designed a twostage device that has a combined access and operation time of 0.5 nanosecond. (Allowing a“clock speed’” of about 2 GHz.) The first stage consists of an RSFQ circuit that operates down to 30 mK right next to the Josephson qubit. (See illustration above.) It doesn’t touch the qubit. The circuit and the qubit communicate by electromagnetic induction, acting as two coils in the same tiny transformer.
This arrangement is connected by transmission lines to a second stage that runs at a comparatively balmy 4.2 K, the temperature of liquid helium. The second stage is the main processing unit and controls the pulses, which are used both to send signals for error correction and to query the qubit and register its state. The “0” and“1” conditions of the qubit are manifested in the clockwise or counterclockwise directions in which a current travels through the Josephsonjunction. The RSFQ circuit detects that direction in its inductively coupled flux comparator (see diagram below) and sends the result back to the processor as a digital signal.
The RSFQ system can rapidly read out as many as 20 separate qubits thanks to a fortunate quirk of quantum physics: As a single pulse quantum ( called a “fluxon”) passes near a qubit, it changes speed depending on the qubit’s state. So if it passes a long row of qubits, the fluxon’s propagation time will be a sensitive measure of the collective states of all the qubits it passed.
The next steps for Herr and the consortium involve scaling up the system, creating a working prototype with multiple qubits, and finding ways to transfer the state of one qubit to another one -- a process essential for control and error correction. Whatever the final design, one thing is certain: It will be very cool.