On The Road To Quantum Computing

Demonstrating a new paradigm for quantum computing, Yale University researchers have built what they call QED integrated circuits to manipulate quantum bits.

While the almost mystical allure of quantum computing has been verified time and again using qubits in a physics lab, building real circuitry on silicon chips has had only sporadic success, until now.

The QED-for quantum electrodynamics-circuits operate on quantum bits by using a superconducting "Cooper box" to store oscillating microwave photons that can be read and written without disturbing their quantum states.

Quantum computers promise to outpace digital computers by using qubits, which can represent a superposition of simultaneous values, thereby achieving parallel processing without parallel hardware.

"I think that EEs understand how qubits involve a superposition of quantum states, but they may not know that you can build integrated circuits that way," said Steven Girvin, a professor of physics at Yale.By superpositioning quantum states that simultaneously perform parallel operations, quantum computers can break encryption codes and work other technological miracles that a digital computer would find impossible. Many quantum state mechanisms, some of them potential building blocks for future quantum computers, have been demonstrated in physics labs. But Yale's demonstration of how to build chips using what it calls "qutons"-a qubit on a photon-enable quantum computers using QED circuits to be put onto chips today. Girvin's work, performed with Yale associate research scientist Andreas Wallraff, was funded by the National Science Foundation.

Twelve-year journey

The history of qutons began in 1992, when a roomful of experimental equipment demonstrated that a coupled atom and an optical cavity could store qubits. The current Yale team (www.eng.yale.edu/rslab/Andreas/content/science/CircuitQEDSpecial.html) improved on that approach, downsizing the apparatus by more than 1 million times to a 1-centimeter-square chip and by switching to microwave transmission lines instead of optical. As a result, the current resonant coupling between a microwave photon and the atoms of an on-chip Cooper box is a thousand times stronger than 1992's original experimental setup.

"Circuits [using] QED offer a new paradigm for EEs-one in which quantum optics experiments can be performed on-chip with electrical circuitry using microwaves instead of visible photons from lasers," said Girvin.

For EEs, the advantages of Yale's method include the relatively small size of its qubit repositories-about a square micron-and the ability to read a qubit's state without disturbing it-the bane of quantum computers to date. Yale's invention stores qubits in a Cooper box that has more than a billion superconducting aluminum atoms acting together, providing a kind of quantum momentum that allows a "probing" photon to read out a qubit's state from the Cooper box without changing it, Girvin said.So far, researchers have concentrated on using visible-light photons from lasers to attempt quantum computers, but switching to microwaves provides a cookbook for computing circuitry using quantum electrodynamics. Soon, the researchers predict, electrical engineers will be setting up experiments in their own labs to test quantum circuitry for a new breed of quantum integrated circuits using optics with microwave photons that interface on-chip with traditional electronic circuits.

"I think the thing that is out of the ordinary for electrical engineers, is that this superconducting electrical circuit has currents and voltages that are uncertain. Their quantum probabilities must be described using the Schrodinger equation of quantum mechanics," said Girvin.

QED circuits are made possible by the coupling between a trapped photon and the atoms of the Cooper box containing it. A Cooper box is a small island on a chip acting as one electrode of a Josephson junction-a superconducting gate in which special quantum coherence properties enable the electrons to travel in "Cooper pairs." By making the Cooper box extremely small, quantum confinement effects dominate its behavior, enabling qubits to be stored there.

In action, the Cooper box acts somewhat like a normal DRAM-type repository for digital bits-a capacitor-except that the qubit stored there is not a static charge but an oscillation. The oscillatory transfer of a single Cooper pair through the Josephson tunnel junction forces the Cooper box to change its quantum energy state, as if it were a single atom. By absorbing and re-emitting the microwave photon-as a single Cooper pair tunnels back and forth across the Josephson junction-a strong coupling between the atoms of the box and the photon trapped there maintains the qubit's state.

When the researchers injected a microwave photon into the superconducting Cooper box, it oscillated, as predicted, by repeatedly absorbing and re-emitting the photon while maintaining the qubit's nebulous state of quantum superposition. Because the approximately 1 billion atoms in the Cooper box were acting together, due to superconductivity, they behaved as if the oscillation was between a single atom and a photon. "EEs can use our work to build real circuits, whereas physicists' experiments using ordinary atoms are just too small and hard to control. Since our atoms are made artificially, they are large enough to engineer with," said Girvin.The oddness of the quton practically defines the field of QED circuitry. One part light-a tiny microwave photon 100,000 times less energetic than a photon of visible light-and one part matter, the Josephson junction acts like a single atom but is 10,000 times larger. At the microwave wavelength of 1 centimeter, the wave properties of the photon are damped out by quantum confinement in the micron-sized Cooper box-a volume equal to one-millionth of its cubic-centimeter wavelength.

Likewise, the particle properties of the superconducting atoms are damped out in favor of a wavelike uniformity among them-prompting coupled behavior as if they were a single atom. As a result, this odd couple oscillates between wave/particle properties in perfect synchronization-at 12 MHz-maintaining the state of its qubit for about one microsecond.

"We observed the formation of a novel quantum state that is partly photon and partly atom excitation-what our research assistant dubbed a quton," said Girvin. "Heisenberg's uncertainty principle says you can't simultaneously measure the velocity and position of a particle, and likewise in QED circuits you can't measure the voltage and the current at the same time."

The novelty of the confined oscillating microwave photon is that the Heisenberg uncertainty principle applies here to prevent an observer from measuring whether the photon is currently absorbed or freshly re-emitted. Quantum mechanics can say only that it is oscillating between those two states, but cannot say when it is in one or the other without destroying its delicate balance. However, the experimenters were able to accurately determine whether the photon was currently absorbed by virtue of the synchronicity among the billion atoms of the superconductor.

'Helper' bitIn other quantum computer designs, elaborate mechanisms involving entangled "helper" bits are required to perform constant error correction, but not so for QED circuits. Instead, quantum computers based on QED circuits should be able to directly read out the state of Cooper-boxed qubits without disturbing their state.

"The coupling of the microwaves to the artificial atom is so strong that it should be relatively easy to construct simple devices to detect the state of individual microwave photons-the analog of a photomultiplier. Now researchers can do detailed experiments using the nonlinear optical effects of QED circuits where photons interact with each other through the atom," said Girvin.

The researchers plan next to construct adjacent qubit repositories on a chip so that multiple qubits can interact with one another to create quantum gates. The researchers also noted one downside-that the circuit needs to be cooled to just two hundredths of a degree above absolute zero (10,000 times colder than room temperature).