October 20, 2012

Quantum Bus to enable connections between quantum processors

In yet another step toward the realization of a practical quantum computer, scientists working at Princeton and the Joint Quantum Institute (JQI) have shown how a major hurdle in transferring information from one quantum bit, or qubit, to another might be overcome. Their so-called "quantum bus" provides the link that would enable quantum processors to perform complex computations.

The finding, by a team led by Princeton physicist Jason Petta, could eventually allow engineers to build quantum computers consisting of millions of quantum bits, or qubits. So far, quantum researchers have only been able to manipulate small numbers of qubits, not enough for a practical machine.

Qubits are unlike a classical bit because they can be not only a 1 or 0 but also both, simultaneously. This property of qubits, called superposition, helps give quantum computers a tremendous advantage over conventional computers when doing certain types of calculations. But these quantum states are fragile and short-lived, which makes designing ways for them to perform basic functions, such as getting qubits to talk to one another—or "coupling"—difficult.

"In order to couple qubits, we need to be able to move information about one to the other," says NIST physicist Jacob Taylor. "There are a few ways that this can be done and they usually involve moving around the particles themselves, which is very difficult to do quickly without destabilizing their spins—which are carrying the information—or transferring information about the spins to light. While this is easier than moving the particles themselves, the interaction between light and matter is generally very weak.

Micrograph of a quantum bus device similar to the one measured for this experiment. Note the Princeton tiger is 1 mm from head to tail. The spin-orbit qubits are located at the nexus of the seven gate electrodes.
Credit: K. Petersson/Princeton

Nature - Circuit quantum electrodynamics with a spin qubit

Taylor says you can think of their solution sort of like playing doubles tennis.

"Whether or not a team will be able to return a serve depends entirely on how well they play together," says Taylor. "If they are complementing each other, with one playing the front half of the court and the other playing the back half, they will be able to return the serve to the other set of players. If they are both trying to play in the front court or the back court they won't be able to return the serve and the ball will go past them. Similarly, if the spins of the electrons are complementary, their field will affect the field of the photon as it goes past, and the photon will carry the information about the electrons' spin to the other qubit. When the spins are not coupled, they will not affect the photon and no information will go to the other qubit."

The Princeton/JQI team's quantum bus is a hybrid system that marries two known quantum technologies—spin-orbit qubits and circuit quantum electrodynamics—with some tweaks. The spin-orbit qubits are a pair of indium-arsenide quantum dots that have been engineered to enable strong coupling between the spins of the electrons trapped inside the dot and the electrons' positions within the dot. This in turn allows the magnetic field of the qubit, comprising spins, to couple with the field of microwave photons traveling through a connected superconducting cavity.

The structure makes it possible for information about the qubits' spin to be transferred to the microwave cavity, which, with some additional tweaks could be transferred to another qubit.

The experiment, which was the culmination of five years of effort, took place at Princeton University. NIST/JQI provided assistance with the quantum theory.

Hybrid DQD / superconducting resonator device. Optical micrograph of a device similar to the one measured. Shown in the expanded views are ,i, the spiral inductor at the cavity node ,ii, the input / output coupling capacitors and, iii, the nanowire device contact layer.

ABSTRACT - Electron spins trapped in quantum dots have been proposed as basic building blocks of a future quantum processor. Although fast, 180-picosecond, two-quantum-bit (two-qubit) operations can be realized using nearest-neighbour exchange coupling, a scalable, spin-based quantum computing architecture will almost certainly require long-range qubit interactions. Circuit quantum electrodynamics (cQED) allows spatially separated superconducting qubits to interact via a superconducting microwave cavity that acts as a ‘quantum bus’, making possible two-qubit entanglement and the implementation of simple quantum algorithms. Here we combine the cQED architecture with spin qubits by coupling an indium arsenide nanowire double quantum dot to a superconducting cavity. The architecture allows us to achieve a charge–cavity coupling rate of about 30 megahertz, consistent with coupling rates obtained in gallium arsenide quantum dots. Furthermore, the strong spin–orbit interaction of indium arsenide allows us to drive spin rotations electrically with a local gate electrode, and the charge–cavity interaction provides a measurement of the resulting spin dynamics. Our results demonstrate how the cQED architecture can be used as a sensitive probe of single-spin physics and that a spin–cavity coupling rate of about one megahertz is feasible, presenting the possibility of long-range spin coupling via superconducting microwave cavities.

7 pages of supplemental information

Eurekalert - "The methods we are using here are scalable, and we would like to use them in a larger system," Petta said. "But to make use of the scaling, it needs to work a little better. The first step is to make better mirrors for the microwave cavity."

To make the quantum dots, the team isolated a pair of electrons on a small section of material called a "semiconductor nanowire." Basically, that means a wire that is so thin that it can hold electrons like soda bubbles in a straw. They then created small "cages" along the wire. The cages are set up so that electrons will settle into a particular cage depending on their energy level.

This is how the team reads the spin state: electrons of similar spin will repel, while those of different spins will attract. So the team manipulates the electrons to a certain energy level and then reads their position. If they are in the same cage, they are spinning differently; if they are in different cages, the spins are the same.

The second step is to place this quantum dot inside the microwave channel. This allows the team to transfer the information about the pair's spin state – the qubit.

Petta said the next step is to increase the reliability of the setup for a single electron pair. After that, the team plans to add more quantum dots to create more qubits. Team members are cautiously optimistic. There appear to be no insurmountable problems at this point but, as with any system, increasing complexity could lead to unforeseen difficulties.

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