qubits

Quantum computers are potentially revolutionary devices and the basis of a growing industry. However, their technology isn’t standardized yet, and researchers are still studying the physics behind the diverse ways to build these quantum devices. Even the most basic building blocks of a quantum computer—qubits—are still an active research topic.

Today’s networks use electronic circuits to store information and optical fibers to carry it, and quantum networks may benefit from a similar framework. Such networks would transmit qubits – quantum versions of ordinary bits – from place to place and would offer unbreakable security for the transmitted information. But researchers must first develop ways for qubits that are better at storing information to interact with individual packets of light called photons that are better at transporting it, a task achieved in conventional networks by electro-optic modulators that use electronic signals to modulate properties of light. Now, researchers in the group of Edo Waks have struck upon an interface between photons and single electrons that makes progress toward such a device.

A big part of the burgeoning science of quantum computation is reliably storing and processing information in the form of quantum bits, or qubits.  One of the obstacles to this goal is the difficulty of preserving the fragile quantum condition of qubits against unwanted outside influence even as the qubits interact among themselves in a programmatic way. 
Spin qubits are one of the most promising candidates for the purpose.  Besides being charged, electrons possess spin, a kind of magnetic axis that can only assume specific quantized values.  An atom with a single unpaired electron can serve as a qubit if that electron can be tickled into residing in both of two allowed quantum states (usually called spin up and down) at the same time.  Likewise, a carefully contrived small puddle of electrons known as a quantum dot can also serve as a qubit.  The dot’s spin consists of the aggregate spin of the small number of electrons (two, three, four, etc.) residing in the dot.  In this way the dot acts as a sort of artificial atom.
JQI scientists, publishing in Physical Review Letters, show how both of these qubit types — atomic spins and quantum dot spins — can be combined into a workable quantum system.  According to Jacob Taylor, the leader of the JQI work, their suggestion for a new qubit interaction protocol uses the dot’s spin to turn on what is essentially a chemical reaction between two atomic spins which sit as much as 50 or 100 nm apart.   In the proposed scheme, the atoms designated for qubit duty are phosphorus impurities sitting in a silicon crystal.

 A new experiment conducted at the University of California at Berkeley used quantum information techniques for a precision test of a cornerstone principle of physics, namely Lorentz invariance.  This precept holds that the results of a physics experiment do not depend on its absolute spatial orientation.  The work uses quantum-correlated electrons within a pair of calcium ions to look for shifts in quantum energy levels with unprecedented sensitivity.   JQI Adjunct Fellow and University of Delaware professor Marianna Safronova, who contributed a theoretical analysi

How do you build a universal quantum computer? Turns out, this question was addressed by theoretical physicists about 15 years ago. The answer was laid out in a research paper and has become known as the DiVincenzo criteria [See Gallery Sidebar for information on this criteria]. The prescription is pretty clear at a glance; yet in practice the physical implementation of a full-scale universal quantum computer remains an extraordinary challenge.

Encoding information using quantum bits—which can be maintained in a superposition of states—is at the heart of quantum computing. Superposition states offer the advantage of massive parallelism compared to conventional computing using digital bits---which can assume only one value at a time.  

What will a quantum computer look like? It will probably not resemble your laptop.  Yet, the semiconductor industry, residing at the heart of modern computing, has features that may carry over. Semiconductors are scalable: today’s processors (2.5 centimeters on a side) hold billions of transistors. Another benefit: semiconductor technology integrates with existing electronic and photonic elements. These practical considerations have led some researchers to construct semiconductor quantum devices.