December 28, 2015

Adding to Quantum Computing Toolkit with Mixed-Atom Logic Operations

Physicists at the National Institute of Standards and Technology (NIST) have added to their collection of ingredients for future quantum computers by performing logic operations—basic computing steps—with two atoms of different elements. This hybrid design could be an advantage in large computers and networks based on quantum physics.

The NIST experiment manipulated one magnesium and one beryllium ion (charged atom) confined in a custom trap (see photo). The scientists used two sets of laser beams to entangle the two ions—establishing a special quantum link between their properties—and to perform two types of logic operations, a controlled NOT (CNOT) gate and a SWAP gate. The same issue of Nature describes similar work with two forms of calcium ions performed at the University of Oxford.

“Hybrid quantum computers allow the unique advantages of different types of quantum systems to be exploited together in a single platform,” said lead author Ting Rei Tan. “Many research groups are pursuing this general approach. Each ion species is unique, and certain ones are better suited for certain tasks such as memory storage, while others are more suited to provide interconnects for data transfer between remote systems.”

Ion trap used in NIST quantum computing experiments demonstrating logic operations with two different types of ions (charged atoms). One magnesium ion and one beryllium ion are trapped 4 micrometers apart near the cross-shaped opening at the center of both photos. The larger-scale photo shows the gold-on-alumina trap inside a case that protects against electrical interference. National Institute of Standards and Technology. Blakestad/NIST

Nature - Multi-element logic gates for trapped-ion qubits



Gates are used to build circuits or programs. As in classical computing, a quantum bit (qubit) can have a value of 0 or 1. But unlike classical bits, a qubit can also be in a “superposition” of both 0 and 1 values at the same time. In the NIST experiment, the qubits are based on the ions’ spin directions (spin up is 1 and spin down is 0). A CNOT gate flips the second (target) qubit if the first (control) qubit is a 1; if it is a 0, the target bit is unchanged. If the control qubit is in a superposition, the ions become entangled. A SWAP gate interchanges the qubit states, including superpositions.

The two types of ions vary in their response to light, so lasers can be tuned to manipulate one without disturbing the other. This minimizes interference. But getting the whole setup to operate coherently was a challenge. The researchers developed a technique to track and stabilize the laser beam phases, that is, the exact positions of the undulating light waves.

“For the logic gate to work, the phase has to be at the correct values. Also, these phases have to be stable, so we can apply the same condition over many repetitions,” Tan said.

If they can be built, quantum computers could solve problems now considered intractable, such as breaking today’s best data encryption codes. The same NIST group has demonstrated many other building blocks for quantum computers based on trapped ions. For example, the group demonstrated the first quantum logic gate (a CNOT gate) on individual qubits in 1995 using a single beryllium ion.

NIST’s latest techniques provide a complete or “universal” set of quantum gates—meaning they could perform any possible computation—using ions of multiple elements. A universal set of quantum gates is one of the so-called DiVincenzo criteria, which describe the elements needed to build a practical quantum computer.

NIST’s new mixed-atom gates could also help make better simulators to model quantum systems and could enable faster and simpler measurements in applications such as NIST’s experimental quantum logic clock.

The mixed-atom gates rely on NIST’s technique for entangling ions demonstrated more than a decade ago. Multiple carefully tuned laser beams apply an oscillating force to a pair of ions. If the ions are in different internal states, they feel different laser forces that alter the ions’ external motions. This coupling of internal states with external motions has the effect of entangling the ions.

The research was supported by the Office of the Director of National Intelligence, Intelligence Advanced Research Projects Activity, and the Office of Naval Research.

Abstract

Precision control over hybrid physical systems at the quantum level is important for the realization of many quantum-based technologies. In the field of quantum information processing (QIP) and quantum networking, various proposals discuss the possibility of hybrid architectures1 where specific tasks are delegated to the most suitable subsystem. For example, in quantum networks, it may be advantageous to transfer information from a subsystem that has good memory properties to another subsystem that is more efficient at transporting information between nodes in the network. For trapped ions, a hybrid system formed of different species introduces extra degrees of freedom that can be exploited to expand and refine the control of the system. Ions of different elements have previously been used in QIP experiments for sympathetic cooling, creation of entanglement through dissipation, and quantum non-demolition measurement of one species with another. Here we demonstrate an entangling quantum gate between ions of different elements which can serve as an important building block of QIP, quantum networking, precision spectroscopy, metrology, and quantum simulation. A geometric phase gate between a 9Be+ ion and a 25Mg+ ion is realized through an effective spin–spin interaction generated by state-dependent forces induced with laser beams. Combined with single-qubit gates and same-species entangling gates, this mixed-element entangling gate provides a complete set of gates over such a hybrid system for universal QIP. Using a sequence of such gates, we demonstrate a CNOT (controlled-NOT) gate and a SWAP gate. We further demonstrate the robustness of these gates against thermal excitation and show improved detection in quantum logic spectroscopy. We also observe a strong violation of a CHSH (Clauser–Horne–Shimony–Holt)-type Bell inequality on entangled states composed of different ion species.

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