Multi-element logic gates for trapped-ion qubits

• Published in: Nature (London) Vol. 528; no. 7582; pp. 380 - 383
• Main Authors: Tan, T. R., Gaebler, J. P., Lin, Y., Wan, Y., Bowler, R., Leibfried, D., Wineland, D. J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 17-12-2015; Nature Publishing Group
• Subjects: 639/766/36/1121; 639/766/483/1255; 639/766/483/2802; 639/766/483/481; Analysis; Beryllium; Gates; Humanities and Social Sciences; Introduced species; Ions; Lasers; letter; Medical care; Methods; multidisciplinary; Networking; Photon correlation; Quantum physics; Quantum theory; Science; Spectroscopy; United States; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature16186

Harnessing the entanglement of different ionic species could bring new flexibility in quantum computing, and now two groups independently demonstrate entanglement between different atomic species; Tan
et al
. achieve entanglement between different elements, whereas the related paper by Ballance
et al
. shows entanglement between different atomic isotopes, together demonstrating a first step towards mixed-species quantum logic.
A mixed bag of entangled particles
In quantum-computing architectures, not all physical systems are equally good at completing each task. For example, in trapped-ion quantum computers, one specific element might be an excellent memory qubit, while another element is more suited to transporting information between nodes. However, a crucial prerequisite to harness these advantages is the entanglement of different atomic species. Now, two groups have independently achieved this. Ting Rei Tan
et al
. showed entanglement between different elements
9
Be
+
and
25
Mg
+
, and Christopher Ballance
et al
. achieved entanglement between different atomic isotopes,
40
Ca
+
and
43
Ca
+
. These studies represent a first step towards mixed-species quantum logic, and from a fundamental perspective they show that particles that are distinguishable by many internal properties can indeed be entangled and violate Bell's inequality.
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 architectures
1
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
2
, creation of entanglement through dissipation
3
, and quantum non-demolition measurement of one species with another
4
. 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
9
Be
+
ion and a
25
Mg
+
ion is realized through an effective spin–spin interaction generated by state-dependent forces induced with laser beams
5
,
6
,
7
,
8
,
9
. 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
10
,
11
,
12
. Using a sequence of such gates, we demonstrate a CNOT (controlled-NOT) gate and a SWAP gate
13
. We further demonstrate the robustness of these gates against thermal excitation and show improved detection in quantum logic spectroscopy
14
. We also observe a strong violation of a CHSH (Clauser–Horne–Shimony–Holt)-type Bell inequality
15
on entangled states composed of different ion species.