ISSN 2186-7437

NII Shonan Meeting Report

No. 2011-6

Hybrid Quantum Devices
Gerard MilburnJörg Schmiedmayer
Yasuhiro Tokura
Kae Nemoto
November 5–9, 2011

National Institute of Informatics
2-1-2 Hitotsubashi, Chiyoda-Ku, Tokyo, Japan

Shonan Workshop Report on “Hybrid Quantum Devices”
Organizers: Gerard Milburn, (UQ)
Jörg Schmiedmayer, (TU-Wien)
Yasuhiro Tokura, (NTT)
Kae Nemoto, (NII)
In recent years techniques for quantum control have dramatically improved and resulted in a number ofdemonstrations in different physical systems at the few qubit level. Based on these new developments, many
proposals to implement quantum devices have been mushrooming both in number and variety. At this stage ofquantum information device research, it is essential to make sure the fundamental quantum control has its future asa practical device for the future technologies. The implementation of quantum devices, however, is usually not
carefully considered as a fundamental device for larger scale quantum information processing and could potentiallylimit the scalability of the system. Recent theoretical analysis shows that hybrid and distributed quantum devices
exhibit better scalability and so are promising for scalable architectures in large-scale quantum information systems.
In this workshop, we focused on such hybrid and distributed devices bringing such state of the art quantum
information systems together. In particular, devices from optics to microwave were extensively investigated.

We divided our efforts into the following six sessions and focused on current problems, future directions and what
can be achieved in the next few years.

Each session was targeted a different aspect of hybrid quantum devices.

Even though hybrid quantum nature in these topics is common, the background knowledge for each topic is rather
different.

We spent enough time to share the recent progress and problems in each field in the beginning of eachsession.

Session: Hybrid Quantum Systems – Theory
Speakers: G J Milburn (session leader), “Hybrid quantum systems”
W Munro, “ Heterotic quantum systems”
S. Devitt, “Large scale NV diamond based quantum computing”
Milburn gave many examples of hybrid quantum systems that included; optomechanical systems, superconducting
devices coupled mechanical resonators and 2DEG quantum dots and spins in NV diamond, cold atoms coupled tomechanical resonators and quantum opto-electronic systems. He then gave a detailed description of a optical to
microwave quantum interface based on coupling both light and microwaves to a common mechanical element.

Heconcluded with some general comments on the promise of hybrid quantum systems and some open questions posedby such systems. These included;
Understanding decoherence and dephasing at the systems-level
Understanding how to integrate very different time scales for subsystems.
Developing ways to deal with a mix of discrete (error correction) and continuous dynamics (master equations).
Developing a systems-level descriptions of complex hybrid dissipative dynamics.

Munro discussed the concept of a heterotic quantum systems --- somewhat more general that a hybrid quantumsystem. He stressed that a key element in complex multi-element hybrid systems will be defect tolerance, citing theHP Teramac project [2] as an example form the field of conventional computing,

He discussed in detail a numberof specific examples that combine spins in NV diamond and optics. He also briefly mentioned the coupling an NV
ensmeble to a flux qubit.

Devitt gave a detailed high-level description of an optical quantum computing system based on NV diamond andintegrating topological codes into the systems design. He stressed that such systems have an inherent problem
arising form the probabilistic nature of the connections.

[1] “Entangling optical and microwave cavity modes by means of a nanomechanical resonator”, Sh. Barzanjeh, D.
Vitali, P.Tombesi, G.J. Milburn, Phys. Rev. A 84, 042342 (2011).
[2] J. R. Heath et al., Science 280, 1716 (1998);[3] Michael Trupke, William J. Munro, Kae Nemoto, and Joerg Schmiedmayer, Enhancing photon collection fromquantum emitters in diamond, Progress in Informatics 8 3337 (2011).
[4] Alp Sipahigil, Michael L. Goldman, Emre Togan, Yiwen Chu, Matthew Markham, Daniel J. Twitchen, Alexander S.
Zibrov, Alexander Kubanek, Mikhail D. Lukin,

Quantum interference of single photons from remote

nitrogen-vacancy centers in diamond, arXiv:1112.3975[5] Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi Karimoto, Hayato Nakano, William J.
Munro, Yasuhiro Tokura, Mark S. Everitt, Kae Nemoto, Makoto Kasu, Norikazu Mizuochi and Kouichi Semba,Coherent coupling of a superconducting flux-qubit to an electron spin ensemble in diamond, Nature 478, 221-224(2011).
[6] Simon J. Devitt, Austin G. Fowler, Ashley M. Stephens, Andrew D. Greentree, Lloyd C.L. Hollenberg, William J.
Munro and Kae Nemoto, Architectural design for a topological cluster state quantum computer, New J. Phys. 11,083032 (2009).

[6] Simon J. Devitt, William J. Munro, and Kae Nemoto, High performance quantum computing, Progress inInformatics 8 4955 (2011).
[7] Simon J. Devitt, Ashley M. Stephens, William J. Munro and Kae Nemoto, Integration of highly probabilistic
sources into optical quantum architectures: perpetual quantum computation, New J. Phys. 13, 095001 (2011).

Session: Optomechanics
Speakers: Simon Gröblacher (session leader), “Hybrid optomechanical systems”
Kai Stannigel, “Optomechanical transducers for quantum information processing”
Matthew Rakher , “Realization of an Optomechanical Interface between Ultracold Atoms anda Micromechanical Membrane”
In the session on optomechanics, the three speakers presented different aspects of coupling light to a mechanicaloscillator and other (quantum) systems.

The work Simon Gröblacher presented focused on the efforts of cooling mechanical resonators into their quantum
ground-state using the radiation-pressure force of light. He described different approaches realized at the Universityof Vienna and the California Institute of Technology, where at the latter they recently achieved ground-state coolingusing an optomechanical crystal [1]. In Vienna, Gröblacher et al. also realized a strongly coupled opto-mechanicalsystem, in which coherent energy exchange between the optical and the mechanical subsystems is possible [2]. The
efforts at both institutions ultimately aims at realizing quantum experiments with massive macroscopic systems,
which with the recently demonstrated experiments is now within reach.

Kai Stannigel presented a proposal for a qubit-light interface based on a cavity-optomechanical system [3]. Theinterface (also termed 'optomechanical transducer', OMT) is primarily intended for 'dark' qubits that do not couple
coherently to light, such as phosphor donors in silicon or superconducting qubits, and converts quantum states ofsuch qubits into propagating photons. Thereby, the OMT enables long-distance quantum communication for thisclass of quits, but may also be used, e.g., as an alternative read-out channel. In particular, the realization of astate-transfer between two distant qubits has been discussed, and for numbers corresponding to present-day
technology fidelities of up to 0.9 are predicted. As an outlook, the speaker presented an alternative scheme forentanglement distribution in quantum networks that does not rely on a specific pulse-sequence, but realizes pureentangled states as steady-states of a driven-dissipative dynamics, also within large networks [4].

Matthew Rakher talked about optomechanical systems that exploit optical forces for cooling and control ofmechanical oscillators. Such systems can be used in precision force and displacement sensing as well as forfundamental studies of quantum physics at macroscopic scales. This burgeoning field has many similarities with thestudy of ultracold atoms, where radiation pressure forces are routinely used for laser cooling and optical dipole forces
are used for trapping and manipulation of atomic motion in optical lattices. Matthew Rakher and co-workers haverecently performed an experiment which aims to combine these systems in a meaningful way. In particular, they haverealized a hybrid optomechanical system where the center of mass motion of an ultracold atomic ensemble iscoupled to the motion of a micromechanical membrane by optical forces [5]. This coupling is accomplished byretroreflecting a laser beam off of the dielectric membrane to create an optical lattice potential for the atoms. In doing
so, Rakher et al. have demonstrated both the effect of the membrane vibrations onto the atoms as well as thebackaction of the atomic motion onto the membrane [6]. Their work represents an initial step towards the quantum
control of mechanical motion by coupling to ultracold atoms.

[1] J. Chan et al., Nature 478, 89 – 92 (2011)[2] S. Gröblacher et al., Nature 460, 724 – 727 (2009)[3] K. Stannigel et al., PRL 105, 220501 (2010)[4] K. Stannigel et al., arXiv: 1112.1690 (2011)[5] K. Hammerer et al., PRA 82, 021803 (2010)[6] S. Camerer et al., PRL 107, 223001 (2011)

Session: “Crystal optics and Memory”Speakers: Wolfgang Tittle (session leader), "Mapping quantum states between photons and RE crystals"
Jevon Longdell, "Quantum memories using photon echoes"
Masahide Sasaki, “Next generation technologies for quantum communication”

This session included presentations by Drs. Wolfgang Tittel from the University of Calgary/Canada, and Jevon
Longdell from the University of Otago/New Zealand. The presentation by Dr. Masahide Sasaki from the National
Institute of Information and Communications Technology/Japan was also loosely attached to this session. However,
due to it covering a different topic, it will be discussed elsewhere.

The session started with an invited presentation by Dr. Tittel, who gave a general introduction why rare-earth-ion
doped crystals at cryogenic temperatures are currently of large interest for quantum information procession. Morespecifically, they allow the reversible mapping of quantum information between different carriers, light and matter,and thereby constitute a key element for quantum repeaters, which promise overcoming the current distance barrier
for quantum communication (including quantum key distribution). Dr. Tittel then discussed the current state-of-the-artof quantum memory research worldwide, and gave an overview (with examples) about different ways to achieve
reversible mapping. This part was followed by the presentation of a particular implementation of quantum memory,which has allowed the storage and unperturbed retrieval of entangled photons in and from a Thulium doped Lithium
Niobate waveguide.

Dr Tittel’s presentation was followed by the presentation by Dr. Longdell, who focused on some new aspects related
to the use of rare-earth-ion doped crystals in quantum information science. More specifically, he discussed the use ofusing optical pulses for rephasing optical coherences. While a simple use of optical rephasing pulses for quantum
memories leads to noise in the recalled light there are ways around this. Dr. Longdell showed preliminary
experiments

working towards quantum memories using this technique as well as a method for creating time

separated entangled light fields. Dr Longdell also described the work his group is doing with rare earth ion dopants
coupled to whispering gallery mode resonators.

The complementary presentations were very well received. Discussions arose during and after the presentations,and were continued during the subsequent days of the workshop (we point out that discussion already happened
before the presentations as quantum memory using rare-earth-ion-doped crystals has become known in the scientificcommunity, including the workshop participants, over the past few years). A lot of interest was in further hybridization(i.e. hybridization beyond light and impurities in crystals), which may lead to subsequent mapping of quantum
information between atomic levels and superconducting qubits. The latter have been another topic featured in thisworkshop.

Session: “Superconducting systems
Speakers: Yasunobu Nakamura (session leader), “Superconducting qubits and circuits”
Semba Koichi, “Toward hybrid quantum systems --- A Quest for a Quantum Memory”
Robert Amsüss, "Coupling NV Centers to a Superconducting Resonator".

Yasu Nakamura (NEC/RIKEN) gave an introductory talk on superconducting qubits and circuits. Recent progress inthe field was reviewed with an emphasis on versatility of the superconducting systems in quantum opticalexperiments in the microwave domain. In the end of the talk, prospects towards hybrid quantum systems werepresented.

Kouichi Semba (NTT-BRL) reported their recent achievement of coherent coupling between a superconducting flux
qubit and an ensemble of electron spins in diamond NV centers. Thanks to its localized distribution of the zero-point
current fluctuations, flux qubit allows larger coupling strength to each spin compared with a coplanar waveguideresonator. Vacuum Rabi oscillations between the flux qubit and a collective excitation among 3107 spins wasobserved. As the decoherence time in the spin ensemble is still limited to a few tens of nanoseconds, improving thecoherence is one of the next targets.

Robert Amsüss (TU Wien) discussed strong coupling between superconducting coplanar waveguide resonator andan ensemble of electron spins of NV centers. Scaling of the coupling strength with N was confirmed experimentally.
It was also impressive to see the effect of hyperfine interaction resolved clearly in the spectrum. There was a good
discussion on the effect of inhomogeneous broadening. In solid-state spin ensembles inhomogeneous broadening
seems to be one of the most important issues to be overcome.

In the general discussion at the end of the session, there was an argument about the necessity of spatial mode
matching in the future interface between optics and microwave. How to implement spin-echo in spin ensemble
experiments were also discussed.

Session: “Diamond and solid state devices”
Speakers: Boris Naydenov (session leader), “NV Centers in Diamond”
Mark Everitt, "A single NV- Centre"
In this session there were two talks dealing with the Nitrogen-Vacancy defect centers (NV) in diamond. In the first talkBoris Naydenov gave a general introduction to the NVs, where he showed their remarkable properties which makethem ideal candidates for solid state quantum bits (qubits) and high sensitive nano-scale magnetic sensor. Since he
was asked by many participants of the workshop how to improve the production efficiency, stability and coherencetime T2 of the NVs, he addressed these topics in the rest of his presentation. The main results are that theimplantation of carbon ions additionally to the nitrogen ones increases the NV creation yield [1], annealing thediamonds at high temperature in vacuum improves both T 2 and the stability of implanted NVs [2] and that for as
grown NVs in ultra pure diamonds by using dynamical decoupling T2 can reach the relaxation time T1 [3].
Mark Everitt discussed in his presentation how we can efficiently use the nuclear spin of the NV center as a qubit.

[1] B. Naydenov et al., App. Phys. Lett. 96, 163108 (2010)

[2] B. Naydenov et al., App. Phys. Lett. 97, 242511 (2010)[3] B. Naydenov et al., Phys. Rev. B 83, 081201(R) (2011)

Session: “Optical systems”
Speakers: Jeremy O'Brien (session leader), “Integrated Quantum Photonics”
Keiichi Edamatsu, “Multi-photon Quantum Interference and Quantum Information”
Stefanie Barz, “Blindfolding a quantum computer”

O’Brien gave an overview of optical approaches to quantum information processing , including cluster state andcircuit based approaches, summarizing theoretical and experimental developments over the last five to ten years. He
then described the integrated quantum photonic approach, starting with circuits for quantum communication ,quantum logic gates , quantum algorithms , quantum walks , and quantum metrology . He then described how theoutstanding issues of scaling , miniaturisation , and reconfigurability
nonlinearand diamond single emitterare being addressed. Finally he discussed

single photon sources, and superconducting single photon detectors .

Prof Edamatsu described a technical tour-de-force experimental demonstration of four-photon quantum interference
using telecom-wavelength photons: Realization of multi-photon quantum interference is essential to linear optics
quantum information processing and measurement-based quantum computing. The Edamatsu group havedeveloped a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom wavelength
region, and have demonstrated the quantum interference exhibiting the reduced fringe intervals that correspond to
the reduced de Broglie wavelength of up to the four photon ‘NOON’ state. These results should open a path topractical quantum information processing using telecom-wavelength photons. He also described experimental
activation of bound entanglement: Entanglement is one of the essential resources in quantum information andcommunication technology (QICT). The entanglement thus far explored and applied to QICT has been pure anddistil- lable entanglement. Yet there is another type of entanglement, called ‘bound entanglement’, which is notdistillable by local operations and classical communication (LOCC). The Edamatsu group have demonstrated theexperimental ‘activation’ of the bound entanglement held in the four-qubit Smolin state, unleashing its immanent
entanglement in distillable form, with the help of auxiliary two-qubit entanglement and LOCC. This is anticipated toopen the way to a new class of QICT applications that utilize more general classes of entanglement than ever,including bound entanglement.

Stefanie Barz described an experimental demonstration of blind quantum computing . Quantum computers, besidesoffering substantial computational speedups, are also expected to provide the possibility of preserving the privacy ofa computation. Barz showed the first such experimental demonstration of blind quantum computation where the input,computation, and output all remain unknown to the computer. They exploited the conceptual framework ofmeasurement- based quantum computation that enables a client to delegate a computation to a quantum server.
They demonstrated various blind delegated computations, including one- and two-qubit gates and the Deutsch andGrover algorithms. Remarkably, the client only needs to be able to prepare and transmit individual photonic qubits.
This demonstration is crucial for future unconditionally secure quantum cloud computing and might become a key
ingredient for real-life applications, especially when considering the challenges of making powerful quantum
computers widely available.

[1] For reviews see: Science, 318, 5856 (2007), Nature Photonics, 3, 687 (2009), Nature 464, 45 (2010)[2] Phys. Rev. A 82, 012304 (2010)[3] Science 320, 5876 (2008)[4] Science 325 1221 (2009)[5] Science 329 1500 (2010)[6] Nature Photonics 3, 346 (2009), Phys. Rev. Lett. 107, 163602 (2011), arXiv:1009.3128[7] Nature Communications 2, 413 (2011), arXiv:1110.4276, arXiv:1111.4147[8] Nature Communications 2, 224 (2011), arXiv:1201.6537[9] Nature Photonics 6, 45 (2012), Phys. Rev. Lett. 108, 053601 (2012)[10] Appl. Phys. Lett. 99, 081110 (2011), Appl. Phys. Lett. 98, 051101 (2011)[11] Appl. Phys. Lett. 97 241901 (2010), Appl. Phys. Lett. 98 133107 (2011)[12] Appl. Phys. Lett. 96, 211101 (2010)[13] arXiv:1112.2019[14] Science 335, 303 (2012)