Quantum teleportation and nonlocality: the puzzling predictions of entanglement are coming of age
QQuantum teleportation and nonlocality:the puzzling predictions of entanglement are coming of age
Nicolas Gisin ∗ Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland
S´ebastien Tanzilli † Universit´e Nice Sophia Antipolis, Laboratoire de Physique de la Mati`ere Condens´ee,CNRS UMR 7336, Parc Valrose, 06108 Nice Cedex 2, France.
Wolfgang Tittel ‡ Institute for Quantum Science and Technology,and Department of Physics and Astronomy,University of Calgary, Canada (Dated: August 26, 2015)
Keywords: Entanglement, Nonlocality, Quantum Teleportation
Note: This paper is intended to be published in the 2015 fourth issue of Europhysics News as a“Feature Article”.
I. FOREWORD
Entanglement is the physical property that marks themost striking deviation of the quantum from the classicalworld. It has been mentioned first by the great AustrianPhysicist Erwin Sch¨odinger in 1935 (an introduction tothis and other quantum phenomena is [1]). Yet, despitethis theoretical prediction now being 80 years old, andthe famous experimental verifications by Alain Aspectdating back to the early eighties [2], entanglement andits use entered mainstream physics as a key element ofquantum information science [3] only in the 1990’s.
II. INTRODUCTION
The academic research into entanglement nicely illus-trates the interplay between fundamental science and ap-plications, and the need to foster both aspects to advanceeither one. For instance, the possibility to distribute en-tangled photons over tens or even hundreds of kilometersis fascinating because it confirms the quantum predic-tions over large distances, while quantum theory is oftenpresented to apply to the very small (see Figure 1). Onthe other hand, entanglement enables quantum key dis-tribution (QKD) [1]. This most advanced application ofquantum information processing allows one to distributecryptographic keys in a provably secure manner. Forthis, one merely has to measure the two halves of anentangled pair of photons. Surprisingly, and being ofboth fundamental and practical interest, the use of en-tanglement removes even the necessity for trusting most ∗ [email protected] † [email protected] ‡ [email protected] FIG. 1. Long-distance entanglement distribution. A source,located at the Geneva main railway station, distributes en-tangled photon pairs emitted at a telecom wavelength to tworemote villages (Alice’s and Bob’s station located in Belle-vue and Bernex, respectively, in the Geneva back-country).Those two user stations are more than 10 km away from eachother, and are connected to the source through commercialfiber optics quantum channels. This experiment reported thefirst tests of entanglement distribution and nonlocality in thereal world [4]. Entanglement can be further exploited for es-tablishing secret sequences of bits ( i.e. , secret keys) findingapplications in cryptography. EPPS: entangled photon pairsource. equipment used for the measurements [5]. Furthermore,entanglement serves as a resource for quantum telepor-tation (see Figure 2) [1]. In turn, this provides a tool forextending quantum key distribution to arbitrarily largedistances and building large-scale networks that connectfuture quantum computers and atomic clocks [6].In the following, we describe the counter-intuitiveproperties of entangled particles as well as a few recentexperiments that address fundamental and applied as- a r X i v : . [ qu a n t - ph ] A ug pects of quantum teleportation. While a lot of work isbeing done using different quantum systems, includingtrapped ions, color centers in diamond, quantum dots,and superconducting circuits, we will restrict ourselvesto experiments involving photons due to their suitabilityfor building future quantum networks. III. ENTANGLEMENT – A PUZZLINGCONSEQUENCE OF QUANTUM THEORY
Entanglement manifests itself through highly, if notperfectly, correlated results of measurements performedon particles that can in principle be arbitrarily far away- say, one on the earth, and one on the moon. If weconsider two photons and their polarization as an exam-ple, quantum theory describes, and experiments confirm,that photon one might be found horizontally polarized,and photon two vertically polarized. Or vice versa. Whatis more, one might also find that one photon is polarizedat 45 ° and the other one at -45 ° . All that is known in ad-vance is that the two photons are orthogonally polarized.What is particularly discomforting about these corre-lations is that they cannot be explained by attributingproperties to the individual photons that determine, ir-respectively of what happens to the other photon, howeach will respond to its measurement. Entangled parti-cles behave in unison, regardless of their separation andeven if they are measured simultaneously. The result ofa quantum measurement is random, but, somehow, thisrandom event manifests itself at both locations. Einsteincalled this “spooky action at a distance”, though there isno real action from one side onto the other [7]! But whilethe question of how to understand this invisible, nonlo-cal tie remains intriguing, the tie enables applications ofquantum communication such as teleportation. IV. QUANTUM TELEPORTATION – ASURPRISING POSSIBILITY
Suppose you see a beautiful sculpture in a museum andyou would like to have the same at home. Unfortunately,you can’t take it with you. However, you can accu-rately measure all its properties - e.g. , its shape (height,length and depth) - and then reproduce an identical copyfor your living room. But this “measure-and-reproduce”strategy would fail if the sculpture was a photon. Indeed,for the case of a quantum object, the quantum no-cloningtheorem [1] tells us that perfect copying of, say, the pho-ton’s polarization is impossible.Quantum teleportation (see [8] for a recent review ar-ticle), proposed in 1993 and first experimentally demon-strated at the Universities of Rome and Innsbruck in1997, allows the flawless transfer of a property betweentwo quantum particles - e.g. , two photons - without run-ning into a contradiction with the no-cloning theorem.As explained in Figure 2a, it requires three particles - one
FIG. 2. Quantum teleportation. a) The original scheme. Pho-ton A emitted on Alice’s side, whose polarization, representedby | Ψ (cid:105) A , should be teleported onto photon B at Bob’s, is mea-sured jointly with photon C. This joint measurement, calledBell-state measurement (BSM) reveals, loosely speaking, thedifference in the electric field directions, without telling theindividual directions. For instance, if we find that A and Care orthogonally polarized, and knowing from the original en-tanglement that the polarization of photon C is orthogonalto that of photon B, we find that the electric field of pho-ton B must point into the same direction as that of photonA before the measurement. Note that the outcome of theBSM could also have been different, for example, A and Care identically polarized. Similar reasoning would then leadto the conclusion that photon B’s polarization is orthogonalto that of photon A. Therefore, rotating it back would also al-low one to perfectly recover the original polarization encodedinto photon A. In short, the BSM, possibly followed by a well-defined rotation of the (unknown) polarization of photon B,allows one to teleport without error the property “polariza-tion” from photon A onto photon B. EPPS: entangled photonpair source; SPS: single photon source; R: polarization rota-tor. b) Teleportation of 2 properties. This scenario is compa-rable to that described in a) with, however, the possibility ofteleporting 2 quantum properties coded on the same originalsingle photon. In this case, the single BSM is replaced by twocascaded joint measurements, one for each property, but thesecond one is conditioned on the success of the first one. whose property is to be teleported (A), and two that areoriginally entangled (C and B). The comparative mea-surement of the property of the first photon with thatof one member of the entangled pair allows transferringit from the first photon onto the second member of thepair. This measurement - the so-called Bell-state mea-surement - is named after the Northern Irish PhysicistJohn Bell, who has played a seminal role in establishingthe science of entanglement. The first photon loses itsproperty during this measurement, that is, there is no’copying’ during teleportation. Moreover, the transferdoes not happen instantaneously (something that is of-ten claimed erroneously in the non-scientific literature),and hence there is no contradiction with another pillarof modern physics either - that of special relativity. V. AN APPLICATION OF QUANTUMTELEPORTATION – EXTENDING THE REACH
Only five years after its discovery, it was realized thatquantum teleportation is not only an intriguing mani-festation of the puzzling predictions of quantum theory.Together with the possibility to transfer properties fromflying photons onto stationary particles ( i.e. , to createquantum memory for light), it allows establishing entan-glement over theoretically arbitrarily long distances bymeans of quantum repeaters [8]. This would allow build-ing ultra-long-distance QKD links as well as quantumnetworks across countries, continents, or even the globe.The first step in this direction was demonstrated in2003 at the University of Geneva, when the photon thatreceived the teleported property (photon B in Figure 2a)was sent over 2 km of spooled, standard telecommunica-tion fiber before being measured [9]. This demonstrationwas extended in 2007 to more than 800 m of deployedfiber that was part of the Swisscom fiber network [10](see Figure 3a). In this experiment the receiving photonwas already hundreds of meters away when the qubit tobe teleported was prepared. More recently, the trans-mission distance of photon B was extended to more than100 km of air by researchers at the University of Viennaas well as the University of Science and Technology inHefei [8].The next step is to extend the distance over which theother two photons (photon A and C in Figure 2a) travelbefore meeting for the Bell-state measurement. How-ever, due to the difficulty of avoiding any modificationof either photon during transmission, this challenge hasnot yet been met outside a well-controlled laboratory.An important step in this direction has been the recentdemonstration by researchers at the University of Cal-gary of a Bell-state measurement - not a full quantumteleportation experiment - with photons that have beencreated at different places within the city of Calgary andtravelled through the standard telecommunication fibernetwork before being measured [11] (see Figure 3b).Another key achievement on the path towards a quan-tum network has been last year’s teleportation at theUniversity of Geneva of a property from a photon intoa rare-earth-ion doped crystal, which stored it for 50nanoseconds [12]. Notably, both photons that took partin the Bell-state measurement travelled over 12.5 km ofspooled fiber, thereby also meeting the above-describedrequirement for building extended quantum networks.This demonstration built on the previous observationthat such crystals are suitable for storing members fromentangled photon pairs, which was reported in 2011 byresearchers in Geneva as well as in Calgary [13].
FIG. 3. Extending the reach. a) The 2007 Geneva teleporta-tion experiment as an example in which photon B is alreadyoutside the lab when photon A is created, and continues totravel before being measured at Bob’s [10]. b) The CalgaryBSM as an example in which photons A and C are travellinga long distance before being jointly measured.
VI. A FUNDAMENTAL QUESTION –TELEPORTATION OF MULTIPLE PROPERTIES
With very few exceptions, all quantum teleportationexperiments to date demonstrated the transfer of a sin-gle property of one particle, e.g. , the polarization of aphoton. However, objects encountered in our every-daylife are composed of many elementary building blocks, e.g. , many atoms, each of which being described by sev-eral properties. A natural question is therefore if one canteleport more complex quantum systems as well. Indeed,this is possible. A first guess may lead to the idea of tele-porting all properties individually in a straightforwardgeneralization of the scheme shown in Figure 2a. Thisguess is correct - most surprisingly even if the propertiesare entangled!
FIG. 4. On-chip teleportation. The on-chip teleporter de-veloped in Nice in 2012, as a telecom-compatible elementaryplug-in. A source of entangled photon pairs and routing cir-cuitry are integrated on a single photonic substrate enablingon-chip teleportation of an incoming photon.
However, in the case of teleporting several propertiesencoded into the same particle, there is an interestingtwist - at least in the scheme employed by researchersfrom the University of Science and Technology in Hefeiin 2015 to transfer the angular orbital momentum andthe polarization of a single photon [14]. As shown inFigure 2b, it requires, as an intermediate step, the con-firmation that exactly one photon traveled along each ofthe two paths connecting the measurement that teleportsthe first property (the orbital angular momentum) withthe measurement that teleports the second one (the po-larization). Obviously, the use of standard single photondetectors is not a viable solution, as the photons wouldbe destroyed during the measurements. But interestinglya standard ’single property teleporter’ does the job. In-deed, a successful Bell-state measurement does not onlytransfer a property from one photon onto another pho-ton (a process that is not distinguishable from the di-rect transmission of the original photon through the tele-porter), but also confirms that the photon existed! Thisidea generalizes from two to any number of properties:an n-property teleporter requires an (n-1)-property tele-porter as a plug-in, which itself requires an (n-2)-propertyteleporter, etc.
Figure 4 shows a fully integrated ’on-chip’ realization of a one-property teleporter, developedat the University of Nice [15], that would constitute agood starting point for building such a nested scheme.As one can see from these insights into current re-search, many fundamental problems related to entangle-ment and teleportation remain to be solved. Further-more, on the application side, the connection of distantnodes into quantum networks, which will enable provablysecure communication and networked quantum comput-ers, will remain an important challenge for many years tocome. One thing is clear: this highly multi-disciplinaryfield will continue to be exciting!
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