Experimental Implementation of a Quantum Game Diplomarbeit an der FakultÄat furÄ Physik der Ludwig-Maximilians-UniversitÄat MuncÄ hen Arbeitsgruppe Prof. Dr. Harald Weinfurter Carsten Schuck 22. Juli 2003 Erstgutachter: Prof. Dr. Harald Weinfurter Zweitgutachter: Prof. Dr. Klaus Sengstock 2 Contents 1 Introduction 5 2 Theory 9 2.1 Quantum Information . 10 2.2 Game Theory . 21 2.3 Optics . 33 3 Implementation 39 3.1 Initial State Preparation . 40 3.2 Local Unitary Operations . 47 3.3 Complete Bell State Analysis . 51 4 Experiment 65 4.1 Characterization of the Photon Pair Source . 66 4.2 The Hong-Ou-Mandel Dip . 75 4.3 Identi¯cation of Bell States . 81 4.4 Results of a Complete Bell measurement . 93 5 Conclusion & Outlook 99 A Appendix 103 A.1 Detectors . 104 A.2 De¯nitions & Explanations . 106 List of Figures 115 Bibliography I 3 Contents 4 1 Introduction The combination of game theory and quantum information led to the new ¯eld of quantum games. The classical theory of games, which is a well established disci- pline of applied mathematics, and quantum communication are both concerned with information and how information is processed and utilized. Game theory analyzes situations of interactive decision making and provides models of conflict and cooperation [1]. It has numerous applications, most im- portantly among many others in economics, social sciences, politics and biology. However, in every game the players have to inform the other players or the game's arbiter about the decisions they take for the game to evolve. In fact the players need to communicate information. Quantum mechanical systems o®er new possibilities for the communication and processing of information. The basis for the theory of quantum information and computation [2] will be shortly reviewed in section 2.1. Accordingly, one can provide the players of a game with quantum mechanical systems as carriers of information they need to communicate. As this extends the possibilities for communication, novel strategies become possible in quantum games. It turns out that entanglement and linear combinations of the classical strategies can enable the players to reach higher bene¯ts in the quantum mechanical version of a game than classically achievable (s. section 2.2). Yet, quantum communication problems also can be viewed in the light of game theory. Just as new solutions to classically unsolvable games can be found in quantum game theory, it is the aim of some quantum communication protocols to provide a quantum solution to an otherwise unsolvable classical problem. It is thus possible to rephrase problems in quantum communication or computation as games. Consider for example eavesdropping in quantum cryptography: To analyse how vulnerable a quantum cryptography protocol is, one can try to ¯nd the optimal quantum strategy for an eavesdropper, trying to obtain as much information as pos- sible in a given setup, playing against Alice and Bob, who are trying to communicate securely. Likewise, searching a database for a particular entry can be formulated as game between two players, both trying to ¯nd this entry ¯rst. The optimal (quan- tum) strategy in this game then corresponds to an optimal (quantum) algorithm to solve the problem. 5 1 Introduction As a contribution to the new ¯eld of quantum games I will show in chapter 3 how the quantum version of a well known classical game, the prisoner's dilemma, can be implemented. The dilemma of the classical game is that it is not possible to maximize the payo® of all rational players with optimal single player strategies. In the quantum version of the game, however, the players are supplied with one sub- system of an entangled state each. If they may perform any quantum mechanically allowed operation to encode their choice of strategy on their respective subsystem before sending it to an arbiter determining each player's bene¯t, the dilemma can be resolved. It is the goal of this work to realize the idea in practice. In the experimental part, I will present the ¯rst optical realization of this quantum game using polarization-time-entangled photon pairs on which arbitrary (quantum-) strategies can be performed. The entangled photon pairs are generated with high temporal de¯nition in a parametric down-conversion process, explained in section 2.3. It will be shown how a compact and highly e±cient entangled photon pair source is set up to generate the initial state of the game (section 3.1). The need for such highly e±cient sources of entangled photons grew with the interest in fundamental tests of quantum mechanics (tests of Bell's inequality) and the re- alization of quantum computation and communication schemes, for example dense coding and teleportation. To experimentally demonstrate the e±ciency of quantum strategies, two players can perform single-qubit operations corresponding to the angular settings of a com- bination of ¸=2- and ¸=4-waveplates, as discussed in section 3.2. To determine the player's bene¯ts, the ¯nal state is transformed into a Bell state and projected onto the complete set of maximally entangled basis-states of the two-particle polarization Hilbert space. This so-called Bell measurement is very important for many quantum communication experiments (e.g. dense coding, teleportation and quantum cryp- tography [3]), and will be covered in section 3.3. LutkÄ enhaus et al. presented a no-go theorem for a perfect Bell state analysis of two polarization-entangled qubits involving only linear optical elements [4]. Nevertheless, in this experiment, it will for the ¯rst time be possible to perform a complete Bell measurement with a method presented by P. Kwiat and H. Weinfurter [5]; the results are presented in chapter 4. The idea behind this measurement is to make use of simultaneous entanglement in multiple degrees of freedom. If the photons are entangled in more than one degree of freedom one can operate in a higher dimensional Hilbert space. In our case the intrinsic time-energy entanglement of the down-converted photons will be employed to achieve additional arrival-time correlations of the photons after having passed an asymmetric interferometer with polarizing beam splitters to temporally separate horizontal and vertical polarizations. Since the photons are entangled in the polar- ization and the time degrees of freedom, i.e. they are embedded in a larger Hilbert space, and a complete Bell state analysis to determine the payo®s is possible. This will for example allow to realize optimal dense coding, where the full two bits of information are encoded in the polarization of the two qubits of a Bell state. This 6 class of state transformations can be regarded as a realization of a simple non-trivial two-qubit quantum logic network. Apart from the possibility to distinguish between all four Bell states and the implementation of single qubit gates it is noteworthy that our quantum network involves a controlled NOT operation. The importance of a controlled NOT gate stems from the fact that it is (together with a set of single-qubit gates) a universal quantum gate [6, 7]. Single qubit gates are relatively simple to realize (s. section 3.2), but the controlled-NOT is generally much more di±cult to implement, because it requires two separated carriers of quantum information to interact in a controlled manner. Here the advantage of the optical approach, namely the photon's intrinsic lack of decoherence when transported through free space or optical ¯bers, turns into a drawback. Since the photon-photon interaction is extremely weak and hence takes place with a negligible probability of success, an optical implementation of a two-qubit gate necessarily requires strong non-linearities. One idea to avoid the need for a direct nonlinear coupling between photons is to mediate the interaction by something else, for example an atom, of course at the expense of introducing noise into the system. Therefore, current experiments are trying to map the state of photons onto atom or ion states and vice versa to implement a non-trivial two-particle gate with these systems. Nevertheless it is desirable to have an optical implemention of such gates with linear elements to avoid all the problems (e.g. decoherence, noise, more complex setups, etc.) that come with schemes involving atoms or ions. A circumvention of the necessity for a strong non-linearity to implement a controlled-NOT gate has been proposed by E. Knill, R. Laflamme and G.J. Milburn (KLM). They showed that scalable quantum information processing with linear op- tical elements "is possible in principle but technically di±cult" [8]. Their scheme for "e±cient quantum computation" requires phase shifters, beam splitters, determin- istic (i.e. on-demand) single-photon sources and the intrinsic non-linearity of highly e±cient discriminating single photon detectors. They propose that it is possible to increase the probability of success asymptotically (as n=(n + 1)) at the expense of an increasing number (n) of auxiliary photons which have to be independently prepared in any desired entangled state. Using beamsplitters it is possible to couple the additional auxiliary modes to the initial input state so that the resulting state is living in the direct product of the two Hilbert spaces and therefore, a larger Hilbert space as well. Of course, our approach is not su±cient for the implementation of general pur- pose controlled-NOT operation in universal quantum networks, because it requires entanglement in more than one degree of freedom, which will generally be di±cult to achive. But since the generation of the necessary entangled ancilla states in the KLM scheme is very challenging, it is interesting to apply the presented idea of entangled states embedded in higher dimensional Hilbert spaces as an extension.