# Information analysis of quantum nondemolition measurement of a photon in a resonator

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<ul><li><p>ISSN 00271349, Moscow University Physics Bulletin, 2009, Vol. 64, No. 6, pp. 611616. Allerton Press, Inc., 2009.Original Russian Text D.N. Yanyshev, B.A. Grishanin, V.N. Zadkov, 2009, published in Vestnik Moskovskogo Universiteta. Fizika, 2009, No. 6, pp. 5054.</p><p>611</p><p>INTRODUCTION</p><p>In connection with the rapid progress of the methods of quantum information processing, storage, andtransmission [2] a number of the foundations of thetraditional quantum mechanics and quantum physicshave lately found new understanding. In particular, thenotion of quantum measurement has begun to beunderstood in a different way than its traditional definition [3, 4], this being that the establishment of anunivocal correspondence between the proper states ofthe measured variable of a quantum object and thereadings of a quasiclassical device does not correspond to the modern understanding of the role ofquantum effects in a physical experiment. Along withthe problem of singlestage direct mining of the datafrom a quantum system in the classical form in problems where the peculiarities of quantum informationare used, the problem of its mapping becomes the keyone, whereas its dequantification is not obligatoryand, on the contrary, in many cases the practical valueof this mapping is attributed to the preservation of thecoherent properties of quantum states.</p><p>The concept of socalled quantum nondemolitionmeasurements is one of the most attractive theories fromthe physical point of view [5, 7]. As is shown in [8, 9] thenotion of nondemolition quantum measurement itself,when introduced in a more general form of entanglingmeasurement, can be considered irrespective of thedegree of quantum coherence loss, i.e. the degree ofdequantification of device readings (and, simultaneously, objects) in the representation used during mea</p><p>surements in the measured quantum</p><p>states of the object in the state of the objectdevice</p><p>system . Even in case of entirely coherent, i.e.,purely quantum representation of the resultant information, the most fundamental quality of classical information related to k indices in this measurement is, nevertheless, adequately represented. This is due to thesimultaneous membership of the k index in two different physical systems A and B, i.e., to the copying andmultiplication of the classical part of the initial quantum information related to a set of states in the initial quantum ensemble of the source.</p><p>This concept of quantum measurement can bedemonstrated by the fundamental example of the nondemolition quantum measurement of a photon in aresonator using a probe atom, which was used in theexperiment described in [1] (Fig. 1). Here a probeatom (A) performs the entirely coherent entanglingmeasurement of the initial state of a number of resonator mode quanta (C). This state is further dequantifiedas the result of the atominduced emission of a photon, which is then recorded by the photodetector (D).</p><p>k| A k| A k| Bk| A</p><p>k| A k| B</p><p>k| A</p><p>Information Analysis of Quantum Nondemolition Measurementof a Photon in a Resonator</p><p>D. N. Yanysheva, B. A. Grishanin, and V. N. Zadkovb</p><p>Faculty of Physics and International Laser Center, Moscow State University, Moscow, 119991 Russiaemail: a yanyshev@physics.msu.ru; b zadkov@phys.msu.ru</p><p>Received June 12, 2009, approved August 28, 2009</p><p>AbstractThe concept of entangled quantum measurement was demonstrated by the example of the fundamental value of the experiment on quantum nondemolition measurement of a photon in a resonator with theuse of a probe atom [1]. The quantuminformation analysis of this experiment was performed. A mechanismof information transmission in the nondemolition measurement scheme in the case of the classical and quantum formalism was demonstrated. The results were shown to coincide in both cases. This is the result of copying the classical part of the initial quantum information attributed to a set of quantum object states.</p><p>Key words: theory of measurement, entangled states, nondemolition measurement</p><p>DOI: 10.3103/S0027134909060101</p><p>ACD</p><p>R1 R2 e</p><p>g</p><p>i</p><p>R1, 2</p><p>Fig. 1. Scheme of the experiment on quantum nondemolition measurement of a photon (C) in a resonator using aprobe atom (A) passing through the resonator.</p><p>C</p><p>Deceased.</p></li><li><p>612</p><p>MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 64 No. 6 2009</p><p>YANYSHEV et al.</p><p>The state of the latter is the result of measurement in atraditional entirely dequantified form. Entanglingmeasurements of a more general form correspond tothe timeintermediate state of the atomphotondetector system when the emitted quantum is not yetfully absorbed and a corresponding quantum derivative of this complex system, which transmits information on the measured number of resonator quanta,performs an entangled measurement with an intermediate degree of coherence.</p><p>Taking into account the importance of this experiment and its close relationship to the problem of quantum information processing, the authors of this studyperformed information analysis of quantum communication channels corresponding to this experiment,viz., they calculated the classical Shannon information applicable to classical channel resonator quantumnumberphotodetector readings and the quantumcoherent information applicable to the quantumchannel quantum resonator statesquantum atomstates.</p><p>1. A MODEL OF QUANTUM NONDEMOLITION MEASURMENT</p><p>To study the process of the quantum nondemolition measurement of a photon in a resonator we analyzed the fundamental experimental scheme [1]. Herethe probe atom (A), which is considered in the elementary threelevel model, serves as the device. Theprobe atom in the experiment drifts through a resonator C, whose mode frequency is quasiresonant to the e</p><p> g transition of the atom. In this case, the atom issubject at the points R1 and R2 to the effects of auxiliarypulses of radiation with a frequency that is quasiresonant to the atom transition frequency ig. The measurement procedure is fourstaged: preparation of theatom in a specified state (interaction of the atom withthe field of the auxiliary radiation pulse), interactionwith the resonator field, and measurement of the stateafter interaction with the second auxiliary pulse anddetection. Let us give a mathematical description ofthese processes.</p><p>First of all, let us determine the structure of thestate space HA HC of the atom (A)resonator (C)system [10]. For that let us define the following basis</p><p>(1)</p><p>using three possible states of the atom (indices a) andtwo resonator states (is there a photon in the resonatoror not; indices c). In case the atom is in the lower state</p><p>1| a 0| c1| a 1| c2| a 0| c2| a 1| c3| a 0| c3| a 1| c </p><p>,</p><p> and there is one photon in the resonator, i.e., the</p><p> resonator state, when the atom is irradiated withthe first auxiliary 1 radiation pulse we get the following state of the system:</p><p>(2)</p><p>For the first stage, namely for preparation of theatom in the state required for performing the measurement, the auxiliary radiation pulse (R1), whose frequency is tuned to the frequency of atom transitioni g, is used. If the time of interaction correspondsto the /2 pulse the atom will pass to the ( +</p><p>)/ state. In the matrix form it can be written asfollows:</p><p>(3)</p><p>where t1 = 1/ig is the time of atoms interaction withthe field of the first auxiliary radiation pulse (R1) andthe vector (c1, c2, c3, c4, c5, c6) in basis (1) is the initialstate of the system. If there is no photon in the resonator that corresponds to the resonator state, we getthe following state of the system:</p><p>(4)</p><p>The second stage is interaction of the probe atomwith the resonator field. The photon frequency in theresonator corresponds to the frequency of the atomictransition g e and here will be assumed to bezero. In this case, the state of the system will have theform:</p><p>1| a1| c</p><p>1</p><p>0</p><p>1/2( )cos</p><p>0</p><p>i 1/2( )sin</p><p>0</p><p>0 </p><p>.=</p><p>1| </p><p>2| 2</p><p>c1 1/2( )cos ic3 1/2( )sin</p><p>c2 1/2( )cos ic4 1/2( )sin</p><p>c3 1/2( )cos ic1 1/2( )sin</p><p>c4 1/2( )cos ic2 1/2( )sin</p><p>c5c6 </p><p>,</p><p>0| c</p><p>1</p><p>1/2( )cos</p><p>0</p><p>i 1/2( )sin</p><p>0</p><p>0</p><p>0 </p><p>.=</p></li><li><p>MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 64 No. 6 2009</p><p>INFORMATION ANALYSIS OF QUANTUM NONDEMOLITION MEASUREMENT 613</p><p>(5)</p><p>where t2 = 2/eg is the time of the atoms interactionwith the resonator field and = 0. For the case where2 = 2 we get</p><p>2</p><p>0</p><p>1/2( )cos</p><p>0</p><p>i 2/2( ) 1/2( )sincos</p><p>2/2( ) 1/2( )sinsin</p><p>0 </p><p>,= (6)</p><p>If the photon frequency in the resonator is tunedout by from the frequency of the probe atom transition g e, we get a more complex state of the system, which is described as follows:</p><p>2</p><p>0</p><p>1/2( )cos</p><p>0</p><p>i 1/2( )sin</p><p>0</p><p>0 </p><p>.=</p><p>(7)2</p><p>0</p><p>1/2( )cos</p><p>0</p><p>i 1/2( )e</p><p>12 2i ( )t2</p><p>2i ( )2</p><p> e</p><p>12 2i +( )t2</p><p>2i +( )2</p><p>+</p><p>sin</p><p>i 1/2( )ie</p><p>12 2i ( )t2</p><p>22t2</p><p>ie</p><p>12 2i +( )t2</p><p>22t2</p><p>sin</p><p>0 </p><p>,=</p><p>where = . If there is no photon inthe resonator, it does not actually change the state ofthe system and it will remain the same as that in (4).</p><p>The third experimental stage is analogous to thefirst one, i.e., the probe atom is subject to the effects of</p><p>422 22/t2</p><p>2 the field of the second auxiliary radiation pulse (R2)</p><p>with a frequency that is resonant with that of theatomic transition i g. If the time of atoms interaction with the field is t1 = 1/ig and there is a photon inthe resonator, the atom will pass to the state</p><p>(8)3</p><p>0</p><p>1/2( )2</p><p>cos 2/2( ) 1/2( )2</p><p>sincos</p><p>0</p><p>i 1/2( )cos 1/2( )sin i 1/2( ) 2/2( ) 1/2( )sincoscos</p><p>2/2( ) 1/2( )sinsin</p><p>0 </p><p>.=</p><p>If there was no photon in the resonator, the state ofthe system will be as follows:</p><p>(9)4</p><p>1/2( )2</p><p>cos 1/2( )2</p><p>sin</p><p>0</p><p>i 1/2( ) 1/2( )sincos</p><p>0</p><p>0</p><p>0 </p><p>.=</p><p>Thus, if 1 = /2 and 2 = 2 and = 0, the systemstate (in the presence or in the absence of a single photon) will be</p><p>(10)</p><p>0</p><p>1</p><p>0</p><p>0</p><p>0</p><p>0 </p><p>or</p><p>0</p><p>0</p><p>i</p><p>0</p><p>0</p><p>0 </p><p>,</p></li><li><p>614</p><p>MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 64 No. 6 2009</p><p>YANYSHEV et al.</p><p>respectively. It follows from Eq. (10) that at = 0 andif the conditions 1 = /2 and 2 = 2 are met, theatom passes either to the state i or g depending onwhether there is a photon in the resonator or not. Inthis case, the photon after such a measurementremains in the resonator if it was there before the interaction or is absent in the opposite case, i.e., the state ofthe probe atom after the measurement depends on theresult of quantum nondemolition measurement of thephotons presence in the resonator.</p><p>The last and fourth stage corresponds to classicaldequantification of quantum information during itsmeasurement with a classical detector (D), which isdescribed in detail in [1].</p><p>2. CLASSICAL INFORMATION ANALYSIS</p><p>2.1 Calculation of the Conditional Probability of the Measurement Event </p><p>In our experiment the amount of information specified by the Shannon formula, which is defined asinformation entropy serving as a measure of uncertainty of the messages of a given source (the messagesare described by the totality of magnitudes x1, x2, , xnand corresponding probabilities p1, p2, , pn of x1,x2, , xn the appearance in the message) will be used asthe criteria for evaluating the amount of the mineddata for a single measurement of the photons presence/absence in the resonator with the use of a probeatom. In the case of a definite (discrete) statistical distribution of the probabilities pk the information entropyor Shannon information (classical information) is thefollowing magnitude</p><p>(11)</p><p>if = 1. In our case we have a conditional</p><p>probability of the measurement event which can bepresented as follows:</p><p>Ish pk pklnk 1=</p><p>n</p><p>=</p><p>pk1k 1=n</p><p>(12)</p><p>where pc is the probability of the photons presence inthe resonator. Then the Shannon information willhave the form:</p><p>(13)</p><p>Figure 2 shows classical information as a functionof dimensionless tuning out of the photon frequencyin the resonator at the frequency of the atomic transition g e. It is apparent from the plot that the maximum is reached at zero tuningout and is equal to 1bit. The sign of the frequency tuning out does notdepend on the result and thus only the positive part ofthe dependence is plotted in the figure.</p><p>Variations in the time of the atoms flight throughthe resonator yield the dependence of Ish on thedimensionless time of the atoms drift through the resonator shown in Fig. 2b. It is apparent from the plotthat the maximum is reached at the drift time corresponding to 2 Rabi pulse and is equal to 1 bit.</p><p>2.2. Taking Resonator Quantum Efficiency into Account </p><p>A detector can determine the state of the atom gwith some probability pd. To calculate the amount ofinformation transmitted by the detector let us construct a matrix of the conditional probability of determining the atomic state</p><p>pijpc 3 4[ ]</p><p>21 pc( ) 4 3[ ]</p><p>2</p><p>pc 1 3 4[ ]2</p><p>( ) 1 pc( ) 1 4 3[ ]2</p><p>( ) </p><p>,=</p><p>Ishpij</p><p>plj pikk 1=</p><p>2</p><p>l 1=</p><p>2</p><p>pij.2logj 1=</p><p>2</p><p>i 1=</p><p>2</p><p>=</p><p>543210/eg, rel. un.</p><p>1.0</p><p>0.8</p><p>0.6</p><p>0.4</p><p>0.2</p><p>0</p><p>Ish, bit</p><p>(a)</p><p>5432102/eg, rel. un.</p><p>1.0</p><p>0.8</p><p>0.6</p><p>0.4</p><p>0.2</p><p>0</p><p>Ish, bit</p><p>(b)</p><p>6 7 1.00.80.60.40.2pd, rel. un.</p><p>1.0</p><p>0.8</p><p>0.6</p><p>0.4</p><p>0.2</p><p>0</p><p>Ish bit</p><p>(c)</p><p>0</p><p>Fig. 2. The amount of classical information as a function of dimensionless photon frequency tuning out in the resonator from thefrequency of atomic transition e g (a), dimensionless time of the atoms passage through the resonator (b) and quantumdetector efficiency pd (c).</p></li><li><p>MOSCOW UNIVERSITY PHYSICS BULLETIN Vol. 64 No. 6 2009</p><p>INFORMATION ANALYSIS OF QUANTUM NONDEMOLITION MEASUREMENT 615</p><p>(14)</p><p>Using the Shannon formula (13) from the matrix ofconditional probability we get the dependence of theamount of information for all the states on the quantum efficiency of the detector pd shown in Fig. 2c.</p><p>3. QUANTUM INFORMATION ANALYSIS</p><p>3.1. Calculation of Coherent Information in the ResonatorAtom System Taking the Presence </p><p>of an Idle Atomic Level Into Account</p><p>Transformation in the entire HA HC system withspace dimensions dimHA = 3, dimHC = 2 is unitary inthe considered model and thus the transformed state ispure and its wave function has the form</p><p>(15)</p><p>where, according to the standard assumption on thestructure of the measuring device (a probe atom in ourcase) its initial state is considered to be preset.</p><p>For the informational characteristics of coherentinformation contained at the system output to bedetermined it is necessary to take into account thatonly two atomic levels iA = 1,2 are used to further process the measured quantum information. At theatomic output this information is presented in theform of quantum entanglement with the measuredobject, which is an atom. This means that the information related to the noninformative level iA = 3 can berepresented only noncoherently using the procedureof projection [11] at the atom output to correspondingtwodimensional and onedimensional subspaces =</p><p> + .</p><p>For the coherent information functional Ic[ ]</p><p>[11], where = t , taking into account the</p><p>pij = 1 pd( ) 3 2[ ]</p><p>2pd 3 4[ ]</p><p>21 pd( ) 3 6[ ]</p><p>2</p><p>1 pd( ) 4 1[ ]2</p><p>pd 4 3[ ]2</p><p>1 pd( ) 4 5[ ]2</p><p>.</p><p>t UCA t( )0, 0 C 0| A,= =</p><p>P1 2+ P1 2+ P3 P3</p><p>CA</p><p>CA t+</p><p>orthogonality of the projection components in , thefollowing obvious relation occurs</p><p>Here the second term vanishes since for it (with theequation Ic = taken into account)both terms of this equation coincide due to the onedi...</p></li></ul>