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Quantum State Tomography of Single Qubit Using Density Matrix PROC. INTERNAT. CONF. SCI. ENGIN. ISSN 1504607797 Volume 4, February 2021 E-ISSN 1505707533 Page 27-32 Quantum State Tomography of Single Qubit Using Density Matrix Syafi’i Fahmi Bastian1, Pruet Kalasuwan2, Joko Purwanto1 1Physics Education Department, Faculty of Science and Technology, Universitas Islam Negeri Sunan Kalijaga 2Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Thailand Email: [email protected] Abstract. The quantum state tomography is a fundamental part in the development of quantum technologies. It can be used to know the signal characterization of small particle called photon in the nanoscale. In this study, photon number has been measured in order to produce the states tomography. Optical devices and quantum-mechanical approaches were explored to obtain the quantum state tomography. Due to a single qubit state density matrix can be revealed by Stokes parameters, so there are four set-ups to measure the Stokes parameters for each sample. The density matrix is used because the pure state only appear theoreticaly. In the real experiment, It always exibits a mixed state. The samples of tomography measurements consist of linear state, cicular state and the IR 808nm. In this study, state tomography is shown by 2x2 density matrix. This experiment also provides the fidelities of experiment result. And it shows the good agreement. From this experiment, the state of IR 808nm has been detected. The laser that examined is showing a vertical state with fidelity F=97,34%. Keywords: a single qubit, density matrix, quantum state INTRODUCTION |퐻⟩ is acting for horizontal state and |푉⟩ is acting for Quantum physics is capable to reveal the behavior of vertical state. These vectors, |퐻⟩ and |푉⟩ are called kets, a particles in nanoscale that classical physics cannot describe name proposed by Paul Dirac. It is part of the word bracket. (Gillespie, 1988). A Research in the nanophotonics field In quantum mechanics, it is called Dirac Notation. The has positive contribution for future technology. In this polarization state in general is: digital era, technology is getting smaller and smaller in size, a H b V but it has more efficient performance. Lack of nanotechnology manufacturing and analysis tools are the where a and b are in general complex with 22 roadblock to innovation, as is lack of modeling tools. New |ab | | | 1 characterization methodes are required to study nanoscale interaction. (Pomrenke, 2004). Then |휓⟩ is normalized. In the case of linear polarization a Quantum computing is the use of a superposition and and b are real numbers. Those numbers can be written as entanglement. In analogy to the primary unit of information cos θ and sin θ. |퐻⟩ and |푉⟩ are the basis states. And those in computer science, the state of a quantum system is two basis are an orthonormal basis because |퐻⟩ and |푉⟩ are labeled a “quantum bit” or “qubit”. (Niggebaum, 2011). perpendicular. Another orthonormal basis can be described This concept, a qubit, is a component of quantum as follow: mechanics. A pure qubit state is a coherent superposition cos HV sin of the basis states. This means that a single qubit can be expressed by a linear combination of |0⟩ and |1⟩. Improvement in realization of the properties of state, sin HV cos especially about state tomography, the quantum computer can be developed. The tomography of a quantum state is a With θ ⊥ φ (perpendicular). In other case, the type of process of creating an image of a signal. The laser which is polarized light can be circular polarization. There are two fired certainly has a quantum state and the image can be types in circular polarization. The first is right circular produced. The observer can not see the quantum state of the polarization and the second is left circular polarization. laser directly and also can not measure it by using an instan Right circular polarization can be dicribed as: tool. So, the optical set-up will be needed. 1 R H i V 2 MATERIALS AND METHODS And left circular polarization can be described as follow: Polarization 1 Light, such as any other electromagnetic wave, roughly L H i V always propagates as a transverse wave, with both electric 2 and magnetic fields oscillating perpendicularly to the direction of propagation. The direction of the electric field The circular polarizations have complex coefficients in is called the polarization of the wave (Monteiro, 2016). A this case. And that is the difference between linear and wave polarized linearly by a wave plate has the same circular polarization. To represent the state vectors, it needs electric field direction. The polarization states can be matrices representation. In this case, Jone’s calculus can represented in a two dimensional vector space (Rothberg, represent the polarization states (Fowles, 1975) as follows: 2008). Polarized light in the x (horizontal) direction can be represented by |퐻⟩ and in the y (vertical) direction by |푉⟩. 27 PROC. INTERNAT. CONF. SCI. ENGIN. 3: 27-32, February 2021 1 0 1 1 The effect of the half-wave plate (HWP) and quarter- H ; V ; D ; wave plate (QWP) can be writen the matrix as follow 0 1 2 1 (James, On the Measurement of Qubit, 2008): 1 1 1 1 ˆ cos 2 sin A ; R ; HWP 2 1 2 i sin 2 cos 2 1 1 ˆ 1 i cos 2 sin 2 L QWP i 2 sin 2i cos 2 2 The density matrix or density operator can be used to The QWP converts linearly polarized light into represent the state of a quantum system. The density matrix circularly polarized light. To make sure that the calculation is used because it is a practical tool when dealing with is compatible with the optical equipment, simulating by mixed states. A pure state is a state characterized by a single experiment is needed to prove the functions. Probabilities wave function. Whereas mixed states are a mixture of calculation can be completed using mathematical statistics that have imperfect information or mix preparations. For example, the representation of information about systems, which are used to obtain polarization phenomenon is shown that the light passes quantum states. The density matrix of pure state is formally through an optical device as operator. It can be described as defined as the outer product of the wavefunction and its bra-ket notation with normalized state (Bjork, 2003) as conjugate (Henao, 2017): follows: ˆ 10 HOHH 1 H HH 00 If the system is orthogonal, the equation will equal zero, 00 as follows: V VV VOHˆ 0 01 H 11 Ôρh is an operator which represents the optical device D DD 11 like polarizer beam splitter gives two output as matrix because polarizer beam splitter can reflect and transmit the 11 light. the transmitted light will be a horizontal state and the A AA reflected light will be a vertical state (Rizea, 2011). So, the 11 matrices as follow: 1 1 i 10 00 RR ˆ ˆ R OH ; OV 2 i 1 00 01 1 1 i And |퐻⟩ and |푉⟩ is a ket vector of polarizing L LL 2 i 1 beamsplitter which tell us the polarization type. A single qubit state density matrix can be related by four In other case, when the polarized light |퐻⟩ passes parameters called Stokes Parameters (James, On the throught the quarter waveplate and polarizing beamsplitter, Measurement of Qubit, 2008), As follow, we can write as normalized state sa follows: 1 3 HOOOHˆ† ˆ ˆ S H 2S ii 0 i0 However, all measurable or observable entities correspond to observables, Ô, which are Hermitian are the standart Pauli matrices ( , , ) plus the 푖 1 2 3 operators. (Bjork, 2003) The definition of Hermitian identity ( ). And S are the Stokes parameters (S , S , S , 0 i 0 1 2 operator is: S3). ˆˆ† We can represent the polarization states by state vectors OO in a two-dimensional vector space (Rothberg, 2008). In this case, an optical setup consisting of waveplates and polarizers are used, to perform projective measurements to different prepared polarization states. The half-wave plate is an optical device which changes the direction into linear polarization. 28 PROC. INTERNAT. CONF. SCI. ENGIN. 3: 27-32, February 2021 The probabilities from that calculation are square of the And the fuction is: result. The relation between the probability and rotaion 1 cos2 2 angle of wave plate will shown in this graph: 2 This fuction represent the red curve (V to QWP). The fourth graph is described by this form: ˆ† ˆ ˆ ROOORQWP H QWP When the equations are subtituted into that form, It can be written as: 1 1iicos 2 sin 2 1 0 1 cos 2 sin 2 1 1 1 i 2 2sin 2i cos 2 0 0 2 sin 2 i cos 2 2 i And the fuction is: 1 sin 2 Figure 1. The graph of relation between probability and angle of waveplat (base on calculation) 2 The graph shows that probabilities will be changed This fuction represent the black curve (R to HWP). when the angle of wave plate is rotated. The first curve can Preparation be writen with the formulations as follow: 1. Optical devices HOOOHˆ† ˆ ˆ In this work, there are four calculations to HWP H HWP callibrate our theories and our experiments. Before If the equations are subtituted to the form above, it will doing the measurement, we need to callibrate our be: optical device. Some main optical devices are needed to measure state tomography are Beam Splitter (BS), cos 2 sin 2 1 0 cos 2 sin 2 1 10 Polarizing Beam Splitter (PBS), Photon Detector, sin 2 cos 2 0 0 sin 2 cos 2 0 Half Wave Plate (HWP), Quarter Wave Plate (QWP), Laser beam.
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