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# Proceedings of the Japan Academy, Series BVol. 92 (2016) No. 1 p. 29-43

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http://doi.org/10.2183/pjab.92.29

Reviews

# Abstract

Recent technological progress in the generation, manipulation and detection of individual single photons has opened a new scientific field of photonic quantum information. This progress includes the realization of single photon switches, photonic quantum circuits with specific functions, and the application of novel photonic states to novel optical metrology beyond the limits of standard optics. In this review article, the recent developments and current status of photonic quantum information technology are overviewed based on the author’s past and recent works.

# 1. Introduction

The hypothesis of ‘light quanta’ was advocated by Einstein in 1905,1) which was two decades earlier than the formalism of quantum theory by Shrödinger2) and Heisenberg.3) However, the validity of the basic concepts of quantum theory was still controversial. The nonlocality of quantum theory, which was named ‘quantum entanglement’ by Shrödinger,4) was deeply debated by Einstein, Podorsky, Rosen (EPR)5) and Bohr.6) However, these discussions were purely theoretical and only employed ‘Gedankenexperiment (thought experiment)’.

A change in the situation occurred around the time of the invention of laser in 1960.7) The second order intensity correlation first observed by Hanbury-Brown and Twiss (HBT)8) opened the door for experiments in photon statistics. Kimble, Dagenais and Mandel used the HBT method to observe evidence for single photon emission from sodium atoms in 1977.9) On the other hand, the effort for experimental verification of the nonlocality of the quantum theory had been continued by Clauser and his collaborators10) from the late 1960s. Finally, Aspect and his collaborators experimentally confirmed the existence of quantum nonlocality.11)

Another important technological invention is the generation of twin photons using parametric down–conversion in a nonlinear crystal. This phenomenon was first observed by Harris, Oshman and Byer12) when they attempted to realize a color change of laser beams. In 1970, the correlation of these twin photons was clearly demonstrated by Burnham and Weinberg;13) however, the stream of this research entered a resting stage for a decade, until Mandel’s series of experiments in the middle of the 1980s. The famous two photon interference was experimentally demonstrated by Hong, Ou and Mandel in 1987,14) and the first realization of an entangled photon source using parametric down–conversion was demonstrated by Ou and Mandel in 1988.15)

Simultaneous with this progress in physics, a new trend of information science based on quantum physics emerged in the 1970s when Weisner proposed to use the uncertainty principle to prevent the copying of bank notes.16) Later in 1984, Bennett and Brassard proposed ‘quantum cryptography’,17) which would enable two distant parties to share a secret key of random digits. In 1985, computation based on a ‘quantum bit’, which can exist in a superposition of the ‘0’ and ‘1’ states, was proposed by Deutsch,18) and in 1992, Deutsch and Jozsa showed that such a computer can solve a certain problem intrinsically faster than current computers.19) In 1994, Shor proposed an algorithm for quantum computers to efficiently solve factoring.20) The security of public key cryptosystems, which are widely used in daily life, relies on the difficulty of factoring, so that Shor’s algorithm attracted much attention and a new paradigm of quantum information technology was established.

Photons are appealing in quantum information technology due to their low noise, light-speed transmission and ease of manipulation. In this review article, the development and current status of quantum information technology using photons are overviewed based mainly on the author’s past works. In section 2, we discuss the generation of a single photon and entangled photon using parametric down–conversion. Advanced single photon detectors will also be introduced. In section 3, the proposal and experimental demonstration of a quantum computation algorithm using a single photon are discussed. In section 4, the realization of quantum gates between two photons using two photon interference at a beam splitter is explained. In section 5, photonic quantum circuits combining photonic gates for photons are introduced. The application of photonic quantum information technology to metrology and nanofabrication is discussed in sections 6 and 7. In section 8, conclusions and future prospects are given.

# 2. Single photon and entangled photon sources and photon detectors

The technology for the generation of photons in desired states and the detection of photons is crucially important. Before we discuss the details of photonic quantum information experiments, here we briefly explain the concept of spontaneous parametric down–conversion (SPDC), the generation of heralded single photons and entangled states, and the various types of photon detectors.

## 2.1. SPDC.

The creation of twin photons by SPDC has been a very important tool to explore the nonlocal features of quantum mechanics, and for experiments in quantum information technology. The demonstration of quantum teleportation using photons21) is one example. In the field of quantum cryptography, SPDC is an indispensable technique for the generation of single photon pulses.

SPDC can be regarded as a reverse process of second harmonic generation (SHG). In the case of SHG, two photons in the pump beam with a frequency ωp are converted by a nonlinear optical crystal into a single photon with a frequency 2ωp when the phase matching (momentum conservation) condition is satisfied. In contrast, for SPDC, single photons in the pump beam with a frequency ωp are converted into a pair of photons, so called signal photons and idler photons, when the phase matching condition is satisfied. The two photons are created simultaneously in the crystal13) and are thus quite useful for the generation of heralded single photons22) or two-photon interference experiments,14) which will be discussed later.

Figure 1 shows a CCD camera image of photons generated via SPDC. In this case, parametric fluorescence is generated in two small spots23) with the appropriately selected phase matching condition. The generated twins are concentrated onto the spots, so that a high single count rate and coincidence count rate per pump power was observed. In this case, the ratio of the coincidence count rate to the single count rate was 80%, where the loss at filters and the quantum efficiency of the detectors were compensated. The shape of the image changes from a spot to an empty circle as the phase matching condition changed. A detailed description of SPDC can be found elsewhere.24)

 View Details Fig. 1. Image of the one of the two small spots of the parametric fluorescence observed with a Peltier-cooled CCD camera. The intensity distribution of the image is shown on the right side. Reprinted with permission from Shigeki Takeuchi, Beamlike twin-photon generation using type II parametric down–conversion, Opt. Lett. 26, 843.

## 2.2. Entangled photons.

Quantum entanglement is used for the superposition of different states of correlated particles, where the state of each particle cannot be described independently. For example, the following state,

 \begin{equation*} |\phi \rangle = \frac{1}{\sqrt{2}}(|\text{H} \rangle_{\text{A}} \otimes |\text{V} \rangle_{\text{B}} - |\text{V} \rangle_{\text{A}} \otimes |\text{H} \rangle_{\text{B}}), \end{equation*}
is an entangled state, where |H⟩A denotes the photon A is horizontally polarized. Entanglement was proposed by Schrödinger to discuss the philosophical concept of quantum mechanics; however, it has become an important resource for various protocols of information science.

SPDC has been widely used for the generation of entangled photon pairs. Photons entangled in polarization25),26) have been used for various types of experiments. Recently, the entangled states between the polarization states of two-photon Fock states have attracted attention, which can be regarded as the entanglement between spin 1 systems.27),28)

Note that photons can be entangled in other degrees of freedom or physical properties, such as angular momentum29)31) and frequency.32),33) These various entangled states are particularly useful for quantum metrology, as will be introduced later.

## 2.3. Single photon detectors.

Single photon detectors generate electric pulses upon the detection of single photons and are a basic tool in photonic quantum information technology.34) In the early experiments performed in the late 1980s, photomultiplier tubes (PMT) were used. The large open aperture is one of the advantages of PMTs, so that they are still widely used for the sensing of photons, for example, neutrino detectors. The disadvantage is the moderate quantum efficiency (approximately up to 30%) and dark counts (approximately hundreds to thousands per second), which is caused by false output pulses without photon injection.

Later, silicon avalanche photodiodes (Si-APD) operated in Geiger mode became popular due to their high detection efficiency at visible wavelengths (∼70% for a wavelength of 700 nm) and relatively low dark counts (tens of counts per second). Si-APDs are still widely used in experiments on photonic quantum information and metrology using visible wavelengths. For telecommunication wavelengths, indium gallium arsenic (InGaAs) avalanche photodiodes are used in Geiger mode, where the typical detection efficiency is approximately 20% with relatively high dark count rates. However, APD detectors operated in Geiger mode have one drawback for some applications, in that they do not distinguish the number of simultaneously incident photons.

With regard to this issue, we developed a photon detection system that has high quantum efficiency (∼88%) and is able to discriminate the number of incident photons.35) In this system, single-photons were detected using a visible light photon counter (VLPC) operated at 6.9 K. The VLPC is a solid state device that makes use of avalanches across a shallow impurity conduction band in silicon. Threefold tight shielding and viewports that function as infrared blocking filters were used to eliminate the dark count caused by room-temperature radiation. Corrected quantum efficiencies as high as 88.2 ± 5% (at 694 nm) were observed, which was the highest reported value for a single-photon detector at that time.

The VLPC also featured noise-free avalanche multiplication and narrow pulse height distribution for single photon detection events; therefore, we investigated the simultaneous multi-photon detection capability using twin photons generated by parametric down–conversion.36) The result is shown in Fig. 2, which shows the output electrical signal with the change in the arrival time interval (delay) between the photons in a pair generated by SPDC. The resulting pulse height for the two-photon incidence is twice as high as the single photon incidence. In addition, there is no deadtime for this detector. However, the drawback of this device is the dark count, which was 2.0 × 104 cps at the highest quantum efficiency.

 View Details Fig. 2. Real-time trace of a photon detection signal recorded with a 5 GS/s digitizing oscilloscope. The time delay between the two beams is changed by modifying the optical path length. The traces are shifted vertically for clarity. (a) Single-photon detection signal, (b) Zero delay, (c) 3 ns delay, (d) 5 ns delay.

Recently, single photon detectors using superconductors have attracted attention. There are two types of such detectors: superconducting nanowire single photon detectors (SNSPDs)37) and transition edge sensors (TESs).38) A typical SNSPD uses a superconductor nanowire with a width of tens of nanometers, which covers a certain region (approximately 10 × 10 µm2) with meander-like pattern. When a single photon is absorbed by the nanowire in the superconducting state, the excitation of pseudoparticles causes a phase transition to the normal state, which results in a sudden change in the resistivity of the device. SNSPDs are sensitive over a broad bandwidth from visible to infrared. Recently, detection efficiencies greater than 90% have been reported using a device with a special cavity structure. Another advantage of SNSPDs is the small dark counts. We have recently observed ultralow dark counts (1.5 × 10−3 cps) with an SNSPD.39) The drawback of this type of device is the operation temperature (∼2 to 3 K), which requires special refrigeration system. TESs have also attracted attention for their higher detection efficiency (>95%) and photon-number distinguishability; however, they have a drawback in their slower detection speed (up to MHz).

# 3. Quantum computation using a single photon and linear optics

In the following sections, we explain past and recent progress in experiments on photonic quantum information. The first example is the experimental demonstration of quantum algorithms using a single photon and linear optics.

Quantum computation is a new concept that utilizes quantum superposition states for ultrafast parallel processing.18) There have been several proposals for the realization of quantum computers. Of these, nuclear magnetic resonance quantum computation (NMR-QC) has played the role of a testbed for quantum algorithms.40),41) However, NMR-QC has the following restrictions: the input qubits are prepared in ‘hot mixed states’, and the results are always given by an average over a large number of quantum systems. Therefore, there is a class of algorithm that cannot be performed by NMR-QC.

Quantum computation using linear optics is an alternatively important testbed for quantum computing. If a single photon is used for computation, then the result is given not by an average, but by a single quantum computation. In quantum algorithms, appropriate unitary transformations are applied to quantum registers. Reck and Zeilinger42) found that linear optics can be used to realize any unitary transformation. The theoretical proposal of quantum computation using linear optics was given by Takeuchi,43) whereby the quantum register was realized by modes and polarization of photons, and the unitary transformation was implemented with linear optics. An example of an optical circuit for the 4-bit Deutsch-Jozsa quantum algorithm was presented, where for each quantum computation, the answer to the Deutsch-Jozsa problem is given by a single-photon detection. This is in striking contrast with the NMR quantum computer, where the results are always given by an average over a large number of quantum systems. A similar idea was suggested independently.44)

We also reported an experimental demonstration of the Deutsch-Jozsa quantum computation algorithm using linear optics and a single photon.45),46) First, let us introduce the problem of the Deutsch-Jozsa algorithm. Suppose we are given 2N digits. We call the arrays “even” when they include as many 1’s as 0’s (e.g., 1,0,1,0 for N = 2), and “uniform” when they are filled with only ‘0’s or ‘1’s (e.g., 1,1,1,1). The problem for the Deutsch-Jozsa algorithm is to find the correct answer between “the given array is not even” and “the given array is not uniform”. When the array satisfies both cases, either of them can be the answer. A classical computer requires N + 1 steps in the worst case. However, a quantum computer can find the answer with O(log(N)) steps.19)

In the experiment (Fig. 3), an incident single photon becomes a superposition of being in four optical paths (Path 1 to Path 4). The polarization of the state at each path is then rotated by electro-optic (E/O) modulators upon the oracle function f(i), of which the value is either 0 or 1. When the photon is detected by the photon detector, the answer is that the given oracle f(i) is not even. If the photon is not detected, then the answer is that the given oracle f(i) is not uniform. For the input single photons, strongly attenuated light from a laser diode (LD) was used, where the average number of photons present in the optical system was 3 × 10−3, and the probability of finding two or more photons in the coherent length of 10 cm was less than 3 × 10−4. In this sense, the computation was performed using the quantum phenomenon of single-photon interference. A reference light was used for precise control of the path lengths.

 View Details Fig. 3. Schematic diagram of the optical system used for the Deutsch-Jozsa algorithm with 4-bit inputs. Reprinted with permission from Shigeki Takeuchi, Physical Review A, Vol. 62, 032301, 2000, Copyright (2000) by the American Physical Society.

The experimental results are shown in Fig. 4. The vertical axis shows the four-bit digits given to the quantum computer and the horizontal axis shows the probability P of photon detection at the output port. The theoretical values are shown by the solid lines and the experimental values are plotted as black dots. This result shows that we can determine whether the statement “the given oracle f(i) is not even” or “the given oracle f(i) is not uniform” is correct with small average error rates of 2.7% and 4.0%, respectively, by the detection of a single photon. Here, the initial state was a pure state and the answer was given by single photon detection. Thus, the key aspect of the Deutsch-Jozsa algorithm to obtain the answer with a single quantum computation was fully demonstrated for the first time. The experiment was equivalent to 3 qubits, which was the largest size of a quantum computer when reported. Our demonstration suggested that quantum computation using linear optics is not only useful as a testbed for quantum algorithms, but also practical for small scale quantum processing, namely for quantum communication.

 View Details Fig. 4. Photon detection probability for the given 4-bit digits f(i). The theoretical values without errors are shown by solid lines and the experimental values are plotted as black dots. Reprinted with permission from Shigeki Takeuchi, Physical Review A, Vol. 62, 032301, 2000, Copyright (2000) by the American Physical Society.

Although this scheme is useful to implement small scale quantum processing, there is a problem in that the number of required modes increases exponentially as the number of qubits increase. To overcome this difficulty, multiple photons must be used for multiple qubits and realize two-qubit gates between single photon inputs, which is discussed in the next section.

# 4. Realization of a photon-photon switch — a photonic controlled NOT gate

In one of the earliest proposals to implement quantum computation,47) each qubit was encoded in a single photon existing in two optical modes. The main advantage of the photonic implementation of qubits is the robustness against decoherence and the availability of one-qubit operations. However, the difficulty of realizing the nonlinear interactions between photons that are required for the implementation of two-qubit operations has been a major obstacle. Knill, Laflamme and Milburn (KLM)48) have shown that this obstacle can be overcome using linear optics, single photon sources, photon number detectors, and multi-photon interference at a beam splitter.

Here, let us explain the concept of multi-photon interference. In quantum mechanics, when physical processes share the same initial and final states, ‘interference’ occurs. To calculate the probability of observing such a phenomenon, it is necessary to evaluate the probability amplitude of each process and then calculate the square of the absolute value of their sum. This results in an unusual phenomenon when we consider the case where two photons are incident to a beam splitter (Fig. 5). Suppose two ‘indistinguishable’ photons are incident to a beam splitter with a reflectivity of 50% (50 : 50 BS). If the photons behave like classical particles, then there would be four cases: (1) the left photon is reflected and the right one is transmitted, (2) the opposite case (the right photon is transmitted and the left photon is reflected), (3) both photons are reflected, and (4) both photons are transmitted. A 50 : 50 BS reflects a photon with a probability of 50%; therefore, it is logical to assume a probability of 50% that a photon is emitted from each output port simultaneously. However, this probability is actually 0 due to quantum interference; the probabilities for cases (3) and (4) have the same amplitude but opposite sign, and thus destructively interfere with each other. Hong, Ou and Mandel experimentally demonstrated this phenomenon using pairs of photons generated via an SPDC process.14) This phenomenon is also called Hong-Ou-Mandel (HOM) interference.

 View Details Fig. 5. Hong-Ou-Mandel two-photon interference. When two indistinguishable photons enter a half mirror, the two cases shown on the right side do not occur due to quantum interference.

Following KLM’s proposal, the authors49) and Ralph et al.50) have independently shown that a ‘compact’ CNOT gate can be realized by interaction at a single beam splitter with a reflectivity of 1/3 and post-selection of the output (Fig. 6(a)). The beam splitter sitting in the center of the circuit is the essential splitter that realizes the quantum phase gate operation, where the phase of the quantum state of two photons that are horizontally polarized are flipped, or shifted by pi, due to two-photon interference. The other two beam splitters with reflectivity of 1/3 are inserted in each of the interferometer paths to balance the amplitudes between vertically and horizontally polarized components. The gate operation is successful when both of the photons input to Cin and Tin are output to each of Cout and Tout. The success probability of this operation is 1/9. This gate requires no ancillary photon inputs or additional detectors; therefore, it should be especially useful for the experimental realization of photonic quantum circuits.

 View Details Fig. 6. Schematics of the ‘compact’ CNOT gate. (a) Optical circuit in the original proposals,49),50) where the polarizing beam splitters (PBSs) reflect (transmit) photons with vertical (horizontal) polarization. (b) Optical circuit without any path interference using partially polarizing beam splitters. Reprinted with permission from Ryo Okamoto, Holger F. Hofmann, Shigeki Takeuchi, and Keiji Sasaki, Physical Review Letters, Vol. 95, 210506, 2005, Copyright (2005) by the American Physical Society.

However, there have been two crucial difficulties. In the original scheme,49),50) the polarization sensitivity of the operation was achieved by separating the paths of the orthogonal polarizations, essentially creating two interacting two-path interferometers. Therefore, experimental realization based on the original proposal may be very sensitive to environmental noise (thermal drifts and vibrations), which would make it necessary to control and stabilize nanometer order path-length differences. In addition to these problems, perfect mode-matching is required in each output of the interferometer. Thus, it is very difficult to construct quantum circuits using devices based on those experimental setups.

These problems were solved in our realization of the ‘compact’ optical CNOT gate49),50) without any path interference51) (Fig. 6(b)). It was shown that the CNOT gate can be implemented using three partially polarizing beam splitters (PPBSs) with suitable polarization-dependent transmittance and reflectance, where the essential interaction is realized by a single intrinsic PPBS (PPBS A), while the other two supplemental PPBSs (PPBS B) act as local polarization compensators on the input qubits. The intrinsic PPBS A implements the quantum phase gate operation by completely reflecting vertically polarized light and reflecting (transmitting) 1/3 (2/3) of horizontally polarized light. The two supplemental PPBS Bs are inserted to adjust the amplitudes of the local horizontal and vertical components of the photonic qubits by transmitting (reflecting) 1/3 (2/3) of vertically polarized light and completely transmitting horizontally polarized light. Figure 6(b) shows that the use of PPBSs enable the four optical paths in Fig. 6(a) to be reduced to only two optical paths, and path interferometers are no longer required.

The measurement results for the input-output probabilities of the CNOT gate are shown in Fig. 7. Figure 7(a) shows the measured probabilities where the control and target qubits are set to either |0⟩ or |1⟩ (ZZ basis). For the inputs ‘00’ (‘01’), where the control qubit is |0⟩ and the target qubit is |0⟩ (|1⟩), the output state is the same as the input state with high probability. On the other hand, for the ‘10’ input, the output state is ‘11’, where the target qubit is flipped from |0⟩ to |1⟩ with high probability. The fidelity of the CNOT operation in the ZZ basis was 0.85. Figure 7(b) shows the result for the input/output qubits in the superposition states between |0⟩ and |1⟩ (XX basis). The fidelity of the operation on an XX basis was 0.87, which is in good agreement with that for ideal operation.52)

 View Details Fig. 7. (a) Bar graph of the experimental results for the CNOT operation in the ZZ basis. (b) Bar graph of the experimental results for the reverse CNOT operation in the XX basis. Reprinted with permission from Ryo Okamoto, Holger F. Hofmann, Shigeki Takeuchi, and Keiji Sasaki, Physical Review Letters, Vol. 95, 210506, 2005, Copyright (2005) by the American Physical Society.

These results have opened a door to the realization of more complex quantum circuits for quantum computing, as discussed in the next section. Similar work from Germany and Australia has also been independently published.53),54)

# 5. Photonic quantum circuits

Next, let us introduce photonic quantum circuits, in which two-photon interference is used as the source of required gate operations.

## 5.1. An entanglement filter.

The first example is the photonic quantum circuit for entanglement filter. Filters, which allow passage of the desired and reject the unwanted (material, signal, frequency, polarization, etc.) are one of the most important scientific and technological tools available to us. Quantum information science and technology is concerned with harnessing quantum mechanical effects to gain exponential improvement and new functionalities for particular tasks in communication, computation, measurement, and lithography. Perhaps the most unique of these quantum mechanical features is entanglement. Filters that act on the quantum correlations associated with entanglement must operate non-locally on multiple quantum systems, typically two-level qubits.

Figure 8(a) shows a case where a single photon is incident to a conventional polarization filter. In this example, vertically polarized photons can pass the filter, while horizontally polarized photons are rejected. When diagonally polarized photons are injected, only the horizontal components of the photons are transmitted. Figure 8(b) shows the function of the entanglement filter. The filter transmits photons only when they share the same vertical or horizontal polarization. Suppose that two diagonally polarized photons are injected to the quantum filter (Fig. 8(c)). A diagonally polarized state |D⟩ is a superposition of a vertically polarized state |V⟩ and a horizontally polarized state |H⟩; therefore, the state of the two incident photons is a superposition of four combinations: |HH⟩, |HV⟩, |VH⟩ and |VV⟩. Only |HH⟩ and |VV⟩ are transmitted and the quantum coherence between these two components is preserved during the process, so that the output state is entangled.

 View Details Fig. 8. Functions of polarization and entanglement filters. (a) Polarization filters pass only certain polarization components of single photons. (b) Entanglement filters pass a pair of photons only if they share the same horizontal or vertical polarization. (c) The quantum coherence between these two possibilities is preserved during the process, so that the output state is entangled when two diagonally polarized photons are input. Reprinted with permission from Ryo Okamoto, Jeremy L. O’Brien, Holger F. Hofmann, Tomohisa Nagata, Keiji Sasaki, and Shigeki Takeuchi, Science, Vol. 323, 483 (2009).

We proposed a photonic quantum circuit for such an entanglement filter55) (Fig. 9). Two input photons are injected to modes S1 and S2, and output from S1OUT and S2OUT. The quantum filter operation is successful only when both of the two ancillary photons input from A1 and A2 are detected simultaneously at detectors D1 and D2. Note that although the success probability of the operation is 1/32, the operation is, in principle, always successful when the coincidence detection events between D1 and D2 occur. For a detailed explanation of the mechanism, please refer to the original paper.55)

 View Details Fig. 9. Photonic quantum circuit for the nondestructive entanglement filter.55) This circuit includes concatenated path-interferometers together with four quantum interferences. PBS: polarizing beam splitter that reflects (transmits) the vertical (horizontal) polarization component, BS: beam splitter, of which the reflectance/transmittance is 1/2 for horizontal/vertical polarization. (Inset) Two photon interference at BS2 and BS3. Blue (red) lines indicate optical paths for vertically (horizontally) polarized components. Reprinted with permission from Ryo Okamoto, Jeremy L. O’Brien, Holger F. Hofmann, Tomohisa Nagata, Keiji Sasaki, and Shigeki Takeuchi, Science, Vol. 323, 483 (2009).

However, there was a serious technical problem to overcome before this quantum filter could be realized. The proposed optical circuit (Fig. 9) required two ancillary photons and multiple quantum gates, which require both quantum interference and classical interference in several nested interferometers. Later, the entanglement filter was demonstrated56) by combining two key recent technological approaches; a displaced-Sagnac architecture57) and partially polarizing beam splitters51) (Fig. 10(a)).

 View Details Fig. 10. (a) Schematic of the experimental setup used, where the system now includes one super-stable optical path-interferometer realized using displaced Sagnac architecture. (b) Experimental results. Input signal photons are prepared with horizontal (H) or vertical (V) polarization and are measured on an H/V basis. (A) Theoretically predicted probabilities. (B) Fourfold coincidence count rates [counts/800 s] of the four detectors, D1 to D4. Note that the events where two pairs of photons are simultaneously incident to the ancillary inputs and no photons are incident to the signal inputs are subtracted, as confirmed by a reference experiment without input photons. (6 coincidence counts/800 s for each of the four cases with HH outputs.) Reprinted with permission from Ryo Okamoto, Jeremy L. O’Brien, Holger F. Hofmann, Tomohisa Nagata, Keiji Sasaki, and Shigeki Takeuchi, Science, Vol. 323, 483 (2009).

We first checked the essential operation of the filter circuit by preparing input signal photons in the four combinations of horizontal (H) and vertical (V) polarizations (which we call the Z-basis states) and observed how these input states were filtered by the circuit (Fig. 10(b)). It is clear from the experimental data that the photon pairs are transmitted through the filter when the two input photons share the same polarization (HH or VV), and most of the pairs are filtered out when the two input photons have different polarization (HV or VH). The observed fidelity of this process was FZZ = 0.80. The device was also evaluated for input and output states on a different basis and the entangling capability of the filter was verified as distinct from classical filters.

The filter can be used for the generation as well as the purification of entanglement, which will be important in realizing quantum relays and repeaters for long distance quantum communication.

## 5.2. Knill-Laflamme-Milburn controlled-NOT operation.

As discussed in Sec. 4, the lack of highly efficient optical Kerr nonlinearities at single photon level was a major obstacle. In a breakthrough, KLM showed that such an efficient nonlinearity can be achieved using only linear optical elements, auxiliary photons, and measurement.48) They proposed a heralded controlled-NOT (CNOT) gate for scalable quantum computation using a photonic quantum circuit to combine two such nonlinear elements. We have experimentally demonstrated a KLM CNOT gate,58) where a stable architecture was developed to realize the required four-photon network of nested multiple interferometers based on a displaced-Sagnac interferometer and several PPBSs (Fig. 11). The result confirms the first step in the original KLM ‘recipe’ for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. Photonic quantum circuits that combine giant optical nonlinearities may find wide applications in quantum information processing, communication and sensing.

 View Details Fig. 11. Experimental scheme for the KLM CNOT gate. The gate is constructed of two nonlinear sign-shift (NS) gates; the output is accepted only if the correct heralding signal is observed for each NS gate. Stable optical setup was realized using PPBSs and a displaced Sagnac architecture. The blue lines indicate optical paths for vertically polarized components and the red lines indicate optical paths for horizontally polarized components. Reprinted with permission from Ryo Okamoto, Jeremy L. O’Brien, Holger F. Hofmann, and Shigeki Takeuchi, Realization of a Knill-Laflamme-Milburn controlled-NOT photonic quantum circuit combining effective optical nonlinearities, PNAS 108, 10067–10071 (2011).

# 6. Quantum metrology

Quantum metrology involves using quantum mechanics to realize more precise measurements than can be achieved classically.59) The canonical example uses entanglement of N particles to measure a phase with a precision of Δϕ = 1/N, the Heisenberg limit. Such a measurement outperforms the $\Delta \phi = 1/\sqrt{N}$ precision limit possible with N unentangled particles, the standard quantum limit (SQL). As for photons, high-precision optical phase measurements have many important applications, including microscopy, gravity wave detection, measurements of material properties, and medical and biological sensing. Although a reduced de Broglie wavelength60) has been reported for three,61) four,62),63) and even six64) photons, the SQL has only been beaten with two photons.65),66)

We demonstrated an entangled four photon phase measurement with a visibility that exceeds the threshold to beat the SQL.57) An ultrastable displaced-Sagnac implementation of a scheme with high intrinsic efficiency (Fig. 12) is used to achieve a four photon interference visibility of 91% (Fig. 13). We also demonstrated that measurement of a reduced de Broglie wavelength does not mean beating the SQL, via another experiment, which shows high-visibility multi-photon fringes, but cannot beat the SQL.

 View Details Fig. 12. An optical interferometer used to beat the SQL. (Inset) Schematic of a Mach-Zehnder (MZ) interferometer consisting of two 50 : 50 beam splitters (BS1 and BS2). Photons are input in modes a and/or b, and detected in modes e and/or f; after a phase shift (PS) is applied to mode d. (Main figure) Schematic of the intrinsically stable displaced-Sagnac architecture used to ensure that the optical path lengths in modes c and d are sub-wavelength (nm) stable. A frequency-doubled 780 nm femtosecond pulsed laser (repetition interval: 13 ns) pumps a type-I phase-matched beta barium borate (BBO) crystal to generate the state |22⟩ab via SPDC. Interference filters (not shown) with a 4 nm bandwidth were used. The photons are guided via polarization maintaining fibers (PMFs) to the interferometer, which has the same function as the MZ interferometer in the inset. A variable phase shift in mode d is realized by changing the angle of the phase plate (PP) in the interferometer. Photons are collected in single mode fibers (SMFs) at the output modes and detected using a single photon counting module (SPCM; detection efficiency 60% at 780 nm) in mode f and three cascade SPCMs in mode e. Reprinted with permission from Tomohisa Nagata, Ryo Okamoto, Jeremy L. O’Brien, Keiji Sasaki, and Shigeki Takeuchi, Science, Vol. 316, 726 (2007).

 View Details Fig. 13. Beating the SQL with four entangled photons. (A) Single photon count rate in mode e as a function of phase plate (PP) angle with single photon input |10⟩ab. (B) Two photon count rate in modes e and f for input state |11⟩ab. (C) Four photon count rate of 3 photons in mode e and 1 photon in mode f for the input state |22⟩ab. Accumulation times for one data point were (A) 1, (B) 300, and (C) 300 s. Reprinted with permission from Tomohisa Nagata, Ryo Okamoto, Jeremy L. O’Brien, Keiji Sasaki, and Shigeki Takeuchi, Science, Vol. 316, 726 (2007).

As a real application of the quantum metrology beating the SQL, we proposed and demonstrated an entanglement-enhanced microscope,67) which is the differential confocal microscope where an entangled photon pair source is used for illumination. We showed that the signal to noise (S/N) ratio of the image obtained by the entanglement microscope is 1.35 times better than that limited by the SQL.

The high-precision multi-photon quantum-interference demonstrated here is key, not only to quantum metrology and quantum lithography,68) but also to other photonic quantum technologies.

# 7. Quantum lithography

Optical lithography is an indispensable technique for the mass production of microstructures such as semiconductor devices. However, there exists a resolution limit; the minimum period of the interference fringe is λ/2 for a conventional light source.69) As the design rule for integrated circuits becomes smaller, the diffraction limit becomes a more significant problem. Quantum lithography68) is a method to overcome this limit completely, because the resolution limit for the N-photon entangled state is reduced to λ/2N. Several experiments have observed a reduced de Broglie wavelength of photons in the time-domain.57),66),70) In the spatial-domain, the reduction of a double-slit interference diffraction pattern by a factor of 2 has been reported using two-photon interference.71),72) However, the direct observation of spatially formed interference-fringe periods smaller than λ/2 in the spatial-domain, which is an indispensable step for the realization of quantum lithography, had not been reported before we reported the direct observation of interference fringes beating the diffraction limit.73) We used a high-fidelity entangled photon source,74) a stable interferometer without active stabilizer,75) and a specially developed near-field scanning optical microscope (NSOM) probe76) (Fig. 14). The experimental result is shown in Fig. 15. We succeeded in the observation of the two-photon interference fringe with a period of 328.2 nm, which is smaller than the diffraction limit of 351.1 nm. Our result confirmed quantum lithography as a viable technique and opened up new possibilities for the use of quantum-optical phenomena in nanotechnology.

 View Details Fig. 14. Schematic of experimental setup. Photon pairs entangled in polarization, $(|VV\rangle _{a} + |HH\rangle _{a})/\sqrt{2}$, were generated from two BBO crystals and passed through an interference filter (IF) and a single mode fiber (SMF). The entangled photons are then spatially separated into paths b and c depending on their polarization in a calcite crystal (Cal). The polarization in path b is rotated by 90° using a half wave plate (WP), which gives a two-photon NOON state $((|2\rangle _{d}|0\rangle _{e} + |0\rangle _{d}|2\rangle _{e})/\sqrt{2} )$. An aspheric lens (L) is used to focus the photons to a small spot and form an interference fringe in the focal plane (FP). A near-field scanning optical microscope (NSOM) probe with elliptical opening (inset) is scanned with a piezo-actuator along the focal plane. The SMF output of the probe is divided by a 50 : 50 fiber coupler and detected by SPCMs. Reprinted with permission from Yoshio Kawabe, Hideki Fujiwara, Ryo Okamoto, Keiji Sasaki, and Shigeki Takeuchi, Quantum interference fringes beating the diffraction limit, Opt. Exp., Vol. 15, 14244–14250 (2007).

 View Details Fig. 15. (a) Interference fringe of single photons. A polarizer is placed before the SMF to select only horizontally polarized photons. The polarization was then rotated 45°. The effective detection efficiency (<1 × 10−3) including the collection efficiency of the probe is so small, so that the generated fringe is the same as that for a single-photon state in two modes: $(|1\rangle _{d}|0\rangle _{e} + |0\rangle _{d}|1\rangle _{e})/\sqrt{2}$ when only the output of the one of the two detectors is recorded. The black lines are sinusoidal curves weighted with a Gaussian function fitted to the experimental data. (b) Interference fringe of entangled photons beating the diffraction limit. The vertical axes indicate coincidence count rates (red dots, left axis). The fringe period of 328.2 nm is smaller than the diffraction limit of 351.1 nm. Single count rates of one of the two detectors measured simultaneously are shown for reference (blue dots, right axis). For (a) and (b), the NSOM probe was scanned with 25 nm steps. The accumulation time for one data point was 5 s. The error bars are shown assuming Poisson statistics. Reprinted with permission from Yoshio Kawabe, Hideki Fujiwara, Ryo Okamoto, Keiji Sasaki, and Shigeki Takeuchi, Quantum interference fringes beating the diffraction limit, Opt. Exp., Vol. 15, 14244–14250 (2007).

The next step is the fabrication of patterns that beat the diffraction limit using real multi-photon absorbing photoresist materials. In addition to the experimental technologies developed here, ultrabright sources of entangled photons and multi-photon absorbing photoresist materials with large cross-sections are required for such experiments, which still remain as challenging problems.

# 8. Conclusions and future prospects

In this review article, we have discussed the current status of photonic quantum information science and technology: single photon and entangled photon generation, photon number discriminating detectors, demonstrations of quantum computation algorithms, photonic quantum gates and photonic quantum circuits, photonic quantum metrology and lithography.

It should be noted that photonic quantum information technology is not only for quantum computers. This technology will certainly be applied to advance quantum key distribution systems, which have already become commercial products; however, this technology will also lead to innovations in the fields of sensing and metrology. One example is quantum optical tomography (Q-OCT),77) by which the monitoring of biomedical tissues such as the retina will become possible with much more improved resolution. It is predicted that photonic quantum information technology will be used in very broad fields of science, spanning from astronomy and particle physics to medicine.

For each of these specific applications, further developments of photonic quantum information technology will be required. For example, an entangled photon source with a very broad spectrum32) is required to improve the resolution of Q-OCT. Such a source may also be useful for a quantum key distribution multiplexed in a frequency region. Many efforts have been devoted to realize solid state single photon sources that generate indistinguishable single photons. When the visibility of two photon interference using these photons becomes comparable to the best (96%) of that using heralding single photon sources with parametric down–conversion,78) the research on photonic quantum technology will be dramatically enhanced.

Another problem to be solved is loss of photons. For example, the enhancement of sensitivity is strongly limited by the loss in optical paths or detectors. Although the effect of loss differ between the applications, the improvement in detection efficiencies of single photon sources, coupling efficiencies of single photon sources to photonic circuits are common key issues. Exploration of loss tolerant schemes for each application is also quite important.

As bulk electric parts such as transistors or registers have disappeared in electric products, such items are now combined in integrated circuit chips. Similarly, all the bulk optics, single photon sources, and detectors will be packaged on a chip. In this sense, progress in optical integrated circuits will have a significant impact on the future of photonic quantum information technology.79)

# Profile

Shigeki Takeuchi was born in Osaka in 1968. He received his B.Sc., M.Sc., and Ph.D. degrees in Physics from Kyoto University in 1991, 1993 and 2000 respectively. He became a researcher of the Central Research Lab., Mitsubishi Electric Co., Japan in 1993, and a lecturer, associate professor, and professor of Research Institute for Electronic Science, Hokkaido University, Japan, in 1999, 2000, and 2007 respectively. He also served as an Invited Professor (residing) at the Institute of Scientific and Industrial Research, Osaka University, Japan from 2007 to 2014. Since 2014, he has been in the current position, a Professor of the Department of Electronic Science and Engineering, Kyoto University. His interest lies in understanding and controlling the nature of light quanta (photons). One of the directions of his research is to invent new concepts and technologies that use photons for quantum information protocols, i.e., quantum computing and quantum key distribution. He is also interested in nano-photonics, namely, the application of solid state micro-cavities and nano-optical fibers to quantum information science and biology. He has received several awards including the Young Scientist Award from the Minister of Education, Culture, Sports, Science and Technology (2005), The Scientific American 50 award, awarded by Scientific American (2007), the 6th Japan Society for the Promotion of Science (JSPS) Prize (2010), the Daiwa Adrian Prize awarded by the Daiwa Adrian Foundation (2010), the Hokkaido University President's Award for Research Excellence (2012), and Osaka Science Prize (2015).

# Acknowledgements

I would like to thank the main co-authors of the works presented in this article; Keiji Sasaki, Holger F. Hofmann, Jeremy L. O’Brien, Hideki Fujiwara, Kenji Tsujino, Ryo Okamoto, Daisuke Kawase, Yoshio Kawabe, Tomohisa Nagata, Masayuki Okano, Masato Tanida, Yoshihisa Yamamoto, Jungsang Kim, Henry H. Hogue, Mitsuo Takeda, and Yoko Miyamoto, for their contribution, assistance, and suggestions. I would also like to thank all other collaborators and colleagues, and friends for their kind support.

The works reported here were supported in part by the Japan Science and Technology Agency (JST)-PRESTO ‘Field and Reaction’ and ‘Light and Control’ projects, JST-CREST projects, Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS), MIC-SCOPE projects, the Quantum Cybernetics Project, the FIRST Program of JSPS, the Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the GCOE program, Mitsubishi Electric Corporation, the International Communication Foundation, and the Research Foundation for Opto-Science and Technology.

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