Among the applications of optical phase measurement, the differential interference contrast microscope is widely used for the evaluation of opaque materials or biological tissues. However, the signal to noise ratio for a given light intensity is limited by the standard quantum limit (SQL), which is critical for the measurements where the probe light intensity is limited to avoid damaging the sample. The SQL can only be beaten by using N quantum correlated particles, with an improvement factor of √ N . Here we report the first demonstration of an entanglement-enhanced microscope, which is a confocal-type differential interference contrast microscope where an entangled photon pair (N=2) source is used for illumination. An image of a Q shape carved in relief on the glass surface is obtained with better visibility than with a classical light source. The signal to noise ratio is 1.35±0.12 times better than that limited by the SQL.Quantum metrology involves using quantum mechanics to realize more precise measurements than can be achieved classically [1]. The canonical example uses entanglement of N particles to measure a phase with a precision ∆φ = 1/N , known as the Heisenberg limit. Such a measurement outperforms the ∆φ = 1/ √ N precision limit possible with N unentangled particles-the standard quantum limit (SQL). Progress has been made with trapped ions [2-4] and atoms [5], while high-precision optical phase measurements have many important applications, including microscopy, gravity wave detection, measurements of material properties, and medical and biological sensing. Recently, the SQL has been beaten with two photons [6][7][8][9][10] and four photons [11][12][13].Perhaps the natural next step is to demonstrate entanglement-enhanced metrology [14][15][16]. Among the applications of optical phase measurement, microscopy is essential in broad areas of science from physics to biology. The differential interference contrast microscope [17] (DIM) is widely used for the evaluation of opaque materials or the label-free sensing of biological tissues [18]. For instance, the growth of ice crystals has recently been observed with a single molecular step resolution using a laser confocal microscope combined with a DIM [19]. The depth resolution of such measurements is determined by the signal to noise ratio (SNR) of the measurement, and the SNR is in principle limited by the SQL. In the advanced measurements using DIM, the intensity of the probe light, focused onto a tiny area of ∼ 10 −13 m 2 , is tightly limited for a noninvasive measurement, and the limit of the SNR is becoming a critical issue.In this work, we demonstrated an entanglementenhanced microscope, consisting of a confocal-type differential interference contrast microscope equipped with an entangled photon source as a probe light source, with an SNR of 1.35 times better than that of the SQL. We use an entangled two-photon source with a high fidelity of 98%, resulting in a high two-photon interference visibility in the confocal microscope setup of 95.2%. An i...

We show that the quantum interference between down-converted photon pairs and photons from coherent laser light can produce a maximally path entangled N-photon output component with a fidelity greater than 90% for arbitrarily high photon numbers. A simple beam splitter operation can thus transform the two-photon coherence of down-converted light into an almost optimal N-photon coherence.

Two path interferometry with coherent states and squeezed vacuum can achieve phase sensitivities close to the Heisenberg limit when the average photon number of the squeezed vacuum is close to the average photon number of the coherent light. Here, we investigate the phase sensitivity of such states in the presence of photon losses. It is shown that the Cramer-Rao bound of phase sensitivity can be achieved experimentally by using a weak local oscillator and photon counting in the output. The phase sensitivity is then given by the Fisher information F of the state. In the limit of high squeezing, the ratio (F − N )/N 2 of Fisher information above shot noise to the square of the average photon number N depends only on the average number of photons lost, n loss , and the fraction of squeezed vacuum photons µ. For µ = 1/2, the effect of losses is given by (F −N )/N 2 = 1/(1+2n loss ). The possibility of increasing the robustness against losses by lowering the squeezing fraction µ is considered and an optimized result is derived. However, the improvements are rather small, with a maximal improvement by a factor of two at high losses.

Quantum information science addresses how the processing and transmission of information are affected by uniquely quantum mechanical phenomena. Combination of two-qubit gates has been used to realize quantum circuits, however, scalability is becoming a critical problem. The use of three-qubit gates may simplify the structure of quantum circuits dramatically. Among them, the controlled-SWAP (Fredkin) gates are essential since they can be directly applied to important protocols, e.g., error correction, fingerprinting, and optimal cloning. Here we report a realization of the Fredkin gate for photonic qubits. We achieve a fidelity of 0.85 in the computational basis and an output state fidelity of 0.81 for a 3-photon Greenberger-Horne-Zeilinger state. The estimated process fidelity of 0.77 indicates that our Fredkin gate can be applied to various quantum tasks.