Petr M. Anisimov

Quantum Metrology with Two-Mode Squeezed Vacuum: Parity Detection Beats the Heisenberg Limit

We study the sensitivity and resolution of phase measurement in a Mach-Zehnder interferometer with two-mode squeezed vacuum (n photons on average). We show that superresolution and sub-Heisenberg sensitivity is obtained with parity detection. In particular, in our setup, dependence of the signal on the phase evolves n times faster than in traditional schemes, and uncertainty in the phase estimation is better than 1/n, and we saturate the quantum Cramer-Rao bound.

Objectively discerning Autler-Townes splitting from electromagnetically induced transparency.

Autler-Townes splitting (ATS) and electromagnetically induced transparency (EIT) both yield transparency in an absorption profile, but only EIT yields strong transparency for a weak pump field due to Fano interference. Empirically discriminating EIT from ATS is important but so far has been subjective. We introduce an objective method, based on Akaikes information criterion, to test ATS vs EIT from experimental data for three-level atomic systems and determine which pertains. We apply our method to a recently reported induced-transparency experiment in superconducting-circuit quantum electrodynamics.

Super-resolution at the shot-noise limit with coherent states and photon-number-resolving detectors

There has been much recent interest in quantum optical interferometry for applications to metrology, subwavelength imaging, and remote sensing such as in quantum laser radar (LADAR). For quantum LADAR, atmospheric absorption rapidly degrades any quantum state of light, so that for high-photon loss the optimal strategy is to transmit coherent states of light, which suffer no worse loss than the Beer law for classical optical attenuation, and which provides sensitivity at the shot-noise limit. We show that coherent light coupled with photon-number-resolving detectors can provide a super-resolution much below the Rayleigh diffraction limit, with sensitivity no worse than shot noise in terms of the detected photon power.

Parity detection achieves the Heisenberg limit in interferometry with coherent mixed with squeezed vacuum light

The interference between coherent and squeezed vacuum light can produce path entangled states with very high fidelities. We show that the phase sensitivity of the above interferometric scheme with parity detection saturates the quantum Cramer-Rao bound, which reaches the Heisenberg-limit when the coherent and squeezed vacuum light are mixed in roughly equal proportions. For the same interferometric scheme, we draw a detailed comparison between parity detection and a symmetric-logarithmic-derivative-based detection scheme suggested by Ono and Hofmann.The interference between coherent and squeezed vacuum light effectively produces path entangled N00N states with very high fidelities. We show that the phase sensitivity of the above interferometric scheme with parity detection saturates the quantum Cramer–Rao bound, which reaches the Heisenberg limit when the coherent and squeezed vacuum light are mixed in roughly equal proportions. For the same interferometric scheme, we draw a detailed comparison between parity detection and a symmetric-logarithmic-derivative-based detection scheme suggested by Ono and Hofmann.

Quantum random walks with multiphoton interference and high-order correlation functions

We show a simulation of quantum random walks (QRWs) with multiple photons using a staggered array of 50/50 beam splitters with a bank of detectors at any desired level. We discuss the multiphoton interference effects that are inherent to this setup, and introduce one, two, and threefold coincidence detection schemes. Feynman diagrams are used to intuitively explain the unique multiphoton interference effects of these QRWs.

An invisible quantum tripwire

We present here a quantum tripwire, which is a quantum optical interrogation technique capable of detecting an intrusion with very low probability of the tripwire being revealed to the intruder. Our scheme combines interaction-free measurement (IFM) with the quantum Zeno effect in order to interrogate the presence of the intruder without interaction. The tripwire exploits a curious nonlinear behaviour of the quantum Zeno effect we discovered, which occurs in a lossy system. We also employ a statistical hypothesis testing protocol, allowing us to calculate a confidence level of IFM after a given number of trials. As a result, our quantum intruder alert system is robust against photon loss and dephasing under realistic atmospheric conditions and its design minimizes the probabilities of false positives and false negatives as well as the probability of becoming visible to the intruder.

Optimizing the multiphoton absorption properties of maximally path-entangled number states

In this paper we examine the N-photon absorption properties of maximally path-entangled number states (N00N states). We consider two cases. The first involves the N-photon absorption properties of the ideal N00N state, one that does not include spectral information. We study how the N-photon absorption probability of this state scales with N, confirming results presented by others in a previous paper by a different method. We compare this to the absorption probability of various other states. The second case is that of two-photon absorption for an N=2 N00N state generated from a type-II spontaneous down-conversion event. In this situation we find that the absorption probability is both better than analogous coherent light (due to frequency entanglement) and highly dependent on the optical setup. We show that the poor production rates of quantum states of light may be partially mitigated by adjusting the spectral parameters to improve their two-photon absorption rates. This work has application to quantum imaging, particularly quantum lithography, where the N-photon absorbing process in the lithographic resist must be optimized for practical applications.

Thwarting the photon number splitting attack with entanglement enhanced BB84 quantum key distribution

We develop an improvement to the weak laser pulse BB84 scheme for quantum key distribution, which utilizes entanglement to increase the security of the scheme and enhance its resilience to the photon-number-splitting attack. This protocol relies on the non-commutation of phase and number to detect an eavesdropper performing quantum nondemolition measurement on photon number. The potential advantages and disadvantages of this scheme are compared to the coherent decoy state protocol.

Single and biphoton imaging and high dimensional quantum communication

Here, we present recent developments in the field of quantum imaging focusing on the high dimensionality aspects of single and biphoton imaging. We discuss some systems that have a “quantum advantage” over classical counterparts. We highlight some recent experiments in single-photon image discrimination, large alphabet quantum key distribution and image buffering.

Qudit communication network

Optical coherent states can be interpreted as d-dimensional quantum systems, or qudits of even superposition of pseudo-number states. Cross-Kerr nonlinear interaction can generate the maximal entanglements of pseudo- phase and pseudo-number states from two opticl coherent states. Extended network of these entangled coherent states is a qudit cluster state and can be used as qudit communication network for d-dimensional teleportation or multi-user quantum cryptographic network.