Johan Ahrens

Two Fundamental Experimental Tests of Nonclassicality with Qutrits

In classical physics, there is no contradiction in considering that systems like balls and coins have preestablished properties like position and velocity that are independent of whether one actually measures them or not. However, according to quantum mechanics, the results of experiments on systems such as atoms and photons do not correspond to preestablished properties. A natural and fundamental question is: Which is the simplest quantum system in which this difference between classical and quantum physics can be observed?

Experimental device-independent tests of classical and quantum dimensions

Hilbert space is made up of a potentially infinite number of dimensions that correspond to all the parameters needed to fully define a system. The idea is seen as an important resource for quantum information processing. A technique for estimating the number of dimensions in an unknown system based on the results of measurements performed on it—a so-called dimension witness—is now experimentally demonstrated.

Sagnac secret sharing over telecom fiber networks

We report the first Sagnac quantum secret sharing (in three-and four-party implementations) over 1550 nm single mode fiber (SMF) networks, using a single qubit protocol with phase encoding. Our secret sharing experiment has been based on a single qubit protocol, which has opened the door to practical secret sharing implementation over fiber telecom channels and in free-space. The previous quantum secret sharing proposals were based on multiparticle entangled states, difficult in the practical implementation and not scalable. Our experimental data in the three-party implementation show stable (in regards to birefringence drift) quantum secret sharing transmissions at the total Sagnac transmission loop distances of 55-75 km with the quantum bit error rates (QBER) of 2.3-2.4% for the mean photon number micro?= 0.1 and 1.7-2.1% for micro= 0.3. In the four-party case we have achieved quantum secret sharing transmissions at the total Sagnac transmission loop distances of 45-55 km with the quantum bit error rates (QBER) of 3.0-3.7% for the mean photon number micro= 0.1 and 1.8-3.0% for micro?= 0.3. The stability of quantum transmission has been achieved thanks to our new concept for compensation of SMF birefringence effects in Sagnac, based on a polarization control system and a polarization insensitive phase modulator. The measurement results have showed feasibility of quantum secret sharing over telecom fiber networks in Sagnac configuration, using standard fiber telecom components.

Experimental Observation of Hardy-Like Quantum Contextuality

Contextuality is a fundamental property of quantum theory and a critical resource for quantum computation. Here, we experimentally observe the arguably cleanest form of contextuality in quantum theory [A. Cabello et al., Phys. Rev. Lett. 111, 180404 (2013)] by implementing a novel method for performing two sequential measurements on heralded photons. This method opens the door to a variety of fundamental experiments and applications.

Experimental Tests of Classical and Quantum Dimensionality

We report on an experimental test of classical and quantum dimension. We have used a dimension witness that can distinguish between quantum and classical systems of dimensions two, three, and four and performed the experiment for all five cases. The witness we have chosen is a base of semi-device-independent cryptographic and randomness expansion protocols. Therefore, the part of the experiment in which qubits were used is a realization of these protocols. In our work we also present an analytic method for finding the maximum quantum value of the witness along with corresponding measurements and preparations. This method is quite general and can be applied to any linear dimension witness.

Two fundamental experimental tests of nonclassicality with qutrits

In classical physics, there is no contradiction in considering that systems like balls and coins have preestablished properties like position and velocity that are independent of whether one actually measures them or not. However, according to quantum mechanics, the results of experiments on systems such as atoms and photons do not correspond to preestablished properties. A natural and fundamental question is: Which is the simplest quantum system in which this difference between classical and quantum physics can be observed?

Secure multiparty quantum communication over telecom fiber networks

Splitting a secret message in the way that a single person is not able to reconstruct it is a common task in information processing and high security applications. For instance, let us assume that the launch sequence of a nuclear missile is protected by a secret code and it should be ensured that a single person is not able to activate it but at least two persons need to cooperate in order to carry out the launch. Another example is a joined banking account. The account is set in such a way that withdrawing cash is possible when all of parties cooperate by generating a code required by an automated teller machine (ATM) or by a banker. A solution for this problem and its generalization, including several variations, is provided by classical cryptography and is called secret sharing. It consists of a way of splitting the message using mathematical algorithms and the distribution of the resulting pieces to two or more legitimate users by classical communication. However, all ways of classical communication currently used are susceptible to eavesdropping attacks. As the usage of quantum resources can lead to unconditionally secure communication, a protocol introducing quantum information scheme to Secret Sharing has been developed. This protocol provides information splitting and eavesdropping protection. However, his scheme is in practice non-scalable since it used multipartite entangled states that are difficult to generate and transmit. Furthermore the use of polarization encoding is impractical for applications over commercial birefringent single mode fibers (SMF) networks. A new protocol solving the above mentioned problems was proposed in [1]. The protocol requires only a single qubit for quantum information transmission, which allowed for its practical experimental realization and scalability.

Sagnac quantum key distribution and secret sharing

We present the first Sagnac quantum secret sharing (in three- and four-party implementations) as well as Sagnac two-user quantum key distribution (QKD) over 1550 nm single mode fiber (SMF) networks, using the BB84- protocol with phase encoding. The secret sharing experiment has implemented a single qubit protocol, which allows for a practical secret sharing implementation over fiber telecom channels and in free-space. The previous quantum secret sharing proposals were based on multiparticle entangled states, not scalable and diffcult in the experimental implementations. Our experimental data show stable, in regards to birefringence drift, quantum secret sharing transmissions at the total Sagnac transmission loop distances of 45-55 km with the quantum bit error rates (QBER) of 3.0-3.7 % for the mean photon number μ = 0.1. In the QKD experiment we have achieved the total Sagnac transmission loop distances of 100-150 km with quantum bit error rates (QBER) of 5.84-9.79 % for μ = 0.1. The distances were much longer and rates much higher than in any other published Sagnac QKD experiments. The stability of quantum transmission in both secret sharing and QKD experiments has been achieved thanks to our new concept for compensation of SMF birefringence effects in Sagnac, based on a polarization control system and a polarization insensitive phase modulator. The measurement results have showed feasibility of quantum secret sharing and QKD over telecom fiber networks in Sagnac confi;guration, using standard fiber telecom components. Our birefringence compensation in SMF Sagnac open the door to other Sagnac-based applications over SMF links such as precise optical sensing, dispersion characteristics of optical fibers, acoustic and strain sensing, and generally sensing of any time varying phenomenon.