We establish several converse bounds on the private transmission capabilities of a quantum channel. The main conceptual development builds firmly on the notion of a private state [Horodecki et al., PRL 94, 160502 (2005)], which is a powerful, uniquely quantum method for simplifying the tripartite picture of privacy involving local operations and public classical communication to a bipartite picture of quantum privacy involving local operations and classical communication. This approach has previously led to some of the strongest upper bounds on secret key rates, including the squashed entanglement and the relative entropy of entanglement.

Here we use this approach along with a “privacy test” to establish a general meta-converse bound for private communication, which has a number of applications. The meta-converse allows for proving that any quantum channel’s relative entropy of entanglement is a strong converse rate for private communication. For covariant channels, the meta-converse also leads to second-order expansions of relative entropy of entanglement bounds for private communication rates. For such channels, the bounds also apply to the private communication setting in which the sender and receiver are assisted by unlimited public classical communication, and as such, they are relevant for establishing various converse bounds for quantum key distribution protocols conducted over these channels. We find precise characterizations for several channels of interest and apply the methods to establish several converse bounds on the private transmission capabilities of all phase-insensitive bosonic channels.

This is joint work with Mario Berta and Marco Tomamichel.

Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement, and generation of completely secure keys. For all these tasks there is a crucial question to answer: What are their optimal rates without quantum repeaters?

Our work solves this basic question for any two parties connected by a quantum channel, without any restriction on their classical communication, which can be unlimited and two-way. We design a method which reduces the most general protocol of quantum communication to the computation of a novel quantity, identified as the channel’s relative entropy of entanglement. In this way, we bound the ultimate rates that are achievable over the most important bosonic and qubit channels, computing a number of exact formulas for their two-way capacities.

In particular, we determine the fundamental rate-loss scaling which affects any quantum optical communication. By setting these limits, we establish the most general and correct benchmarks for testing the performance of quantum repeaters.

Photonic integrated circuits (PICs) have become increasingly important in classical communications applications over the past decades, including as transmitters and receivers in long-haul, metro and datacenter interconnect. Many of the same attributes that make PICs attractive for these applications— stability, compactness, high bandwidth, and integration with electronics—also make them appealing for quantum communications.

The first part of this talk will review our recent progress in adapting one of the leading PIC architectures—silicon photonics—for various quantum secure communications protocols. The second part of the talk will consider how photonic integrated circuits technology can extend the reach of quantum communications through all-optical and memory-based quantum repeater protocols.

Experimental implementations of quantum computers are in their infancy, but already we are faced with the following conundrum: If quantum computers are exponentially more powerful than their classical counterparts, how can we verify the outcome of a quantum computation? In this context, the scientific method of “predict and verify” appears to fail dramatically: these computations are so complex that they are impossible to predict. For a solution to this problem, we turn to theoretical computer science, where it is well established that interaction dramatically increases the power of a verification process.

We thus give a new interactive protocol for the verification of quantum computations in the regime of high computational complexity. Our results are given in the language of quantum interactive proof systems. Specifically, we show that any language in BQP has a quantum interactive proof system with a polynomial- time classical verifier (who can also prepare random single-qubit pure states), and a quantum polynomial- time prover. Here, soundness is unconditional—i.e. it holds even for computationally unbounded provers. Compared to prior work, our technique does not require the encoding of the input or of the computation; instead, we rely on encryption of the input (together with a method to perform computations on encrypted data), and show that the random choice between three types of input (defining a computational run, versus two types of test runs) suffice. Because the overhead is linear, this shows that verification could be achieved at minimal cost. We also present a new soundness analysis, based on a reduction to an entanglement-based protocol.

Construction and operation of a distributed quantum network can enable exciting applications, such as scalable quantum computer and long-distance secure quantum communication systems, but remains a major experimental challenge. Practical realization of a quantum network can be accomplished by the integration of high quality quantum memories and quantum information processing elements with photonic communication channels. Other than identifying physical quantum systems to implement the qubits and logic operations, new protocols for maintaining quantum coherence through a range of quantum operations, classical controllers that keep track of quantum coherence, and new enabling technologies to bridge the hybrid quantum systems are required.

I will describe main challenges and ideas for research directions in enabling practical quantum networks, and discuss examples of relevant research efforts in trapped ion and single photon experiments. Specifically, experimental progress in trapped ion systems in microfabricated traps will be presented.

Quantum key distribution (QKD) between moving users is an important step toward realizing a secure network applicable to special use-cases in the field as well as for reaching global distances via quantum satellites. While free-space systems are conceptually similar to fiber-optic systems, the intrinsically variable free-space quantum channel poses unique challenges for the quantum link, including the alignment of reference frames between Alice and Bob, optics and telescopes with active pointing and tracking, blocking background signals, and coping with atmospheric turbulence which makes the beams multi-modal.

Typically, free-space systems have been based on polarization encoding, and I will present our experimental results for transmitting quantum signals from a stationary transmitter to a moving quantum receiver located on a moving truck. I will also present our prototype satellite payload, which has the form- fit-function of the final system, and is currently undergoing outdoor trials.

As a viable alternative to polarization based systems, I will present our recent achievements on implementing time-bin encoded photon analyzers, which demonstrate high interference visibility between time bins despite the highly multi-modicity in spatial and temporal degrees of freedom of the received photons. In conclusion, I will illustrate that time-bin encoding of photons is now applicable to highly multimodal beams, and could lead to interesting advances and applications for quantum communications not possible with polarization encoding.

Quantum hacking threatens the security of practical quantum key distribution (QKD) systems by exploiting their real-life imperfections.

Here, I survey recent practical methods to foil quantum hackers together with their strengths and weaknesses. Device-independent QKD, which is now close to experimental realization, promises ultimate security, but with current technology, it will give a low-key rate at metropolitan distances and can be vulnerable to memory attacks. Measurement-device-independent (MDI)-QKD is automatically immune to all attacks on detectors—the most vulnerable part of a QKD system—and has a high key rate. MDI-QKD has been demonstrated over long distances (e.g. 404 km. of low loss optical fiber and 311 km. of standard optical fiber), as well as in a network setting. With MDI-QKD, the source of a QKD system may become Eve’s main interest. Fortunately, encoding flaws can be taken care of by the loss-tolerant protocol. Recently, a loss-tolerant MDI-QKD experiment has been performed, thus addressing both encoding flaws and detector flaws.

Nevertheless, I will argue that significant challenges remain in the security research of practical QKD systems. Those challenges include, for example, quantum random number generation, phase randomization, side channels and covert/subliminal channels. Recent progress on addressing those challenges will also be presented.

In recent years, the computer-aided verification of cryptographic schemes has seen great progress. For example, various state-of-the-art cryptographic schemes were analyzed using the EasyCrypt tool using a probabilistic relational Hoare logic (pRHL). However, existing tools and logics are unsuited for analysis of quantum cryptographic protocols—be it protocols using quantum mechanics or protocols secure against quantum adversaries

In this talk, we explain why pRHL is not sound for quantum cryptography, and show how to lift the ideas from pRHL to the quantum setting.