The laws of quantum mechanics pose stringent constraints on the amplification of a quantum signal. Deterministic amplification of an unknown quantum state always implies the addition of a minimal amount of noise. In principle, linear and noiseless amplification is allowed provided it works only probabilistically [1,2].
The state comparison amplifier  is an approximate probabilistic amplifier that amplifies a coherent state chosen at random from a set of coherent states with known mean photon number. The amplification process works as follows: Alice picks uniformly at random an input state and passes it to Bob. He desires to amplify the state so he mixes it with a guess coherent state at a beam splitter in an attempt to achieve destructive interference in one of the output arms. This output is fed into an APD detector.
The lack of trigger at the detector is an imperfect indication that Bob’s guess is right and that the output contains the correct amplified state. On the other hand, if the first detector fires Bob knows that his guess was wrong but he can still correct the output by changing the input state for a second amplification stage via a feed-forward loop.
In summary, Bob declares success when both the detectors do not fire or when the first detector does fire and state correction is performed. We generalize this mechanism for an arbitrary number of input states and beam splitters, using an on-line learning strategy based on maximum a posteriori probability.
The success probability-fidelity product  of the SCAMP is the joint probability of success and of passing a measurement test on the output comparing it to right amplified state.
Our figures of merit compare favorably with other schemes. The success probability-fidelity product of the SCAMP is always bigger than that of a USD based amplifier  that, when inconclusive, delivers a conveniently chosen random output.
The SCAMP can be realized with classical resources (i.e., lasers, linear optics and APD detectors), the ability to switch between input states on the fly requires delay lines and fast switching but it can still be achieved with classical resources and the loss introduced by the delay can be offset at the second stage. Similar systems, with no state correction, proved to achieve high-gain, high fidelity and high repetition rates, e.g. [4, 5].
Due to its simplicity, the system we propose might represent an ideal candidate either as a recovery station to counteract quantum signal degradation due to propagation in a lossy fibre or across the turbulent atmosphere or as a quantum receiver to improve the key-rate of continuous-variable quantum key distribution with discrete modulation. The system is also suitable for on-chip implementation.
 T.C. Ralph & A.P. Lund, Proceedings of the 9th QCMC Conference 2009.
 S. Pandey, et al., Phys. Rev. A 88, 033852 (2013).
 E. Eleftheriadou et al., Phys. Rev. Lett. 111, 213601 (2013).
 R. Donaldson et al., Phys. Rev. Lett. 114, 120505 (2015).
 R. Donaldson et al., in preparation.
There is ongoing research into information-theoretically secure digital signature schemes. Mathematically based approaches typically require additional resources such as anonymous broadcast and/or a trusted authority to achieve information-theoretical security. The principles of quantum mechanics can be applied to the problem to create the approach known as quantum digital signatures, which does not have these limitations. This presentation will provide an overview of the development of experimental quantum digital signatures. The evolution of experimental test-beds will be charted from small scale demonstrators to long distance implementations with commercial prototypes, along with overviews of the theoretical background of each stage.
As light propagates through a transmission media, such as an optical fiber, it experiences a length-dependent loss which can reduce the communication efficiency as the transmission distance increases. In conventional telecommunications, optical signals can be transmitted over inter-continental distances, due to deterministic all-optical amplifiers. However, quantum communications are still limited to transmission distances of typically a few 100’s km since deterministic amplifiers cannot be used to amplify quantum signals. The use of deterministic amplification on a quantum signal will introduce noise that will mask the original quantum properties of the signal, introducing uncertainty or errors to any measurement. Nondeterministic methods for amplifying quantum signals via post-selection can be used instead, providing a solution to create a low noise quantum amplifier. Several methods for nondeterministic amplification have already been experimentally demonstrated. However, these devices rely on “quantum resources” which makes implementation challenging. Here we present an overview of experimental demonstrations for amplifying coherent states using a method called state comparison amplification. This is a nondeterministic protocol that performs amplification of known sets of phase-encoded coherent states using two modular stages. The outcome of each stage is recorded using single-photon detectors and time-stamped electronics to enable post-selection. State comparison amplification is a relatively simple technique, only requiring “off-the-shelf” components. The presentation will show several demonstrations of state comparison amplification including an amplifier which has high gain, fidelity, and success rate with the added advantage of being robust to channel noise and easily reconfigurable. Finally, we will discuss the effect of introducing a feedforward mechanism allowing for unsuccessful state amplifications.
Quantum mechanics imposes stringent constraints on the amplification of a quantum signal. Deterministic amplification of an unknown quantum state always implies the addition of a minimal amount of noise. Linear and noiseless amplification is allowed in principle provided that it only works probabilistically. Here we present a probabilistic amplifier that combines two quantum state comparison amplifiers (SCAMP) together with a feed-forward state correction strategy. Our system outperforms the unambiguous state discrimination (USD) measure-and-resend based amplifier in terms of the success probability-fidelity product and requires a more complex experimental setting.
Classical digital signatures are commonly used in e-mail, electronic financial transactions and other forms of electronic
communications to ensure that messages have not been tampered with in transit, and that messages are transferrable. The
security of commonly used classical digital signature schemes relies on the computational difficulty of inverting certain
mathematical functions. However, at present, there are no such one-way functions which have been proven to be hard to
invert. With enough computational resources certain implementations of classical public key cryptosystems can be, and
have been, broken with current technology. It is nevertheless possible to construct information-theoretically secure
signature schemes, including quantum digital signature schemes. Quantum signature schemes can be made information theoretically
secure based on the laws of quantum mechanics, while classical comparable protocols require additional
resources such as secret communication and a trusted authority.
Early demonstrations of quantum digital signatures required quantum memory, rendering them impractical at present.
Our present implementation is based on a protocol that does not require quantum memory. It also uses the new technique
of unambiguous quantum state elimination, Here we report experimental results for a test-bed system, recorded with a
variety of different operating parameters, along with a discussion of aspects of the system security.
As society becomes more reliant on electronic communication and transactions, ensuring the security of these interactions becomes more important. Digital signatures are a widely used form of cryptography which allows parties to certify the origins of their communications, meaning that one party, a sender, can send information to other parties in such a way that messages cannot be forged. In addition, messages are transferrable, meaning that a recipient who accepts a message as genuine can be sure that if it is forwarded to another recipient, it will again be accepted as genuine. The classical digital signature schemes currently employed typically rely on computational complexity for security. Quantum digital signatures offer the potential for increased security. In our system, quantum signature states are passed through a network of polarization maintaining fiber interferometers (a multiport) to ensure that recipients will not disagree on the validity of a message. These signatures are encoded in the phase of photonic coherent states and the choice of photon number, signature length and number of possible phase states affects the level of security possible by this approach. We will give a brief introduction into quantum digital signatures and present results from our experimental demonstration system.