Category Archives: Story of the month

Story of the month: Chip-based technologies for Quantum Communications

The need for Quantum Communications

The rise of quantum computers will break public key cryptography and consequently render obsolete existing secure communication infrastructures our modern society relies upon. This imminent threat prompts development of new counter technologies, and one of the most promising candidates is quantum communication. Quantum physics provides the ideal background to work with, due to the inherent uncertainty of quantum properties. Such uncertainty is crucial to generate randomness, which is the main ingredient of secure communications.

Quantum Key Distribution

Quantum Key Distribution (QKD) is one of the several ideas which exploit quantum randomness. Generating an encryption key, shared between two parties and unknown to any attackers, is the goal of the different QKD protocols. The security of the generated key is guaranteed by the laws of quantum physics: an eavesdropper can not do better than just guess the encryption key, no matter their computational power.

The BB84 protocol was the very first QKD protocol, introducing for the first time the idea of using the quantum states of photons to distribute a key between two parties. After that, a myriad of new protocols were proposed, each with their own advantages and disadvantages. The underlying concept, however, remains unchanged: one of the two parties, Alice, sends the other, Bob, a random sequence of bits encoded in a quantum property of a train of photons (polarisation, phase, time); an eavesdropper intercepting the signal inevitably disturbs the quantum state and, consequently, introduces noise at the receiving end that will highlight the presence of eavesdropping.

Since that first proposal in 1984, QKD has attracted a great deal of interest in the scientific community. Its experimental implementations have improved substantially: the communication distance has risen from a mere 32cm in free space in the very first QKD experiment, to 421km of optical fibre in a recent demonstration. This impressive distance can even be extended much further with the newly discovered TF-QKD protocol (which will be covered in the next story of the month).

QKD has gained popularity outside academia as well, with companies like QCALL partners Toshiba and ID Quantique developing their own QKD systems.

QKD Systems
QKD systems developed by Toshiba (left) and ID Quantique (right).

Chip-based QKD

Large-scale deployment of QKD systems is yet to become a reality. One of the obstacles is that existing QKD equipment is space and power consuming, and very expensive.

To mitigate these constraints and ease the QKD path to market, it is necessary to miniaturise and mass-produce the QKD devices. This is the main focus of QCALL project #1, aiming at the development of integrated photonic devices for Quantum Communications.

Integrated Photonics

Photonic Integrated Circuits (PICs) are already a widespread technology in classical communications. They have the capability of embedding a plethora of optical components on a very small form factor device.

Chip-Finger
An InP chip with lasers, modulators and output waveguides. Such elements integrated on a chip sit on the top of a fingertip, whereas they would result in a 40 to 50 times bigger setup using ordinary components.

In addition to this, mass production of photonic chips is significantly cheaper than assembling from discreet, bulky components, thanks to generic integration technology and multi-project wafers: like with electronic printed circuit boards, the foundries release a set of basic building blocks, which their clients will assemble to make their own circuits. This way, all components can be grown and processed on a wafer monolithically, i.e. in a single run: this allows for multiple circuits to be implemented at the same time, drastically lowering development and production costs for both the foundry and their clients.

Chip-based quantum communications

Integrated photonics seems the natural choice for nxet’generation Quantum Communications devices since their performance is well established in classical communications.

Among the building blocks needed for a QKD transmitter, there are lasers, waveguide couplers, phase modulators and photodiodes. All these can easily be implemented on a chip, and indeed there are several examples of chip-based devices implementing QKD protocols.

Our goal at Toshiba is to make smaller, cheaper and more efficient QKD devices based on photonic integration.

Chip-based quantum communications at Toshiba

Chip-based Quantum Random Number Generator

In order to correctly implement any QKD protocol, a prerequisite is that the initial bit sequence, sent from Alice to Bob, has to be truly random, otherwise an eavesdropper could exploit correlations among the bits to guess the bit sequence.

Generating true randomness is difficult and even fundamentally impossible by only classical means. The deterministic laws of electrodynamics always deliver a predictable outcome, in principle. However, the outcome becomes different in quantum physics. In our Quantum Random Number Generator (QRNG), we generate pulses from two independent laser and have them interfere on a beam splitter. The spontaneous emission process triggering the lasing action is of quantum nature. This guarantees that the phase of the optical pulses emitted by the lasers is inherently random. Hence, when the two pulses interfere, their interference amplitude is not predictable and we can use it to extract random numbers.

Layout of our Quantum Random Number Generator chip
Layout of our Quantum Random Number Generator chip. DC: Direct Current; RF: Radio Frequency; VOA: Variable Optical Attenuator; MMI: Multi-Mode Interferometer; PD: Photodiode.

The novelty of our QRNG, as shown in this JOSA B paper, is in its plug-and-play format: as soon as it is assembled, it is ready to be used. All the optical components (the two lasers and a photodiode to detect the interfering pulses) are embedded onto a 6x2mm photonic chip, which is then connected to bespoke electronics. This is composed of an analog-to-digital converter reading the analog signal of the photodiode, and an FPGA that will post-process the data by removing any remaining information that an adversary can use. As an additional check, we ran the NIST test suite on subsets of our generated numbers, and observed that they passed all 17 tests. The output string of random numbers can then be used as input for the QKD modules.

A modulator-free quantum key distribution transmitter chip

The random key obtained from the QRNG is fed into a QKD transmitter. The transmitter chip developed by Toshiba removes the need for power-hungry phase modulators by the use of  Master-Slave paired lasers. The information is encoded in the phase difference between pulses from the Slave laser, and can then be decoded by a receiving interferometer.

The working principle of the transmitter is based on combining the well-known phenomena of direct phase modulation and optical injection locking, both techniques already in use in classical optical communications. The idea of combining them for Quantum Communications was first introduced by Toshiba in 2016.

Direct phase modulation, as the name suggests, exploits the fact that the phase of a laser’s output is directly related to its driving signal: modulating such electrical current allows direct control over the phase of light emitted by a laser.

Direct phase modulation
Direct phase modulation

Optical injection locking is a phenomenon where one laser, namely the Master, injects light into a second, Slave, laser. This will cause the light from the Master to trigger emission from the Slave laser: light emitted from the Slave will then be “locked” to the same properties, in particular the same phase, as the injected light.

Optical Injection Locking
Optical Injection Locking

Combining these two techniques, we can then generate pulses from the Slave laser that will feature the phase we want to encode by modulating the Master.

Modulation for the DPS (left) and BB84 (right) protocols
Modulation for the DPS (left) and BB84 (right) protocols

This removes the need for phase modulators, which are extremely power consuming, while still allowing us to encode all the relevant information in our photons.

Implementing this setup into a photonic chip results in a versatile, compact QKD transmitter. Our QKD chip was tested in an experiment that is described in this npj Quantum Information paper.

QKD Chip
Layout of the Toshiba QKD transmitter. DFB: Distributed Feed-Back laser; MMI: Multi-Mode Interferometer; TOPS: Thermo-Optical Phase Shifter; VOA: Variable Optical Attenuator; PD: Photodiode; SSC: Spot-Size Converter;

The simple layout of our device allows us to achieve results in line with state-of-the-art bulk implementations for both the BB84 and the DPS protocol.

Results QKD chip
Performance of our QKD chip for the DPS (left) and BB84 (right) protocols. The blue lines represent QBER, the green lines represent the raw counts from the detectors, the red lines represent the secure key rates. The fibre length assumes an equivalent loss of 0.2 dB/km. The yellow markers indicate points obtained on a real fibre link.

This shows that our devices are suitable for being implemented into QKD systems. The small form factor and the lower cost associated with the generic integration process, combined with the lack of phase modulators, make our QKD chips a candidate for large-scale implementation of Quantum Communication systems.

Story of the month: Solid state crystals for quantum repeaters

Antonio works on Solid state crystals for quantum repeaters, do you like to know more? Please continue to read!

Quantum physics has some peculiar properties which can enhance the security and privacy of long-distance communication, a topic very relevant today and likely to become even more so in the near future. Moreover, quantum computers will need to be connected in a way compatible with their quantum nature. This is why preparing the ground for an implementation of quantum communication over future quantum networks is among the goals of the QCALL project. This task includes working on the physical devices that one day will make a quantum network functional.

Actually, the backbone of a quantum network already exists. Optical fibres are extensively used for high speed internet connection, and this is no coincidence. Light is an excellent carrier of information, given its speed that no other particle can reach, and it can be easily manipulated with modern technology, especially thanks to the laser. In addition, photons are among the most versatile physical systems in which a quantum state can be created, controlled and transferred. It seems only logical to exploit the existing fibre network to transmit also quantum states.

Quantum memories and repeaters

However, the present network is not ready yet for quantum communication. One of the reasons is that the quantum state of a photon is more fragile than the classical analogue of a simple pulse of light. Optical fibres are not perfect, and loss of information becomes a serious problem when speaking of long distances. In a state-of-the-art optical fiber, half of the photons in a laser pulse are lost every 17 km. While for classical information this can be compensated by repeater stations, where light pulses are amplified, this is not possible with quantum states as a consequence of the more fundamental no-cloning theorem.

We therefore need to rethink the concept of quantum repeater and adapt it to the rules of quantum physics. So far, several protocols have been proposed which exploit another feature typical of the quantum world: entanglement. In simple words, two or more particles are said to be entangled when they exist in a global state that cannot be simply described as a product of the states of the individual particles. A consequence of the entanglement is that interacting with one of these particles will also affect the result of any measurement of the properties of the others, which can be used for instance to prove the security of a communication channel. A quantum repeater then needs to be able to distribute entangled states between the network nodes, but also store these states in order to maintain a link with a node active for some time and allow the synchronization of multiple direct links that will finally connect the nodes at the extreme ends of the channel.

This is where quantum memories enter the game. At the core of a quantum repeater, these devices interact with incoming photons, store their state in some internal degrees of freedom, and release it after some time that can be chosen on-demand. All this while preserving the quantum properties of the stored state, including the entanglement with other photons and nodes of the network.

This is how a first quantum network might look. Links on the ground could also be integrated with satellite technology for intercontinental communication.
This is how a first quantum network might look. Links on the ground could also be integrated with satellite technology for intercontinental communication.

The quest for better memories

My task in the QCALL project, and as a PhD student in the group supervised by Mikael Afzelius at the University of Geneva in Switzerland, is to work on the practical side of the implementation of a quantum repeater, in particular by studying some novel candidate materials for quantum memories and to demonstrate storage of entanglement in such memories. Several physical systems have been proposed already for the purpose, most of them relying on groups (or more properly, ensembles) of atoms that can collectively interact with photons and potentially store the information they carry in their internal degrees of freedom. The strength of using a collection of many atoms is that, like a team, they can work together to enhance the probability that incoming photons will interact with any of them, and together keep trace of the information stored until the moment is chosen for it to be released as another photon. Another great advantage of ensemble-based memories is that an ensemble of atoms can store many photons at a time, and this multiplexing ability is key in order to reach reasonable communication rates. Here in Geneva we decided to work with a specific family of atoms, called rare earth elements, embedded in solid state crystals.

Rare earth elements are, despite the name, relatively common in the Earths’ crust and used widely in modern technology. For instance, one can find them in tv screens, smartphones and computer components. But how can they be useful in our quest for a quantum memory?

Examples of rare earth elements, with the most successful in quantum memories research highlighted in blue.
Examples of rare earth elements, with the most successful ones in quantum memories research highlighted in blue.

Advantages and challenges

As mentioned before, a quantum state is a delicate thing, be it a photon or an ensemble of rare earth ions in a crystal. Even if one manages to prepare a quantum state in a particle, in normal conditions it won’t last long. Most probably, our particle will interact with its environment very quickly, modifying the state in unpredictable ways. This is why, in our system, rare earth ions are embedded in specific types of crystals, which contain mostly elements that interact only weakly with the ions themselves or with the environment, on the condition that their temperature is of just few Kelvin above absolute zero, which is easily obtained with commercially available cryostats.

But how can we make sure our system is resilient to perturbations? A way of quantifying this is to measure the coherence lifetime: in other words, for how long our group of ions can keep working as a team in preserving the stored information without them “losing coherence” between themselves and with respect to the intial state of the photon absorbed. This is the crucial metric for a system to be able to preserve a quantum state.

When cooled, rare earth ions can easily record coherence lifetimes ranging from hundreds of microseconds to even a few seconds, depending on the specific element. This numbers might sound small, but from the point of view of a photon it’s actually a lot: to give an idea, in 100 microseconds a photon covers about 20 km in an optical fibre! However, a long coherence lifetime is not the only property we need. Other questions we have to answer are: is the colour (or frequency) of the light we want to use for communication compatible with our ions? How much information can we store in a given timeframe (multiplexing and bandwidth)? Can a state of light be efficiently absorbed and re-emitted without being distorted?

Unfortunately, so fare none of the physical systems proposed as a quantum memory can answer affirmatively to all these questions at the same time. In the specific case of rare earths, for instance, erbium is the most compatible with the infrared photons that are currently used in telecommunications (1500 nm of wavelength), but it suffers from high sensitivity to external perturbations. Praseodymium is very efficient in absorbing photons, but mainly for red light (600 nm), and its coherence lifetime is good but far from the best. Europium is very resistant to perturbations, but it is also much less efficient than praseodymium, and again at the wrong colour (yellow, 580 nm).

All these issues are being tackled with constant progress by various groups of researchers around the globe. For example, frequency conversion can solve the “wrong colour” problem at a price in efficiency; optical cavities can help the efficiency, but also reduce the bandwidth; and other techniques involving active control of the ions’ environment can reduce their sensitivity to perturbations and increase the effective coherence time, while adding more complexity to the storage protocol and demanding more resources. In any case, the exploration of new materials that can help improve all these aspects is still ongoing.

Our new entry: ytterbium

This is what the first part of my PhD was about. We studied a relatively unexplored rare earth element, ytterbium, in a host crystal called yttrium orthosilicate (YSO in short). It absorbs in the near infrared (980 nm), but its main strength, to begin with, is the bandwidth. Its electrons and nucleus can be addressed with photons at several frequencies, a bit like a cabinet with multiple shelves, which are well separated by at least half a GHz. This ultimately sets the bandwidth limitation of the memory! This is an advantage with respect to many of the most successful rare earths so far, for which this range is limited to tens of MHz of bandwidth only! (as in europium and praseodymium). In principle, increasing the bandwidth of the memory will lead to a larger operating speed of the repeater, and thus a faster connection rate in a quantum network.

The big unknown was the coherence lifetime. But we unexpectedly discovered that ytterbium can reach more than 1 ms if we choose to store a light pulse in specific internal states, created by an interplay of the intrinsic property of electrons and nuclei, called spin. At first this was very surprising, since usually materials with a non-zero electronic spin are extremely sensitive to perturbations due to magnetic field fluctuations, normally present in the environment and in the host crystal. In other materials with similar sensitivity to magnetic fields, this problem is typically suppressed by techniques involving huge external magnetic fields (around some Teslas), which are impossible to achieve without additional expensive equipment such as superconducting magnets. However, the internal states of ytterbium in our crystal were found to have a nice “stability valley” for magnetic field values close to zero. Its sensitivity to fluctuations is dependent on the value of the magnetic fields, and it happens to be null when no field is present at all!

The spin property of ytterbium’s electrons offers another useful feature. Despite the insensitivity to the slow field fluctuations due to the environment, the quantum information stored within the electrons can efficiently be manipulated using microwave pulses. This opens the possibility that a future quantum memory based on ytterbium could interact directly with superconducting qubits, which are at the core of the most commercially exploited quantum computing platforms. These qubits are typically controlled with signals in a similar range of frequencies and cannot be exposed to the high magnetic fields required by other rare earths.

Our results were published in two papers, one highlighting the applicative potential of ytterbium in Nature Materials and one reporting the underlying spectroscopic properties of the material in Physical Review. We also had the chance to present the work at several conferences, and to a broader public thanks to scientific news outlets around the web (for instance, see Science Daily).

Our workhorse: europium

While the ytterbium project progressed towards the implementation of the first storage experiments, I switched to another rare earth element being studied for a longer time in our labs: europium. Its main strength is the long coherence lifetime, which made it one of the main contenders for implementing a quantum repeater, together with praseodymium. In europium, a group in Australia obtained the record coherence time (original paper here) extrapolated from measured data: 6 hours, achieved with a sophisticated method of suppression of magnetic noise at an applied 7 Teslas of magnetic field. We do not need to reach those values for practical repeaters, but it shows the potential of europium.

A europium-doped YSO crystal, of about 1 cm in length, as visibile from a window of our cryostat, while our yellow laser shines through it. The coil enveloping the crystal is used to address the spin states of europium via an oscillating magnetic field.
A europium-doped YSO crystal, of about 1 cm in length, as visibile from a window of our cryostat, while our yellow laser shines through it. The coil enveloping the crystal is used to address the spin states of europium via an oscillating magnetic field.

In our group, europium in a YSO host was shown (original paper here) to be able to create pairs of single photons correlated in time, with a time delay between them. More precisely, after being excited by a weak laser pulse, an ion of europium can change its spin state by emitting a single photon. This photon heralds a successful excitation of a single spin of the ensemble. After some time, which amounted to 1 ms in our last implementation, this excitation is released as a second photon. Using certain test, one can prove that the second photon really originated from the same excitation that produced the first one. The experiment outlined forms the basis of the DLCZ quantum repater (named after its inventors, Duan, Lukin, Cirac and Zoller), which we adapted to our crystals, and that can be used to entangle two distant nodes in a quantum network. Based on these results, we have implemented several improvements of the setup used to control our quantum memory, focused on the reduction of the noise that limited the previous experiment.

Right now, we find ourselves at the point where all these improvements are being combined and tested, in order to show that pairs of entangled photons can be produced in europium, and afterwards converted into telecom frequencies. In the meantime, ytterbium is still progressing and being used in our first storage protocols. As mentioned before, all materials have their own strengths and weaknesses, making them suitable for different applications depending on which features we want our repeater to have. In any case, we will continue working towards the goal of preparing a functional quantum repeater unit and implementing it in a fibre network.