Editors' Picks Electrical Engineering Issue I Physics Volume XVII

Encrypt the Future – Quantum Cryptography

About the Author: Xiaoyu Huang

Xiaoyu Huang is a junior majoring in electrical engineering and minoring in cinematic arts at USC. He enjoys discovering fun gadgets and new technologies that involve electrical engineering.

The inherent weakness of traditional cryptography has exposed its unreliability to modern computing technology. To overcome this issue, scientists used the laws of quantum mechanics to create quantum cryptography, which is invincible to conventional hacking. By developing the quantum cryptography system, conventional hacking will eventually be eliminated.


The internet, as one of the greatest inventions in the world, has served many functions in our society. Without the internet, people will not be able to communicate freely with each other, and more importantly, the internet connects everything together to create the ultimate digital world – a place where everything is expressed as zeros and ones. The internet is composed of various data such as personal information, online banking data, private records, and even confidential government data. It is essential that the sensitive information can be protected so that unauthorized people cannot access the data. Fortunately, we have the solution of modern cryptography, which is the digital safe protecting massive amount of data that is being accessed by millions of people daily. However, as scientists discover more and more about advanced computing technology, the inherent weakness of traditional cryptography exposes its unreliability to modern computing technology. Nowadays, we are at the calm before the storm of confidentiality crisis because evidence has shown that the crisis in confidentiality is approaching us.

Confidentiality: Crisis of Traditional Encryption

Because of the unreliability of traditional cryptography, scientists are searching for a way to replace the traditional cryptography, a solution that is invincible to be hacked. In the traditional encryption, information can be encrypted and decrypted using a unique secret key – a mathematical algorithm that could overwhelm the current computing ability. However, emerging technologies have spawned new computers that possess more powerful computing ability, such as quantum computers. In January 2014, The Washington Post published an article revealing that the National Security Agency (NSA) is racing to build a quantum computer that could break nearly every kind of encryption used to protect banking, business, and government records around the world [1]. Moreover, according to Yahoo! NEWS, China is also working on building a quantum computer to rival the one being constructed by the NSA [2]. Once the quantum computer is built, it will be capable of performing the most complex calculations in a relatively short amount of time, which will also put 99.9% of private information and financial transactions at risk of being hacked [2]. In order to prevent valuable information from being hacked by quantum computers, scientists need to find a new encryption method that does not depend on mathematical algorithms. After years of research, scientists have begun to consider quantum cryptography as a replacement for traditional cryptography because of its unique advantages in quantum mechanics.
Enter the Quantum World
What is a small object? Different people may give different answers based on their personal experiences – a pen is small, a needle is smaller, and the dust is even smaller! However, when zooming into the quantum world, scientists find a place on a much smaller scale than anything that can be seen by human eyes – a place that is dominated by atoms. Comparing to normal world, atoms are unimaginably small, and the normal physical rules in the microscope world will behave in a totally different way in the quantum level [3]. Considering the example of classic projectile motion, people know that particles move in a trajectory that obeys Newton’s laws of motion. In order to observe the motion of a particle, we see the reflection of the light to locate the position of the particle and then draw a relationship between time and position. However, it is very hard to observe particles in the quantum world since electrons possess both particle and wave properties, and the collision of an electron with a photon will actually change an electron’s motion; in other words, we cannot simultaneously know the position and velocity of an electron, which means we cannot obtain the exact trajectory of an electron [3]. Therefore, in quantum mechanics, traditional trajectories are replaced with probability distribution maps, as shown in Fig. 1 [3].

Tro, Nivaldo J/Pearson Education
Figure 1: Trajectory vs. Probability, the dots indicate the possible position of the electron.

It is a fact that an atom is composed by the neutrons and protons within a nucleus and electrons that orbit around that nucleus. Since atoms are always in motion, scientists use quantum numbers to specify the orientation of the orbital – the spinning motion of the particle [3]. Although quantum numbers could be found arbitrarily, the relationship between each quantum number will always follow the rules of quantum mechanics. After years of studying on quantum mechanics, scientists were able to use the uncertainty of quantum particles to make quantum cryptography – an encryption method that is free of mathematical algorithms; that is to say, quantum cryptography is invincible to hacking, not even quantum computers could decrypt quantum cryptography.

Quantum Cryptography

In order to compare quantum cryptography to traditional cryptography, a basic knowledge of how quantum cryptography actually works is imperative. Quantum cryptography is based on the spinning orientation, or polarization, of photons, which represents the digital zeros and ones in binary. In general, the horizontally-polariz​ed photons (↔) represent “0” and the vertically-polarized​ photons (↨) represent “1”; in addition, photons can also be twisted 45 degrees clockwise to create a diagonal polarization mode, which uses right-tilt direction (/) to represent “1” and left-tilt direction (\) to represent “0” [4,5] [Fig. 2].

Torah Kachur/Science In Seconds
Figure 2: Polarization of Light Waves Source: Science in Seconds.

Here is an example of how quantum cryptography works: the sender, Alice, and the receiver, Bob, will establish a secured connection using quantum cryptography. First, Alice will transfer data by sending a series of optical pulses through a fiber wire with each pulse containing a single photon with a specific polarization mode; however, the polarization state of each photon is chosen randomly by Alice, which means no mathematical algorithms are involved in this process. Additionally, Alice will also randomly select half of the sending photons and twist them 45 degrees clockwise before sending them to Bob. Bob then will randomly twist half of the photons he receives 45 degrees clockwise before evaluating their polarization states. Then, Bob and Alice will tell each other which photons they have twisted and compare their photons’ polarization results [4]. Finally, Bob and Alice will discard those results that do not match and keep the polarization represented bits that match exactly the same. Thus, Alice and Bob have created a unique secret key that cannot be hacked since the secret key does not depend on logical algorithms. However, what if there is another person, Eve, who tries to spy on the data they are transmitting? In order for Eve to hack their information, she has to measure the polarization of the photons sent by Alice, but based on the laws of quantum mechanics, attempting to measure the polarization state of a photon will alter its polarization state [5]. As a result, Alice and Bob will notice that the photons received have different polarization states than the photons has been sent, which exposes the fact that Eve has been spying on this connection [4]. Therefore, by using quantum cryptography, people can establish a secured connection that is immune to being hacked.

The Limitations
Although quantum cryptography is ideally invincible, there are still some limitations in building the quantum cryptography system using existing technology. In reality, there is no device can reliably produce only one photon per pulse; instead, some of the pulses will contain two photons while others contain no photons. If two photons were produced in a single pulse, the hacker would simply measure only one of those two photons while leaving the other undisturbed; as a result, the sender and receiver would not notice the presence of the hacker [4]. Fortunately, experiments have demonstrated that a low frequency pulse, such as 10 Hz, will reduce the probability of getting two photons in a single pulse, which eliminate the chance for a hacker to spy on the connection [4]. However, in communication, people expect higher frequency of pulses to transfer large amount of data in a short time; in addition, a reasonable price for quantum cryptography systems is critical for it to be successful in the market. Therefore, scientists must continue perfecting quantum cryptography both technically and financially.


The motivation of building quantum computers was to handle complex calculation processes such as simulating and analyzing molecular activities, mapping out DNA-sequencing data, and discovering distant planets. However, as scientists discover more and more about the potential of quantum computing, this powerful computing ability triggers the crisis in traditional cryptography. Daniel Lidar, a professor from the University of Southern California said that “the irony of quantum computing is that if you can imagine someone building a quantum computer that can break encryption a few decades into the future, then you need to be worried right now [1].” Indeed, it will be too late to starting concerning about confidentiality when our data is no long under the protection of encryption. The potential threat of unsecured data makes both governments and civilians need a better and stronger cryptography – just like quantum cryptography to protect valuable information. Although, in theory, quantum cryptography provides the invincible encryption, the unique and weird properties of quantum physics remain the challenge that needs to be understood and overcome first. But is quantum cryptography still very far away from us? The answer might be “No.” In such a technology-dominated​ era, it is reasonable to believe that the days of quantum cryptography will come sooner rather than later.


    • [1] Rich, Steven, and Barton Gellman. “NSA Seeks to Build Quantum Computer That Could Crack Most Types of Encryption.” WashingtonPost.com. The Washington Post, 03 Jan. 2014. Web. 23 Feb. 2014.
    • [2] Hall, Chris. “China Takes on U.S. in Quest to Be First to Create a Quantum Computer.”Yahoo News UK. Yahoo News, 10 Jan. 2014. Web. 25 Feb. 2014.
    • [3] Tro, Nivaldo J. “The Quantum-Mechanical Model of the Atom.” Chemistry: A Molecular Approach. Second Edition ed. New Jersey: Pearson Education, 2011. 277-309. Print. Mt. San Antonio College Edition.
    • [4] Benjamin, Simon. “Single Photons “on Demand”” Science 290.5500 (2000): 2273-2274.AAAS. Web. 28 Feb. 2014.
    • [5] Dettmer, R., “Light holds the key [quantum cryptography],” IEE Review , vol.51, no.7, pp.32,36, July 2005

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